The impact of both multiplicative and additive hardware impairments (HWIs) on multiple-input multiple-output affine frequency division multiplexing (MIMO-AFDM) systems is investigated. For small-scale MIMO-AFDM systems, a tight bit error rate (BER) upper bound associated with the maximum likelihood (ML) detector is derived. By contrast, for large-scale systems, a closed-form BER approximation associated with the linear minimum mean squared error (LMMSE) detector is presented, including realistic imperfect channel estimation scenarios. Our first key observation is that the full diversity order of a hardware-impaired AFDM system remains unaffected, which is a unique advantage. Furthermore, our analysis shows that 1) the BER results derived accurately predict the simulated ML performance in moderate-to-high signal-to-noise ratios (SNRs), while the theoretical BER curve of the LMMSE detector closely matches that of the Monte-Carlo based one. 2) MIMO-AFDM is more resilient to multiplicative distortions, such as phase noise and carrier frequency offset, compared to its orthogonal frequency division multiplexing (OFDM) counterparts. This is attributed to its inherent chirp signal characteristics; 3) MIMO-AFDM consistently achieves superior BER performance compared to conventional MIMO-OFDM systems under the same additive HWI conditions, as well as different velocity values. The latter is because MIMO-AFDM is also resilient to the additional inter-carrier interference (ICI) imposed by the nonlinear distortions of additive HWIs. In a nutshell, compared to OFDM, AFDM demonstrates stronger ICI resilience and achieves the maximum full diversity attainable gain even under HWIs, thanks to its intrinsic chirp signalling structure as well as to the beneficial spreading effect of the discrete affine Fourier transform.

Generalized spatial modulation-aided affine frequency division multiplexing (GSM-AFDM) is conceived for reliable multiple-input multiple-output (MIMO) communications over doubly selective channels. We commence by proposing several low-complexity detectors for large-scale GSM-AFDM systems to meet the diverse requirements of heterogeneous receiver designs in terms of detection complexity and reliability. Specifically, we introduce the linear minimum mean square error (LMMSE) equalizer-based maximum likelihood detector (LMMSE-MLD). By exploiting the GSM properties, we then derive the LMMSE-based transmit-antenna activation pattern (TAP) check-based log-likelihood ratio detector (LMMSE-TC-LLRD). In addition, we propose a pair of new detectors, namely the greedy residual check detector (GRCD) and the reduced space check detector (RSCD). We also derive a bit error rate (BER) upper-bound by considering the MLD. Our analytical results are also available for multiple-input multiple-output (MIMO)-AFDM, since MIMO-AFDM can be regarded as a special case of the proposed GSM-AFDM. Our simulation results demonstrate that 1) the BER upper bound derived is tight for moderate to high signal-to-noise ratios (SNRs), 2) the proposed GSM-AFDM achieves lower BER than its conventional orthogonal frequency division multiplexing (OFDM), orthogonal time frequency space (OTFS) and AFDM counterparts. Specifically, at a BER of 10−4and a velocity of 540 km/h, the proposed GSM-AFDM is capable of attaining about 6 dB SNR gain compared to GSM-OFDM, and 3) the conceived detectors strike a compelling trade-off between the BER and complexity.

In this paper, a novel network architecture that deploys the multi-functional reconfigurable intelligent surface (MFRIS) in low-Earth orbit (LEO) is proposed. Unlike traditional RIS with only signal reflection capability, the MF-RIS can reflect, refract, and amplify signals, as well as harvest energy from wireless signals. Given the high energy demands in shadow regions where solar energy is unavailable, MF-RIS is deployed on LEO to enhance signal coverage and improve energy efficiency (EE). To address this, we formulate a long-term EE optimization problem by determining the optimal parameters of MF-RIS, including amplification, phase-shifts, energy harvesting ratios, and LEO transmit beamforming. To address the complex non-convex and non-linear problem, a federated learning enhanced multi-agent deep deterministic policy gradient (FEMAD) scheme is designed. Multi-agent deep deterministic policy gradient (DDPG) of each agent can provide the optimal action policy from its interaction with environments, whereas federated learning enables the hidden information exchange among multi-agents. In numerical results, we can observe significant EE improvements compared to the other benchmarks, including centralized deep reinforcement learning as well as distributed multi-agent DDPG. Additionally, the proposed LEO-MF-RIS architecture has demonstrated its effectiveness, achieving the highest EE performance compared to the scenarios of fixed/no energy harvesting in MF-RIS, traditional reflection-only RIS, and deployment without RISs/MF-RISs.

Visible Light Positioning and Communication (VLPC) is a promising candidate for implementing Integrated Sensing And Communication (ISAC) in the unlicensed 400 THz to 800 THz band. The current Visible Light Positioning (VLP) systems mainly operate based on the Received Signal Strength (RSS) of the Line-of-Sight (LoS) path. However, its accuracy is degraded by interferences from Non-LoS (NLoS) paths. Furthermore, in Visible Light Communication (VLC) systems, the estimation of Channel State Information (CSI) also becomes challenging, when the optical channel becomes dispersive. Against this background, we propose a new VLPC scheme using Direct Current (DC) biased Optical Orthogonal Frequency-Division Multiplexing (VLPC-DCO-OFDM), where OFDM-based sensing is applied for the sake of improving the resolution of the estimated Channel Impulse Response (CIRs) exploited for positioning functionality. The CIRs estimated by sensing are further exploited to provide enhanced CSI for communication data detection. Moreover, we propose a hybrid Radar-RSS based solution, where the conventional RSS-aided VLP method is invoked for the sake of refining OFDM radar. Our simulation results demonstrate that the proposed VLPC-DCO-OFDM scheme – which simultaneously supports the triple functionalities of illumination, bi-static sensing and communication – is capable of achieving centimeter-level positioning accuracy and Giga-bits-per-second data rate.

Low Earth orbit (LEO) has a significant potential to provide ubiquitous global coverage with high capacity data services in sixth generation (6G) wireless networks. Due to the denser deployment of LEO satellites, it becomes mandatory to mitigate the interference induced by LEO beams. The high mobility of LEOs further stirs up a complex interference scenario different from conventional terrestrial networks. Therefore, we conceive a multi-LEO constellation that incorporates multi-beamforming and handover by using only the information of signal-to-interference-plus-noise ratio (SINR) and beam indexes. In this paper, we propose a joint LEO handover and fast beam switching (HOBS) algorithm that performs handover, beam search, and beam/power resource allocation. Our goal is to maximize energy efficiency (EE) while satisfying the SINR requirement of each user. We evaluate our proposed HOBS scheme in terms of different user densities, time frame sizes, and beam-sweeping schemes. Benefiting from a comparatively smaller beam search space, HOBS is capable of providing lower latency, as well as higher SINR and EE compared to conventional exhaustive beam search and fixed power control.

Three-dimensional (3D) positioning technology plays an important role in millimeter wave (mmWave) non-terrestrial or integrated sensing and communication (ISAC) networks in sixth-generation (6G) systems. However, the complexity of joint range and orientation estimation in mmWave hybrid beamforming (HBF) systems forms a technical hurdle to its practical realization. In view of the recent advancement in green learning (GL) technology, a low-complexity GL architecture is developed herein for 3D positioning in mmWave HBF systems. The entire architecture only consists of one layer of unsupervised representation learning, followed by a supervised feature learning stage and a regression layer for parameter estimation. Compared to the typical deep learning method, the complexity of the proposed method is at least 3 order lower, while the performance is comparable to that of the maximum likelihood estimations of individual parameters given perfect knowledge of the other parameters. This presents the potential of GL in ISAC for future$6G$systems.

We demonstrate that the proposed optical orthogonal time frequency space (O-OTFS) is capable of improving the bandwidth-/power-/energy-efficiencies of optical orthogonal frequency-division multiplexing (O-OFDM). The bandwidth-efficiency is improved because only a single cyclic prefix (CP) is needed for an entire O-OTFS frame. The power-efficiency is enhanced thanks to the diversity gain achieved by its symplectic finite Fourier transform (SFFT), which also leads to a reduced peak-to-average power ratio (PAPR), hence improving its energy-efficiency. These features are facilitated by the proposed layered asymmetrically clipped O-OTFS (LACO-OTFS), which is capable of removing the direct current (DC) bias while retaining the full optical throughput. Nonetheless, there exists an inherent trade-off, where increasing the O-OTFS frame size leads to a commensurately reduced CP percentage at the cost of an increased PAPR. In order to mitigate this, we propose to perform discrete Fourier transform based spreading (DFT-S) in the delay-Doppler (DD)-domain. Furthermore, we demonstrate that regardless of the choice of domain in which the information is modulated (i.e. O-OFDM/O-OTFS with/without DFT-S), the frequency-selectivity of the quasi-static but dispersive optical channel can always be equalized by single-tap frequency-domain equalization (FDE). Moreover, the channel estimation techniques are conceived to operate in the time-/frequency-/DD-domains for both O-OFDM and O-OTFS. Our simulation results demonstrate that for a multi-user optical wireless system associated withM= 64 subcarriers and the OTFS frame length ofN= 64, LACO-OTFS is capable of achieving a 7 dB power-efficiency gain over LACO-OFDM, where the CP overhead is reduced by a factor ofN= 64. DFT-S-LACO-OTFS is also capable of providing a 7 dB power-efficiency gain over DFT-S-LACO-OFDM, where the low PAPR of single-carrier transmission is retained.

To liberate indoor industrial cell-free massive multiple-input multiple-output (CF-mMIMO) networks from wired fronthaul, this paper proposes a multicarrier-division duplex (MDD)-enabled two-tier terahertz (THz) fronthaul scheme. In our scheme, two layers of fronthaul links rely on the mutually orthogonal subcarrier sets in the same THz band, while access links are implemented over sub-6G band. However, the proposed scheme leads to a complicated mixed-integer nonconvex optimization problem incorporating access point (AP) clustering, device selection, the assignment of subcarrier sets and the resource allocation at both the central processing unit (CPU) and APs. Hence, in order to address the formulated problem, we first resort to the low-complexity but efficient heuristic methods thereby relaxing the involved binary variables. Then, the overall end-to-end optimization is implemented by iteratively optimizing the assignment of subcarrier sets and the number of AP clusters. Furthermore, an advanced MDD frame structure consisting of three parallel data streams is tailored for the proposed scheme. Simulation results demonstrate the effectiveness of the proposed dynamic AP clustering approach in dealing with the networks of varying sizes. Moreover, benefiting from the well-designed frame structure, MDD is capable of outperforming TDD in the two-tier fronthaul networks. Additionally, the effect of the THz bandwidth on system performance is analyzed, and it is shown that empowered by sufficient bandwidth, our proposed two-tier fully-wireless fronthaul scheme can achieve a comparable performance to the fiber-optic based systems. Finally, the superiority of the proposed MDD-enabled fronthaul scheme is verified in a practical scenario with realistic ray-tracing simulations.

In this paper, we consider a prospective receiving hybrid beamforming structure consisting of several radio frequency (RF) chains and abundant antenna elements in multi-input multi-output (MIMO) systems. Due to conventional costly full connections, we design an enhanced partially connected beamformer employing a low-density parity-check (LDPC)-based structure. As a benefit of the LDPC-based structure, information can be exchanged among clustered RF/antenna groups, which results in a low computational complexity order. Advanced message passing (MP) capable of inferring and transferring information among different paths is designed to support the LDPC-based hybrid beamformer. We propose a message-passing enhanced antenna and RF chain selection (MARS) scheme for minimizing the operational power of antennas and RF chains of the receiver as well as hybrid beamforming. Furthermore, sequential and parallel MP schemes for MARS are designed, namely, MARS-S and MARS-P, respectively, to address the convergence speed issue. A heuristic genetic algorithm is designed for receiving hybrid beamforming, comprising gene generation initialization, elite selection, crossover, and mutation. Simulations validate the convergence of both the MARS-P and the MARS-S algorithms. Due to the asynchronous information transfer of MARS-P, it requires higher power than MARS-S, which strikes a compelling balance among power consumption, convergence, and computational complexity. It is also demonstrated that the proposed MARS scheme outperforms the existing benchmarks using the heuristic method of fully/partially connected architectures in the open literature by requiring the lowest power and realizing the highest energy efficiency.

Wireless sensing and wireless energy are enablers to pave the way for smart transportation and a greener future. In this paper, an intelligent reflecting surface (IRS) assisted integrated sensing and wireless power transfer (ISWPT) system is investigated, where the transmitter in transportation infrastructure networks sends signals to sense multiple targets and simultaneously to multiple energy harvesting devices (EHDs) to power them. Recognizing the inherent tradeoff between energy harvesting and sensing performance, we propose to jointly optimize the system performance via optimizing the beamforming and IRS phase shift. However, the coupling of optimization variables makes the formulated problem non-convex. Thus, an alternative optimization approach is introduced and based on which two algorithms are proposed to solve the problem. Specifically, the first algorithm involves the semi-positive definite programming techniques, and the second algorithm is based on the successive convex approximations and majorization minimization to design the closed form solutions of the optimization variables, which can effectively reduce the computational complexity. Our simulation results validate the proposed algorithms and demonstrate the advantages of using IRS to assist wireless power transfer in ISWPT systems. This research contributes to the integration of wireless sensing and wireless energy in intelligent transportation systems and underscores the optimization of system performance through the introduction of IRS.

In order to maximize the spectral efficiency (SE) in multicarrier-division duplex (MDD) enabled cell-free massive MIMO (CF-mMIMO), a heterogeneous graph neural network (HGNN), referred to as CF-HGNN, is specifically introduced to optimize the power allocation (PA). To efficiently manage the interference invoked, a meta-path based mechanism is applied in CF-HGNN to enable individual access point (AP) and mobile station (MS) nodes to aggregate information from the interfering and communication paths with different priorities during message passing. Moreover, the proposed CF-HGNN employs the adaptive node embedding layer and adaptive output layer to make it scalable to the various numbers of APs, MSs and subcarriers. For comparison, a quadratic transform and successive convex approximation (QT-SCA) algorithm is proposed to solve the PA problem in classic way. Numerical results show that CF-HGNN is capable of achieving 99% of the SE achievable by QT-SCA but using only 10−4 times of its operation time, and it can outperform the conventional learning-based and greedy unfair methods in terms of SE performance. Furthermore, CF-HGNN exhibits good scalability to the CF networks with various numbers of nodes and subcarriers, and also to the large-scale CF networks when assisted by user-centric clustering.

In orthogonal time sequency multiplexing (OTSM) modulation, the information symbols are conveyed in the delay-sequency domain upon exploiting the inverse Walsh Hadamard transform (IWHT). It has been shown that OTSM is capable of attaining a bit error ratio (BER) similar to that of orthogonal time-frequency space (OTFS) modulation at a lower complexity, since the saving of multiplication operations in the IWHT. Hence we provide its BER performance analysis and characterize its detection complexity. We commence by deriving its generalized input-output relationship and its unconditional pairwise error probability (UPEP). Then, its BER upper bound is derived in closed form under both ideal and imperfect channel estimation conditions, which is shown to be tight at moderate to high signal-to-noise ratios (SNRs). Moreover, a novel approximate message passing (AMP) aided OTSM detection framework is proposed. Specifically, to circumvent the high residual BER of the conventional AMP detector, we proposed a vector AMP-based expectation-maximization (VAMP-EM) detector for performing joint data detection and noise variance estimation. The variance auto-tuning algorithm based on the EM algorithm is designed for the VAMP-EM detector to further improve the convergence performance. The simulation results illustrate that the VAMP-EM detector is capable of striking an attractive BER vs. complexity trade-off than the state-of-the-art schemes as well as providing a better convergence. Finally, we propose AMP and VAMP-EM turbo receivers for low-density parity-check (LDPC)-coded OTSM systems. It is demonstrated that our proposed VAMP-EM turbo receiver is capable of providing both BER and convergence performance improvements over the conventional AMP solution.

The recently-developed orthogonal time frequency space (OTFS) modulation is capable of transforming the time-varying fading of the time-frequency (TF) domain into the time-invariant fading representations of the delay-Doppler (DD) domain. The OTFS system using orthogonal frequency-division multiplexing (OFDM) as inner core naturally requires the subcarrier spacing (SCS) Δfto be larger than the maximum Doppler frequency ϑmax, i.e. Δf> ϑmax, when perfect channel state information (CSI) knowledge is assumed. However, for the first time in literature, we explicitly demonstrate that the practical OFDM-based OTFS systems have to double their SCS in order to facilitate CSI estimation, requiring Δf′ = 2Δf> 2ϑmax. In order to mitigate this loss, we propose a novel noncoherent OTFS system, which is capable of operating at Δf> ϑmax. The major challenge in this context is the mitigation of the DD-domain interference without CSI. Against this background, we draw an analogy between the input-output model of OTFS and that of V-BLAST, where V-BLAST’s blind inter-antenna interference mitigation technique is invoked. Moreover, we propose to partition the DD-domain modulated symbols into groups, where space-time block coding is invoked in order to eliminate the DD-domain interference within each group. Our simulation results demonstrate that the proposed noncoherent OTFS is capable of substantially outperforming its coherent counterparts relying on CSI estimation.

For millimeter or Tera-Hertz communications, channel capacity is often found limited by its channel diversity due to the sparse or line-of-sight transmission conditions in these frequency bands. This diversity problem exacerbates particularly in future non-terrestrial communications. Wireless communication that exploits the diversity of orbital angular momentum (OAM) in electro-magnetic waves is considered one of the potential technologies to push the limit despite the skepticism for its long-rang transmissions in free space. Inspired by the recent advances on hybrid beamforming, we propose a novel OAM beam focusing method to explore the feasibility of multiple input multiple output (MIMO) OAM wireless transmissions. With the proposed optimal beam focusing and power allocation scheme and the use of a concentric uniform circular array (CUCA) of diameter 1.2 meters (m), we show that signals of up to three OAM modes can be delivered to a receive uniform circular array of diameter 20 cm at a distance of 500 m, which presents the potential of MIMO OAM for future 6th generation communication systems.

Low-Earth orbit (LEO) satellites have been prosperously deployed for various Earth observation missions due to its capability of collecting a large amount of image or sensor data. However, traditionally, the data training process is performed in the terrestrial cloud server, which leads to a high transmission overhead. With the recent development of LEO, it is more imperative to provide ultra-dense LEO constellation with enhanced on-board computation capability. Benefited from it, we have proposed a collaborative federated learning for low Earth orbit (FELLO). We allocate the entire process on LEOs with low payload inter-satellite transmissions, whilst the low-delay terrestrial gateway server (GS) only takes care for initial signal controlling. The GS initially selects an LEO server, whereas its LEO clients are all determined by clustering mechanism and communication capability through the optical inter-satellite links (ISLs). The re-clustering of changing LEO server will be executed once with low communication quality of FELLO. In the simulations, we have numerically analyzed the proposed FELLO under practical Walker-based LEO constellation configurations along with MNIST training dataset for classification mission. The proposed FELLO outperforms the conventional centralized and distributed architectures with higher classification accuracy as well as comparably lower latency of joint communication and computing.

Low Earth orbit (LEO) satellite constellation is capable of providing global coverage area with high-rate services in the next sixth-generation (6G) non-terrestrial network (NTN). Due to limited onboard resources of operating power, beams, and channels, resilient and efficient resource management has become compellingly imperative under complex interference cases. However, different from conventional terrestrial base stations, LEO is deployed at considerable height and under high mobility, inducing substantially long delay and interference during transmission. As a result, acquiring the accurate channel state information between LEOs and ground users is challenging. Therefore, we construct a framework with a two-way transmission under unknown channel information and no data collected at long-delay ground gateway. In this paper, we propose hierarchical multi-agent multi-armed bandit resource allocation for LEO constellation (mmRAL) by appropriately assigning available radio resources. LEOs are considered as collaborative multiple macro-agents attempting unknown trials of various actions of micro-agents of respective resources, asymptotically achieving suitable allocation with only throughput information. In simulations, we evaluate mmRAL in various cases of LEO deployment, serving numbers of users and LEOs, hardware cost and outage probability. Benefited by efficient and resilient allocation, the proposed mmRAL system is capable of operating in homogeneous or heterogeneous orbital planes or constellations, achieving the highest throughput performance compared to the existing benchmarks in open literature.

To accomplish complex tasks, several nano-machines may need to communicate via multiple-access channel with one access point, where information fusion is carried out. However, multiple-access diffusive molecular communications (DMC) systems suffer from severe multiple-access interference (MAI) and intersymbol interference (ISI), which should be effectively mitigated at receiver in order to achieve acceptable performance. Built on two fundamental single-user detection schemes, namely, threshold-assisted majority vote detection (TMVD) and equal gain combination detection (EGCD), we first propose three low-complexity interference cancelation schemes, which are the TMVD-assisted iterative interference cancelation (TMVD-IIC), TMVD-based minimum-distance decoding-assisted interference cancelation (TMVD-MDDIC) and the EGCD-assisted$N$-order iterative interference cancelation (EGCD-NIIC), for operation in the molecular-type spread-assisted molecular shift keying (MTS-MoSK) DMC systems. Then, following the principle of maximum likelihood (ML) detection, we propose a simplified approximate ML (SAML) detection scheme. The error performance of the MTS-MoSK DMC systems employing, respectively, the considered detection schemes is comprehensively investigated and compared. Furthermore, the complexities of the detection schemes are analyzed and discussed in terms of the complexity-performance tradeoff. Our studies and results show that, compared with the single-user TMVD and EGCD schemes, the proposed interference cancelation schemes are capable of mitigating efficiently the effect of MAI and enabling significant performance improvement at the slightly increased complexity.

The recently developed reconfigurable intelligent surfaces (RISs) are capable of improving the coverage of space–air–ground integrated networks (SAGINs), where the signals can be reflected in the desired direction without relying on power-thirsty radio-frequency (RF) chains. However, in the face of the substantially increased Doppler frequency, the classic orthogonal frequency-division multiplexing (OFDM) becomes inadequate in supporting RIS for the following reasons. First, the detrimental doubly selective fading leads to intersymbol interference (ISI) and intercarrier interference (ICI), which result in error floors for OFDM operating in the time–frequency (TF) domain. Second, it is far from trivial to configure RIS based on the time-varying fading channels. Third, the interpolation-based TF-domain channel estimation methods become impractical for the high-Doppler and high-dimensional RIS systems. Against this background, in this article, we propose the powerful 2-D orthogonal time–frequency space (OTFS) modulation for RIS-aided SAGINs, which transforms the time-varying fading encountered in the TF-domain to the time-invariant fading in the delay-Doppler (DD) domain. More explicitly, first, for the first time in the literature, we devise the DD-domain channel model of RIS-assisted SAGINs in the face of doubly selective fading. Second, in order to facilitate the RIS configuration in the DD-domain, we propose to create “virtual” Doppler frequencies that guide the phase changes at the RIS, even though the RIS phase rotations do not suffer from Doppler effects. Third, we conceive an attractive DD-domain RIS channel estimation method that can support both OFDM and OTFS, where the TF-domain interpolation is eliminated. Our simulation results demonstrate that the proposed DD-domain RIS configuration and channel estimation methods for both OFDM and OTFS are capable of mitigating the error floors encountered in the TF-domain. Furthermore, our simulation results confirm that OTFS-based RIS-assisted SAGIN systems are capable of outperforming their OFDM counterparts and exhibit excellent performance across a wide range of SAGIN channel parameters including the Ricean K factor, Doppler frequency, delay spread, coverage distance, and carrier frequency.

Space-time shift keying-aided orthogonal time frequency space modulation-based multiple access (STSK-OTFS-MA) is proposed for reliable uplink transmission in high-Doppler scenarios. As a beneficial feature of our STSK-OTFS-MA system, extra information bits are mapped onto the indices of the active dispersion matrices, which allows the system to enjoy the joint benefits of both STSK and OTFS signalling. Due to the fact that both the time-, space- and DD-domain degrees of freedom are jointly exploited, our STSK-OTFS-MA achieves increased diversity and coding gains. To mitigate the potentially excessive detection complexity, the sparse structure of the equivalent transmitted symbol vector is exploited, resulting in a pair of low-complexity near-maximum likelihood (ML) multiuser detection algorithms. Explicitly, we conceive a progressive residual check-based greedy detector (PRCGD) and an iterative reduced-space check-based detector (IRCD). Then, we derive both the unconditional single-user pairwise error probability (SU-UPEP) and a tight bit error ratio (BER) union-bound for our single-user STSK-OTFS-MA system employing the ML detector. Furthermore, the discrete-input continuous-output memoryless channel (DCMC) capacity of the proposed system is derived. The optimal dispersion matrices (DMs) are designed based on the maximum attainable diversity and coding gain metrics. Finally, it is demonstrated that our STSK-OTFS-MA system achieves both a lower BER and a higher DCMC capacity than its conventional spatial modulation (SM) and its orthogonal frequency-division multiplexing (OFDM) counterparts. As a benefit, the proposed system strikes a compelling BER vs. system complexity as well as BER vs. detection complexity trade-offs.

Molecular communication (MC) is an emerging field aiming at realizing information exchange via chemical signals between nanomachines in nanonetworks. Information transmission that is energy-efficient and relies on relatively low-complexity transceiver techniques is of practical importance for MC systems. Based on the fact that constellation shaping can improve energy efficiency, in this paper, we propose a molecular shell mapping (MSM) scheme to implement the probabilistic constellation shaping for MC. The MSM method is designed to exploit the concentration sequences with the lowest sum sequence weight, which results in an energy-efficient signal constellation. Furthermore, we propose an algorithm for selecting and sorting the concentration sequences to mitigate inter-symbol interference. For information detection, we design a genie-aided maximum-likelihood (ML) detector and a realistic ML detector to leverage the constructive effect of intra-sequence interference, as well as derive their bit error rates and achievable rates. Additionally, for the applications using large blocklength sequences, a low-complexity ML detection method is proposed. Numerical simulation results confirm that the shaped signaling using the MSM method is more energy-efficient than the conventional equiprobable signaling, achieving shaping gains of up to 1.5 dB at an ultra-short blocklength of 4.

Long-term chlorophyll-a (Chl-a) prediction has the potential to provide an early warning of red tide, and support fishery management and marine ecosystem health. The existing learning-based Chl-a prediction methods mostly predict a single point or multiple points with monitoring data. However, the monitoring data are subject to sparse sampling and difficult to be measured in a large-scale and synchronous way. Moreover, the advanced learning-based models for point Chl-a prediction, such as long short-term memory (LSTM) and convolutional neural network (CNN)-LSTM, are unable to fully mining the spatio-temporal correlation of Chl-a variations. Therefore, by using the satellite remote sensing data with extensive coverage, we design a framework, namely Ca-STANet, to simultaneously predict the Chl-a of all the locations in a large-scale area from the perspective of spatio-temporal field. Specifically in our method, the original data are firstly divided into multiple sub-regions to capture the spatial heterogeneity of large-scale area. Then, two modules are respectively operated to mine the spatial correlation and long-term dependency features. Finally, the outputs from the two modules are integrated by a fusion module to fully mine the spatio-temporal correlations, which are exploited to attain the final Chl-a prediction. In this paper, the proposed Ca-STANet is comprehensively evaluated and compared with the legacy methods based on the OC-CCI Chl-a 5.0 data of the Bohai Sea. The results demonstrate that the proposed Ca-STANet is highly effective for Chl-a prediction and achieves higher prediction accuracy than the baseline methods. Moreover, as the OC-CCI Chl-a 5.0 data have many missing areas, we introduce DINEOF method to fill the data gaps before using them for prediction.

Changes in oceanic variables, such as sea surface temperature (SST) and chlorophyll-a (Chl-a), have important implications for marine ecosystems and global climate change. The deep learning methods relying on convolutional neural networks can be employed to extract the spatial correlation for the prediction of oceanic variables. However, these methods are inflexible in the cases where some regions, e.g., land and islands, are invalid for the prediction of oceanic variables. By contrast, the graph convolutional network (GCN) is capable of capturing the large-scale spatial dependency existing in the irregular data. Owing to this, in this paper, we propose a GCN-based method for the prediction of oceanic variables, including SST and Chl-a, with high accuracy, which is referred to as OVPGCN. The proposed OVPGCN consists of three modules aiming to fully extract the spatial correlation and temporal dependency via modeling the multi-characteristics of the spatio-temporal dynamic evolution. In particular, three modules are implemented to extract the stationary and non-stationary variations in the recent spatio-temporal sequences, the spatial differences between different sites, and the periodic features in historical data, respectively. The well-designed OVPGCN is applied to the monthly SST and Chl-a prediction in the Bohai Sea and the Northern South China Sea (NSCS). The performance demonstrates that the proposed OVPGCN is highly effective and enables to achieve much higher prediction accuracy than the state-of-the-art methods.

This paper proposes a multi-cluster wireless powered Internet of Things (WP-IoT) network assisted by multiple intelligent reflecting surfaces (multi-IRS). In this network, a power station (PS) first broadcasts wireless energy to the distributed IoT devices grouped into multiple clusters. The IoT devices then use the harvested energy to convey their information to an access point (AP), based on a hybrid time- and frequency-division multiple access (TDMA-FDMA) protocol. Furthermore, multiple IRSs are deployed to perform anomalous reflection for energy and information transfer, to improve energy harvesting and data transmission capabilities. Under the constraints of the unit-modulus phase shifts, the transmission time shared among clusters and the bandwidth shared by the devices in each cluster, the considered system is optimized by maximizing its sum throughput. The optimization problem is non-convex and with complicatedly coupled variables. To solve this problem, we propose to first apply the Lagrange dual method and the Karush-Kuhn-Tucker (KKT) conditions to derive closed-form solutions for transmission scheduling and bandwidth allocation, then the quadratic transformation (QT) and the alternating optimization (AO) algorithm are introduced to solve the downlink and uplink IRS phase shifts, whilst the Majorization-Minimization (MM) and Riemannian Manifold Optimization (RMO) methods are applied to iteratively derive their closed-form solutions. Additionally, we provide a benchmark scheme to facilitate the system design, where each IRS can control its “on/off” state to aid the downlink and uplink transmissions in the condition of at most one activated IRS during one certain time duration. Finally, simulation results are presented to verify the optimality of our proposed scheme and highlight the beneficial role of the IRS.

Reconfigurable intelligent surface (RIS) is capable of improving the wireless system performance by steering the reflected signal in the desired direction. One of the major challenges is that both the transceiver and RIS have to be jointly optimized, where the optimization problems have to be reformulated for different system models and scenarios. To circumvent this challenge, new low-complexity antenna selection (AS) algorithms for transceiver-agnostic RIS configuration are proposed. Given a multiple-input multiple-output (MIMO) channel, the proposed RIS-AS opts for accurately aligning the RIS both with the transmit antenna (TA) and receive antenna (RA) for the sake of maximizing the MIMO channel’s overall output power. The proposed RIS-AS only has to configure the RIS alone, i.e. without iterations with the transceiver optimization. As a result, the proposed RIS-AS has the compelling benefit that they are generically applicable, regardless of the specific transceiver architecture. Our simulation results confirm that the proposed RIS-AS is capable of supporting any MIMO configuration, regardless of their closed/open-loop, single-/ full-RF and multiplexing-/diversity-oriented setups.

Indoor positioning is one of the core technologies of Internet of Things (IoT) and artificial intelligence (AI) and is expected to play a significant role in the upcoming era of AI. However, affected by the complexity of indoor environments, it is still highly challenging to achieve continuous and reliable indoor positioning. Currently, 5G cellular networks are being deployed worldwide, the new technologies of which have brought the approaches for improving the performance of wireless indoor positioning. In this article, we investigate the indoor positioning under the 5G new radio (NR), which has been standardized and being commercially operated in massive markets. Specifically, a solution is proposed and a software-defined radio (SDR) receiver is developed for indoor positioning. With our SDR indoor positioning system, the 5G NR signals are first sampled by universal software radio peripheral (USRP), and then, coarse synchronization is achieved via detecting the start of the synchronization signal block (SSB). Then, with the assistance of the pilots transmitted on the physical broadcasting channel (PBCH), multipath acquisition and delay tracking are sequentially carried out to estimate the Time of Arrival (ToA) of received signals. Furthermore, to improve the ToA ranging accuracy, the carrier phase of the first arrived path is estimated. Finally, to quantify the accuracy of our ToA estimation method, indoor field tests are carried out in an office environment, where a 5G NR base station (known as gNB) is installed for commercial use. Our test results show that, in the static test scenarios, the ToA accuracy measured by the 1-$\sigma $error interval is about 0.5 m, while in the pedestrian mobile environment, the probability of range accuracy within 0.8 m is 95%.

This paper investigates the first resource management problem for vehicular-to-infrastructure (V2I) networks under the in-band full-duplex (IBFD) backhauling scheme with guaranteed ultra-reliable and low-latency (URLLC) service. The considered networks suffers from interference caused by three node transmission in IBFD scheme and the mobility of vehicular user equipments (VUEs). The resource allocation problem is formulated to jointly optimize VUE association, resource block assignment (RA) and power allocation (PA), while satisfying the reliability and latency constraints at the same time. The formulated problem is a mixed integer non-linear problem with non-convex objective function and constraints. Finding globally optimal solution for this type of problem is still an open problem. To develop tractable solutions, the original problem is firstly simplified using derived equivalent expression for the objective function. The proposed method then decomposed it into RA and PA sub-problems. In each iteration, the PA sub-problem is solved for a set of given PA solution; while the solution for PA sub-problem is searched under determined RA results. The RA and PA sub-problems are solved iteratively until the converging condition is achieved. Theoretical analysis proves that the proposed method achieves Nash-stable equilibrium and local optimality for RA and PA sub-problems respectively. Simulation results verify the effectiveness of the derived performance analysis and demonstrate that the proposed algorithm outperforms the state-of-the-art algorithms in the literatures.

In molecular communication via diffusion (MCvD), information is conveyed by diffusing molecules. MCvD enjoys high energy efficiency but suffers from low date rates due to the long tail of the channel impulse response. To this end, the multiple-input multiple-output (MIMO) technique has been introduced to MCvD. However, the inter-symbol interference (ISI) and inter-link interference (ILI) deteriorate the bit error rate (BER) performance of MIMO MCvD systems. In this paper, a novel molecular modulation scheme, called molecular type permutation shift keying in the spatio-temporal domain, is proposed for MIMO MCvD systems to improve the BER performance by combating ILI and ISI. A low-complexity detector without requiring channel impulse response information is proposed. Furthermore, a complementary coding scheme that can effectively reduce ILI is designed. Additionally, the BER upper bound is analyzed. Numerical simulations on BER corroborate the analysis and show that the proposed scheme is a promising multi-molecule modulation alternative, which outperforms the existing MIMO MCvD modulation schemes.

Index modulation (IM) has been recognized as a compelling candidate for the next generation wireless networks. This paper considers index modulation-orthogonal frequency division multiplexing (IM-OFDM) with single subcarrier activation, where a single subcarrier is activated for transmitting an amplitude-phase modulated (APM) symbol in each IM group. Furthermore, a reduced-complexity approximate message passing aided symbol detector (AMPD) is proposed for reducing the detection complexity at the receiver side. The system performance of IM-OFDM with single subcarrier activation is investigated in terms of spectral efficiency (SE), peak-to-average power ratio (PAPR), bit error ratio (BER), as well as detection complexity. Our studies show that IM-OFDM with single subcarrier activation is capable of attaining low PAPR. Moreover, IMOFDM with single subcarrier activation outperforms the classic OFDM using APM for a SE of 1 bit/s/Hz, when high power amplifier (HPA) is employed. Furthermore, our investigation shows that low detection complexity can be achieved by the proposed AMPD for IM-OFDM with single subcarrier activation at the cost of a certain BER performance loss.

Communication between nanomachines is still an important topic in the construction of the Internet of Bio-Nano Things (IoBNT). Currently, molecular communication (MC) is expected to be a promising technology to realize IoBNT. To effectively serve the IoBNT composed of multiple nanomachine clusters, it is imperative to study multiple-access MC. In this article, based on the molecular division multiple access technology, we propose a novel multiuser MC system, where information molecules with different diffusion coefficients are first employed. Aiming at the user fairness in the considered system, we investigate the optimization of molecular resource allocation, including the assignment of the types of molecules and the number of molecules of a type. Specifically, three performance metrics are considered, namely, min-max fairness for error probability, max-min fairness for achievable rate, and weighted sum-rate maximization. Moreover, we propose two assignment strategies for types of molecules, i.e., best-to-best (BTB) and best-to-worst (BTW). Subsequently, for a two-user scenario, we analytically derive the optimal allocation for the number of molecules when types of molecules are fixed for all users. In contrast, for a three-user scenario, we prove that the BTB and BTW schemes with the optimal allocation for the number of molecules can provide the lower and upper bounds on system performance, respectively. Finally, numerical results show that the combination of BTW and the optimal allocation for the number of molecules yields better performance than the benchmarks.

Molecular communication (MC) is a bio-inspired communication paradigm, which lays the foundation for the Internet of Bio-NanoThings (IoBNT) in the medical field. As a high energy-efficient information transfer method, MC via diffusion (MCvD) is envisioned as a promising candidate for IoBNT but suffers from low date rates due to the long tail of the channel impulse response (CIR). To this end, the multiple-input–multiple-output (MIMO) technique has been introduced to MCvD. However, the intersymbol interference (ISI) and interlink interference (ILI) deteriorate the bit error rate (BER) performance of MIMO MCvD systems. In this article, molecular type permutation shift keying in the space domain (MTPSK-SD) and time-interleaved MTPSK-SD are proposed for MIMO MCvD systems to improve the BER performance by reducing ILI. The principle of MTPSK-SD is further generalized to the spatiotemporal domain, yielding three spatiotemporal modulation schemes, which can provide desirable BER performance without requiring any CIR information in the communication scenarios affected by different levels of ISI and ILI. Two low-complexity detectors are proposed to obtain different tradeoffs between anti-ILI and anti-ISI performance. Furthermore, a complementary coding scheme, which can effectively reduce the ILI under the considered symmetrical system topology, is designed and applied to all the proposed modulation schemes. Additionally, the BER upper bound is analyzed. Numerical simulations on BER corroborate the analysis and show that the proposed schemes are promising multimolecule modulation alternatives, which outperform the existing MIMO MCvD modulation schemes.

An edge-cancellation-aided iterative detection and decoding (EC-IDD) algorithm is proposed for polar-coded sparse code multiple access (SCMA), which jointly performs Gaussian-approximated message passing (GA-MP) detection of SCMA supported by the soft list decoding (SLD) of polar codes. A reduced-edge factor graph is formulated in each consecutive iteration with the aid of the cyclic redundancy check (CRC) and EC. Based on the simplified factor graph, the EC-IDD gradually reduces its complexity in each subsequent iteration, while improving the bit error rate (BER) performance, compared to the state-of-the-art joint detection and decoding (JDD) of polar-coded SCMA. Furthermore, an embedded decision-directed channel estimator (DD-CE) is proposed for our polar-coded SCMA system under realistic imperfect channel state information (CSI). Our simulation results demonstrate that the proposed EC-IDD achieves better BER performance than the state-of-the-art JDD under both perfect and imperfect CSI, despite achieving a complexity reduction of 92%. Finally, the BER of the proposed joint DD-CE and EC-IDD algorithm under imperfect CSI converges to that of EC-IDD operating under perfect CSI.

Space-time coded generalized spatial modulation-aided sparse code division multiple access (STC/GSM-SCDMA) is proposed, which exploits the two-dimensional transmit diversity potential of both the spatial and of the frequency domain. Hence, it constitutes a promising solution for the pervasive connectivity of devices in next-generation nonorthogonal multiple access (NOMA) systems. More explicitly, our STC/GSM scheme achieves diversity in the spatial-domain, while the sparse signal-spreading action of SCDMA results in frequency-domain (FD) diversity. A single-user bit error rate (BER) bound is derived as the benchmark of the BER performance of our STC/GSM-SCDMA system. Furthermore, a pair of novel detectors, namely a bespoke message passing aided (MPA) detector and a tailor-made approximate message passing (AMP) detector are conceived by designing a new factor graphs for our proposed STC/GSM-SCDMA system. The performance of these detectors is characterized in terms of their BER vs. complexity. Our simulation results show that the proposed AMP detector is capable of operating within 2 dB of the MPA detector's signal-to-noise ratio (SNR) requirement, while supporting a normalized user load of 150%, despite its appealing low complexity, which is about 1000 times lower than the MPA detector.

This paper is focused on code-domain non-orthogonal multiple access (CD-NOMA), which is an emerging paradigm to support massive connectivity for future machine-type wireless networks. We take a comparative approach to study two types of overloaded CD-NOMA, i.e., sparse code multiple access (SCMA) and dense code multiple access (DCMA), which are distinctive from each other in terms of their codebooks having sparsity or not. By analysing their individual diversity orders (DO) in Rayleigh fading channels, it is found that DCMA can be designed with the aid of generalized sphere decoder (i.e., a nonlinear multiuser detector) to enjoy full DO which is equal to the maximum number of resource nodes in the system. This is in contrast to SCMA whose error rate suffers from limited DO equal to the codebook sparsity (i.e., the effective number of resource nodes occupied by each user). We conduct theoretical analysis for the codebook design criteria and propose to use generalized sphere decoder for DCMA detection. We numerically evaluate two types of multiple access schemes under “ 4×6” (i.e., six users communicate over four subcarriers) and “ 5×10” NOMA settings and reveal that DCMA gives rise to significantly improved error rate performance in Rayleigh fading channels, whilst having decoding complexity comparable to that of SCMA.

As the enabling technologies move up to the mmWave and even to the TeraHertz bands for the next-generation wireless systems, the signal processing of high-bandwidth orthogonal frequency division multiplexing (OFDM) becomes increasingly power-thirsty, owing to the following OFDM deficiencies: (1) the high peak-to-average power ratio (PAPR); (2) the bandwidth efficiency loss due to the cyclic prefix (CP) overhead; (3) the sensitivity to carrier frequency offset; (4) the complex out-of-band (OOB) filtering. Over the past six decades, a variety of waveforms have been developed in order to mitigate these deficiencies, which are generally achieved at the cost of compromising some of OFDM’s beneficial properties, such as its subcarrier (SC) orthogonality, its high throughput and its straighforward adoption to multiple-input multiple-output (MIMO) systems. Against this background, we propose a new waveform termed as multi-band discrete Fourier transform spread-OFDM with index modulation (MB-DFT-S-OFDM-IM), where the component multi-carrier techniques are conceived to constructively function together in order to mitigate the OFDM deficiencieswithout compromising the beneficial OFDM properties. More explicitly, first of all, the PAPR is reduced by the DFT-precoding. Secondly, thanks to the IM design, MB-DFT-S-OFDM-IM is capable of achieving a high throughput that is strictly equal to or higher than the OFDM throughput. Thirdly, MB-DFT-S-OFDM-IM achieves a beneficial frequency diversity gain, which leads to a higher tolerance to carrier frequency offset. Fourthly, the OOB filters are placed in each sub-band before DFT, so that the SC orthogonality remains intact, which is unique to the proposed MB-DFT-S-OFDM-IM structure. Last but not least, we extend the proposed MB-DFT-S-OFDM-IM to support a variety of MIMO schemes, where the IM philosophy is integrated with the space-, time- and frequency-domains within a singleunifiedplatform.

To improve angular resolutions and precision in radar object detection and estimation, designs of radar systems tend to employ more antenna elements or modules, for instance the millimeter wave (mmWave) radar for autonomous driving. However, as the number of antennas increases, the computational complexity for radar signal processing also increases drastically. An effective way to resolve such a dilemma is to select optimal antenna subarrays for individual objects under detection and estimation, which not only can constrain the computational complexity for subsequent radar signal processing, but also can maintain the precision of angular estimation. In view of the complexity of optimal subarray selections, existing results typically use greedy approaches to search for suboptimal solutions. More recently, deep learning (DL) methods are proposed to improve the correctness of subarray selection. Motivated by the recent advancements in DL, we propose herein a novel DL method of subarray selection for radar multi-target detection and angles estimations based on a one-dimensional Saak transformation. Compared to the existing DL method based on convolutional neural networks (CNN), the proposed method not only has a higher correctness in subarray selection, but also has a lower computational complexity. Simulation results also show that signal collections for subarray selection can be limited to two chirp signal durations, which significantly reduce the time delays for subsequent radar signal processing.

The authors analyse and investigate the bit error rate (BER) performance of the diffusive molecular communication (DMC) systems employing binary molecular shift keying (BMoSK) modulation. The DMC systems both without and with inter‐symbol interference compression (ISIC) are considered. Specifically, by modelling the number of molecules of one type presenting in a three‐dimensional (3D) detection space as a Poisson or Gaussian distributed random variable, they derive the BER expressions of the BMoSK‐modulated DMC systems without/with ISIC. However, the strong inter‐symbol interference (ISI) present in DMC systems makes the computation of the above‐mentioned BER formulas highly involved or even impossible. Therefore, they introduce some simplified approaches, namely the Monte–Carlo and the simplified Poisson (or simplified Gaussian) approaches, to reduce the computation complexity in the cases of long ISI. Furthermore, based on the statistics of decision variables, they analyse the effect of decision threshold on the BER performance. Finally, the BER performance of the BMoSK‐modulated DMC systems without/with ISIC is investigated by considering the impact from different aspects, while the different analytical approaches are compared in terms of the accuracy of the BER evaluated based on these approaches.

This study investigates the equalisation techniques for the diffusive molecular communication (DMC) systems with binary molecular‐shift keying (BMoSK) modulation, referred to as the BMoSK‐DMC systems, in order to mitigate the effect of inter‐symbol interference (ISI). The authors first show that in terms of receiving equalisation, a BMoSK‐DMC system is in fact equivalent to a conventional binary phase‐shift keying modulated radio communication system encountering ISI. Hence, a wealth of equalisation techniques developed with the conventional radio communication systems may be introduced for equalisation of BMoSK‐DMC signals. However, considering the limited capability of molecular transceivers on computation and storage, in this study, the authors investigate only the linear equalisers (LEQ) in the principles of matched‐filtering, zero‐forcing (ZF) and minimum mean‐square error (MMSE). They characterise the effects of the different aspects related to DMC signalling and propagation on the achievable performance of the BMoSK‐DMC systems with these LEQ. Their studies and performance results demonstrate that both the ZF‐ and MMSE‐LEQ are capable of efficiently suppressing the ISI and attaining promising performance, while still have low‐complexity to facilitate implementation.

Massive multiple‐input multiple‐output (MIMO) systems with hundreds of correlated antennas at base station are capable of offering abundant spatial resources. When spatial‐domain modulation is applied to these systems, the spatial modulation (SM) using only one radiofrequency chain benefits practical implementation, but suffers from significantly reduced multiplexing gain offered by the massive number of antennas. By contrast, the generalised SM (GSM) allows to simultaneously activate multiple transmit antennas, which improves spatial multiplexing gain, but degrades the achievable error performance, as its design pays no attention to the antenna correlation. The authors propose an antenna grouped SM (GrSM) scheme, which is capable of circumventing the shortcomings of both the SM and GSM schemes suffered in massive MIMO scenarios. In the proposed GrSM scheme, transmit antennas are partitioned into multiple groups, where the relatively strongly correlated antennas within individual groups are used to implement component SM schemes, while the relatively weakly correlated antennas in different groups are beneficial to obtain multiplexing gain. In order to further improve the spectral efficiency, adaptive modulation is integrated with GrSM to form the adaptive GrSM (AGrSM). The achievable error and spectral efficiency performance of GrSM and AGrSM systems are investigated based on both mathematical analysis and Monte‐Carlo simulations, which are also compared with that of the conventional SM and GSM schemes, when massive MIMO communication scenarios are considered. Their studies and performance results show that GrSM is a promising transmit scheme for massive MIMO, which can outperform both the SM and GSM schemes.

This paper proposes a novel spatial-modulated multicarrier sparse code-division multiple access (SM/MC-SCDMA) system for achieving massive connectivity in device-centric wireless communications. In our SM/MC-SCDMA system, the advantages of both MC signalling and SM are amalgamated to conceive a low-complexity transceiver. Sparse frequency-domain spreading is utilized to mitigate the peak-to-average power ratio (PAPR) of MC signalling, as well as to facilitate low-complexity detection using the message passing algorithm. We then analyze the single-user bit error rate performance of SM/MC-SCDMA systems communicating over frequency-selective fading channels. Furthermore, the performance of SM/MC-SCDMA systems is evaluated based on both Monte-Carlo simulations and analytical results. We demonstrate that our low-complexity SM/MC-SCDMA transceivers are capable of achieving near-maximum likelihood (ML) performance even when the normalized user-load is as high as two, hence constituting a variable solution to support massive connectivity in device-centric wireless systems.

In this paper, we present the space shift keying based molecular communication (SSK-MC), where the space symbol is of interest. For micro-scale SSK-MC, the information is embedded into the index of a single activated transmitter nanomachine, while the index of the sensor is considered for the macro-scale SSK-MC. The cancellation of the strongest inter-link interference in multiple-input multiple-output (MIMO) based MC is available with the implementation of SSK-MC, in which only one transmitter is activated during each symbol duration. We derive the symbol error rate (SER) of SSK-MC, which shows a perfect match with its numerical simulations. Apart from the theoretical analysis, we further design an underwater prototype under the 4×4 MIMO-MC configuration for SSK-MC, demonstrating its SER performance with different detection algorithms.

Diffusion-based Molecular Communication (DMC) is a feasible method for information transmission in some nano-networks operated in gas or liquid environments. In this paper, we first propose an information modulation scheme for DMC, which is referred to as the Type-Spread Molecular Shift Keying (TS-MoSK). Considering that DMC signals usually experience severe inter-symbol interference (ISI), our TS-MoSK is characterized by introducing extra types of molecules for ISI mitigation (ISIM). Furthermore, we introduce two ISIM methods to the TS-MoSK modulated DMC systems, which are the active ISIM and passive ISIM. We detail their operation principles, and investigate as well as compare their achievable performance. Our studies show that, aided by the extra types of molecules, TS-MoSK outperforms the MoSK without spreading. Both ISIM approaches are effective for further improving the performance of TS-MoSK.

Simultaneous wireless information and power transfer (SWIPT) has gained significant popularity in the recent past owing to its applications in a wide range of use cases. Although SWIPT has been fairly well investigated in the literature, the existing work has mainly focused on attaining the optimal rate energy (RE) tradeoff assuming Gaussian input alphabet. However, practical systems operate with finite input alphabets such as quadratic-amplitude modulation (QAM)/phase-shift keying. We characterize the attainable RE tradeoff in SWIPT systems employing finite input alphabet for transmission over parallel Gaussian channels of say orthogonal frequency-division multiplexing subcarriers or multiple-input multiple-output streams. Some of the key results in the literature that assume Gaussian input alphabet are shown to be special cases of our results. Furthermore, we provide insights into our results with the aid of graphical illustrations, which throw light on the optimal power allocation policy for various energy-harvesting constraints. Furthermore, we consider practically relevant time-sharing and power-splitting schemes operating with finite input alphabet and characterize their RE tradeoff. Their optimal solutions in the asymptotic regime are obtained, which serve as low-complexity solutions suitable for practical implementation. Our simulation studies have demonstrated that the Gaussian input assumption significantly over-estimates the attainable RE tradeoff, especially when the signal set employed is small. Furthermore, it is observed through numerical simulations that the proposed optimal power allocation performs significantly better than the power allocation based on the Gaussian input assumption. Specifically, as much as 30% rate improvement is observed when employing the classic 4-QAM signal set.

Index modulation (IM) is an attractive concept for next generation communication systems. However, as an emerging technique, there are still some challenges need to be tackled in this frontier. In this paper, we consider two aspects of IM, which are diversity and detection complexity. Specifically, we propose a linear precoding assisted index modulation (LPIM) scheme for orthogonal frequency division multiplexing (OFDM) systems. We commence by analyzing the diversity and coding gains of the proposed scheme. Moreover, a detailed codebook design criterion is proposed. Then, our LPIM codebook is designed based on the maximum diversity and coding gain criteria. In contrast to the signaling model of the existing full diversity precoder designed for OFDM, we introduce a modeling method to link the zero-valued IM symbols to the origin of a Lattice for implementing our full diversity precoder designed for OFDM-IM. Both analytical and computer simulation results are provided for characterizing the attainable performance of OFDM-LPIM, demonstrating that it is capable of achieving full diversity as well as an attractive coding gain. However, the maximum-likelihood (ML) detection complexity of OFDM-LPIM is excessive, hence a low-complexity generalized iterative residual check detector (GIRCD) is proposed, which is inspired by the existing sparse recovery algorithms. Finally, computer simulation results are provided for demonstrating that GIRCD can provide a beneficial trade-off between bit error ratio performance and complexity.

In order to take the advantages of spatial modulation (SM) and support the multiple-access (MA) communications requiring high connectivity, we integrate the SM with DS-CDMA to propose a SM/DS-CDMA scheme, which is in favor of attaining both antenna diversity and frequency diversity. Associated with the SM/DS-CDMA scheme, a range of linear and nonlinear multiuser detectors (MUDs) are proposed and studied. Specifically, for linear MUDs, both minimum mean-square-error-aided joint spatial demodulation (MMSE-JSD) and MMSE-aided separate spatial demodulation (MMSE-SSD) are introduced. For nonlinear MUDs, we first present the joint maximum-likelihood MUD (JML-MUD) for demonstrating the potential of the SM-/DS-CDMA systems. Then, the MMSE-relied iterative interference cancellation (MMSE-IIC) is suggested, and the associated two types of low-complexity high-efficiency reliability measurement schemes are proposed. In this paper, we mathematically analyze both the single-user average bit error rate (ABER) of the SM/DS-CDMA systems employing joint spatial demodulation (JSD) and the approximate ABER of the SM/DS-CDMA systems employing the MMSE-JSD. We investigate and compare the ABER performance of the SM/DS-CDMA systems with various detection schemes and also with some legacy schemes. Our studies and performance results show that the SM/DS-CDMA systems in conjunction with appropriate MUD schemes are capable of providing a desirable tradeoff among the number of users supportable, implementation complexity, and the ABER performance.

In order to enhance the secrecy performance of the generalized space shift keying (GSSK) visible light communication (VLC) system, in this paper, an optical jamming-aided secrecy enhancement scheme is proposed, in which the source transmitter (S) simultaneously sends both the confidential desired signal and optical jamming signals under the amplitude and power constraints. The optical jamming signals obey the truncated Gaussian distribution for satisfying the constraints. Given the discrete set of channel inputs, the optical jamming-aided GSSK-VLC system's secrecy performance is analyzed. Explicitly, the average mutual information (AMI), the lower bound of AMI and its closed-form approximation as well as the achievable secrecy rate are formulated analytically. Furthermore, the optimal power sharing strategy of the proposed GSSK-VLC systems relying on optical jamming is derived. Closed-form expressions are provided for the optimal power sharing in both the low- and high-SNR regions. Finally, the extensive simulation results are presented to validate our analytical results.

In order to support high-user-load multiple-access (MA), we propose a non-orthogonal MA scheme based on a beneficial amalgam of spatial modulation (SM) and sparse code-division multiple-access (SCDMA), which is termed the SM-SCDMA. Hence, SM-SCDMA inherits both the merits of SM with single radio-frequency MIMO transceiver implementation and the advantages of SCDMA relying on low-complexity signal detection. In this paper, we evaluate the potential of SM-SCDMA as well as its low-complexity near-optimum signal detection. Given these objectives, we consider both the maximum likelihood detection and the message passing algorithm aided detection (MPAD) that is derived based on the maximum a posteriori principles. In order to evaluate the performance of large SM-SCDMA without relying on time-consuming simulations, we propose new approaches for analyzing the performance of SM-SCDMA systems. A range of formulas that are valid in the signal-to-noise ratio region of practical interest are derived. Finally, the performance of SM-SCDMA systems is investigated by addressing diverse design concerns. Our studies and performance results show that SM-SCDMA constitutes a promising MA scheme for the future ultra dense systems. Assisted by the MPAD, it is capable of supporting high-user-load MA transmission associated with a normalized user-load factor of two.

In this paper, we study a multi-pair bidirectional (or two-way) full-duplex (FD) massive MIMO relay (FDMMR) system, where the relay employs massive antennas and is operated in the amplify-and-forward (AF) mode. We analyze its spectral efficiency (SE) performance, when both imperfect channel estimation and channel aging effect are considered. We propose four power-scaling schemes based on the zero-forcing reception/zero-forcing transmission (ZFR/ZFT) relaying processing. Assuming that the number of relay antennas approaches infinity, the SE is analyzed in the context of the proposed power scaling schemes. Our analytical results show that the inter-pair interference caused by the other user pairs as well as the self-interference can be completely eliminated by the ZFR/ZFT processing at the relay. The self-loop interference and the inter- user interference can also be cancelled, if a power scaling scheme is carefully selected. Furthermore, our studies show that the channel aging may significantly degrade the SE of the system.

Activating direct communications among mobile users (MUs) for sharing a content of common interest (CoCI) becomes an essential paradigm for realising the efficient content distribution in densely populated scenarios. Relying on the cooperative multicast among the MUs, a centralised fair resource allocation algorithm is proposed in this paper for improving the attainable content distribution delay. Apart from the physical wireless transmission, social aspects of MUs are also taken into account, where a MU only multicasts the CoCI to its social contacts. The content distribution process is further modelled by a discrete-time pure-birth-based Markov Chain (DT-PBMC). Relying on the DT-PBMC, we minimise the state-retention probability during each transmission frame in order to reduce the content distribution delay to its smallest possible. The classic branch-and-bound algorithm is invoked for obtaining the optimal resource allocation scheme. The simulation results demonstrate that the proposed algorithm outperforms its existing counterparts, while our novel resource allocation scheme may simultaneously achieve better fairness and reduced content distribution delay.

In this paper, we propose a cognitive Go-Back-N Hybrid Automatic Repeat reQuest (CGBN-HARQ) scheme for a cognitive radio (CR) system to opportunistically transmit data over a primary radio (PR) channel. We model the activity of PR users (PRUs) occupying the PR channel as a Markov chain with two states: `ON' and `OFF'. In order to use the PR channel, the CR system first senses the availability/unavailability of the PR channel. Once it finds that the PR channel is free, the CR system transmits data packets over the PR channel's spectrum, whilst relying on the principles of GBN-HARQ. In this paper, we investigate both the throughput and delay of CGBN-HARQ, with a special emphasis on the impact of various system parameters involved in the scenarios of both perfect and imperfect spectrum sensing. Our studies demonstrate that the activity of PRUs, the transmission reliability of the CR system as well as the number of packets transmitted per time-slot may have a substantial impact on both the throughput and the delay of the CR system.

We propose a novel spatial modulation (SM) scheme, which is a joint of two index modulation schemes. One is the M-ary spatial shift-keying (MSSK) modulation operated in the space domain, while the other one is the M-ary orthogonal shift keying (MOSK) modulation, which usually requires more bandwidth to implement, when its modulation level increases. As both MSSK and MOSK belong to the energy-efficient modulation schemes, the joint MSSK-MOSK modulation is energy-efficient, which is suitable for providing reliable communications in low signal to noise ratio (SNR) region. We also propose two approaches for detection of the MSSK-MOSK signals. The first approach is the joint maximum likelihood detection (MLD), which simultaneously detects the component MSSK symbol and MOSK symbol in MLD principle. The second approach is the successive MLD, which first detects the component MOSK symbol using non-coherent MLD, and then the component MSSK symbol using coherent MLD. Finally, we investigate the error performance of the MSSK-MOSK modulation, when assuming communications over flat Rayleigh fading channels with receiver diversity. Our studies demonstrate that the joint MLD slightly outperforms the successive MLD, but requires much higher complexity for implementation. With the resources provided from both the space and frequency domains, the MSSK-MOSK with high modulation levels may be readily implemented, which makes it is possible to provide reliable communications at very low energy consumption, even when the low-complexity successive MLD is employed.

The recently developed orthogonal frequency-division multiplexing relying on index modulation (OFDM-IM) is adopted to the in-vehicle power line communications (PLCs) in order to combat the deleterious effects of frequency-selective fading and impulsive noise, whilst improving the energy efficiency of data communications. Furthermore, the low density parity check (LDPC) coding is invoked to further enhance the reliability of in-vehicle PLCs, which is of particular importance in light-weight airborne vehicles. For aiding LDPC decoding, a reduced complexity soft-decision detection scheme is proposed. The performance of the LDPC-coded OFDM-IM system is studied by simulation, when assuming communications over in-vehicle PLC channels. Our studies show that LDPC-coded OFDM-IM is capable of combating frequency-selective fading, mitigating impulsive noise, as well as striking a compelling trade-off between the spectral efficiency and energy efficiency for in-vehicle PLCs.

In this study, the authors consider a two‐hop network, where multiple source nodes (SNs) transmit to a destination node (DN) with the aid of a relay node (RN). The RN is equipped with a buffer, which is capable of storing multiple frames received from the SNs. During each time slot, the proposed protocol activates either the SN–RN hop or the RN–DN hop, depending on the channel quality of each hop and the buffer state at the RN. To optimise the hop activation for the network, they design a hop quality metric and propose a multi‐user buffer‐aided‐relaying uplink (MU‐BR‐UL) protocol, with the aid of the minimum signal‐to‐noise power ratio approximation. The benefits of the proposed protocol are analysed in terms of the end‐to‐end (e2e) outage probability and the e2e transmission delay. Then, the optimal power allocation is proposed for minimising the e2e outage probability under the total power constraint. The results indicate that the outage performance is significantly improved when the proposed power allocation is utilised in the MU‐BR‐UL protocol.

Hidden semi‐Markov modelling of the asynchronous impulsive noise (IN) encountered in indoor broadband power line communications (PLCs) is investigated by considering the statistical distributions of both the inter‐arrival time and the duration of asynchronous IN components. Then, the bit error ratio (BER) of orthogonal frequency division multiplexing systems using Q ‐ary quadrature amplitude modulation is analysed with the aid of the proposed noise model, when communicating over dispersive indoor power line channels inflicting asynchronous IN in addition to the background noise. The authors’ simulation results confirm the accuracy of the analysis and quantify the impact of various factors on the achievable BER performance. The grave impact of asynchronous IN on indoor broadband PLCs suggests that efficient techniques have to be designed for mitigating its effects.

Based on the common interest of mobile users (MUs) in a social group, the dissemination of content across the social group is studied as a powerful supplement to conventional cellular communication with the goal of improving the delay performance of the content dissemination process. The content popularity is modeled by a Zipf distribution to characterize the MUs' different interests in different contents. The factor of altruism (FA) terminology is introduced for quantifying the willingness of content owners to share their content. We model the dissemination process of a specific packet by a pure-birth-based Markov chain and evaluate the statistical properties of both the network's dissemination delay as well as of the individual user-delay. Compared to the conventional base station (BS)-aided multicast, our scheme is capable of reducing the average dissemination delay by about 56.5%. Moreover, in contrast to the BS-aided multicast, increasing the number of MUs in the target social group is capable of reducing the average individual user-delay by 44.1% relying on our scheme. Furthermore, our scheme is more suitable for disseminating a popular piece of content.

Developed from the recently emerged generalized pre-coding aided spatial modulation (GPSM) concept, a novel non-linear GPSM scheme based on the powerful vector perturbation philosophy is proposed, where a particular subset of receive antennas is activated and the specific activation pattern itself conveys useful implicit information in addition to the conventional modulated and perturbed symbols. Explicitly, both the infinite and finite alphabet capacities are derived for the proposed non-linear GPSM scheme. The associated complexity, energy efficiency, and error probability are also investigated. Our numerical results show that, as the only known non-linear realization within the spatial modulation family, the proposed scheme constitutes an attractive solution to the flexible design of green transceivers, since it is capable of striking a compelling compromise amongst the key performance indicators of throughput, energy consumption, complexity, and performance. In particular, in the challenging full-rank scenario, conveying information through receive antenna indices exhibits a lower complexity, a higher energy efficiency, and a better error resilience than that of the conventional arrangement.

This work considers the physical-layer security of a wiretap channel, where a transmitter communicates with a receiver using the precoding-aided spatial modulation (PSM) in the presence of an unauthorized passive eavesdropper. We first analyze the security capability of the PSM and show that the PSM has the property of low-probability-of-interception (LPI). Based on the observations, we then design a secret PSM (SPSM), which represents a generalization of the PSM. We derive the upperbounds for the bit error rate (BER) of both the desired receiver and the eavesdropper when assuming communications over Rayleigh fading channels. Both the numerical results evaluated from the BER upper-bounds and the simulation results are provided to demonstrate the secrecy performance of the SPSM. Our studies show that the SPSM can significantly enhance the security of the PSM, in addition to inheriting all its merits.

Accurate localization of railway vehicles is one of the key requirements for development of high-efficiency automatic train control systems. This paper proposes an angle offset-assisted positioning (AOAP) scheme for improving the localization of railway vehicles in tunnel environments. Our AOAP system consists of a wireless sensor network (WSN), where anchor nodes or sensors are distributed along the railway tracks to collect the signals transmitted by a target node installed a train. Based on the information obtained by the anchor nodes from the target node, the position of the target node or the train is initially estimated. However, radio signals experience transmission delay, which makes this estimated position have a bias from the real position. Hence, the initially estimated train's position is updated with the aid of the angle offset estimated. In this contribution, we address the effect of angle offset error on the positioning accuracy. It can be shown that our AOAP approach is capable of providing more accurate positioning than the conventional least square approach without considering angle offset.

Detecting spectrum holes and efficiently accessing them are the two basic functions that enable a cognitive radio (CR) to make use of the licensed spectrums of a primary radio (PR). In this paper, we consider a CR scheme, which opportunistically accesses a PR channel for communication between a pair of nodes based on the stop-and-wait hybrid automatic repeat request (SW-HARQ). Hence, it is referred to as the cognitive SW-HARQ (CSW-HARQ) arrangement. In our CSW-HARQ system, the PR channel is modelled as a two-state Markov chain having `On' and `Off' states. The CR may only access the PR channel in its `Off' state. In this paper, we analyze both the throughput and delay performance of the CSW-HARQ system, for which a range of closed-form formulas are derived that are also validated by simulation results. Our performance results show that both the activities of PR users and the reliability of the CR channel have a substantial impact on the achievable performance of the CR system.

In echoing Varshney's seminal concept of jointly transmitting information and energy, we propose the concept of an energy pattern aided Simultaneous Wireless Information and Power Transfer (SWIPT) system, where in addition to power transfer, information is conveyed both by the specific Receive Antenna (RA) indices to which the power is delivered as well as by the particular intensity of the power assigned to that particular RA pattern. By embedding information into energy patterns rather than imposing it by modulating classic radio waveforms, our proposed solution is capable of operating both in an integrated receiver mode and in a power-split mode, while relying on a low-complexity two-stage non-coherent detection algorithm. Both our analysis and simulations show that our energy pattern aided SWIPT system exhibits a beneficial immunity to any potential performance degradation imposed by power-conversion. Moreover, the achievable rate versus power conversion trade-off bounds are characterized, demonstrating that our proposed energy pattern aided SWIPT system leads to a beneficial wireless information and power transfer convergence.

In multihop diversity (MHD) aided multihop links, the nodes are assumed to have buffers for temporarily storing their received packets for further transmission at instances of good channel quality. Since adaptive modulation is employed, the number of packets in each time slot (TS) is affected both by the channel quality and the buffer fullness. During each time-slot (TS), the criterion used for activating a specific hop is that of transmitting the highest number of packets. When more than one hop is capable of transmitting the same number of packets, the particular hop having the highest channel quality (reliability) is activated. Hence, we refer to this regime as the maximum throughput adaptive rate transmission (MTART) scheme. The bit error ratio (BER), the outage probability, the throughput as well as the bandwidth-efficiency of the MTART scheme is analyzed. Our results demonstrate that our MTART regime has the potential of significantly outperforming conventional adaptive modulation. Naturally, the BER is improved by the buffering scheme advocated at the cost of an increased delay. Hence, the distribution of the end-to-end packet delay will also be characterized.

In this contribution, we propose and study a bufferaided opportunistic routing (BOR) scheme, which combines the benefits of both opportunistic routing and multihop diversity (MHD) aided transmissions. It was conceived for a buffer-aided three-node network (B3NN) composed of a source node (SN), a buffer-aided relay node (RN) and a destination node (DN). In this network, there are three channels namely the SN-RN, RN-DN and SN-DN channels. The key motivation is that we are aiming for activating the specific channels requiring a recuded energy dissipation. In order to study this problem, a three-dimensional (3D) transmission activation probability space (TAPS) is proposed, which is divided into four regions representing each of the three channels plus an outage region. In a specific time slot (TS), the instantaneous channel fading values may be directly mapped to a specific point in this 3D channel space. The BOR scheme then relies on the position of this point to select the most appropriate channel for its transmission. Both the energy dissipation and the outage probability (OP) are investigated for transmission in this network. The results show that when the system is operated at a normalized throughput of 0.4 packet/TS, the energy dissipation was reduced by 24.8% to 77.6% compared to three different benchmark schemes. Alternatively, our technique is capable of reducing the OP by 89.6% when compared to conventional opportunistic routing. As in all buffer-aided system, the performance improved with the cost of higher packet delay, which is also studied.

The recently proposed multiple input multiple output (MIMO) transmission scheme termed as generalized pre-coding aided spatial modulation (GPSM) is analyzed, where the key idea is that a particular subset of receive antennas is activated and the specific activation pattern itself conveys useful implicit information. We provide the upper bound of both the symbol error ratio (SER) and bit error ratio (BER) expression of the GPSM scheme of a low-complexity decoupled detector. Furthermore, the corresponding discrete-input continuous-output memoryless channel (DCMC) capacity as well as the achievable rate is quantified. Our analytical SER and BER upper bound expressions are confirmed to be tight by our numerical results. We also show that our GPSM scheme constitutes a flexible MIMO arrangement and there is always a beneficial configuration for our GPSM scheme that offers the same bandwidth efficiency as that of its conventional MIMO counterpart at a lower signal to noise ratio (SNR) per bit.

In this paper, we propose a routing protocol for mobile social networks (MSNs) by exploiting the fact that people's daily lives have community preferences. The proposed protocol is capable of efficiently integrating the merits of high delivery ratio, low delivery latency and low resource consumption of the Epidemic, LABEL and BUBBLE protocols for MSNs, while circumventing their respective shortcomings. In our routing protocol, an approach for measuring people's community preference is proposed, both direct meetings in one community and indirect contacts via different communities are considered. The performance of the proposed protocol and that of the three routing protocols as above-mentioned are investigated in the context of our proposed and more realistic mobility model, which jointly considers people's social characteristics, including randomness, Waypoint effect and mutual-community-based relationship. Our studies and performance results show that the proposed protocol is capable of attaining a good trade-off between the average delay of information delivery and the cost of resources for operation of the protocol.

In multihop diversity aided multihop links, the number of bits transmitted in each Time Slot (TS) is affected by both the Channel Quality (CQ) and the Buffer Fullness (BF), when adaptive modulation is employed. We assume that every node has buffers for temporarily storing its received packets for further transmission at instances of good CQ. For the sake of improving the throughput, a Maximum Throughput Adaptive Rate Transmission (MTART) scheme was proposed, where the specific hop having the capability of transmitting the highest number of bits (packets) will be activated. If more than one hops are capable of transmitting the same number of bits, the particular hop having the highest CQ (reliability) is activated. We demonstrate that the MTART scheme has 8 dB gain at the Outage Probability (OP) of 10-3and has 3 dB gain in terms of the throughput attained in comparison to the conventional adaptive modulation aided scheme.

In this paper buffer-aided Opportunistic Routing (OR) was designed with the aid of the novel concept of linear Transmission Activation Probability Space (TAPS) partitioning invoked for relay-assisted networks, which combines the benefits of both OR [1] and of buffer-aided transmissions [2]. More specifically, a packet may be transmitted from the Source Node (SN) to the Destination Node (DN) either directly or indirectly via one of theMRelay Nodes (RNs), depending on the instantaneous channel qualities. The above-mentioned linear multi-dimensional TAPS partitioning concept is proposed for partitioning the transmission space into (2M+1) transmission regions plus an outage region, while ensuring that the number of input packets is equal to the number of output packets at each RN's buffer. The benefit of having a buffer and tolerating the associated delay is that the best channel is activated for transmission based on our linear TAPS partitioning method.

A novel Popularity-assisted Routing Protocol (PRP) is proposed for Mobile Social Networks (MSNs). The PRP is designed by exploiting the social characteristics of mobile nodes, which are reflected by the friendship derived from regular node meetings. Moreover, the strength of a node's friendship is specified by its popularity. In detail, four novel metrics are introduced to reflect the social characteristics of mobile nodes, which include friendship, popularity, popularity rank and relationship. Based on these metrics, the PRP is proposed, which makes use of these metrics for message delivery. The performance of the PRP is investigated and compared with that of some existing routing protocols for MSNs under a Random Waypoint Mobility (RWM) model. Our studies and results show that the metrics for social behaviors are efficient for route discovery. The proposed PRP is capable of achieving the total message reception with considerably low average delay. Additionally, the PRP requires significantly less resource, such as, bandwidth, buffer space and energy, than the other routing protocols considered.

This paper concerns the problem of dynamic routing in mobile social networks (MSNs). By exploiting the social characteristics of mobile nodes (MNs) in MSNs, a routing protocol is proposed, which is referred to as the Social Contact Probability assisted Routing (SCPR) protocol. This protocol can efficiently integrate the merits of high delivery ratio, low delivery latency and low resource consumption of the existing Epidemic, Prophet and SimBet protocols for MSNs, while circumventing their shortcomings. The performance of our SCPR protocol and of the above-mentioned other three routing protocols are evaluated in the context of our proposed mobility model, which jointly takes into account of MNs' social characteristics, including randomness, waypoint and community behaviors.

A buffer-aided two hop link is studied, where the RN is capable of temporarily storing the received packets. We commerce by defining the concept of a two-dimensional Channel Probability Space (CPS) based on the source-relay and relay-destination channel. Specifically, a non-linear CPS division method is proposed, which partitions the CPS into several regions representing the quality of the specific channels plus an outage region. Then the best channel is activated for the sake of minimizing the system's energy dissipation. Finally, the proposed buffer-aided transmission scheme relying on our non-linear CPS division regime is investigated and the results show that at given average end-to-end energy dissipation, the outage probability was reduced by 33.5% compared to the benchmark scheme.

Since mobile communications exhibit strong social characteristics, based on the potential common interests of mobile users, mobile social networks (MSNs) are capable of mitigating the tele-traffic bottleneck. By multicasting the content of common interest from a content owner to content seekers within the owner's transmission range, a distributed MSNs architecture is proposed, which is capable of mitigating the tele-traffic imposed on network operators. In this contribution, the social relationship between a pair of MSN users is defined according to their geographic characteristics. By jointly considering the geographic social relationships and the wireless propagation environment, we derive the closed-form equations for evaluating both the throughput and delay of the social unicast/multicast transmissions. Simulation results are provided, both for supporting our theoretical analysis, as well as for investigating the impact of social relationships on the achievable network performance. Based on the results presented, we conclude that a more socially-minded content owner is particularly efficient in multicasting the content of common interest to content seekers.

A novel hybrid information dissemination scheme is proposed for picocellular systems. At the first stage of our scheme, some of the mobile users (MUs) successfully receive the information of common interest via BS-aided multicast. At the second stage, the information is cooperatively multicast (co-multicast) by the information owners (IOs) in a self-organized ad hoc network until all the MUs receive it. Since limited resources are provided for the second stage of the spontaneous co-multicast, some of the IOs are appointed for relaying the information to the hitherto unserved MUs. Several relay selection protocols are conceived for the sake of improving the performance of our hybrid information dissemination scheme. The simulation results demonstrate that our scheme may reduce the average dissemination delay and the average energy dissipation by 20% and 70%, respectively, when compared to the conventional BS aided single-hop multicast. Furthermore, we demonstrate that relying on the IOs associated with the best links plays a crucial role in facilitating spontaneous information dissemination.

This study proposes a security‐oriented cooperation (SOC) scheme, which first determines one of the three possible cooperation scenarios, namely the jammer only, relay only and the relay–jammer pair, according to the security requirement and network's operational conditions. After determining a cooperation scenario, then, the selection of a relay or/and a jammer as well as their transmit power are jointly optimised with the objective to attain a good trade‐off between security performance and energy consumption. The performance of the networks employing the proposed SOC is investigated here over finite‐state Markov channels. The studies and results explain that the proposed SOC scheme constitutes one of the promising SOC schemes. It belongs to a generalised cooperative security scheme, which adapts according to the specific security requirement and communication environments.

Most of the nodes in ad hoc networks rely on batteries, which requires energy saving. Hence, numerous energy-efficient routing algorithms have been proposed for solving this problem. In this paper, we exploit the benefits of cross-layer information exchange, such as the knowledge of the Frame Error Rate (FER) in the physical layer, the maximum number of retransmissions in the Medium Access Control (MAC) layer and the number of relays in the network layer. Energy-consumption-based Objective Functions (OF) are invoked for calculating the end-to-end energy consumption of each potentially available route for both Traditional Routing (TR) and for our novel Opportunistic Routing (OR), respectively. We also improve the TR and the OR with the aid of efficient Power Allocation (PA) for further reducing the energy consumption. For the TR, we take into account the dependencies amongst the links of a multi-hop route, which facilitates a more accurate performance evaluation than upon assuming the links that are independent. Moreover, two energy-efficient routing algorithms are designed based on Dijkstra's algorithm. The algorithms based on the energy OF provide the theoretical bounds, which are shown to be close to the bound found from exhaustive search, despite the significantly reduced complexity of the former. Finally, the end-to-end throughput and the end-to-end delay of this system are analyzed theoretically and a new technique of characterizing the delay distribution of OR is proposed. The simulation results show that our energy-efficient OR outperforms the TR and that their theoretical analysis accurately matches the simulation results.

This paper addresses the subcarrier allocation in downlink multicarrier direct-sequence code-division multiple access (MC DS-CDMA) systems, where one subcarrier may be assigned to several users who are then distinguished from each other by their unique direct-sequence spreading codes. We first analyze the advantages and shortcomings of some existing subcarrier-allocation algorithms in the context of the MC DS-CDMA. Then, we generalize the worst subcarrier avoiding (WSA) algorithm to a so-called worst case avoiding (WCA) algorithm, which achieves better performance than the WSA algorithm. Then, the WCA algorithm is further improved to a proposed worst case first (WCF) algorithm. Furthermore, we propose an iterative worst excluding (IWE) algorithm, which can be employed in conjunction with the WSA, WCA, and the WCF algorithms, forming the IWE-WSA, IWE-WCA, and the IWE-WCF subcarrier-allocation algorithms. The complexities of these algorithms are analyzed, showing that they are all low-complexity subcarrier-allocation algorithms. The error performance is investigated and compared, demonstrating that we can now be very close to the optimum performance attained by the high-complexity Hungarian algorithm.

In this contribution, we investigate the signal detection in the multicarrier direct-sequence code-division multiple-access (DS-CDMA) systems employing both time (T)-domain and frequency (F)-domain spreading, which are referred to as the TF/MC DS-CDMA systems. A multicell scenario with universal frequency reuse is considered, where signals experience frequency-selective Rayleigh fading. A decision-feedback multiuser detector (MUD), which represents the extension of a so-called receiver multiuser diversity aided multi-stage minimum mean-square error MUD (RMD/MS-MMSE MUD), is employed to cope with both the intracell multiuser interference (MUI) and intercell interference (ICI), when base-station (BS) cooperation is or is not assumed. Furthermore, when BS cooperation is assumed, symbols detected at different BSs are interchanged and combined to enhance the detection performance. Our studies and performance results demonstrate that the RMD/MS-MMSE MUD constitutes one of the promising multicell processing (MCP) schemes, making it possible for each cell to support heavily overloaded users, even when a frequency-reuse factor one is applied.

This contribution investigates the achievable error performance of transmitter preprocessing in the downlink multicell multicarrier direct-sequence code-division multiple-access (MMC/DS-CDMA) systems employing both time (T)-domain and frequency (F)-domain spreading. Three types of multiuser transmitter preprocessing (MUTP) schemes are studied and compared, when assuming communications over frequency-selective Rayleigh fading channels. The first one is a single-cell minimum mean-square error MUTP (SMMSE-MUTP1), which only aims at suppressing the intracell interference (IntraCI). The second one is also a single-cell MMSE-MUTP (SMMSE-MUTP2), which tries to suppress both the IntraCI and intercell interference (InterCI). The final one is the multicell cooperative MMSE-MUTP (CMMSE-MUTP), which exploits the multicell diversity (or macro-diversity) for interference suppression and performance enhancement. Furthermore, power-allocation in these schemes is considered. Our studies show that the CMMSE-MUTP is capable of achieving the best error performance among the three schemes considered. However, it demands an extremely high complexity, as it requires information exchange among the base-stations (BSs) with the aid of a backhaul system. By contrast, when the number of mobile terminals (MTs) supported by each cell is not very high, the SMMSE-MUTP2, which does not require any intercell cooperation, can effectively mitigate both the IntraCI and the InterCI.

In this work, we propose a minimum mean squared error (MMSE) robust base station (BS) precoding strategy based on switched relaying (SR) processing and limited transmission of side information for interference suppression in the downlink of multiuser multiple-input multiple-output (MIMO) relay systems. The BS and the MIMO relay station (RS) are both equipped with a codebook of interleaving matrices. For a given channel state information (CSI) the selection function at the BS chooses the optimum interleaving matrix from the codebook based on two optimization criteria to design the robust precoder. Prior to the payload transmission the BS sends the index corresponding to the selected interleaving matrix to the RS, where the best interleaving matrix is selected to build the optimum relay processing matrix. The entries of the codebook are randomly generated unitary matrices. Simulation results show that the performance of the proposed techniques is significantly better than prior art in the case of imperfect CSI.

In network multiple‐input–multiple‐output (MIMO) systems, cooperative base stations (BSs) and mobile terminals (MTs) are in general at different geographic locations. Correspondingly, the channel gains with respect to different BSs and/or MTs may obey different distributions, which resulted from, different pathlosses, different strength of line‐of‐sight (LOS) components and so on. Furthermore, future wireless communication systems are expected to be equipped with multiple antennas for transmission/receiving. In these multiantenna systems, signals received by the antennas of one BS from one MT may be correlated. Against the above‐mentioned scenarios, in this contribution, the authors study the capacity of the generalised uplink network MIMO systems with multicell cooperation (MCoP), when propagation pathlosses, fast Rician fading with various LOS components and spatial correlation are simultaneously considered. Specifically, the theory of operator‐valued free probability is introduced to derive the approximate eigenvalue distribution (AED) of the correlation matrix of the equivalent channels. Based on the AED, the approximate capacity of uplink network MIMO systems is then studied. Furthermore, a range of special cases are analysed by specialise the authors model and modifying their results. Finally, both numerical and simulation results are provided for characterising the capacity of network MIMO systems with MCoP.

The delay characteristics of network coding nodes and Butterfly network are investigated. First, the probability mass function (PMF) of delay and the average delay of general network coding nodes having multiple input links and one output link are analysed. It can be shown that the PMF and average delay can be readily evaluated from the recursive equations derived. Then, the delay performance of the Butterfly network is investigated. Specifically, the authors consider four different scenarios in the context of the packets transmitted over the Butterfly network. The PMFs of delay as well as the average delays of these four scenarios are analysed and, correspondingly, a range of formulae are obtained. Finally, the delay performance of network coding nodes and Butterfly network is investigated based on the numerical results evaluated from the formulae derived. The authors studies demonstrate that network coding nodes embedded in a network generate a big impact on the network's delay performance. Usually, in a network including some network coding nodes, the longest path with some network coding nodes may be chosen for approximately estimating the network's average delay.

The articles in this special issue focus on the technology and applications supported by virtual multiple antennas, or VMIMOs. The impetus for this has been spurred by the strong desire to understand VMIMO, which is a rapidly growing research area. VMIMO is believed to be a key technology for beyond 4th generation mobile communications technologies (B4G). It enables one to make use of all the neighboring terminals and amortize the cost of multiple antennas; hence, a large MIMO channel can be created to increase capacity significantly as well as improve error rate performance. Nevertheless, fundamental roadblocks need to be addressed in order to take full advantage of VMIMO.

In this contribution, we propose and study a signal transmission scheme, namely the multihop diversity (MHD) scheme, conceived for transmission of information over multihop links (MHLs). During each time-slot (TS), the MHD scheme activates the specific hop transmission, whose signal-to-noise ratio (SNR) cumulative distribution function (CDF) gives the highest ordinate value amongst all the available hops. The next packet is then transmitted over the selected hop. Our studies show that this CDF-aware MHD scheme represents a generalized MHD scheme suitable for operation in the scenarios, where the different hops may have different distances, hence resulting in different average SNRs, and/or experience different types of fading. This MHD scheme is also capable of achieving the full diversity gain provided by the independent fading experienced by the different hops. In this paper, our attention is dedicated to the error probability, outage probability as well as to the achievable diversity order, when BPSK and M-ary quadrature amplitude modulation (MQAM) are employed. Both the error probability and the outage probability of the MHLs are investigated for transmission over Nakagami-m fading channels, both of which demonstrate a significant diversity gain. Furthermore, the impact of the various system parameters on the achievable error probability and outage performance is characterized.

In this paper, we propose a novel low-complexity single-user noncoherent combining scheme, named as the erasure-supported equal-gain combining assisted detector (ES-EGCD), for signal detection in fast frequency-hopping M-ary frequency-shift keying (FFH/MFSK) systems. In the proposed ES-EGCD, the decision variables are formed based on the principles of equal-gain combining (EGC), after erasing a number of entries having the largest values from each of the rows of a so-called detection matrix formed during detection. We derive the single-user bit-error rate (BER) of the FFH/MFSK systems employing the ES-EGCD and investigate by simulation the BER performance of the multiuser FFH/MFSK systems employing either the conventional EGC-assisted detector (EGCD) or the proposed ES-EGCD, when assuming communications over Rayleigh fading channels. Both random and optimum FFH addresses are considered in our studies. Our studies and performance results illustrate that the ES-EGCD is capable of significantly outperforming the conventional EGCD in multiuser scenarios. Although the ES-EGCD is a low-complexity single-user detector, it employs the capability to effectively mitigate multiuser interference (MUI).

A new Multiple Input Multiple Output (MIMO) transmission scheme termed as Generalised Pre-coding aided Spatial Modulation (GPSM) is proposed, where the key idea is that a particular subset of receive antennas is activated and the activation pattern itself conveys useful information. This is in contrast to the previously proposed Spatial Modulation (SM) schemes, which operated by activating a specific subset of transmit antennas. We provide both analytical and numerical results characterizing the GPSM scheme proposed for both conventional as well as for large-dimensional MIMO configurations subject to both realistic imperfect Channel State Information at the Transmitter (CSIT) and to the low-rank approximation invoked for large-dimensional MIMO configurations. We also design a so-called reinforcement matrix for attaining substantial performance improvements for our proposed GPSM scheme. Our investigations show that the GPSM scheme constitutes a flexible alternative to the state-of-the-art MIMO transmission schemes, especially because it is capable of achieving a high throughput. Moreover, the benefits of mapping information to the spatial domain rather than relying on conventional modulation has substantial benefits in the medium to high Signal to Noise Ratio (SNR) region. Quantitatively, GPSM is capable of supporting the same throughput as the conventional full-multiplexing gain based MIMO arrangements at an SNR gain of about 1dB at the same receiver complexity. Furthermore, the reinforcement matrix aided GPSM scheme attains a further 3-4dB performance improvement as compared to the conventional GPSM scheme.

As a promising technique of improving the attainable bandwidth efficiency, cognitive radio (CR) has attracted substantial attention from both the academic and industrial communities. In order to improve the performance of the secondary user (SU), a novel hybrid CR system is introduced, which combines the conventional interweave and underlay paradigms for enhancing the chances of the SU to access the spectrum. Queuing theory is invoked in this paper for analysing the impact of the primary user's (PU) delay tolerance on the performance of the SU. Multiple queues are assumed for the SU engaging in video communication. Besides the Poisson traffic generation, we model a Rayleigh fading channel as a Poisson service process with the aid of the outage probability in the presence of cochannel interferences. Two valuable goals are achieved, namely that of maximizing the average service rate and minimizing the overall average delay of the SU's multiple queues. As our numerical results demonstrate, the overall average delay of the SU becomes 27% and 34% lower than that of the Proportional as well as that of the Round-Robin schemes respectively.

In this paper we exploit the benefits of the diversity gains arising from a cluster of opportunistic relays (OR) and from the independently fading subcarriers of multiple users. Our goal is to improve the energy-efficiency of the OR assisted single-carrier frequency-division multiple-access (SC-FDMA) uplink using amplify-and-forward (AF), where the direct transmission (DT) link is unavailable. By assuming that the pilot aided channel quality information (CQI) of all the users may be exchanged amongst the cooperating relays, we propose two joint dynamic resource allocation (DRA) schemes based on the so-called `first-hop quality awareness'. Our results demonstrate that compared to the DT benchmark, the proposed joint DRA schemes are capable of achieving a power reduction of 10dB for a single-antenna base station (BS) receiver, albeit for a multi-antenna BS the power-reduction remains more modest.

The error performance of a two-hop communication link (THCL) supported by a cluster of relay nodes (RNs) is investigated. The THCL includes one source node (SN) and one destination node (DN). The SN sends information to the DN via a cluster of RNs. At the RNs, signals received from the SN are processed based on one of the three relay processing schemes. The three relay processing schemes considered include the distributed relay processing, maximal-ratio combining (MRC)-assisted relay processing and the majority vote and equal-gain combining (MV-EGC) aided relay processing. For the MRC-assisted and MV-EGC aided relay processing schemes, information exchange among the RNs is supported by a local information exchange network, within which communicates are based on the principles of direct-sequence code-division multiple-access (DS-CDMA). In this contribution, the bit error rate (BER) performance of the THCLs is investigated and compared, when assuming that the first and second hops of the THCLs experience flat Rayleigh fading and that the communications within the local information exchange network conflict only Gaussian noise. We address the impact of the energy consumed for RNs' cooperation on the achievable BER performance of the THCLs. Our studies demonstrate that, when the energy spent for cooperation is taken into account, cooperation among the RNs may impose a big trade-off on the achievable BER performance of the THCLs.

In this paper, we propose a novel WSN framework for one fusion center to monitor simultaneously multiple source events (SEs), each of which has multiple states. In the proposed WSN, every SE is observed by a range of local sensors, which convey their observations to the fusion center in the principles of direct-sequence code-division multiple-access (DS-CDMA) associated with M-ary orthogonal modulation (MOM). Therefore, the proposed WSN is referred to as the MOM DS-CDMA WSN. In the MOM DS-CDMA WSN, the local sensors monitoring the same SE transmit their signals to the fusion center in the principles of time-division multiple-access (TDMA), while the local sensors serving different SEs communicate with the fusion center in the principles of DS-CDMA. By making use of the advantages of MOM, the fusion center detects the SEs' states either coherently based on the maximal ratio combing (MRC) fusion rule, or noncoherently based on the equal gain combining (EGC) fusion rule. The detection performance of the fusion center is investigated by simulations. Specifically, the impacts of the number of SEs, the number of local sensors per SE, the observation reliability of local sensors, the reliability of the channels from local sensors to fusion center, etc., on the fusion detection performance are addressed.

In this contribution, a novel wireless sensor network (WSN) assisted by M-ary frequency-shift keying (MFSK) modulation and frequency-hopping (FH), referred to as the FH/MFSK WSN, is proposed to monitor multiple source events (SEs). In the FH/MFSK WSN, the multiple SEs of each with multiple states are observed by a common group of local sensors (LSNs), each of which observes simultaneously all the SEs. The LSNs convey their decisions through wireless channels to the fusion center (FC) with the aid of MFSK and FH. At the FC, the SEs' states are detected using noncoherent fusion rules. In this contribution, two noncoherent fusion rules are investigated. The first one is the equal gain combining (EGC) fusion rule, while the second one uses both the EGC and iterative interference cancellation (IIC), which is referred to as the EGC-IIC fusion rule. The detection performance of the FH/MFSK WSN employing the proposed EGC or EGC-IIC fusion rule is investigated, when assuming the wireless channels experience independent Rayleigh fading. Our studies show that the FH/MFSK WSN may constitute one of the promising WSN schemes for some special application scenarios. It is capable of monitoring simultaneously multiple SEs of each with multiple states, is low-complexity owing to using the noncoherent fusion rule and, furthermore, is capable of achieving promising detection performance, as it can make use of the diversity in both frequency and space domains.

In this paper, we investigate the achievable performance of channel coded single-carrier frequency-division multiple-access (SC-FDMA) systems employing various detection schemes, when communicating over frequency-selective fading channels. Specifically, three types of minimum mean-square error (MMSE) based frequency-domain (FD) turbo equalisers are considered. The first one is the turbo FD linear equaliser (LE). The second one is a parallel interference cancellation (PIC)-assisted turbo FD decision-feedback equaliser (DFE). The final one is the proposed hybrid interference cancellation (HIC)-aided turbo FD-DFE, which combines successive interference cancellation (SIC) with iterative PIC and decoding. The benefit of interference cancellation (IC) is analysed with the EXtrinsic Information Transfer (EXIT) charts. The performance of the coded SC-FDMA systems employing the above-mentioned detection schemes is investigated with the aid of simulations. Our studies show that the IC techniques achieve an attractive performance at a moderate complexity.

The steady-state performance of general network coding node is investigated, when data are transmitted in packets based on the stop-and-wait automatic repeat request error-control scheme. The general network coding node considered has H number of incoming links that provide packets for forming the coded packets transmitted by one outgoing link. Each of the incoming and outgoing links is assumed to have some buffers for temporarily storing the data packets. The state transition of the general network coding node is modelled by a finite state machine, and its steady-state performance is derived based on the properties of discrete-time Markov chain. Specifically, the throughput, blocking probability and the distribution of contents are analysed, when the general network coding node is operated in the steady state. The performance of the general network coding node is investigated either by simulations or by evaluation of the formulas derived in this study. The performance results show that the analytical formulas obtained can be well justified by the simulation results. Furthermore, it can be shown that the packet error rate, the capacity of buffer and the number of incoming links may impose significant impact on the performance of general network coding node.

The rationale of our design is that although much of the literature of cooperative systems assumes perfect coherent detection, the assumption of having any channel estimates at the relays imposes an unreasonable burden on the relay station. Hence, non-coherently detected Reed-Solomon (ReS) coded Slow Frequency Hopping (SFH) assisted M-ary Frequency Shift Keying (FSK) is proposed for cooperative wireless networks, subjected to both partial-band interference and Rayleigh fading. Erasure insertion (EI) assisted ReS decoding based on the joint maximum output-ratio threshold test (MO-RTT) is investigated in order to evaluate the attainable system performance. Compared to the conventional error-correction-only decoder, the EI scheme may achieve an E_b/N_0 gain of approximately 3 dB at the Codeword Error Probability, P_w, of 10^{-4}, when employing the ReS(31,20) code combined with 32-FSK modulation. Additionally, we evaluated the system's performance, when either equal gain combining (EGC) or selection combining (SC) techniques are employed at the destination's receiver. The results demonstrated that in the presence of one and two assisting relays, the EGC scheme achieves gains of 1.5 dB and 1.0 dB at the P_w of 10^{-6}, respectively, compared to the SC arrangement. Furthermore, we demonstrated that for the same coding rate and packet size, the ReS(31,20) code using EI decoding is capable of outperforming convolutional coding, when 32-FSK modulation is considered, whilst LDPC coding had an edge over the above two schemes.

This paper analyzes the steady-state throughput of network coding nodes when data is transmitted based on the stop-and-wait automatic repeat request (SW-ARQ) scheme. The state transition of network coding nodes employing SW-ARQ is analyzed, which shows that the operations of network coding nodes can be modeled by a finite state machine. Therefore, the throughput expressions of network coding nodes can be derived based on the properties of finite state machines. Furthermore, the throughput performance of network coding nodes is investigated either by simulations or by evaluation of the expressions obtained. It can be shown that the simulation results converge closely to the numerical results and justify the effectiveness of our analytical expressions obtained.

In this contribution, we analyze the asymptotic spectral-efficiency (ASE) of multiuser MIMO-CDMA systems, when assuming communications over flat fading channels with arbitrary spatial correlation. Our analysis is built on the operator-valued free probability theory, which is applied to obtain the limit distribution of the correlation matrix's eigenvalues, as the MIMO-CDMA systems' size tends to infinity. The spectral-efficiency (SE) performance of the MIMO-CDMA systems is investigated via both analysis and simulations. Our simulation and numerical results show that the ASE is capable of providing a good measure of the SE achieved by the corresponding realistic MIMO-CDMA systems.

The concept of multihop diversity is proposed, where all the nodes of a multihop link are assumed to have buffers for temporarily storing their received packets. During each time-slot, the best hop having, for example, the highest signal-to-noise ratio (SNR), is selected from the set of those hops, where the corresponding nodes have packets awaiting transmission in their buffer. The packet is then transmitted over the best hop. Explicitly, this hop-selection procedure yields selection diversity. In this paper, we assume having perfect channel knowledge and focus our attention on the principles and performance bounds of the error probability and outage probability, when M-ary quadrature amplitude modulation (MQAM) is employed. The error probability and outage probability of the multihop links operated under our proposed multihop diversity scheme are investigated, when communicating over Rayleigh fading channels. Our studies show that relying on multiple hops has the potential of providing a significant diversity gain, which may be exploited for enhancing the reliability of wireless multihop communications.

In this contribution, we employ the Mellin transform to derive the expressions for probability density function (PDF) of the product of Nakagami-m and Gamma distributed random variables. As the special cases of the Nakagami-m and Gamma family, the PDF of the product of Rayleigh distributed random variables and that of the product of exponentially distributed Random variables are also derived. We exploit the fact that the Mellin transform of a product of independent and identically distributed random variables is the product of the Mellin transforms of the individual random variables. Using this approach, the PDF of the product of random variables is expressed in the form of an easily computable infinite series. Furthermore, application of the PDFs in digital communications using M-ary orthogonal modulation is illustrated.

The concept of multihop diversity is proposed, where all the nodes of a multihop link are assumed to have buffers for temporarily storing their received packets. During each time-slot, the best hop having, for example, the highest signal-to-noise ratio (SNR), is selected from the set of those hops that have packets awaiting transmission in the buffer. The packet is then transmitted over the best hop. This hop-selection procedure yields selection diversity, but it requires the global channel knowledge of the hops' channel quality. In this paper, we assume having perfect channel knowledge and focus our attention on the principles and performance bounds of the error probability and outage probability. Our studies show that relying on multiple hops has the potential of providing a significant diversity gain, which may be exploited for enhancing the reliability of wireless multihop communications.

The rationale of our design is that although much of the literature of cooperative systems assumes perfect coherent detection, the assumption of having any channel estimates at the relays imposes an unreasonable burden on the relay station. Hence, non-coherently detected Reed-Solomon (ReS) coded Slow Frequency Hopping (SFH) assisted M-ary Frequency Shift Keying (FSK) is proposed for cooperative wireless networks, subjected to both partial-band interference and Rayleigh fading. Erasure insertion (EI) assisted ReS decoding based on the joint maximum output-ratio threshold test (MO-RTT) is investigated in order to evaluate the attainable system performance. Compared to the conventional error-correction-only decoder, the EI scheme may achieve a gain of approximately 3 dB at the Eb/N0of 10-4, when employing the ReS(31,20) code combined with 32-FSK modulation. Additionally, we evaluated the system's performance, when either equal gain combining (EGC) or selection combining (SC) techniques are employed at the destination's receiver. The results demonstrated that in the presence of one and two assisting relays, the EGC scheme achieves gains of 1.5 dB and 1.0 dB at the Eb/N0of 10-6, respectively, compared to the SC arrangement.

A multiway relay direct-sequence code-division multiple-access (MR-DS-CDMA) system is proposed for exchanging information among a group of distributed mobile terminals (MTs). In order to implement information exchange, a MT having similar distances from the other MTs is chosen to act as a relay, which assists the whole group of MTs to achieve their information exchange within two time-slots. In our proposed MR-DS-CDMA system, the relay is operated in detection-and-forward (DF) relaying strategy. Signals at the relay and the MTs are detected in the principles of minimum mean-square error multiuser detection (MMSE-MUD) or of the receiver multiuser diversity assisted multi-stage MMSE MUD (RMD/MS-MMSE MUD). The error performance of the MR-DS-CDMA is investigated, when communicating over Nakagami-m fading channels. Our studies show that the MR-DS-CDMA employing the RMD/MS-MMSE MUD at both the relay and MTs constitutes a high-efficiency information exchange system. It has low delay, promising error performance and low-complexity. Furthermore, it is robust to system overloading, where the number of MTs supported is higher than the spreading factor of the DS-CDMA system.

This paper proposes and studies a transmitter preprocessing aided spatial modulation (PSM) scheme, which conveys information jointly by a conventional amplitude-phase modulation (APM) and a preprocessing aided space shift keying (pre-SSK) modulation. In contrast to the existing SSK modulation, which carries information using the indexes of transmitter antennas and assumes channel state information at receiver (CSIR), the pre-SSK modulation extracts the transmitted information using the indexes of receive antennas and assumes channel state information at transmitter (CSIT). This paper addresses the issues of preprocessing optimization and detection of PSM signals. Furthermore, the bit-error-rate (BER) performance of the PSM is investigated, when assuming that the channel from any transmit antenna to any receive antenna experiences independent Rayleigh fading. Our studies show that, when appropriately designed, the pre-SSK and APM invoked can enhance each other, resulting in that the PSM is capable of attaining a better BER performance than the corresponding pure pre-SSK and pure APM.

Redundant residue number system (RRNS)-based multicarrier DS-CDMA (MC/DS-CDMA) is proposed for dynamic multiple-access (DMA) in cognitive radios (CRs). The proposed RRNS-based MC/DS-CDMA DMA has the merits of low-complexity for implementation, high-flexibility for reconfiguration and spectrum handoff, robustness to spectrum varying, and fault-tolerance to errors. Specifically, in our RRNS-based MC/DS-CDMA DMA system, RRNS-based orthogonal modulation aided by MC/DS-CDMA is employed for information transmission. At the receiver, signals are detected subcarrier-by-subcarrier independently based on suboptimum MMSE interference cancellation (SMMSE-IC). In performance study, we model the arrival process of primary users (PUs) in primary radios (PRs) as a Poisson process. Both the bit error rate (BER) performance and throughput performance are investigated. Our studies and performance results show that the RRNS-based MC/DS-CDMA constitutes one of the highly promising DMA schemes for application in CRs. It is capable of achieving a substantial throughput with required quality for the CR systems, while without degrading the quality-of-services (QoS) of the PR systems.

In ad hoc networks the nodes actively and voluntarily participate in constructing a network and act as relays for other nodes. As a result of node-mobility, the Channel State Information (CSI) varies and hence a substantial amount of control messages have to be exchanged across the network to maintain reliable communications between certain pairs of nodes, which potentially imposes a high energy-consumption. Therefore, minimizing the energy consumption and maximizing the throughput of ad hoc nodes is extremely important. This paper analyzes both the energy consumption and the achievable throughput of a multihop route by exploiting both the CSI quantified, for example in terms of the Frame Error Ratio (FER), as well as the number of Medium Access Control (MAC) retransmissions and the number of hops. Since using limited number of MAC retransmissions in a hop-by-hop retransmission mode imposes dependencies amongst the links of a multi-hop route, a more accurate objective function may be formulated than that which assumes the availability of an infinite number of retransmissions and which considers the links to be independent. Our simulations confirm the improved accuracy of the proposed Objective Function (OF), especially when the FER and the number of hops are high and the number of MAC retransmissions is low. Additionally, a low-complexity routing algorithm is designed, which carries out routing decisions based on the energy consumption predicted by the OF, and strikes a compromise between having 'few long-distance hops' and 'many short-distance hops' for the sake of energy minimization and throughput maximization.

The DFT-based beamforming weight-vector codebook is considered as an effective design for spatially correlated channels. In this paper, we demonstrate that when the antenna elements are uniformly spaced as well as linearly arranged, and the channels are spatially correlated, the codewords in a DFT-based beaforming weight-vector codebook approximately match the distribution of the optimal beamforming weight-vectors. As a result, the DFT-based codebook is indeed effective. Furthermore, we also demonstrate that if the antenna elements are uniformly spaced and circulary arranged, the statistical distribution of the optimal beamforming weight-vectors becomes different. We will demonstrate that in this scenario the DFT-based codebook will no longer outperform the Grassmannian codebook, which has not been shown in previous studies. Finally, an algorithm is proposed for constructing the DFT-based precoding matrix, which outperforms the conventional algorithm by ensuring the orthogonality of the precoding matrix.

The performance of multihop links is studied in this contribution by both analysis and simulations, when communicating over Generalized-K (KG) fading channels. The performance metrics considered include symbol error rate (SER), outage probability, level crossing rate (LCR) and average outage duration (AOD). First, the expressions for both the SER and outage probability are derived by approximating the probability density function (PDF) of the end-to-end signal-to-noise ratio (SNR) using an equivalent end-to-end PDF. We show that this equivalent end-to-end PDF is accurate for analyzing the outage probability. Then, the second-order statistics of LCR and AOD of multihop links are analyzed. Finally, the performance of multihop links is investigated either by simulations or by evaluation of the expressions derived. Our performance results show that the analytical expressions obtained can be well justified by the simulation results. The studies show that the KGchannel model as well as the expressions derived in this paper are highly efficient for predicting the various types of performance metrics and statistics for design of multihop communication links.

In this contribution we investigate the spectral-efficiency of a two-hop cooperative network using multicarrier code-division multiple-access (MC-CDMA) transmission scheme. The two-hop network constitutes K source users transmitting signals to K destinations with the aid of N relays. Our focus is on the relay optimization, when assuming that the N relays cooperate or do not cooperate with each other. Specifically, in this contribution the egocentric-altruistic (E-A) optimization is introduced, which constitutes an E-optimization motivating to suppress the multiuser interference (MUI) of the source-relay channels and an A-optimization aiming to pre-mitigate the potential MUI of the relay-destination channels. Both the minimum mean-square error (MMSE) and zero-forcing (ZF) optimization criteria are considered. Furthermore, the spectral-efficiency performance of the two-hop MC-CDMA systems using the proposed E-A relay optimization is investigated by simulations, when assuming communications over frequency-selective fading channels.

In this contribution the asymptotic signal-to-interference-plus-noise ratio (SINR) performance of multicarrier direct-sequence code-division multiple-access systems employing time-frequency-domain spreading, i.e., of the TF/MC DS-CDMA systems, is studied, when separate minimum mean-square error multiuser detection (MMSE-MUD) is considered. The separate MMSE-MUD detects signals first in the time (T)-domain and then in the frequency (F)-domain. Based on random matrix theory, closed-form expressions for the asymptotic SINR of the TF/MC DS-CDMA systems using separate MMSE-MUD is derived, when communicating over additive white Gaussian noise (AWGN) channels. The closed-form expressions show that the asymptotic SINR performance is only depended on the T- and F-domain user load factors as well as noise variance. Hence, they are beneficial to evaluation. Furthermore, our simulation and numerical results show that in most cases the asymptotic SINR can provide a good approximation to the SINR achieved by realistic TF/MC DS-CDMA systems employing separate MMSE-MUD.

This contribution analyzes the steady-state throughput of general network coding nodes, when data is transmitted based on stop-and-wait automatic repeat request (SW-ARQ) scheme. The state transition of network coding nodes employing SW-ARQ is analyzed, which shows that the operations of network coding nodes can be modelled by a finite state machine. Therefore, the throughput expressions of general network coding nodes can be derived based on the properties of finite state machines. Furthermore, the throughput performance of network coding nodes is investigated either by simulations or by evaluation of the expressions obtained. It can be shown that the simulation results converge closely to the numerical results and justify the effectiveness of our analytical expressions obtained.

The steady-state throughput of general network coding node is investigated, when data is transmitted in packets based on the stop-and-wait automatic repeat request (SW-ARQ) error-control scheme. The general network coding node has H number of incoming links that provide packets for forming the coded packets transmitted by one outgoing link. Each of the incoming and outgoing links is assumed to have some buffers for storing the temporary data packets. The state transition of the general network coding node is modelled by a finite state machine, and its steady-state throughput is derived based on the properties of discrete-time Markov chain. Furthermore, the throughput performance of the general network coding node is investigated either by simulations or by evaluation of the expressions derived in this paper. Our performance results show that the analytical expressions obtained can be well justified by the simulation results. Furthermore, it can be shown that the packet error rate, the amount of buffer and the number of incoming links may impose significant impact on the achievable throughput of the general network coding nodes.

In this treatise we propose a three-stage twin-transmit-antenna assisted Multiuser (MU) Direct Sequence Code-Division Multiple Access (DS-CDMA) system employing both a Unitary Rate Code (URC) and a Recursive Systematic Code (RSC) to carry out iterative turbo detection. A Space-Time Block Code (STBC) is used to provide second-order diversity gain in conjunction with a novel soft-output (SO) ant-colony-optimization (ACO) based space-time multiuser detection (ST/MUD) algorithm, which is capable of carrying out STBC decoding, while mitigating the Multiuser Interference (MUI) and generating the Log-Likelihood Ratios (LLR) for facilitating the iterative exchange of extrinsic information between the URC and RSC decoders of each user. With aid of the above mentioned state-of-the-art techniques, the proposed system becomes capable of approaching the performance of the single-user system within about 0.5dB, when K = 32 users are supported with the aid of 31-chip Gold-codes.

In this contribution, a novel Ant Colony Optimization (ACO) based Space-Time (ST) Multiuser Detection (MUD) algorithm was proposed in order to improve the Bit Error Ratio (BER) versus Signal-to-Noise Ratio (SNR) performance achieved by the Space-Time Block Code (STBC) assisted two transmit antenna aided fully loaded DS-CDMA system directly employing an ACO based MUD algorithm. Our simulation results demonstrate that the proposed ACO based MUD algorithm enables the fully loaded system to approach the single user performance associated with a second-order diversity gain. Furthermore, the complexity of the improved ACO based MUD algorithm is a factor of 1018lower than that of the Maximum Likelihood (ML) MUD, when K = 32 users are supported by the STBC Q2asssited DS-CDMA systems employing Gold codes having a length of 31 chips.

Design of low-complexity detection schemes that can approach near-optimum bit error rate (BER) performance in multiuser or other multiple-input multiple-output (MIMO) systems has always been highly challenging. In this paper we propose and investigate a so-called receiver multiuser diversity aided multi-stage minimum mean-square error multiuser detection (RMD/MS-MMSE MUD) scheme operated in the principles of successive interference cancellation (SIC). The BER performance of the RMD/MS-MMSE MUD is investigated in association with both the direct-sequence code-division multiple-access (DS-CDMA) communicating over both Gaussian and Rayleigh fading channels, and the space-division multiple-access (SDMA) communicating over Rayleigh fading channels. Furthermore, we assume that the DS-CDMA and SDMA systems are either full-load or overload. Our studies show that the RMD/MS-MMSE MUD in full-load cases is capable of converging to the optimum BER performance of the maximum likelihood (ML) multiuser detector (MUD). For the overload systems, the RMD/MS-MMSE MUD can make a DS-CDMA or SDMA system support K=2N users, but still achieve much better BER performance than a corresponding DS-CDMA or SDMA system using the conventional MMSE-MUD to support K=N users, where N denotes the spreading factor of DS-CDMA or the number of receive antennas of SDMA.

The so-called receiver multiuser diversity aided multi-stage minimum mean-square error multiuser detector (RMD/MS-MMSE MUD), which was proposed previously by the author, is investigated in the context of the direct-sequence code-division multiple-access (DS-CDMA) and space-division multiple-access (SDMA) systems that employ in- and quadrature-phase (I-Q) modulation schemes. A detection scheme is studied, which is operated in real domain in the principles of successive interference cancellation (SIC). The concept of noise recognition factor (NRF) is proposed for explaining the efficiency of SIC-type detectors and also for motivating to design other high-efficiency detectors. The achievable bit error rate (BER) performance of the RMD/MS-MMSE MUD is investigated for DS-CDMA and SDMA systems of either full-load or overload, when communicating over Rayleigh fading channels for the SDMA and over either additive white Gaussian noise (AWGN) or Rayleigh fading channels for the DS-CDMA. The studies and performance results show that the RMD/MS-MMSE MUD is a highly promising MUD. It has low implementation complexity and good error performance. Furthermore, it is a high-flexibility detector suitable for various communication systems operated in different communication environments.

In this contribution we exploit the benefits of combining the diversity gains arising from cooperation, multiple propagation paths and from the opportunistic scheduling of multiple users, for the sake of supporting a power-efficient single-relay assisted single-carrier frequency-division multiple-access uplink, where each relay supports a single user communicating over dispersive channels subject to large-scale fading. Based on the proposed joint frequency-domain equalisation and diversity combining aided receiver relying on the minimum mean-square error criterion, two different relay selection schemes designed for single-user and multi-user scenarios are investigated, which employ imperfect power control. Our results show that at a bit error ratio of 10-4, the proposed receiver is capable of saving 2 dB power by achieving a higher cooperative diversity gain than the conventional receiver. Moreover, an approximately 6.8 dB and 8 dB transmit power reduction are attainable by invoking the proposed selective diversity oriented single-user and multi-user relay selection schemes, respectively. Most importantly, a power-efficiency improvement is gleaned from our multi-user relay selection scheme, which avoids the effects of deep shadowing and provides 9.7dB power reduction compared to the non-cooperation scenario.

By investigating the conditions required for successive interference cancellation multiuser detectors (SIC-MUD) to achieve near-optimum BER performance, we propose and investigate a so-called receiver multiuser diversity aided multi-stage minimum mean-square error MUD (RMD/MS-MMSE MUD), which is operated in the SIC principles. The BER performance of the RMD/MS-MMSE MUD is investigated in association with both the direct-sequence code-division multiple-access (DS-CDMA) over either Gaussian or Rayleigh fading channels, and the space-division multiple-access (SDMA) over Rayleigh fading channels. Furthermore, we consider both full-load and overload scenarios in comparison with the spreading factor N of DS-CDMA and the number of receive antennas N in SDMA. Our studies show that the RMD/MS-MMSE MUD is highly efficient for both full-load and overload systems. Specifically, the RMD/MS-MMSE MUD for the full-load systems of moderate size is capable of attaining the BER performance similar to that of the optimum maximum likelihood MUD (ML-MUD). For the overload systems, it can allow a DS-CDMA or SDMA system to support K=2N users and still achieve much better BER performance than a corresponding DS-CDMA or SDMA system using conventional MMSE-MUD to support K=N users.

In this paper a range of reduced-rank adaptive multiuser detectors (MUDs) are proposed and investigated for the hybrid direct-sequence time-hopping ultrawide bandwidth (DS-TH UWB) systems. The adaptive MUDs are operated based on the recursive least square (RLS) principles. Three types of reduced-rank techniques are investigated, which are the principal component (PC), cross-spectral metric (CSM) and Taylor polynomial approximation (TPA). These reduced-rank adaptive techniques are beneficial to achieving low-complexity, high spectral-efficiency and robust detection in hybrid DSTH UWB systems. In this contribution bit error rate (BER) performance of the hybrid DS-TH UWB systems using proposed reduced-rank adaptive MUDs is investigated by simulations, when communicating over UWB channels modelled by the Saleh-Valenzuela (S-V) channel model. Our simulation results show that, given a sufficiently high rank of the detection subspace, the reduced-rank adaptive MUDs are capable of achieving a similar BER performance as that of the full-rank ideal minimum mean-square error MUD (MMSE-MUD) but with significantly lower detection complexity. Furthermore, the TPA-based reduced-rank adaptive MUD is capable of yielding a better BER performance than the PC- or CSM-based reduced-rank adaptive MUD, when the same but relatively low rank detection subspace is assumed.

This letter aims to design a low-complexity subcarrier-power allocation scheme to improve the communication reliability of various types of frequency-division multiple-access (FDMA) systems. Both uplink and downlink are considered. Specifically, a low-complexity worst subcarrier avoiding (WSA) subcarrier-allocation scheme is proposed, in order to avoid assigning users the subcarriers experiencing severe fading. After the subcarrier-allocation, channel-inversion assisted powerallocation is employed to assign the subcarriers the corresponding power. Our studies and simulation results show that the achievable error performance of the FDMA systems employing the proposed subcarrier-power allocation algorithm is independent of the multiplexing method. The proposed algorithm outperforms the existing subcarrier-power allocation algorithms that have a similar complexity as the proposed one.

In this paper we consider the low-complexity detection in hybrid direct-sequence time-hopping ultrawide bandwidth (DS-TH UWB) systems. A reduced-rank adaptive LBER detector is proposed, which is operated in the least bit error-rate (LBER) principles within a detection subspace obtained with the aid of the principal component analysis (PCA)-assisted reduced-rank technique. Our reduced-rank adaptive LBER detector is free from channel estimation and does not require the knowledge about the number of resolvable multipaths as well as that about the multipaths' strength. In this paper the bit error-rate (BER) performance of the hybrid DS-TH UWB system is investigated, when communicating over the UWB channels modelled by the Saleh-Valenzuela (S-V) channel model. Our study and simulation results show that this reduced-rank adaptive LBER detector constitutes a feasible detection scheme for deployment in practical pulse-based UWB systems.

In this contribution reduced-rank adaptive minimum mean- square error multiuser detector (MMSE-MUD) is proposed and investigated for the hybrid direct-sequence time-hopping ultrawide bandwidth (DS-TH UWB) systems. The adaptive MMSE-MUD is operated based on the normalised least mean-square (NLMS) principles associated with using Taylor polynomial approximation (TPA)-assisted reduced-rank technique. It can be shown that the reduced-rank adaptive technique is beneficial to achieving low-complexity, high convergence speed and robust detection in hybrid DS-TH UWB systems. In this contribution bit-error-rate (BER) performance of the hybrid DS-TH UWB systems using proposed reduced- rank adaptive MMSE-MUD is investigated, when communicating over UWB channels modelled by the Saleh-Valenzuela (S-V) channel model. Our simulation results show that the TPA-assisted reduced-rank adaptive MMSE-MUD is capable of achieving a similar BER performance as that of the full-rank adaptive MMSE-MUD but with significantly lower detection complexity.

A convolutionally encoded noncoherent M-ary Frequency Shift Keying (MFSK) modulation scheme using diversity reception is considered. A novel technique of generating soft information for Soft Decision Decoding (SDD) of noncoherent MFSK is derived assuming various channel models. Furthermore, the convergence behavior of the MFSK receiver is investigated using Extrinsic Information Transfer (EXIT) charts. It is demonstrated that the expressions derived for SDD of the MFSK scheme are useful for exploiting the beneficial effects of diversity. We also demonstrate that EXIT chart analysis provides accurate insights into the system's iterative decoding behavior.

This contribution considers the joint transmitter-receiver optimization in generalized multicarrier code-division multiplexing (GMCCDM) systems. The joint transmitter-receiver optimization consists of an one-tap post-frequency-domain equalizer (post-FDE) and an one-tap pre-FDE. The one-tap post-FDE is optimized under the criterion of minimum mean-square error (MMSE), while the one-tap pre-FDE is optimized in order to achieve either the highest possible throughput or best possible error performance. Furthermore, an optimum spreading scheme is proposed for the GMC-CDM system, which is capable of achieving certain signal-to-noise ratio (SNR) gain in addition to the promised close-loop diversity gain. In this paper the performance of the GMC-CDM systems is investigated, when communicating over frequency-selective Rayleigh fading channels. It can be shown that our proposed pre-FDE and post-FDE algorithms are highly efficient, which are low-complexity and capable of enhancing significantly the error or throughput performance of the GMCCDM systems.

This paper investigates the effect of different array geometries on the performance of the turbo coding assisted beamforming uplink. More specifically, we focus on the maximum achievable rate as a measure of the system performance, which is calculated with the aid of EXIT chart analysis. Our performance results recorded for K = 4 uplink receiver antenna elements at the Base Station (BS) supporting M = 4 or M = 7 users demonstrated that the Hexagonal Array (HA) slightly outperforms the Uniform Linear Array (ULA) and Uniform Circular Array (UCA), when we have a low angular spread ¿ for the Direction-Of-Arrival (DOA) of each user. It is also demonstrated that the performance difference becomes smaller upon increasing the angular spread ¿.

An iterative detection aided down-link (DL) space division multiple access (SDMA) system is proposed. The base station (BS) requires DL channel state information (CSI) estimated by and fed back from the mobile station (MS) for separating the DL signals to be transmitted to the MSs from the BS. We investigate the achievable system performance subject to finite-accuracy CSI feedback. The achievable receiver performance is investigated with the aid of extrinsic information transfer (EXIT) charts. Furthermore, a novel concept, referred to as the EXIT-chart optimized CSI quantization (ECO-CQ) is proposed. The ECO-CQ scheme assists the system in maintaining the lowest possible CSI feedback overhead, while ensuring that an open EXIT-tunnel is still attainable, which implies maintaining an infinitesimally low BER. Furthermore, we demonstrate that the proposed ECO-CQ may reduce the normalized feedback overhead compared to the conventional CQ. For instance, the ECO-CQ aided iterative DL-SDMA system using an average of q = 2.7 quantization bits per CIR coefficient achieves a 10% normalized overhead reduction at Eb/N0= 5 dB, compared to the conventional CQ aided benchmarker system.

Although multiuser detection (MUD) has received intensive research since its invention by Verdu in 1983, however, the statistical properties in MUD have not received enough attention in research and application. In this contribution, we first study the statistical properties of minimum mean-square error (MMSE)-MUD, maximum likelihood (ML)-MUD and hybrid MMSE/ML-MUD from different aspects in the context of direct-sequence code-division multiple-access (DS-CDMA) communicating over additive white Gaussian noise (AWGN) channels. Both the amplitude statistics and ratio statistics are investigated. Then, the applications of the statistics of MUD are discussed. Our study shows that the statistics embedded in the MUD schemes may be exploited for design of high- efficiency search algorithms in ML-MUD, for design of good spreading sequences, for post-MUD processing enhancement, for design of novel MUD schemes, etc.

This paper proposes a multi-stage minimum mean-square error (MS-MMSE) detector and two low-complexity reliability measurement schemes, namely the Type-L and Type-A schemes, in order for the multiple-input multiple-output (MIMO) systems using binary phase-shift keying (BPSK) modulation to attain near single-user bit-error-rate (BER) performance. In our MS-MMSE detector, signals to be detected are ranked according to their real-time reliabilities measured by the Type-L or Type-A scheme. The Type-L scheme adopts both the signal-to-interference-plus-noise ratio (SINR) and magnitudes of the MSMMSE detector's decision variables to measure the reliabilities, while the Type-A scheme only makes use of the magnitudes of the MMSE detector's decision variables for reliability measurement. The BER performance of the MS-MMSE detector is investigated associated with the proposed Type-L or Type-A scheme. Our study and performance results show that the MS-MMSE detector is a low-complexity and high-flexibility detector. Both the Type-L and Type-A schemes are high-efficiency reliability measurement schemes, which make it possible for the MS-MMSE detector to achieve near single-user BER performance.

The proposed singular value decomposition (SVD) aided spatial multiplexing (SM) based multiple-input-multiple-output (MIMO) system is capable of achieving a high bandwidth efficiency. The SVD operation effectively splits the MIMO channel into orthogonal eigen-beams having unequal bit-error-ratios (BERs). This property may be exploited for the transmission of speech, audio, video and other multimedia source signals, where the source-coded bits have different error sensitivity.

In this treatise, we have derived the lower ergodic channel capacity bound of a closed-loop transmit beamforming system scheme, when considering the effects of channel estimation errors, transmit beamforming vector quantization errors, the effective capacity loss caused by pilot symbols and that imposed by the channel information feedback bits. We intend to answer the question "whether the potential capacity improvement achieved by using a closed-loop design and an increased number of transmit antennas will be completely eroded by the imperfections of the channel information acquisition and signalling?". Based on our investigations, we found that the capacity improvements predicted are indeed achievable for slowly fading channels, but erode for realtively rapidly fading channels. An improved closed-loop transmit beamforming scheme is proposed for rapidly fading channels.

In this contribution we propose and study a novel multiuser detection (MUD) scheme for multicarrier direct-sequence code-division multiple-access systems employing both time (T)-domain and frequency (F)-domain spreading, which are referred to as the TF/MC DS-CDMA systems. Specifically, a reliability-aided MUD scheme is proposed, which consists of a linear MUD and a so-called L-level maximum-likelihood (ML)-MUD. The linear MUD is either a joint TF-domain linear MUD or constituted by two linear MUDs, one of which is operated in the T-domain and the other one in the F-domain. The L-level ML-MUD is a reduced ML-MUD, which only searches in a space of size 2Lin order to find the optimum solutions for the L most unreliable data bits detected by the linear MUD. In this contribution the bit error rate (BER) performance of the TF/MC DS-CDMA using the reliability-aided MUD is investigated, when communicating over additive white Gaussian noise (AWGN) channels or over frequency-selective Rayleigh fading channels. Our simulation results show that the reliability-aided MUD is capable of significantly outperforming the corresponding linear MUD. It can be shown that, provided that the signal-to-noise ratio (SNR) is sufficiently high, the reliability-aided MUD can readily obtain several decibels of SNR gain over the linear MUD at the cost of slight or moderate increase of the detection complexity.

This paper characterises two single-relay assisted single-carrier frequency-division multiple-access (SC-FDMA) amplify-and-forward (AF) based cooperative strategies employed for multiuser uplink transmissions over multipath fading channels. For the sake of exploiting the benefits of cooperative diversity, we investigate both single-dedicated-relaying (SDR) as well as single-shared-relaying (SSR), and propose a frequency-domain (FD) subband-based AF scheme combined with subband remapping, which is benchmarked against conventional time-domain (TD) AF in the context of both of the above-mentioned relaying topologies. Our simulation results show that cooperative diversity is beneficial for both SDR and SSR, both in the absence and in the presence of multipath diversity. More importantly, the proposed FD-AF scheme is capable of attaining a multiuser performance, which is better than that of the conventional single-user TD-AF protocol operating in a multipath environment, because of the subband-based AF and noise suppression capability of the relay.

In this contribution, a novel soft-output ant colony optimization (SO-ACO)-based multiuser detector (MUD)—namely the MUlti-input-Approximation (MUA) assisted SO-ACO-based MUD—is proposed for the synchronous direct-sequence code-division-multiple-access (DS-CDMA) uplink (UL). The previously proposed conventional ACO based MUDs were unable to provide soft log-likelihood ratio (LLR) values for the channel decoder. Hence, to solve this open problem, we commence by proposing the MAximum-Approximation (MAA) assisted SO-ACO algorithm, leading to a novel MUA assisted SO-ACO algorithm, which subsumes the MAA algorithm as a particular case and outperforms the MAA algorithm. More explicitly, at a signal-to-noise ratio (SNR) of 13 dB, the BER performance of the convolutional coding (CC) aided CDMA UL employing the MAA SO-ACO is improved from$5.2\cdot 10^{-6}$to$2.7\cdot 10^{-6}$by employing the MUA SO-ACO. Our numerical results also demonstrate that the MUA assisted SO ACO-MUD is capable of approaching the optimum performance of the Bayesian detector, when$K=32$UL users are supported with the aid of 31-chip Gold codes, while the complexity of the former is a fraction of$10^{-8}$lower than that of the latter.

Conventional two-stage turbo-detected schemes typically suffer from a Bit Error Rate (BER) floor, preventing them from achieving infinitesimally low BER values, especially, when the inner coding stage is of non-recursive nature. We circumvent this deficiency by proposing a three-stage turbo-detected Sphere Packing (SP) aided Space-Time Block Coding (STBC) STBC-SP scheme, where a rate-1 recursive inner precoder is employed to avoid having a BER floor. The convergence behaviour of this serially concatenated scheme is investigated with the aid of 3D Extrinsic Information Transfer (EXIT) Charts. Furthermore, the capacity of the STBC-SP scheme is determined and an algorithm is proposed for calculating a tighter upper bound on the maximum achievable bandwidth efficiency, based on the EXIT charts of the STBC-SP demapper. The proposed three-stage turbo-detected scheme operates within about 1.0 dB of the capacity and within 0.5 dB of the maximum achievable bandwidth efficiency limit.

In this letter we propose and investigate a cooperative downlink transmission scheme for direct-sequence code-division multiple-access (DS-CDMA) systems, in order to achieve relay diversity. Transmitter zero-forcing (TZF)-based preprocessing is employed at the base-station (BS) of our proposed scheme for suppressing the downlink multiuser interference (MUI). At the mobile terminals (MTs) the signals are combined based on the maximal ratio combining (MRC) principles. As a benefit of TZF, the BS decontaminates the relayed signals from MUI without requiring any information from the relays. Furthermore, only low-complexity signal processing is necessary at the relays for forwarding information to the destination MTs (DMTs), while achieving the relay diversity by only exploiting the knowledge of the DMTs' spreading sequences.

A novel singular value decomposition (SVD) aided uplink (UL) multiuser MIMO system is proposed. In contrast to the traditional minimum mean square error (MMSE) or zero- forcing (ZF) multiuser detection (MUD) technique, the proposed method exploits the specific characteristics of the individual users' channel matrix, instead of treating all the users' channels jointly. Furthermore, two different power allocation schemes are investigated in the context of the proposed structure. One of them was designed for achieving the maximum information rate, while the other for maintaining the maximum signal-to-noise ratio (SNR). We demonstrate that the capacity of the proposed scheme using the maximum information rate based power allocation policy is higher than that of the classic ZF receiver for the UL.

Systematic Luby Transform (SLT) codes have shown good performance for single antenna aided systems for transmission over AWGN and uncorrelated Rayleigh fading channels. For the sake of improving both the Bit Error Ratio (BER) performance and the diversity gain of Vertical Bell Laboratories Layered Space Time (V-BLAST) schemes, in this paper we propose a SLT coded V-BLAST system having four transmit and four receive antennas. As a benefit of iteratively exchanging the Log-Likelihood Ratios (LLRs) between the QPSK demapper and the SLT decoder of each antenna-specific stream of the V-BLAST system, the system exhibits an infinitesimally low BER for Eb/N0values in excess of 6.5 dB, when using an interleaver length of L = 1, 200 bits. Additionally, the SLT coded system provides an Eb/N0gain of 5 dB at a BER of 10 6 over its benchmark scheme employing iterative extrinsic information exchange between a Recursive Systematic Convolution (RSC) code and a unity-rate code, having an interleaver length of L= 1,200 bits.

In this contribution, a novel soft-output Ant Colony Optimization (ACO) based Multi-User Detector (MUD) is proposed for the synchronous Direct-Sequence Code-Division- Multiple-Access (DS-CDMA) uplink. The foraging behaviour of the ant colony in nature motivates the employment of reduced- search ACO-based MUDs, which are capable of approaching the optimum Maximum Likelihood (ML) MUD's performance at the cost of a computational complexity, which maybe as low as that of the Mathched Filter (MF) based Single User Detector (SUD). However, the previously proposed conventional ACO based MUDs were unable to provide soft Log-Likelihood Ratio (LLR) values for the channel decoder. Hence in this paper, we present a novel soft-output ACO-MUD capable of delivering soft LLRs, which allows a CDMA system to achieve a near-single-user performance without any additional information feedback from the channel decoder, even when the number of users supported is as high as the number of chip in the spreading sequence. Our numerical results show that at a BER of 10-3, the performance of the currently known ACO-assisted state-of-the-art systems can be improved by about 17 dB with the aid of the proposed soft-output ACO-MUD. More explicitly, the soft-output ACO- MUD is capable of approaching the optimum performance of the Bayesian detector, when K = 32 UL users are supported with the aid of 31-chip Gold codes, while the complexity of the former is only a fraction of 10-8of the latter.

In this contribution an adaptive detection scheme based on least mean square (LMS) principles is proposed and investigated in the context of the hybrid direct-sequence time-hopping ultrawide bandwidth (DS-TH UWB) systems. The bit-error-rate (BER) performance of the hybrid DS-TH UWB system is investigated when communicating over the UWB channels modelled by the Saleh-Valenzuela (S-V) channel model. Furthermore, since both the pure DS-UWB and pure TH-UWB constitute special examples of the hybrid DS-TH UWB, their BER performance is also investigated in this contribution for the sake of comparison with that of the hybrid DS-TH UWB. Our study and simulation results show that the LMS-aided adaptive detection can be a feasible detection scheme for deployment in practical DS-, TH- or hybrid DS-TH UWB systems. It can be shown that, with the aid of a training sequence of reasonable length, the considered UWB schemes are capable of achieving a BER performance which is close to that achieved by the minimum mean-square error (MMSE) detector with perfect channel knowledge.

In this contribution decision-directed adaptive detection scheme based on normalised least mean-square (DD/NLMS) principles is proposed and investigated for the hybrid direct- sequence time-hopping ultrawide bandwidth (DS-TH UWB) system. The bit-error-rate (BER) performance of the hybrid DS-TH UWB system is investigated when communicating over the UWB channels modelled by the Saleh-Valenzuela (S-V) channel model. Since both the pure DS-UWB and pure TH-UWB constitute special examples of the hybrid DS-TH UWB, their BER performance is also investigated in this contribution for sake of comparison with that of the hybrid DS-TH UWB. Our study and simulation results show that the DD/NLMS-aided adaptive detection constitutes a feasible detection scheme for practical DS-, TH- or hybrid DS-TH UWB systems. With the aid of a training sequence of reasonable length, our study shows that the considered UWB schemes are capable of achieving the BER performance that is close to that achieved by the minimum mean-square error (MMSE) detector with perfect channel knowledge. Furthermore, the DD adaptive techniques may be employed by UWB systems in order to improve their spectral-efficiency.

This paper investigates and compares the performance of hybrid direct-sequence time-hopping (DS-TH) ultrawide bandwidth (UWB) systems using full-rank and various reduced-rank detections when minimum mean-square error (MMSE) principles are applied, while communicating over Nakagami-m fading channels. In addition to the full- rank MMSE detection, three classes of reduced-rank MMSE techniques are investigated, which are derived based on the principles of principal components (PC), cross-spectral metric (CSM) and Taylor polynomial approximation (TPA), respectively. Our study and simulation results show that the reduced-rank techniques constitute a family of promising detection techniques for UWB systems, which are capable of achieving similar bit error rate (BER) performance as the full-rank detection, while with a detection complexity that is significantly lower than that of the full-rank detection.

In relay-aided cellular wireless communications mobile terminals (MTs) near the base-station (BS) are usually preferred to being selected as relays for the sake of achieving the relay diversity while simultaneously obtaining a high power-efficiency. However, this strategy may result in a so-called 'bottleneck' problem, when each source MT employs different relays, since the available relays near the BS are limited. In this contribution we propose and study a relay diversity scheme for uplink direct-sequence code-division multiple-access (DS-CDMA) systems. In the proposed scheme all the uplink source MTs share a set of relays chosen in the vicinity area of the BS, and the number of relays is independent of the number of source MTs. Therefore, the proposed relay scheme can significantly mitigate the above-mentioned 'bottleneck' problem. In this contribution the performance of the uplink DS-CDMA using our proposed relay diversity scheme is investigated, when either the maximal ratio combining aided single-user combining (MRC-SUC) or maximum signal-to-interference-plus-noise ratio aided multiuser combining (MSINR-MUC) is employed. We assume that the channels experience both propagation pathloss and fast Nakagami-m fading. Our study and simulation results show that the diversity gain may be overwhelmed by the multiuser interference (MUI), when the MRC-SUC is employed. However, when the MUI is efficiently suppressed with the aid of the MSINR-MUC, then the full diversity gain can be achieved and the bit error rate (BER) performance of the DS-CDMA improves, when increasing the number of relays.

In this contribution we investigate the bit-error-rate (BER) performance of a downlink direct-sequence code-division multiple-access (DS-CDMA) system, where a set of relays are chosen to assist a mobile terminal (MT) in order to improve its communications reliability with the base-station (BS). In our relay-aided DS-CDMA system, the downlink signal is detected at the relays based on the minimum mean-square-error (MMSE) principles, since the MMSE detector can be implemented with the aid of low-complexity adaptive techniques. At the destination MTs (DMTs) the signals are combined according to the maximum signal-to-interference-plus-noise ratio (MSINR) criterion. The BER performance of the relay-aided DS-CDMA downlink is investigated, when the communications channels are assumed to experience both propagation pathloss and fast Nakagami-m fading. Furthermore, we consider the impact of the relays' locations and that of the power-allocation among the BS and relays on the achievable BER performance. From our study and simulation results, it can be shown that full relay-diversity gain can be achieved, after the multiuser interference (MUI) and inter-relay interference (IRI) are suppressed by the MMSE and MSINR detectors.

In this treatise, we propose a novel singular value decomposition (SVD) based downlink (DL) multiuser MIMO system, which takes into account the specific characteristics of the individual users channel matrix, instead of treating all the users channels jointly, as in the traditional minimum mean square error (MMSE) or zero-forcing (ZF) multiuser transmission (MUT) technique. Furthermore, two different power allocation schemes are investigated in the context of the proposed structure. One of them was designed for achieving the maximum information rate, while the other for maintaining maximum signal-to-noise ratio (SNR). The simulation results demonstrate that the BER performance of the proposed structure combined with the maximum SNR based power allocation policy is better than that of the traditional equal-power policy employed in the DL.

Transmit preprocessing employed at the basestation (BS) has been proposed for simplifying the design of the mobile receiver. Provided that the channel impulse response (CIR) of all the BS to mobile station (MS) links is known in advance-even before the signal's transmission-it is plausible that the different users' signals may be differentiated with the aid of their unique, user-specific downlink CIRs. Naturally, this non-causal CIR knowledge is unavailable in practice. Hence a natural design option is to estimate the CIRs at the receiver after the BS's signal was received and convey it using side-information to the BS for its future use. Naturally, the resultant CIR has to be quantized before its transmission. In addition to this quantization error, it also becomes outdated and both imperfections result in an erosion of the achievable transmit preprocessing gain expressed in terms of either the attainable transmit power reduction or the number of users that may be supported. Another attractive design option is to avoid the CIR-signalling latency by invoking the previously received CIRs for predicting their future evolution using CIR-tap prediction. In this paper, Kalman filtering aided CIR prediction is combined with both minimum mean square error (MMSE) and zero forcing based preprocessing. Our simulation results show that the proposed scheme is capable of attaining 6.5 dB gain at a BER of 10-2, when using 6 antennas.

In this contribution we study the spectral-efficiency performance of a multicarrier direct-sequence code-division multiple-access system using both time (T)-domain and frequency (F)-domain spreading, which is referred to as TF/MC DS-CDMA for convenience, when communicating over Nakagami-m fading channels. We consider the TF/MC DS-CDMA scheme, since it is a generalized multiple-access scheme that can be readily configured to some other CDMA schemes, so that we can conveniently compare their spectral-efficiency performance. Furthermore, the Nakagami-m fading channel model makes it possible to study the impact of channel quality on the achievable spectral-efficiency. In this contribution, the spectral-efficiency of the TF/MC DS-CDMA is investigated, when various single-user and multiuser detection schemes are invoked, which include optimum detection, minimum mean-square error (MMSE) detection, zero-forcing (ZF) detection and the matched-filter (MF) based detection. One of our conclusions is that channel quality has insignificant impact on the achievable spectral-efficiency of the TF/MC DS-CDMA, when the number of T-domain resolvable paths of the frequency-selective fading channels is sufficiently high.

In this treatise, a jointly optimized bit interleaved coded modulation using Iterative decoding (BICM-ID) and Luby transform (LT) coding scheme was proposed for the sake of attaining the best possible performance, when communicating over the wireless Internet. This scheme is particularly attractive when handling packetized wireless Internet data traffic received over wireless channels inflicting both statistically multiplexing induced packet erasure and packet corruption imposed by the channel without retransmitting the corrupted packets. A LLR-based estimation packet reliability algorithm was proposed to assist the LT decoder in recovering all the transmitted packets. Furthermore, the bit-to-sphere-packing-symbol mapping scheme was optimized for achieving error free LT decoding at the lowest possible Eb/No value rather than for approaching the channel's capacity. These two design philosophies require different mapping schemes. Finally, differential space time coding (DSTBC) was employed for combatting the effects of fading and provided spatial diversity at the same time. The LT code's packet length was also carefully chosen to strike a good balance between the affordable complexity and the achievable performance.

Invoking SDMA in the down-link (DL) has the potential of increasing the achievable throughput with the aid of linear transmit preprocessing, provided that the channel impulse responses (CIRs) of all users and all antenna elements are known at the DL transmitter. However, in a frequency division duplex (FDD) system, since these CIRs have to be transmitted by the mobile terminals (MTs) to the base station (BS), they are naturally out-dated. Hence, we proposed a periodical CIR update scheme employing a channel predictor at the DL transmitter for predicting the CIR taps for each future symbol transmission instant and hence to mitigate the performance degradation imposed by the associated signalling delays. Moreover, a predictive vector quantizer (PVQ) is used at the MTs for compressing the CIRs before their uplink transmission. Compared to a conventional vector quantizer (VQ), PVQ has significantly reduced the CIR feedback bit rate. Hence, with the aid of the same feedback bit rate, the new PVQ scheme can provide more accurate CIR information or support a channel having a higher Doppler frequency.

Serially concatenated and iteratively decoded Irregular Variable Length Coding (IrVLC) combined with preceded Fast Frequency Hopping (FFH) M-ary Frequency Shift Keying (MFSK) is considered. We employ Extrinsic Information Transfer (EXIT) charts to investigate the 3-stage concatenation of the FFH-MFSK demodulator, the rate-1 decoder and the outer IrVLC decoder. The proposed joint source and channel coding scheme is capable of operating at low Signal-to-Noise Ratio (SNR) in Rayleigh fading channels contaminated by Partial Band Noise Jamming (PBNJ). The IrVLC scheme is comprised of a number of component Variable Length Coding (VLC) codebooks employing different coding rates for encoding particular fractions of the input source symbol stream. These fractions may be chosen with the aid of EXIT charts in order to shape the inverted EXIT curve of the IrVLC codec so that it can be matched with the EXIT curve of the inner decoder. We demonstrate that using the proposed scheme an inflnitesimally low bit error ratio may be achieved at low SNR values.

Recently, the authors of this paper proposed systematic Luby transform (SLT) codes and their soft decoding using the classic loglikelihood message passing algorithm for transmission over hostile Wireless Internet channels, where the transmitted data is affected by both packet loss events and random Gaussian noise. This scheme is further improved here with the aid of a new degree distribution and a novel random integer generator, which are termed as the truncated degree distribution and the conditional random integer generator. The SLT code using the new design is capable of achieving BER les 10-5at low Eb/N0values. For example, the SLT(1200,3600) code attains BER < lCT5 in excess of an Eb/No value of 1.5 dB for transmission over the AWGN channel and above 3.5 dB over the uncorrelated Rayleigh channel if additionally a packet erasure probability Pe of 0.1 is inflicted, an Eb/No value above 2 dB is required for transmission over the AWGN channel and in excess of 4 dB over the uncorrelated Rayleigh channel, when using a maximum of Iter = 20 iterations and quadrature phase- shift keying.

Since the different bits of the compressed video sequence produced by the BBC's wavelet-based video codec exhibit different error sensitivity, efficient unequal error protection (UEP) schemes have to be used to ensure that the perceptually more important bits benefit from more powerful protection. Furthermore, in the context of turbo detection the over-complete mapping aided soft videobit based source decoder should also match the characteristics of the channel codes for the sake of attaining a good iterative decoding performance. In this paper, we address this design dilemma by proposing a novel irregular over-complete mapping (IrOM) scheme constituted by an appropriately weighted different-rate over-complete mapping. Our turbo detection scheme has attractive iterative decoding convergence properties, while maintaining unequal error protection capabilities matched to the requirements of the wavelet encoded video sequence. An EXIT chart based design guideline is presented and used in the context of protecting the different-sensitivity video bits of the BBC wavelet-based video codec. As a benefit, the unequal error protection scheme using rate 2/3 irregular over-complete mapping exhibits a transmit power advantage of about 0.7 dB at an error probability of 0.016% over the equal-protection benchmarker scheme employing rate 2/3 over-complete mapping, when communicating over the uncorrelated Rayleigh-fading channel. This leads to an approximately ldB signal-to-noise ratio gain, when considering the video PSNR performance of our UEP iterative source-channel decoding scheme using an over- complete mapping having the same rate and I = 6 iterations.

In this contribution, the authors investigate the single-user-bound performance of a direct-sequence code-division multiple-access (DS-CDMA) system, where one source mobile terminal (MT) communicates with its base-station (BS) with the assistance of multiple relays. The authors assume that the communications channels experience both propagation pathloss and fast fading, and that the channels from the source MT to the BS and relays as well as that from the relays to the BS may experience different fast fading modelled correspondingly by Nakagami-m distributions. In the study, they assume both the single-user combining using maximal ratio combining and the multiuser combining, which are derived based on the maximum signal-to-interference-plus-noise ratio criterion. The bit-error-rate (BER) performance of the DS-CDMA is investigated associated with considering the locations of the relays as well as the power-allocation among the source MT and relays. From the study and simulation results, it can be shown that the achievable BER performance of the DS-CDMA depends on the locations of the relays and also on the power-allocation among the source MT and relays. When the relays of a source MT are chosen from a different area, the power-allocation should also be adjusted correspondingly in order to achieve the minimum BER. Furthermore, when optimum power-allocation is assumed, the BER performance of the DS-CDMA can be significantly improved, when increasing the number of relays assisting the source MT.

A downlink (base-to-mobile) multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) system employing multiple antennas at both the base station and each of the mobile terminals is investigated, when communicating over fast time-varying fading channels resulting in time-selective fading. In the considered multiantenna MC DS-CDMA, space–time spreading (STS) based on the family of orthogonal variable spreading factor (OVSF) codes is proposed in order to achieve the time diversity because of the time-selective fading, in addition to the transmit/receive diversity. In this contribution, the closed-form expressions for the single-user bit-error rate (BER) are derived, in order to gain insight into the achievable BER performance of the multiantenna MC DS-CDMA. The BER performance of the multiantenna MC DS-CDMA system is investigated, when communicating over correlated time-selective Rayleigh fading channels. The study and performance results show that the multiantenna MC DS-CDMA using OVSF codes-assisted STS constitutes a high-efficiency downlink space–time transmission scheme. It is capable of achieving the full diversity provided by the time-selective fading and multiple transmit/receive antennas. Furthermore, the proposed multiantenna MC DS-CDMA is capable of achieving a self-balance between the time diversity achieved and the multiuser interference suppression capability, when using low-complexity correlation detection.

In this letter a multiuser transmitter preprocessing (MUTP) approach is proposed for downlink wireless communications, such as in spatial-division multiple-access (SDMA) or direct-sequence code-division multiple-access (DS-CDMA) system. In the proposed MUTP the preprocessing matrix is obtained by constructing a subspace that is orthogonal to the interference subspace without relying on eigen-analysis. The proposed MUTP fully removes the downlink multiuser interference, resulting in the zero-forcing (ZF) solution. It is capable of achieving the same bit error rate (BER) performance as the conventional ZF-based MUTP (ZF-MUTP).

In this paper we investigate a turbo detection scheme designed for robust video transmission over an uncorrelated Rayleigh fading channel. We first introduce a novel block error-correcting code, referred to as an extrinsic information transfer (EXIT)-chart optimized block code (ECOBC), which benefits from soft decision decoding. The ECOBC exploits both the residual redundancy which manifests itself in terms of the non-uniform probability distribution ofK-bit strings inherent in the video encoded bit-stream, as well as the intentional redundancy imposed by the ECOBC for mitigating the effects of transmission errors. As our first novel contribution, we address the design of ECOBCs by formulating the necessary and sufficient condition, which ensures that the EXIT function of the softbit ECOBC decoder reaches the (1,1) point in the EXIT chart. Secondly, we propose a novel real-valued free-distance metric for characterizing the ECOBC's performance. Furthermore, the attainable performance and the computational complexity imposed by the ECOBC are investigated. The effects of the interleaver delay on the ECOBC's performance are also quantified. As an additional result, we demonstrate that at a fixed system bandwidth, the turbo detection scheme using the H.264 video codec with the error concealment tools disabled but amalgamated with the proposed ECOBC as well as an additional generalized low-density parity-check (GLDPC) code may lead to an approximately 2.5 dB signal-to-noise ratio gain in comparison to the benchmarker having the same total bit rate, which operated the H.264 scheme with the error concealment tools enabled hence resulting in a higher video rate and amalgamated with the same-rate GLDPC code, but dispensing with our novel ECOBC.

In this paper we propose a novel irregular variable length coding (IrVLC) scheme for near-capacity joint source and channel coding. We employ a number of component variable length coding (VLC) codebooks having different coding rates for encoding particular fractions of the input source symbol stream. These fractions may be chosen with the aid of extrinsic information transfer (EXIT) charts in order to shape the inverted EXIT curve of the IrVLC codec for ensuring that it does not cross the EXIT curve of a serially concatenated channel codec. In this way, an open EXIT chart tunnel may be created even at low Eb/N0values that are close to the capacity bound of the channel. We propose iteratively decoded serially concatenated IrVLC designs amalgamated with Trellis coded modulation (TCM). These schemes are shown to be capable of operating within 0.6 dB of the uncorrelated narrowband Rayleigh fading channel's capacity bound using an average interleaver length of 217,500 bits and an effective bandwidth efficiency of 1.56 bit/s/Hz, assuming ideal Nyquist filtering. By contrast, the equivalent-rate regular VLC-based bench-marker schemes were found to be capable of operating at a higher distance of 1.1 dB from the capacity bound, which is nearly twice that of the proposed IrVLC-TCM scheme. Additionally, an irregular convolutional coiling (IrCC) based bench-marker was found to be capable of operating at 0.8 dB from the capacity bound, owing to its slightly eroded performance when operating with the considered interleaver length.

In this contribution we first derive a transmitter multiuser preprocessing (TMP) scheme for a general multiuser multiple-input-multiple-output (MIMO) system based on the minimum power distortionless response (MPDR) criterion, which minimizes the power in the context of a given downlink mobile terminal (MT) under the distortionless condition. This optimization problem results in a solution, which has the same form as the minimum mean-square error (MMSE) multiuser detection (MUD). Hence, we then extend the well-known rank-reduction techniques in MMSE-MUD to the MPDR-TMP in order to mitigate the possible implementation problems of the MPDRTMP. In our study three classes of rank-reduction algorithms are considered, which are derived, respectively, based on the eigen-analysis methods of principal components (PC) and crossspectral metrics (CSMs) as well as on the Taylor polynomial approximation (TPA) approach, which does not depend on the eigen-analysis. In this contribution both the capacity and error performance of a downlink space-division multiple-access (SDMA) system is investigated, when either the full-rank or reduced-rank MPDR-TMP is invoked. From our study and simulation results, it can be shown that the MPDR-TMP scheme is highly efficient for achieving the capacity and for suppressing the multiuser interference (MUI). The reduced-rank techniques can be employed by the MPDR-TMP, so as to reduce its implementation complexity while achieving the near full-rank performance of the full-rank MPDR-TMP.

The (lower-bound) symbol error rate (SER) performance of cooperative digital communication schemes is investigated, where multiple dual-hop relays are invoked for achieving the cooperative diversity with the aid of the maximal ratio combining (MRC). A range of closed-form expressions are obtained for the probability density functions (PDFs) of the relay-channels' output signal-to-noise ratio (SNR). Accurate and approximated closed-form expressions are derived for computing the SER. Our study shows that, with the aid of the closed-form expressions obtained in this contribution, the SER performance of the relay-assisted digital communications schemes, which may employ various classes of coherent modulations, can be readily evaluated in the context of a variety of fading scenarios that the cooperative signals might experience.

Conventional two-stage turbo-detected schemes typically suffer from a bit error rate (BER) floor, preventing them from achieving infinitesimally low BER values, especially, when the inner coding stage is of non-recursive nature. We circumvent this deficiency by proposing a three-stage turbo-detected sphere packing (SP) aided space-time block coding (STBC) STBC-SP scheme, where a rate-1 recursive inner precoder is employed to avoid having a BER floor. The convergence behaviour of this serially concatenated scheme is investigated with the aid of 3D extrinsic information transfer (EXIT) charts. Furthermore, the capacity of the STBC-SP scheme is shown and an algorithm is proposed for calculating a tighter upper bound on the maximum achievable bandwidth efficiency. The proposed three-stage turbo-detected scheme operates within about 1.0 dB of the capacity and within 0.5 dB of the maximum achievable bandwidth efficiency limit.

It is demonstrated that iteratively decoded variable length space time coded modulation (VL-STCM-ID) schemes are capable of simultaneously providing both coding gain as well as multiplexing and diversity gain. The VL-STCM-ID arrangement is a jointly designed iteratively decoded scheme combining source coding, channel coding, modulation as well as spatial diversity/multiplexing. In this contribution, we analyse the iterative decoding convergence of the VL-STCM-ID scheme using symbol-based three-dimensional EXIT charts. The performance of the VL-STCM-ID scheme is shown to be about 14.6 dB better than that of the fixed length STCM (FL-STCM) benchmarker at a source symbol error ratio of 10-4, when communicating over uncorrelated Rayleigh fading channels. The performance of the VL-STCM-ID scheme when communicating over correlated Rayleigh fading channels using imperfect channel state information is also studied.

Novel concepts are introduced for finding the relationship between multiuser detection (MUD) and multiuser transmission (MUT), so that the study in MUT can benefit from the well-built theory in MUD. Our study shows that, for any given linear MUD scheme, there exists a counterpart linear MUT scheme, which can be readily designed from the original linear MUD. Based on our observations, a range of MUT algorithms are derived in correspondence with the well-known linear processing schemes in the fields of MUD and array processing, including minimum mean-square error (MMSE), minimum variance distortionless response (MVDR), minimum power distortionless response (MPDR) and maximum signal-to-interference-plus-noise ratio (MSINR), respectively. Furthermore, the physical principles behind the considered MUT schemes are explained with the aid of the concepts built in this contribution. In order to gain insight into the effect of noise power on the achievable performance of a MUT scheme, a noise-suppression factor is introduced to those MUT schemes, such as MMSE, requiring the knowledge of noise variance. Our study shows that the achievable performance of the MUT is generally loosely depended on the noise-suppression factor. This finding may result in extremely low-complexity wireless transceivers in time- division duplex (TDD) cellular systems, where the uplink MUD and downlink MUT can be implemented based on one adaptive signal processing scheme.

This paper investigates and compares the performance of various ultrawide bandwidth (UWB) systems when communicating over Nakagami-m fading channels. Specifically, the direct-sequence (DS), time-hopping (TH) and hybrid direct-sequence time-hopping (DS-TH) UWB systems are considered. The performance of these UWB systems is studied, when using the conventional single-user correlation detector or the minimum mean-square error (MMSE) multiuser detector. Our simulation results show that the hybrid DS-TH UWB system may outperform a corresponding pure TH-UWB or pure DS-UWB system in terms of the achievable error performance. Given the total spreading gain of the hybrid DS-TH UWB system, there is an optimal setting of the TH spreading factor and DS spreading factor, which results in the best error performance.

Serially concatenated and iteratively decoded irregular variable length coding (IrVLC) combined with preceded fast frequency hopping (FFH) M-ary frequency shift keying (MFSK) is considered. The proposed joint source and channel coding scheme is capable of low signal-to-noise ratio (SNR) operation in Rayleigh fading channels contaminated by partial band noise jamming (PBNJ). The IrVLC scheme is comprised of a number of component variable length coding (VLC) codebooks employing different coding rates for encoding particular fractions of the input source symbol stream. These fractions may be chosen with the aid of extrinsic information transfer (EXIT) charts in order to shape the inverted EXIT curve of the IrVLC codec so that it can be matched with the EXIT curve of the inner decoder. We demonstrate that using the proposed scheme near-zero bit error ratio may be achieved at low SNR values and the IrVLC based scheme yields a further gain of up to 0.3 dB over the identical-rate single-class VLC based benchmarker scheme

We propose a novel bit error ratio (BER) analysis technique for fast frequency hopping (FFH) assisted M-ary frequency shift keying (MFSK) using product combining (PC), when the channel is contaminated by partial band noise jamming (PBNJ). Exploiting the fact that the Mellin transform of the product of independent random variables is the product of their Mellin transforms, we derive the probability density function (PDF) of the PC's output, when communicating in uncorrelated Rayleigh fading channels contaminated by PBNJ. The derivation of the PDF thus allows computation of the system's BER. It is shown that the Mellin transform substantially simplifies the analysis of the PC receiver and facilitates for the first time the analysis of FFH-MFSK PC receiver for modulation orders of M > 2.

In this contribution, soft-input-soft-output (SISO) decoding aided successive interference cancellation (SIC) is considered in fast frequency hopping (FFH), M-ary frequency shift keying (MFSK) using product combining (PC), when operating in multiple access (MA) Nakagami-m channels. All users' M-ary source symbols are encoded using non-binary LDPC codes and the encoded symbols are transmitted using FFH-MFSK modulation. A sub-optimum soft metric is proposed which is based on self-normalization of the diversity combiner outputs. In the context of the proposed SIC scheme, the receiver exploits the soft information fed back by the LDPC decoder to the demodulator in order to cancel the interference imposed by the reliably detected symbols. Our simulation results show that the proposed scheme is capable of combatting the effects of multiuser interference and outperforms conventional iterative decoding by about 1 dB.

This paper presents a novel generalised Multi-Carrier Direct Sequence Code Division Multiple Access (MC DS-CDMA) system invoking smart antennas for improving the achievable performance in the downlink, as well as employing multi-dimensional Sphere Packing (SP) modulation for increasing the achievable diversity product. In this contribution, the MC DS-CDMA transmitter considered employs multiple Antenna Arrays (AA) and each of the AAs consists of several antenna elements. Furthermore, the proposed system employs both time- and frequency- (TF) domain spreading for extending the achievable capacity, when combined with a novel user-grouping technique for reducing the effects of Multiuser Interference (MUI). Moreover, in order to further enhance the system's performance, we invoke a MultiLevel Coding (MLC) scheme, whose component codes are determined using the so-called equivalent capacity based constituent-code rate-calculation procedure invoking a 4-dimensional bit-to-SP-symbol mapping scheme. Our results demonstrate an approximately 3.8 dB Eb/N0gain over an identical throughput scheme dispensing with SP modulation at a BER of 10-5.

In this contribution we investigate the single-user-bound performance of a direct-sequence code-division multiple-access (DS-CDMA) system, where one source mobile terminal (MT) communicates with its base-station (BS) with the assistance of multiple relays. We assume that the communications channels experience both propagation pathloss and fast fading, and that the channels from the source MT to the BS and relays as well as from the relays to the BS may experience different fast fading modeled correspondingly by the Nakagami-m distributions. In our study we assume a multiuser combining (MUC) scheme, which suppresses the inter- relay interference under the criterion of maximum signal-to-interference-plus-noise ratio (MSINR). The bit-error-rate (BER) performance of the DS-CDMA is investigated associated with considering the locations of the relays as well as the power allocation among the source MT and relays. From our study and simulation results, it can be shown that the achievable BER performance of the DS-CDMA depends on the locations of the relays and also on the power allocation among the source MT and relays. When the relays of a source user are chosen from a different area, the power allocation should also be adjusted correspondingly in order to achieve the minimum BER. Furthermore, when optimum power allocation is assumed, the BER performance of the DS-CDMA can be significantly improved, when increasing the number of relays of a source MT.

In relay-assisted cellular wireless communications the mobile terminals (MTs) near the base-station (BS) are usually preferred to being chosen as the relays for assisting the uplink transmission, for the sake of achieving the relay diversity while maintaining the system power-efficient. However, this strategy may result in the so-called 'bottleneck' problem, when each user chooses different relays. In this case a huge number of relays might be required, but the available relays around the BS are limited. In this contribution we propose a relay strategy for direct-sequence code-division multiple-access (DS-CDMA) systems, where a set of MTs in the vicinity of the BS are chosen to act as the relays for all the uplink users. Explicitly, this relay strategy can significantly mitigate the 'bottleneck' problem. In this contribution the performance of the DS-CDMA using our proposed relay strategy is investigated, when considering that there exist both multiuser and inter-relay interference. The bit error rate (BER) performance is investigated, when communicating over Nakagami-mfading channels, where signals from the MT transmitters to the relays and that from the relays to the BS receiver may experience different fading.

In this contribution projection approximation subspace tracking using deflation (PASTD) is investigated in the context of MIMO transmit preprocessing systems by exploiting the specific property of time division duplexing (TDD) techniques that the uplink and downlink channels are similar, since they both use the same carrier frequency. Hence the channel estimated from the received signal can also be used for transmit preprocessing. More explicitly, based on the received signal, the PASTD algorithm is used for tracking both the left and the right singular vectors of the MIMO channel matrix, which are required by eigenmode transmissions, instead of periodically reestimating the MIMO channel matrix and performing the singular value decomposition (SVD), which would impose a high computational complexity. A specific deficiency of the family of subspace tracking algorithms is their phase ambiguity imposed by the non-unique nature of the SVD, which is resolved in this treatise by employing differential encoding. The efficiency of the proposed subspace tracking scheme is demonstrated by our performance results, indicating that the advocated technique preforms within 1 dB from the BER curve of the perfect channel estimation aided benchmarker.

Eigen-beamforming is capable of attaining attractive performance gains in the context of multiple-input multiple-output (MIMO) systems, provided that accurate channel state information (CSI) is available. However, when a realistic pilot-based channel predictor is used for acquiring the CSI, a significant performance degradation may be imposed by the phase-ambiguity inherent in the estimated eigen-vectors. In this contribution, both higher-complexity coherently detected coded modulation (CM) schemes requiring channel information as well as their lower-complexity differentially encoded counterparts are employed for assisting the operation of the eigen-beamformer, when using a minimum mean square error based pilot-assisted channel predictor. It is shown that differentially encoded CM schemes are capable of assisting the eigen-beamformer in attaining a coding gain of about 6.5 dBs, when communicating over correlated Rayleigh fading channels.

Luby transform codes (LT) were originally designed for the binary erasure channel (BEC) encountered owing to randomly dropped packets in the statistical multiplexing aided Internet, where transmitted packets are not affected by the fading or noise of the environment. For the sake of transmitting data over the BEC routinely encountered in statistical multiplexing aided wireless Internet-style scenarios, LT codes have to be combined with classic channel codes. In this paper we introduce a novel almagamated LT coding and generalized low density parity check (G-LDPC) coding scheme. Upon exploiting the packet erasure information provided by the G-LDPC decoder for the LT decoder, the proposed scheme achieves a low bit error ratio (BER) for packet error ratio (PER) Eb/N0values in excess of 3.2 dB when transmitting data over the AWGN-BEC channel having a packet erasure probability of Pe=0.1. When communicating over the Rayleigh-BEC channel at Pe=0.1, an Eb/N0value of 5.3 dB was required. In comparison to the serially concatenated LT-GLDPC scheme using no information exchange between the G-LDPC decoder and the LT decoder, the amalgamated LT-GLDPC arrangement achieved an Eb/N0gain of up to 1.6-2.5 dB.

In this paper, we characterize the achievable performance of a wavelet encoding-based videophone system, which further compresses and protects the wavelet-based video codec's bit-stream with the aid of reversible variable-length (RVLC) codes. More specifically, the video codec uses RVLC codes for compressing and protecting the video frame header. By contrast irregular convolutional codes (IRCCs) are used for unequal error protection of the video bitstream generated from the wavelet-subbands, when communicating over intersymbol interference (ISI) contaminated channels. At the receiver, the turbo-equalizer, the IRCC decoder and the RVLC decoder all employ the maximum a-posteriori (MAP) algorithm and their operations are performed in an iterative turbo-detection style. The proposed system was designed for maintaining a near-unimpaired video reconstruction quality for channel signal-to-noise ratios (SNR) in excess of about 5 dB, when communicating over a five-path dispersive AWGN channel.

The intentionally imposed or inherent unintentional residual redundancy found in source encoded bitstreams can be exploited for supporting joint SoftBit-source decoding and channel decoding, which has the potential of improving both error correcting capability as well as the subjective audio or video quality of communication systems. As a potent error concealment technique, the softbit-based source decoding, proposed by Adrat Vary and Spittka exploits the residual redundancy or correlation inherent in the source codec parameters for mitigating the effects of transmission errors. However, when using efficient source encoders, limited source redundancy is left in the source- encoded bitstream. In this scenario the SoftBit-source decoder (SBSD) may have a limited extrinsic information contribution which results in no system performance improvements beyond two decoding iterations. In our novel approach, we partition the total available bit budget between the source and channel codecs in order to improve the attainable error correcting capability and hence to maximize the overall system's performance. More explicitly, the inherent redundancy in the encoded bit- stream is intentionally increased with the aid of over-complete mapping, and extrinsic information transfer (EXIT) charts are used for designing a suitable mapping of the source-coded bits to the modulated symbols, leading to an approximately 2dB signal-to-noise gain.

Bit-Interleaved Coded Modulation using Iterative Decoding (BICM-ID) is amalgamated with Luby Transform (LT) coding. The resultant joint design of the physical and data link layer substantially improves the attainable Bit Error Rate (BER) performance. A Cyclic Redundancy Check (CRC) combined with a novel Log-Likelihood Ratio (LLR) based packet reliability estimation method is proposed for the sake of detecting and disposing of erroneous packets. Subsequently, bit-by-bit LT decoding is proposed, which facilitates a further BER improvement at a lower number of BICM-ID iterations. Finally, we revisit the pseudo random generator function used for designing the LT generator matrix.

In this contribution we propose and investigate a transmitter preprocessing scheme designed for downlink transmission in multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) systems using multiple base-station antenna arrays, where each antenna array employs multiple array elements. The transmitter preprocessing scheme is derived based on the singular value decomposition (SVD). Our transmitter preprocessing design is capable of supporting a high number of MC DS-CDMA users, while maintaining a high diversity gain at a low detection complexity at the remote mobile stations (MSs). The characteristics of MC DS-CDMA using the proposed transmitter preprocessing scheme are discussed and the achievable bit-error-rate (BER) performance is investigated, when assuming that each subcarrier experiences flat Rayleigh fading. Our simulation results demonstrate that for a SVD-assisted MC DS-CDMA system using M distinct transmit antenna arrays and time (T)-domain spreading sequences of length Nechips, the number of users supported can be as high as MNe, since Neand M number of users may be distinguished in the T-domain and spatial- or S-domain, respectively. Furthermore, the SVD-assisted MC DS-CDMA system is capable of supporting MNenumber of users at a near single-user BER performance.

In this contribution we present a novel ant colony optimization (ACO) based multiuser detector (MUD) designed for synchronous multi-carrier direct sequence code division multiple access (MC DS-CDMA) systems. The operation of the ACO-based MUD is based on the behaviour of the ant colony in nature. The ACO-based MUD aims for achieving the same bit-error-rate (BER) performance as the optimum maximum likelihood (ML) MUD, without carrying out an exhaustive search of the entire MC DS-CDMA search space constituted by all possible combinations of the received multi-user vectors. We will demonstrate that the system is capable of supporting almost as many users as the number of chips in the spreading sequence, while searching only a small fraction of the entire ML search space. It will also be demonstrated that the number of floating point operations per second is a factor of 108 lower for the proposed ACO-based MUD than that of the ML MUD, when supporting K = 32 users in a MC DS-CDMA system employing 31-chip Gold codes as the T-domain spreading sequence.

This contribution motivates finding the relationship between multiuser detection (MUD) and multiuser transmission (MUT) or multiuser transmitter preprocessing, so that a study in MUT can benefit from the well-built theory in MUD. Our study shows that there exists a counterpart linear MUT scheme for any given linear MUD scheme. The counterpart linear MUT can be readily designed from the original linear MUD associated with considering the power constraint on the transmitted signals as well as the availability of the knowledge about the noise presenting at the remote terminals. With the aid of our obtained theory, then, three types of MUT algorithms are derived and investigated in correspondence with the well-known linear processing schemes in the fields of MUD, including matched-filtering (MF), zero-forcing (ZF) and minimum mean-square error (MMSE).

This contribution considers joint transmitter-receiver design in general multiuser multiple-input multiple-output (MIMO) systems, where uplink and downlink use time-division duplex (TDD). We assume that there exist multiuser interference as well as other jamming signals in the system. The optimization strategy considered in this contribution represents one of the applications of the theory proposed in (L.-L. Yang, 2007), where the transmitters and receivers are respectively optimized in order to achieve their altruistic (O)-optimum and egocentric (E)-optimum solutions. Furthermore, a novel criterion of minimum power distortionless response (MPDR) is proposed for transmitter preprocessing. Our design motivates simple mobile terminals (MTs) with low-complexity. Our study and performance results show that the MPDR-assisted transmitter preprocessing is capable of efficiently suppressing the multiuser interference. However, joint transmitter-receiver optimization is required, in order to suppress the extra jamming signals.

In this contribution we investigate the performance of spatial division multiple access (SDMA) multiple-input multiple-output (MIMO) systems using transmitter preprocessing, when the channel knowledge required for preprocessing is acquired by the receiver and conveyed to the transmitter via noise feedback channels that may also conflict fading. Specifically, in our system the MIMO channel impulse responses (CIRs) are vector quantized. Then, the CIR magnitudes and phases are conveyed to the transmitter via a feedback channel, which is noise contaminated and may also experience Rayleigh fading. At the transmitter, the CIRs used for transmit preprocessing are recovered using a soft estimator, which is optimum in the minimum mean-square error (MMSE) sense, and is implemented based on the so-called Hadamard soft-decoding principles. Our study and simulation results demonstrate that vector quantization combined with soft-decoding constitutes an efficient technique of feeding back the CIRs from the receiver to the transmitter. However, it is also known that the performance of the zero-forcing (ZF) or MMSE transmit preprocessing schemes is highly sensitive to the effect of quantization errors as well as to the feedback channel induced errors.

In this contribution the performance of multiple-input multiple-output space-division, code-division multiple-access (MIMO SCDMA) is investigated, when the low-complexity reduced-rank detection is employed. In L.L. Yang (2006) a MIMO SCDMA system is proposed, aiming at improving the capacity of the existing direct-sequence (DS) CDMA (DS-CDMA) systems. The performance of the SCDMA system in L.L. Yang (2006) is investigated, when the full-rank multiuser detectors (MUDs) are considered. However, since the rank of the detection-related matrices in SCDMA is usually high, the complexity is extremely high, even linear detectors, such as decorrelating, minimum mean-square error (MMSE), etc. are employed. In this paper the reduced-rank detection scheme, which is derived based on the Taylor polynomial approximation (TPA), is proposed for detecting the SCDMA signal. The bit error rate (BER) performance of the MIMO SCDMA is investigated, when communicating over frequency-selective fading channels. It can be shown that the considered reduced-rank detection is capable of achieving a BER performance that is close to that of using full-rank detection, but with significantly lower detection complexity.

Iterative decoding (ID) aided fast frequency hopping (FFH), M-ary frequency shift keying (MFSK) using clipped combining in multiple access (MA) channels is investigated. All users' data are convolutionally encoded and the encoded bits are interleaved and converted to M-ary symbols, which are transmitted using FFH-MFSK modulation. The soft metrics to be passed from the demodulator to the decoder are derived assuming a Rayleigh fading channel. We also propose a novel multiuser detection (MUD) scheme which employs joint soft decoding as well as successive interference cancellation (SIC), the receiver exploiting the soft information fed back by the decoder to the demodulator in order to cancel the interference imposed by reliable symbols. Our simulation results show that the proposed scheme is capable of combatting multiuser interference and outperforms the conventional ID by about 3dB.

This paper presents a generalised multicarrier direct sequence code division multiple access (MC DS-CDMA) system invoking smart antennas for improving the achievable performance in the downlink. In this contribution, the MC DS-CDMA transmitter employs an antenna array (AA) and steered space-time spreading (SSTS). Furthermore, the proposed system employs both time and frequency (TF) domain spreading for extending the capacity of the system, which is combined with a user-grouping technique for reducing the effects of multi-user interference (MUI). Moreover, to eliminate the high- complexity multiple input multiple output (MIMO) channel estimation required for coherent detection, we also propose a Differential SSTS (DSSTS) scheme. More explicitly, for coherent SSTS detection MVNrnumber of channel estimates have to be generated, where M is the number of transmit AAs, V is the number of subcarriers and Nris the number of receive antennas. This is a challenging task, which renders the low-complexity DSSTS scheme attractive.

A joint video and channel coded system employing an iteratively decoded serial concatenation of a vector quantization (VQ) based video codec and a trellis-coded modulation (TCM) scheme is proposed. The video codec imposes VQ-induced code constraints, which may be completely described by a trellis structure, which is employed as the basis for optimal minimum mean-squared-error VQ-encoding and -decoding. In the latter case, the Bahl-Cocke-Jelinek-Raviv (BCJR) algorithm is employed to facilitate the iterative exchange of soft information between the VQ and TCM decoder. An error-free video reconstruction quality is supported using 16-Ievel quadrature amplitude modulation (16QAM) based TCM for transmission over Rayleigh-fading channels at a signal-to-noise ratio (SNR) per bit of 5.25 dB. This value is within 1.29 dB of the Rayleigh channel's capacity at our system's effective bandwidth-efficiency of 2 bits/s/Hz. Owing to its ability to exploit the VQ-induced code constraints during iterative decoding, the joint video and channel coding approach is found to consistently outperform the Shannonian source and channel separation philosophy. This is achieved at the cost of a 1.6 times higher computational complexity. Finally, the convergence of the iterative decoder is investigated with the aid of a novel so-called extrinsic information transfer (EXIT) chart

An iteratively decoded variable length space time coded modulation (VL-STCM-ID) scheme capable of simultaneously providing both coding and iteration gain as well as multiplexing and diversity gain is proposed. Non-binary unity-rate precoders are employed for assisting the iterative decoding of the VL-STCM-ID scheme. The discrete-valued source symbols are first encoded into variable-length codewords that are mapped to the spatial and temporal domains. Then the variable-length codewords are interleaved and fed to the precoder assisted modulator. More explicitly, the proposed VL-STCM-ID arrangement is a jointly designed iteratively decoded scheme combining source coding, channel coding, modulation as well as spatial diversity/multiplexing. As expected, the higher the source correlation, the higher the achievable performance gain of the scheme becomes. Furthermore, the performance of the VL-STCM-ID scheme is about 14.6 dB better than that of the fixed length STCM (FL-STCM) benchmarker at a source symbol error ratio of 10-4

In a wireless system with distributed mobile users, several mobile users may cooperate with each other in order to achieve the transmit diversity. In this contribution we investigate the single-user performance of an uplink direct-sequence code-division multiple-access (DS-CDMA) scheme, where one user is assisted by several other users (relays) so as to achieve the relay-assisted diversity. Specifically, the performance of the relay-assisted DS-CDMA is investigated, when communicating over generalized Nakagami-m fading channels, where signals from the transmitter to the relays and that from the relays to the base-station (BS) receiver may experience different fading. In our study two types of detection schemes are invoked. The first detection scheme is a maximal ratio combining (MRC) assisted single-user detection (SUD) scheme, which maximizes the output signal-to-noise ratio (SNR) without taking the interference among the relays into account. By contrast, the second detection scheme maximizes the output signal-to-interference-plus-noise ratio (MSINR), which is a multiuser detection (MUD) scheme that is capable of suppressing the interference among the relays. Our study and results show that, in a relay-assisted DS-CDMA without using orthogonal spreading codes, the relay-assisted diversity may only be achievable after suppressing the inter-relay interference

A multiple-input multiple-output (MIMO) space-division multiple-access (SDMA) system is presented and investigated, when the space-time block coded (STBC) user signals are transmitted over Rayleigh fading channels. In the proposed SDMA system each mobile uses two transmit antennas, while the base-station (BS) receiver employs multiple receive antenna arrays and each receive antenna array consists of several array elements. The MIMO SDMA signals at the BS are detected with the aid of various reduced-rank MMSE schemes. Specifically, three classes of reduced-rank MMSE techniques are investigated in conjunction with the proposed SDMA system. The three classes of reduced-rank MMSE algorithms are derived, respectively, based on the principles of principal components (PC), cross-spectral metric (CSM) and Taylor polynomial approximation (TPA). Our study and simulation results show that the reduced-rank MMSE detection is capable of achieving a satisfactory trade-off between the affordable detection complexity and the achievable detection BER performance

A time-hopping multicarrier code-division multiple-access (TH/MC-CDMA) scheme is proposed and investigated. In the proposed TH/MC-CDMA each information symbol is transmitted by a number of time-domain pulses with each time-domain pulse modulating a subcarrier. The transmitted information at the receiver is extracted from one of the, say M, possible time-slot positions, i.e., assuming that M-ary pulse position modulation is employed. Specifically, in this contribution we concentrate on the scenarios such as system design, power spectral density (PSD) and signal detection. The error performance of the TH/MC-CDMA system is evaluated, when the channel that each subcarrier signal experiences is modeled as flat Nakagami-m fading in addition to the additive white Gaussian noise (AWGN). According to our analysis and results, it can be shown that the TH/MC-CDMA signal is capable of providing a near ideal PSD, which is flat over the system bandwidth available, while decreases rapidly beyond that bandwidth. Explicitly, signals having this type of PSD is beneficial to both broadband and ultra-wide bandwidth (UWB) communications. Furthermore, our results show that, when optimum user address codes are employed, the single-user detector considered is near-far resistant, provided that the number of users supported by the system is lower than the number of subcarriers used for conveying a information symbol

In this contribution we propose and investigate a high-capacity cellular DS-CDMA wireless communications system, where numerous antennas are distributed in the area covered by the system. The bit error rate (BER) performance of the distributed antenna cellular DS-CDMA system is investigated, when transmission pass-loss, lognormal shadowing slow fading and Nakagami-m fast fading are considered. Our investigation and numerical results suggest that the distributed antenna cellular DS-CDMA system constitutes a high power-efficiency wireless system. For utilizing the same set of system parameters, it is capable of providing an extremely higher capacity, than the cellular DS-CDMA system built on the conventional cellular concepts

In this contribution, we analyze the bit error rate (BER) performance of fast frequency hopping (FFH) assisted M-ary frequency shift keying (MFSK) using product combining. Product combining constitutes an efficient yet low-complexity scheme that may be employed in FFH-MFSK receiver to combat the detrimental effects of interference or jamming. We propose a novel approach to the analysis of this receiver system, which is based on the Mellin transform. Using this approach, the probability density function (PDF) of the product combiner output is expressed in a closed form. Based on the resultant PDF, the BER of the FFH-MFSK product combining receiver operating in Rayleigh fading channel is evaluated analytically. It is shown that the Mellin transform simplifies the analysis of the product combining receiver.

In this contribution, we propose two successive interference cancellation (SIC) schemes for a fast frequency hopping (FFH) multiple access (MA) system using M-ary frequency shift keying (MFSK) and invoking multi-user detection (MUD). One of the proposed schemes invokes clipped combining, while the other scheme employs both product combining and clipped combining. The SIC schemes are adapted from a scheme proposed by U.-C. Fiebig in 1996. The basic principle of the SIC schemes is that detection is carried out in multiple stages and during each stage, only the most reliable symbols are detected. In subsequent stages, the interference contributed by the already detected symbols may be removed. The performance of the proposed schemes is evaluated and compared to that of Fiebig's scheme, when the FFH-MFSK system operates in a Nakagami-m fading MA channel. The simulation results demonstrate that the proposed schemes attain a better bit error rate performance than Fiebig's scheme.

Subspace-based blind and group-blind space-time multiuser detection is invoked for a smart antenna aided generalized MC DS-CDMA system communicating over a dispersive space-time (ST) Rayleigh fading channel, where the channel estimates are attained by using subspace- based blind techniques. In the generalized MC DS-CDMA system considered, smart antennas are employed, in order to provide increased degrees of freedom and hence to improve the attainable performance and user capacity. Furthermore, our simulation results confirm that in the proposed multiple antenna aided generalized MC DS-CDMA system, the blind and group-blind space-time multiuser detectors equipped with the subspace- based blind channel estimator are capable of achieving a similar performance to that exploiting the perfect knowledge of the channel.

In this contribution, the application of fully connected recurrent neural networks (FCRNNs) is investigated in the context of narrowband channel prediction. Three different algorithms, namely the real time recurrent learning (RTRL), the global extended Kalman filter (GEKF) and the decoupled extended Kalman filter (DEKF) are used for training the recurrent neural network (RNN) based channel predictor. Our simulation results show that the GEKF and DEKF training schemes have the potential of converging faster than the RTRL training scheme as well as attaining a better MSE performance.

A wavelet-based videophone scheme proposed, where the video bits are unequal error protection (UEP) using irregular convolutional codes (IRCCs). The proposed system uses adaptive arithmetic coding (AAC) for encoding the motion vectors and individual wavelet subband coefficients. The turbo-equalized IRCC-aided videophone scheme is capable of attaining a near unimpaired video quality for channel signal-to-noise ratios (SNRs) in excess of about 4.5 dB over a five-path dispersive AWGN channel

In bit-interleaved coded modulation (BICM) the coding and modulation schemes were jointly optimized for the sake of attaining the best possible performance when communicating over fading wireless communication channels. The iterative decoding scheme of BICM (BICM-ID) invoking an appropriate bit-to-symbol mapping strategy enhances its achievable performance in both AWGN and Rayleigh channels. BICM-ID may be conveniently combined with Luby transform (LT) codes, which were designed for handling packetized wireless Internet data traffic in erasure channels without retransmitting the corrupted packets. By jointly designing a serially concatenated LT-BICM-ID code, an infinitesimally low bit error rate (BER) is achieved for signal to noise ratios (SNR) in excess of 7.5 dB over wireless Internet type erasure channels contaminated by AWGN

Iterative minimum mean square error (MMSE) equalization and channel decoding is appealing, since it exhibits a lower complexity than maximum a posteriori (MAP) turbo equalization. However, the MMSE equalizer's attainable performance is upper-bounded by that recorded for transmission over the coded AWGN channel, a phenomenon usually referred to as "error shoulder". In order to circumvent this performance limitation, we propose a three-stage concatenated transceiver constituted by inner and outer coding as well as MMSE equalization, which achieves significant iteration gains, despite using low-complexity serially concatenated memory-1 convolutional codes. It is demonstrated that this three-stage scheme outperforms the traditional two-stage MMSE turbo equalization scheme above a certain Eb/N0threshold. Furthermore, the convergence behavior of the proposed scheme is analyzed with the aid of 3D extrinsic information transfer (EXIT) charts and their 2D projections, leading to a number of practical iterative receiver design guidelines.

In this contribution we propose and investigate a high-capacity cellular direct-sequence code-division multiple-access (DS-CDMA) wireless communications system, where numerous antennas are distributed in the area covered by the system. The bit error rate (BER) performance of the distributed antenna cellular DS-CDMA system is investigated, when minimum mean-square error (MMSE) multiuser detection is employed, and when transmission pass-loss, lognormal shadowing slow fading and Nakagami-m fast fading are considered. Our study suggests that the distributed antenna cellular DS-CDMA system constitutes a high power-efficiency wireless system. For utilizing the same set of system parameters, it is capable of providing an extremely higher capacity, than the cellular DS-CDMA system designed based on the conventional cellular concepts. Furthermore, the proposed distributed antenna cellular system can be operated without having to use power-control.

In this contribution an iterative (turbo) channel equalization and source decoding scheme is considered. In our investigations the source is modelled as a Gaussian-Markov source, which is compressed with the aid of vector quantization. The communications channel is modelled as a time-invariant channel contaminated by intersymbol interference (ISI). Since the ISI channel can be viewed as a rate-1 encoder and since the redundancy of the source cannot be perfectly removed by source encoding, a joint channel equalization and source decoding scheme may be employed for enhancing the achievable performance. In our study the channel equalization and the source decoding are operated iteratively on a bit-by-bit basis under the maximum aposteriori (MAP) criterion. The channel equalizer accepts the apriori information provided by the source decoding and also extracts extrinsic information, which in turn acts as a priori information for improving the source decoding performance. Simulation results are presented for characterizing the achievable performance of the iterative channel equalization and source decoding scheme. Our results show that iterative channel equalization and source decoding is capable of achieving an improved performance by efficiently exploiting the residual redundancy of the vector quantization assisted source coding.

Differential space-time modulation (DSTM) schemes were proposed for detection without any knowledge of the channel state information (CSI), which is an attractive feature in the presence of fast fading channels, where accurate channel estimation is a challenging task. In this contribution we invoke two DSTM schemes for improving the achievable performance in the downlink of a generalized MC DS-CDMA system employing smart antennas, namely the differential space-time spreading (DSTS) scheme and the differential steered space-time spreading (DSSTS) scheme. In the MC DS-CDMA system considered the transmitter employs multiple antenna arrays and each of the antenna arrays consists of several antenna elements. Furthermore, the generalized MC DS-CDMA system employs both time- and frequency (TF) domain spreading, where a user-grouping technique is employed for reducing the effects of multiuser interference

In this treatise we demonstrate, how a high spatial and frequency diversity can be achieved by combining differential space-time block coding (DSTBC) and fast frequency hopping (FFH). The proposed FFH assisted DSTBC scheme is capable of achieving a high diversity order given by the product of the frequency diversity order achieved by FFH and by the spatial diversity order offered by the DSTBC arrangement. Hence, the system is capable of exploiting the advantages of both DSTBC and FFH. Furthermore it is less vulnerable to some of their individual disadvantages, such as the performance degradation of DSTBC due to fast fading and the large bandwidth requirement of the FFH system. The proposed system also benefits from the employment of sphere packing and iterative turbo detection

In this contribution, we propose and investigate a multiple-input-multiple-output space-division, code-division multiple-access (MIMO SCDMA) scheme. The main objective is to improve the capacity of the existing direct-sequence (DS)-CDMA systems, for example, for supporting an increased number of users, by deploying multiple transmit and receive antennas in the corresponding systems and by using some advanced transmission and detection algorithms. In the proposed MIMO SCDMA system, each user can be distinguished jointly by its spreading code signature and its unique channel impulse response (CIR) transfer function referred to as spatial signature. Hence, the number of users might be supported by the MIMO SCDMA system and the corresponding achievable performance are determined by the degrees of freedom provided by both the code signatures and the spatial signatures, as well as by how efficiently the degrees of freedom are exploited. Specifically, the number of users supported by the proposed MIMO SCDMA can be significantly higher than the number of chips per bit, owing to the employment of space-division. In this contribution, space-time spreading is employed for configuring the transmitted signals. Three types of low-complexity linear detectors, namely, correlation, decorrelating, and minimum mean-square error (MMSE) are considered for detecting the MIMO SCDMA signals. The bit-error rate performance of the MIMO SCDMA system associated with these linear detectors are evaluated by simulations, when assuming that the MIMO SCDMA signals are transmitted over multipath Rayleigh-fading channels. Our study and simulation results show that MIMO SCDMA assisted by multiuser detection is capable of facilitating joint space-time despreading, multipath combining, and receiver diversity combining, while simultaneously suppressing the multiuser interfering signals.

The novel family of redundant residue number system (RRNS) codes is studied. RRNS codes constitute maximum-minimum distance block codes, exhibiting identical distance properties to Reed-Solomon codes. Binary to RRNS symbol-mapping methods are proposed, in order to implement both systematic and nonsystematic RRNS codes. Furthermore, the upper-bound performance of systematic RRNS codes is investigated, when maximum-likelihood (ML) soft decoding is invoked. The classic Chase algorithm achieving near-ML soft decoding is introduced for the first time for RRNS codes, in order to decrease the complexity of the ML soft decoding. Furthermore, the modified Chase algorithm is employed to accept soft inputs, as well as to provide soft outputs, assisting in the turbo decoding of RRNS codes by using the soft-input/soft-output Chase algorithm

In this paper we investigated the BER performance of DS-CDMA using various chip-waveforms, which include three time-limited chip-waveforms and two band-limited chip-waveforms. Closed-form formulae were derived for evaluating the achievable bit-error rate performance with the aid of the standard Gaussian approximation, when communicating over a Nakagami-m channel. The time-limited waveforms impose a low implementational complexity, since they maybe over sampled and read from a look-up table. However, they are outperformed by the frequency-domain raised-cosine waveform as well as the optimum waveform specifically designed by Cho and Lehnert for achieving the lowest possible bit error rate

A novel sequential estimation method is proposed for the acquisition of m-sequences. This sequential estimation method exploits the principle of iterative soft-in-soft-out (SISO) decoding for enhancing the acquisition performance, and that of differential preprocessing for the sake of achieving an enhanced acquisition performance, when communicating over various communication environments. Hence, the advocated acquisition arrangement is referred to as the differential recursive soft sequential estimation (DRSSE) acquisition scheme. The DRSSE acquisition scheme exhibits a low complexity, which is similar to that of an m-sequence generator, while achieving an acquisition time that is linearly dependent on the number of stages in the m-sequence generator. A low acquisition time is achieved with the advent of the property that the proposed DRSSE scheme is capable of determining the real-time reliabilities associated with the decision concerning a set of, say, S consecutive chips. This set of consecutive chips constitutes the sufficient initial condition for enabling the local m-sequence generator to produce a synchronized local despreading m-sequence replica. Owing to these attractive characteristics, the DRSSE acquisition scheme constitutes a promising initial synchronization scheme for acquisition of long m-sequences, when communicating over various propagation environments.

In this contribution multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) using space-time spreading (STS)-assisted transmit diversity is investigated in the context of broadband communications over frequency-selective Rayleigh-fading channels. We consider the issue of parameter design for the sake of achieving high-efficiency communications in various dispersive environments. Furthermore, in contrast to conventional MC DS-CDMA schemes employing time (T)-domain spreading only, in this contribution we also investigate broadband MC DS-CDMA schemes employing both T-domain and frequency (F)-domain spreading, i.e., employing TF-domain spreading. The bit-error rate (BER) performance of STS-assisted broadband MC DS-CDMA is investigated for downlink transmissions associated with the correlation based single-user detector and the decorrelating multiuser detector. Our study demonstrated that when appropriately selecting the system parameters, broadband MC DS-CDMA using STS-assisted transmit diversity constitutes a promising downlink transmission scheme. This scheme is capable of supporting ubiquitous communications over diverse communication environments without BER performance degradation.

Abstract Digital beamforming (DBF) techniques are capable of improving the performance of communications systems significantly. However, if the transmitted signals are conflicted with strong interference especially in the direction of the transmitted beams, these directional jamming signals will severely degrade the system performance. In order to efficiently mitigate the interference of the directional jammers, in this contribution a beam‐hopping (BH) communications scheme is studied. In the BH communications scheme, only one pair of the beams is used for transmission and it hops from one to the next according to an assigned BH pattern. In this contribution, a range of expressions in terms of the average signal to interference plus noise ratio (SINR) performance have been derived, when both the uplink and downlink are considered. The average SINR performance of the BH scheme and that of the conventional single‐beam (SB) as well as multiple‐beam (MB) assisted beam‐processing schemes have been investigated. Our analysis and results show that the BH scheme is capable of efficiently combating the directional jamming, with the aid of utilizing the directional gain of the beams generated by both the transmitter and the receiver. Furthermore, the BH scheme is capable of reducing the intercept probability of the communications. Therefore, the BH scheme is suitable for communications when several distributed antenna arrays are available around a mobile. Copyright © 2005 John Wiley & Sons, Ltd.

In this contribution, the performance of a space-division multiple-access (SDMA) system is investigated, when the space-time block coded signals based on two transmit antennas are transmitted over Nakagami-m fading channels. In the proposed SDMA systems the receiver employs multiple receive antennas and each of the receive antennas consists of several antenna array elements. The antenna arrays' output signals are combined linearly based on the minimum mean square error (MMSE) principles, which only uses the knowledge associated with the desired user, i.e. we consider a single-user MMSE detector. Specific examples are provided for illustrating the properties of the MMSE detector. Our study and simulation results show that the MMSE detector is capable of facilitating joint space-time decoding, beamforming and receiver diversity combining, while simultaneously suppressing the interfering signals.

A novel sequential estimation method is proposed for the initial synchronization of pseudonoise (PN) signals derived from m-sequences. This sequential estimation method is designed based on the principle of recursive soft-in/soft-out (SISO) decoding, and we refer to it as the recursive soft sequential estimation (RSSE) acquisition scheme. The RSSE acquisition scheme exhibits a complexity similar to that of a conventional m-sequence generator, which increases only linearly with the number of stages in the m-sequence generator. Our simulation results also show that the acquisition time of the proposed RSSE acquisition scheme is also linearly dependent on the number of stages in the m-sequence generator. Owing to the above properties, the employment of the proposed RSSE acquisition scheme is beneficial for the acquisition of long m-sequences.

In multicarrier code-division multiple-access (MC-CDMA) the total system bandwidth is divided into a number of subbands, where each subband may use direct-sequence (DS) spreading and each subband signal is transmitted using a subcarrier frequency. In this contribution we divide the symbol duration into a number of fractional sub-symbol durations also referred to here as fractions, in a manner analogous to subbands in MC- CDMA systems. In the proposed MC-CDMA scheme the data streams are spread at both the symbol-fraction level and at the chip level by the transmitter, and hence the proposed scheme is referred to as the fractionally-spread MC-CDMA arrangement, or FS MC-CDMA. In comparison to conventional MC-CDMA schemes, which are suitable for communications over frequency-selective fading channels, our study demonstrates that the proposed FS MC-CDMA is capable of efficiently exploiting both the frequency-selective and the time-selective characteristics of wireless channels.

In this contribution we propose and study a multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) scheme, which employs both time (T)-domain and frequency (F)-domain spreading. We investigate the achievable detection performance in the context of synchronous TF-domain spread MC DS-CDMA, when communicating over an additive white Gaussian noise (AWGN) channel. The performance of three different detection schemes is investigated, which include the single-user correlation based detector, the joint TF-domain MMSE multiuser detector (MUD) and the separate TF-domain MMSE/decorrelating MUD. Our simulation results show that the separate TF-domain MMSE/decorrelating MUD scheme is capable of achieving a bit error rate (BER) performance that is similar to that of the significantly more complex joint TF-domain MMSE MUD schemes.

A novel sequential estimation method is proposed for the initial synchronization of pseudonoise (PN) signals derived from m-sequences. This sequential estimation method is designed based on the principle of iterative soft-in-soft-out (SISO) decoding, and hence we refer to it as the recursive soft sequential estimation (RSEE) acquisition scheme. The RSSE acquisition scheme exhibits a complexity similar to that of a conventional m-sequence generator, which increases only linearly with the number of stages in the m-sequence generator. Furthermore, our simulation results show that the acquisition time of the proposed RSEE acquisition scheme is also linearly dependent on the number of stages in the m-sequence generator. Hence, the employment of the proposed RSEE acquisition scheme is beneficial for employment in the acquisition of long m-sequences.

In this contribution the family of generalized multicarrier DS-CDMA (MC DS-CDMA) schemes is investigated, when considering time-domain half-sine and raised-cosine chip waveforms, in addition to the rectangular chip waveform. Our results show that for a given subcarrier spacing the particular choice of the chip waveform has a substantial influence on the system's performance. However, in the context of MC DS-CDMA using the optimum subcarrier spacing, all chip waveforms may provide a similar bit error rate (BER) performance.

We extend our investigation of generalized multicarrier direct-sequence code-division multiple access by considering two additional types of chip waveforms, namely, the time-domain half-sine and raised-cosine chip waveforms, in addition to the rectangular chip waveform. A range of closed-form equations are provided for quantifying the effect of both the multipath interference and the multiuser interference, when using partially overlapping subcarriers. These closed-form formulas allow us to evaluate the bit-error rate performance of arbitrary code-division multiple access schemes using overlapping subbands with the aid of the standard Gaussian approximation.

In this contribution the performance of wideband code-division multiple-access (W-CDMA) systems using space-time spreading (STS) based transmit diversity is investigated, when frequency-selective Nakagami-m fading channels, multiuser interference and background noise are considered. The analysis and numerical results suggest that significant diversity gain can be achieved by employing several transmit antennas or/and by combining several multipath components. Furthermore, both the transmit diversity and the frequency-selective diversity appear to have the same order of importance.

Ultra-wideband (UWB) systems having a bandwidth on the order of a Gigahertz have received wide attention both in the USA and in Europe. A novel UWB multiple access scheme based on the well-known family of frequency-hopping multiple access (FHMA) techniques is proposed, where the associated FH is implemented using multistage frequency hopping multiple access, or MS/UWB FHMA. We highlight the principles of the proposed MS/UWB FHMA communication system, investigate the associated spectrum assignment, the FH strategy, the applicable signal detection schemes as well as the achievable bit error rate (BER) performance. Our study shows that MS/UWB FHMA is capable of supporting an extremely high number of users, while employing relatively simple receivers.

We propose a novel space-time coding-assisted double-spread RAKE receiver (STC-DS-RR) based CDMA scheme for downlink (base to mobile) transmissions over dispersive Rayleigh fading channels. The STC-DS-RR scheme invokes turbo convolutional (TC) channel coding and space-time block coding (STBC) for achieving both time and space diversity gains in addition to the path diversity gain provided by the dispersive Rayleigh fading environments. The double-spreading mechanism of the STC-DS-RR scheme is capable of detecting the wanted user's channel-impaired wideband signals with the aid of a low-complexity RAKE receiver, without resorting to complex multi-user detection. The performance of the proposed scheme was investigated using quadrature-phase-shift-keying (QPSK), when communicating over dispersive channels. It was shown that the wideband scheme advocated is capable of achieving E/sub b//N/sub 0/ gains up to 35 dB in comparison to an uncoded narrowband single-transmitter scheme.

In this contribution we analyze the uplink capacity of a TDD/CDMA system. In TDD/CDMA the mobiles suffer from interference inflicted by the other mobile stations (MSs) both in the reference cell the MS is roaming in (intracell interference) as well as due to those in the neighbouring cells (intercell interference). Furthermore, in contrast to FDD/CDMA, where the base stations (BSs) transmit in an orthogonal frequency band, in TDD/CDMA there is additional interference imposed by other BSs of the adjacent cells. In return for this disadvantage TDD/CDMA guarantees the flexible utilization of all the available bandwidth, which meets the demand for the support of asymmetric uplink and downlink services, such as high speed data rate download in mobile Internet services, etc. This study quantifies the amount of uplink interference inflicted by the MSs and BSs. With the aid of characterising the interference, a detailed discussion of the uplink capacity of TDD/CDMA is provided and two extreme teletraffic loading scenarios of the UTRA TDD/CDMA system are considered. Our numerical results show that the capacity of TDD/CDMA is significantly poorer, than that of FDD/CDMA.

An adaptive space-time spreading (ASTS) assisted transmit diversity scheme is proposed for improving the throughput of wideband code-division multiple-access (W-CDMA) systems. The performance of the system is investigated, when frequency-selective Nakagami-m fading channels, multiuser interference and background noise are considered. Our numerical results demonstrate that the proposed ASTS-based transmission scheme is capable of significantly improving the effective throughput of W-CDMA systems. Specifically, the studied W-CDMA system's bitrate can be increased by a factor of three at the modest cost of requiring an extra 0.4 dB or 1.2 dB transmitted power in the context of the investigated urban or suburban areas, respectively.

In this article multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) using space-time spreading (STS) is investigated in the context of broadband communications over frequency-selective Rayleigh fading channels. We consider a range of design issues as well as the achievable bit error rate (BER) performance for the down-link, by assuming synchronous transmission of the user signals. The BER performance of STS assisted broadband MC DS-CDMA using binary phase shift keying (BPSK) modulation is investigated by simulation for a range of parameter values. Our study shows that by appropriately selecting the system parameters, STS assisted broadband MC DS-CDMA is capable of supporting ubiquitous communications in various communication environments including indoor, open rural, suburban and urban areas without BER performance degradation. Furthermore, the STS based transmit diversity schemes can be designed for attaining a certain required diversity gain, while maintaining a near-constant BER in various communication environments, provided that frequency-selective Rayleigh fading channels are encountered.

Ultra-wideband (UWB) systems having a bandwidth on the order of gigahertz have received wide attention both in the US and in Europe. The family of UWB systems may communicate either, by generating ultra-wideband signals or with the aid of innovatively combining conventional narrowband, wideband, or broadband signals. At the time of writing, UWB systems have only been implemented using ultra-wideband signals, such as those known from impulse radio systems. Hence, in this paper, UWB systems using narrowband signals are explored as a design alternative, which are based on the well-known family of frequency-hopping (FH) spread-spectrum multiple-access techniques. In the proposed UWB system, FH is implemented using multistage frequency-hopping multiple access (MS/UWB FHMA). We highlight the principles of the synchronous MS/UWB FHMA communication system, investigate the associated spectrum assignment, and the residue number system (RNS) based FH strategy: detection of the received signal can be achieved with the aid of existing fast FH signal detection schemes. Our study shows that the RNS assisted FH strategy is capable of efficiently dividing the huge number of users supported by the synchronous MS/UWB FHMA system into a number of reduced-size user groups, where the multiuser interference only affects the users within the same group. Since the number of users in each group is only a small fraction of the total number of users supported by the synchronous MS/UWB FHMA system, advanced multiuser detection algorithms can be employed for achieving near-single-user performance at an acceptable complexity. Our results show that MS/UWB FHMA is capable of supporting an extremely high number of users, while employing relatively simple receivers.

We propose two novel low-complexity, low-delay erasure insertion schemes, namely, the output threshold test (OTT) and joint maximum output and ratio threshold test (MO-RTT). The employment of the OTT and MO-RTT is beneficial in the context of the "errors-and-erasures" Reed-Solomon (RS) decoding in a slow frequency-hopping spread-spectrum (SFH/SS) system using M-ary frequency-shift keying (MFSK). The statistics of the erasure insertion related decision variables associated with the OTT, MO-RTT as well as with the ratio threshold test (RTT) are investigated, when the channel of each frequency-hopping (FH) slot is modeled as flat Nakagami-m fading. The transmitted signals also experience both additive white Gaussian noise (AWGN) as well as partial-band Gaussian interference (PBGI). The properties of these erasure insertion schemes are investigated with the aid of their statistics. The performance of the proposed erasure insertion schemes and that of the erasure insertion scheme using the RTT is investigated and compared in the context of RS coded SFH/SS systems using MFSK. Furthermore, the performance of the RS coded SFH/SS systems is also compared both with and without side-information concerning the PBGI.

A class of generalized multicarrier direct sequence code-division multiple-access (MC DS-CDMA) schemes is defined and its performance is considered over multipath Nakagami-m fading channels. The spacing between two adjacent subcarriers of the generalized MC DS-CDMA is a variable, allowing us to gain insight into the effects of the spacing on the bit error rate (BER) performance of MC DS-CDMA systems. This generalized MC DS-CDMA scheme includes the subclasses of multitone DS-CDMA and orthogonal MC DS-CDMA as special cases. We present a unified analytical framework for determining the exact average BER of the generalized MC DS-CDMA system over generalized multipath Nakagami-m (1960) fading channels. The optimum spacing of the MC DS-CDMA system required for achieving the minimum BER is investigated and the BER performance of the system having optimum spacing is evaluated. The resultant BER is compared with that of both multitone DS-CDMA and orthogonal MC DS-CDMA.

We investigate the issue of pseudo noise (PN) code acquisition in single-carrier and multicarrier (MC) direct-sequence code-division multiple-access (DS-CDMA) systems, when the channel is modeled by frequency-selective Nakagami-m (1960) fading. The PN code acquisition performance of single-carrier and MC DS-CDMA systems is analyzed and compared when communicating over Nakagami-m fading channels under the hypothesis of multiple synchronous states (H/sub 1/ cells) in the uncertainty region of the PN code. In the context of MC DS-CDMA, the code acquisition performance is evaluated, when the correlator outputs of the subcarriers associated with the same phase of the local PN code replica are noncoherently combined by using equal gain combining (EGC) or selection combining (SC) schemes. The performance comparison of the above mentioned schemes shows that the code acquisition performance of the MC DS-CDMA scheme, especially when using the EGC scheme, is more robust, than that of single-carrier DS-CDMA schemes communicating over the multipath Nakagami-m fading channels encountered. However, our code acquisition performance comparison also shows that if the detection threshold was set inappropriately, the performance might be degraded, even if the channel fading becomes less severe.

We propose two novel low-complexity, low-delay erasure insertion schemes, namely the output threshold test (OTT) and joint maximum output and ratio threshold test (MO-RTT). The employment of the OTT and MO-RTT is beneficial in the context of 'errors-and-erasures' (EAE) Reed-Solomon (RS) decoding in a slow frequency-hopping based spread-spectrum (SFH/SS) system using M-ary frequency-shift keying (MFSK). The statistics of the erasure insertion related decision variables associated with the OTT and MO-RTT as well as with the ratio threshold test (RTT) are investigated, when the channel of each FH slot is modeled as flat Nakagami-m fading. The transmitted signals also experience both additive white gaussian noise (AWGN) and partial-band Gaussian interference (PBGI). The performance of the proposed erasure insertion schemes and that of the erasure insertion scheme using the RTT is investigated and compared in the context of RS coded SFH/SS systems using MFSK modulation.

In this contribution the conventional error-detection and error-correction algorithms used for RRNS codes are improved and unified in the context of a so-called projection-based 'minimum-distance decoding' algorithm, which can efficiently detect or correct multiple residue digit errors.

Against the backcloth of the explosive expansion of the Internet and the continued dramatic increase in demand for high-speed multimedia wireless services, there is an urgent demand for flexible, broadband transceivers. A versatile broadband multiple access scheme, combining frequency-hopping (FH) with multicarrier DS-CDMA (FH/MC DS-CDMA) is proposed and investigated. The FH/MC DS-CDMA scheme is capable of meeting the requirements of future mobile wireless systems, while also ensuring compatibility with the existing 2nd- and 3rd-generation systems in the context of multirate transmission.

In this contribution we investigate the performance of RS coded DS-CDMA using M-ary orthogonal signalling over multipath Rayleigh fading channels. 'Errors-and-erasures (EAE)' RS decoding is considered, where erasures are declared based on the joint maximum output and ratio threshold test (MOR-TT). Two classes of diversity reception techniques suitable for noncoherent M-ary detection are invoked, namely either equal gain combining (EGC) or selection combining (SC) and their related performance is evaluated for both uncoded and coded DS-CDMA systems. The performance of the proposed MOR-TT technique is compared to that of the ratio threshold test (RTT) and the output threshold test (OTT), as well as that of 'error-correction-only (ECO)' decoding. The numerical results show that when using EAE decoding, RS codes of a given code rate are capable of achieving a significantly higher coding gain than without erasure information. Furthermore, the MOR-TT technique outperforms both the RTT and the OTT techniques.

In this paper residue number system (RNS) arithmetic and redundant residue number system (RRNS) based codes as well as their properties are reviewed. We propose a number of applications for RRNS codes and demonstrate how RRNS codes can be employed in global communication systems, in order to simplify the associated systems by unifying the entire encoding and decoding procedure across global communication systems.

The performance of M-ary orthogonal signaling schemes employing Reed-Solomon (RS) codes and redundant residue number system (RRNS) codes is investigated over frequency-selective Rayleigh fading channels. "Errors-and-erasures" decoding is considered, where erasures are judged based on two low-complexity, low-delay erasure insertion schemes-Viterbi's ratio threshold test (RTT) and the proposed output threshold test (OTT). The probability density functions (PDF) of the ratio associated with the RTT and that of the demodulation output in the OTT conditioned on both the correct detection and erroneous detection of M-ary signals are derived, and the characteristics of the RTT and OTT are investigated. Furthermore, expressions are derived for computing the codeword decoding error probability of RS codes or RRNS codes based on the above PDFs, The OTT technique is compared to Viterbi's RTT, and both of these are compared to receivers using "error-correction only" decoding over frequency-selective Rayleigh-fading channels. The numerical results show that by using "errors-and-erasures" decoding, RS or RRNS codes of a given code rate can achieve higher coding gain than that without erasure information, and that the OTT technique outperforms the RTT, provided that both schemes are operated at the optimum decision thresholds.

A novel multiple access scheme based on slow frequency hopping multicarrier direct-sequence code division multiple access (SFH/MC DS-CDMA) is proposed and investigated, which can be rendered compatible with the existing second-generation narrowband CDMA and third-generation wideband CDMA systems. The frequency hopping patterns are controlled by a set of constant-weight codes. Consequently, multirate communications can be implemented by selecting the corresponding sets of constant-weight codes having the required weights controlling the SFH patterns invoked. Two FH schemes, namely random and uniform FH, are considered and their advantages as well as disadvantages are investigated. We assume that the system operates in a multipath fading environment and a RAKE receiver structure with maximum ratio combining (MRC) is used for demodulation. The system's performance is evaluated over the range of multipath Nakagami (1960) fading channels, under the assumption that the receiver has all explicit knowledge of the associated frequency-hopping (FH) patterns invoked. Furthermore, the performance of the SFH/MC DS-CDMA system is compared to that of the conventional single-carrier (SC) DS-CDMA system and that of the conventional MC DS-CDMA system, under the assumptions of constant system bandwidth and of constant transmitted signal power.

A parallel pseudo-noise (PN) code acquisition scheme is investigated for multicarrier CDMA (MC-CDMA) systems, The channel is modeled by a frequency-selective Nakagami distribution. The parallel acquisition scheme's performance is evaluated, when multiple synchronous states (H/sub 1/ cells) are assumed in the uncertainty region of the PN code, and when the correlator outputs of the subcarriers associated with the same phase of the local PN code replica are noncoherently combined with the aid of equal gain combining (EGC). It is demonstrated that when EGC is employed, the acquisition performance of the MC-CDMA scheme is better than that of the equivalent single-carrier CDMA benchmark scheme over the investigated multipath Nakagami fading channels.

We investigate the issue of pseudonoise (PN) code acquisition in a multicarrier CDMA (MC-CDMA) system, which is suitable for high data rate transmissions. Specially, we investigate the acquisition performance of a pure parallel acquisition scheme over frequency-selective fading channels under the multiple in-phase states (H/sub 1/ cells) hypothesis, considering both noncoherent equal gain combining (EGG) and selection combining (SC) of the correlator outputs associated with the sub-carriers. It is demonstrated that the acquisition performance of the MC-CDMA scheme is better than that of the equivalent single-carrier CDMA (SC-CDMA) benchmarker, when EGC is employed, and it is similar to that of the single-carrier CDMA scheme, when SC is employed.

The performance of Reed-Solomon (RS) coded direct-sequence code division multiple-access (DS-CDMA) systems using noncoherent M-ary orthogonal modulation is investigated over multipath Rayleigh fading channels. Equal gain combining (EGC) diversity reception is invoked and the related performance is evaluated for both uncoded and RS-coded DS-CDMA systems. 'Errors-and-erasures' decoding is considered, where the erasures are based on Viterbi's (1982) so-called ratio threshold test (RTT). The performance of the 'errors-and-erasures' decoding technique employing the RTT is compared to that of 'error-correction-only' decoding over multipath Rayleigh fading channels. The numerical results show that when using RTT-based 'errors-and-erasures' decoding, RS codes of a given code rate can achieve a higher coding gain than without erasure information.

The performance of Reed-Solomon (RS) coded direct-sequence code division multiple-access (DS-CDMA) systems using noncoherent M-ary orthogonal modulation is investigated over multipath Rayleigh fading channels. Diversity reception techniques with equal gain combining (EGC) or selection combining (SC) are invoked and the related performance is evaluated for both uncoded and coded DS-CDMA systems. "Errors-and-erasures" decoding is considered, where the erasures are based on Viterbi's (1982) so-called ratio threshold test (RTT). The probability density functions (PDF) of the ratio associated with the RTT conditioned on both the correct detection and erroneous detection of the M-ary signals are derived. These PDFs are then used for computing the codeword decoding error probability of the RS coded DS-CDMA system using "errors-and-erasures" decoding. Furthermore, the performance of the "errors-and-erasures" decoding technique employing the RTT is compared to that of "error-correction-only" decoding refraining from using side-information over multipath Rayleigh fading channels. As expected, the numerical results show that when using "errors-and-erasures" decoding, RS codes of a given code rate can achieve a higher coding gain than without erasure information.

A novel multiple-access scheme based on slow frequency-hopping multicarrier direct-sequence, code-division multiple access (SFH/MC DS-CDMA) is proposed and investigated, which can be rendered compatible with the existing second-generation narrow-band CDMA and third-generation wide-band CDMA systems. Blind joint soft-detection of the SFH/MC DS-CDMA signals is investigated, assuming that the receiver has no knowledge of the associated frequency-hopping (FH) patterns invoked. The system's performance is evaluated over the range of Nakagami multipath fading channels. The results show that blind joint soft-detection achieves the required bit-error rate performance, while blindly acquiring the FH patterns employed. This is advantageous during the commencement of communications or during soft handover.