This paper proposes an optical reconfigurable intelligent surface (ORIS)-assisted ground-to-unmanned aerial vehicle (UAV) free-space optical (FSO) communication system empowered by simultaneous lightwave information and power transfer (SLIPT). To overcome the line-of-sight (LoS) limitation of FSO-based SLIPT systems, we introduce an ORIS that reflects the laser beam towards a non-LoS UAV receiver. We model and analyze the combined channel characteristics, incorporating atmospheric loss, turbulence-induced fading, pointing error, and angle-of-arrival fluctuations due to UAV hovering. We derive closed-form expressions for harvested energy, outage probability, and symbol error rate (SER). Numerical results show that integrating ORIS improves EH efficiency while maintaining manageable outage and SER performance.

In this paper, we propose denoising autoencoder (DAE)-assisted secret key generation (SKG), where channel noise reciprocity imperfections induced due to wireless channel measurements are suppressed, hence significantly enhancing the reliability and efficiency. More specifically, the DAE is capable of capturing the intrinsic structure of input distributions, reconstructing the original data structure, and removing additive noise while preserving the essential structure of signals. In our analysis, it is demonstrated that the proposed SKG scheme exhibits higher performance than the conventional schemes in terms of key disagreement rate (KDR), secret key capacity (SKC), and randomness of the generated keys.

This paper proposes low-overhead and low-complexity channel estimation (CE) of frequency-domain equalization (FDE)-aided faster-than-Nyquist (FTN) signaling. In the proposed CE scheme, the concept of pilot superimposition is employed, where the FTN block is designed to superimpose pilot symbols with information symbols, thereby eliminating the need for dedicated time and frequency resources or guard bands. Furthermore, interference induced by the pilot superimposition is eliminated by invoking a novel scheme, referred to as spectral interference alignment, where a specially-designed data-dependent sequence is subtracted from the transmitted information symbols to achieve interference-free CE. The theoretical mean-square error (MSE) of the proposed CE is derived, confirming that the MSE is no longer affected by interference from the pilot superimposition. Our performance results demonstrate that the proposed CE achieves lower CE errors, better bit-error rates, higher achievable rate, and spectral efficiency with half of the overhead compared with the conventional benchmark.

This paper proposes precoded multicarrier faster-than-Nyquist (MFTN) signaling for integrated sensing and communication (ISAC) waveform, which improves the information rate. The complexity of MFTN signaling is maintained low with the aid of fast Fourier transform (FFT)-based diagonalization. Furthermore, under the constraints of the total transmit power and subcarrier power ratio, we optimize power allocation by simultaneously maximizing sensing mutual information and communication mutual information. Our performance results demonstrate that the proposed MFTN-based ISAC waveform is capable of enhancing the information rate while achieving a high comparable sensing performance.

This paper proposes low-overhead and low-complexity channel estimation (CE) of frequency-domain equalization aided faster-than-Nyquist (FTN) signaling. In the proposed CE scheme, the concept of pilot superimposition is employed, where the FTN block is designed to superimpose pilot symbols with information symbols, and thus, no dedicated time and frequency resources nor guard bands are required, resulting in a 50% reduction of the overhead. Furthermore, interference induced by the pilot superimposition is eliminated by invoking a novel scheme, referred to as spectral interference alignment, where a data-dependent sequence is subtracted from transmitted information symbols. The theoretical mean-square error (MSE) of the proposed CE is derived, which verifies that the MSE is no longer affected by interference due to the pilot superimposition.

This study aims to enhance the transmission efficiency of microwave power transfer, which has garnered significant attention as a method of wireless power supply for IoT devices and drones, by using Intelligent Reflecting Surface (IRS). IRS is a plate-shaped device that can control the reflection characteristics of radio waves, and it is expected to improve the transmission efficiency when placed between a wireless transmitter and a wireless receiver. However, existing methods of reflection phase optimization for IRS are predominantly based on sequential feedback or a small number of discrete reflection phases, both of which have problems, including feedback overhead and beam shape limitations. In this paper, we propose a novel reflection phase optimization method based on curve fitting. This approach is capable of continuous reflection phase control and optimization of the reflection phase of all reflective elements simultaneously. Through numerical simulation and actual experiments using an implemented IRS prototype, it was observed that the proposed method enhanced the received power by up to 4 dB compared to a conventional sequential optimization method. However, it was also observed that the range limitation of the reflection phase in the implemented IRS hindered the effectiveness of the proposed method.

Large optical reconfigurable intelligent surfaces (ORISs) are proposed for employment on building rooftops to facilitate free-space quantum key distribution (QKD) between highaltitude platforms (HAPs) and low-altitude platforms (LAPs). Due to practical constraints, the communication terminals can only be positioned beneath the LAPs, preventing direct upward links to HAPs. By deploying ORISs on rooftops to reflect the beam arriving from HAPs towards LAPs from below, reliable HAP-to-LAP links can be established. To accurately characterize the optical beam propagation, we develop an analytical channel model based on extended Huygens-Fresnel principles for representing both the atmospheric turbulence effects and the hovering fluctuations of LAPs. This model facilitates adaptive ORIS beam-width control through linear, quadratic, and focusing phase shifts, which are capable of effectively mitigating the detrimental effects of beam broadening and pointing errors (PE). Consequently, the information-theoretic bound of the secret key rate and the security performance of a decoy-state QKD protocol are analyzed. Our findings demonstrate that quadratic phase shifts enhance the SKR at high HAP-ORIS zenith angles or mild PE conditions by narrowing the beam to optimal sizes. By contrast, linear phase shifts are advantageous at low HAP-ORIS zenith angles or moderate-to-high PE by diverging the beam to mitigate LAP fluctuations.

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.

In this paper, we examine a millimeter-wave (mmWave) wiretap scenario where a multiple-antenna base station (Alice) communicates with a legitimate user (Bob) via a reconfigurable intelligent surface, while multiple colluding eavesdroppers (Eves) attempt to passively intercept the communications using the maximal-ratio combining scheme. We develop a framework for achieving the maximum effective secrecy throughput (EST) that meets both reliability and secrecy constraints. Firstly, we derive a generalized and tractable form of the independent fluctuating two-ray (IFTR) distribution for accurately modeling mmWave fading channels. Secondly, we propose a novel approximation method to statistically characterize the end-to-end channels, considering the phase-shift estimation errors at both Bob and the colluding Eves over the IFTR channels. Finally, we derive a new closed-form EST metric and its asymptotic expression.

This paper introduces a novel robust signal-to-noise ratio (SNR) estimation against Doppler shift using a convolutional neural network (CNN)-based multimodal network. SNR estimation is valuable for Beyond 5G (B5G) and 6G since it helps to evaluate the channel state information (CSI) reliability that is fed back from users and schedule radio resources. The proposed scheme increases feature diversity by converting the same received signal into multiple modalities, enabling accurate and robust SNR estimation against Doppler shifts. Simulation results show that the proposed scheme provides the most accurate estimation compared to conventional schemes in the presence of Doppler shifts.

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.

In this paper, we propose a novel physical-layer secret key generation (SKG), which randomly activates a pilot sequence during the channel probing phase. Each legitimate user interpolates the received signals to recover the full channel state information (CSI). Due to the random reduction of the pilot sequence, the proposed scheme can degrade the eavesdropping performance regardless of the correlation between legitimate users and eavesdroppers, thereby improving the secret key capacity (SKC). Furthermore, low power consumption during the channel probing phase can be achieved. Our simulation results demonstrate that the proposed scheme exhibits superior performance in the presence of eavesdroppers.

In this paper, we consider a millimeter-wave (mmWave) wiretap scenario where a multiple-antenna base station (Alice) communicates with a legitimate user (Bob) via a reconfigurable intelligent surface (RIS) amid multiple colluding eavesdroppers (Eves) using maximal-ratio combining. We formulate the maximum effective secrecy throughput (EST) that satisfies reliability and secrecy constraints. Our key contributions are threefold. Firstly, we propose an analytical framework that unifies fluctuating two-ray (FTR) and independent FTR (IFTR) fading distributions into a single model. While FTR/IFTR distributions are recognized as the statistical models suitable for the mmWave fading channel, they have traditionally been treated as separate models. Secondly, we propose a novel approximation method to statistically characterize end-to-end channels, accounting for the combined impact of the non-uniform RIS amplitude response and phase estimation errors at Bob and Eves over the unified FTR/IFTR channels. Finally, a new approximate closed-form expression of the EST is derived, extensively validated through Monte Carlo simulations. Our numerical findings highlight the superior benefits of deploying a larger RIS over increasing the number of antennas at Alice and the improved EST performance in a mmWave scenario compared to the microwave counterpart.

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.

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.

In this paper, we propose a novel precoded non-orthogonal frequency division multiplexing (NOFDM) with subcarrier index modulation (SIM) to increase spectral efficiency for multicarrier systems. The proposed NOFDM-SIM scheme is constituted by a filterbank multicarrier system under reduced subcarrier spacing, where precoded index modulated symbols are mapped onto the subcarriers. The detrimental effects of inter-carrier interference (ICI) resulting from the non-orthogonality between subcarriers are efficiently mitigated with the aid of eigenvalue-decomposition-based precoding. Our simulation results demonstrate that the proposed precoded NOFDM-SIM system exhibits a similar peak-to-average power ratio to OFDM and OFDM-SIM counterparts while achieving the bit error rate limit of zero-ICI OFDM in a channel-uncoded setting. We analyze the power spectral density of the proposed scheme to demonstrate that the proposed scheme is strictly bandlimited, unlike its orthogonal counterpart. Furthermore, we present detailed investigations of the effects on ICI in the proposed scheme with and without precoding as well as power allocation. Finally, we introduce a three-stage iterative decoding architecture for our proposed scheme and show near-capacity error performance.

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.

In this paper, we propose a novel precoded faster-than-Nyquist (FTN) signaling scheme with power-domain-multiplexing in non-orthogonal multiple access (NOMA) down-link for tapping the joint benefits of high spectral efficiency and simultaneous multiuser connectivity. Non-orthogonality is introduced in both the symbol-interval (time) domain as well as in the multiple-access (power) domain to achieve a flexible resource allocation. Eigendecomposition of the FTN-specific intersymbol interference (ISI) matrix is used to achieve efficient cancellation of ISI, while successive interference cancellation is used to elim-inate multiuser interference induced by NOMA. We derive the achievable analytical rate bound and demonstrate the numerical results of the bit error rate performance for the proposed scheme.

In this paper, we propose novel reduced-complexity fast Fourier transform (FFT)-spread faster-than-Nyquist (FTN) signaling with optimal power allocation for a frequency-selective fading channel. The information rate of the proposed FTN signaling is approximately derived by relying on the circulant approximation of the FTN-specific intersymbol interference matrix and noise covariance matrix. This allows us to constitute efficient calculations of precoding and weighting matrices. The power allocation coefficients are optimized such that the approximated information rate is maximized. Our simulation results demonstrate that the proposed scheme achieves the bit error ratio performance close to the conventional eigenvalue-decomposition (EVD)-precoded FTN signaling counterpart that is optimal in terms of an achievable information rate while significantly reducing the computational complexity as low as the order of ${\mathcal{O}}(N\log N)$.

In this paper, we propose a novel precoded non-orthogonal frequency division multiplexing (NOFDM) with sub-carrier index modulation (SIM) to increase spectral efficiency for multicarrier systems. The proposed NOFDM-SIM scheme is constituted by a filterbank multicarrier system under reduced subcarrier spacing, where precoded index modulated symbols are mapped onto the subcarriers. The detrimental effects of inter-carrier interference (ICI) resulting from the non-orthogonality between subcarriers are efficiently mitigated with the aid of eigenvalue-decomposition-based precoding. Our simulation results demonstrate that the proposed precoded NOFDM-SIM system exhibits a similar peak-to-average power ratio to OFDM and OFDM-SIM counterparts while achieving the bit error ratio limit of zero-ICI OFDM.

In this paper, we propose a novel secure buffer-aided relay selection that is capable of operating both virtual full-duplex (VFD) and half-duplex (HD) transmissions in a hybrid manner. The proposed scheme consists of five modes: two unicast modes, a broadcast mode, cooperative beamforming, and a VFD mode. Since the VFD mode has the potential of achieving a higher secrecy capacity than the other four HD modes, the proposed relay selection algorithm gives the highest priority to activating the VFD mode. Our performance results demonstrate that the proposed secure relay selection scheme outperforms the benchmarks in terms of the secrecy outage probability.

In this paper, we propose a precoded faster-than-Nyquist (FTN) signaling technique for time-domain single-carrier index-modulated symbol transmission. More precisely, eigenvalue decomposition (EVD) precoding is adopted for the FTN transmission of data bits modulated by single-carrier time-domain index modulation (IM). While the FTN scheme increases the spectral efficiency and data rate by increasing the density of transmit symbols, the time-domain IM works towards the same objective while maintaining symbol sparsity. We analytically derive the constrained capacity of the proposed system. Our simulation results show that the proposed scheme has better bit error ratio (BER) performance than the conventional FTN-IM scheme, particularly for the scenario of a higher packing ratio. With the proposed scheme, a 2.5 dB performance gain is observed at the BER of 10−4, where the packing ratio is 0.7 and the roll-off factor is 0.5 in the channel-uncoded scenario. We further analyze our scheme’s performance by characterizing its minimum Euclidean distance and inter-symbol interference. Furthermore, we present a three-stage serially concatenated iterative detection architecture for the proposed EVD-FTN-IM signaling scheme. It is demonstrated in our simulations that a near-capacity BER performance is achievable in the proposed turbo-coded FTN-IM scheme.

In this paper, we propose eigenvalue decomposition (EVD)-precoded faster-than-Nyquist (FTN) signaling with power allocation in a frequency-selective fading channel. More specifically, we derive the mutual information associated with the proposed FTN signaling. Then, the optimal power coefficients are calculated such that the derived mutual information is maximized. Our analytical performance results show that the proposed FTN signaling scheme achieves a higher information rate than the conventional FTN signaling scheme without relying on power allocation and the classic Nyquist signaling scheme, under the assumption that all the schemes employ a root-raised cosine shaping filter. Moreover, we derive the proposed scheme’s achievable information rate in the presence of channel estimation errors. Furthermore, our numerical simulation results for the bit error ratio performance and the power spectral density demonstrate that the proposed FTN scheme outperforms the conventional Nyquist signaling scheme without incurring any bandwidth broadening.

In this paper, we propose a precoded faster-than-Nyquist (FTN) signaling technique for time-domain single-carrier index modulated (IM) symbol transmission. More precisely, eigenvalue decomposition precoding is adopted for the FTN transmission of data bits modulated by single-carrier time-domain IM. While the FTN scheme increases the spectral efficiency and data rate by packing more transmit symbols per block duration than those defined in the Nyquist criterion, time-domain IM works towards the same objective while maintaining symbol sparsity. We analytically derive the constrained capacity of the proposed system. Our simulation results show that the proposed scheme has better bit error ratio (BER) performance over the conventional FTN-IM scheme, particularly for the scenario of a higher packing ratio. In the proposed scheme, 2.5-dB performance gain is observed at the BER of = 10−4, employing the packing ratio of 0.7 and the roll-off factor of 0.5 in a channel-uncoded scenario.

In this paper, we investigate the information-theoretic secrecy performance of recent precoded faster-than-Nyquist signaling (FTN) with the aid of optimal power allocation in eigenspace. More specifically, the secrecy rate and secrecy outage probability of a fading wiretap channel, which was derived for classical Nyquist-based orthogonal signaling transmission, is extended to those of our eigendecompsition-based FTN (E-FTN) signaling for a quasi-static frequency-flat Rayleigh fading channel. Our performance results demonstrate that the proposed E-FTN signaling scheme exhibits improvements in secrecy rate and secrecy outage probability over conventional Nyquist-based and FTN signaling transmission, assuming employment of a root-raised cosine filter. We also show that the same benefits as those of single-carrier E-FTN signaling are attainable by its non-orthogonal multicarrier counterpart, where subcarrier spacing is set lower than that of orthogonal frequency-division multiplexing.

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.

This paper proposes a service framework to realize a wide area information-sharing system based on a delay-tolerant network. The proposed system consists of gateway nodes, relay nodes, and application services. The application services push the data to the nearest gateway node. The relay nodes copy the data from the gateway and carry them to another gateway. The proposed system serves an end-to-end data delivery service for application services over DTN. We have implemented a prototype of the proposed system. The prototype system uses Raspberry Pis 3 as the gateway nodes and relay nodes. We have implemented some application services as an iOS application that can send and receive media files using HTTP communication, an iOS message application that uses BLE communication, and a Bulletin Board System that uses HTTP communication. This paper demonstrates a prototype implementation of end-to-end data delivery service based on the proposed service framework.

This paper develops a prototype chat application that employs an automated communication scheme among smart-phones based on Bluetooth Low Energy (BLE) technology. Conventional smartphone applications use a cellular network to communicate with each other even when they can communicate directly. The proposed scheme realizes a direct data transfer over long distances by designing multi-hop communication technology. As communication technology, we employ BLE that is the latest standard of Bluetooth with low-rate communication capability and low-power consumption. We demonstrate the prototype chat application based on the proposed method to confirm the feasibility of the proposed multi-hop communication.

In this paper, we investigate the achievable performance of precoded faster-than-Nyquist (FTN) signaling with truncated power allocation from the information-theoretic perspective. More specifically, the effects of the eigenvalue distribution of the FTN-specific inter-symbol interference (ISI) matrix on the information rate were investigated. In order to evaluate the effects of significantly low eigenvalues, 1024-bit eigendecomposition calculations, which is significantly accurate in comparison to the calculations in the standard double-precision environment, are carried out. It is demonstrated that there is a novel performance tradeoff between the information rate versus the calculation precision of a transmit signal in the precoded FTN signaling scheme with truncated power allocation.

This paper proposes a novel ultra-dense non-orthogonal frequency-division multiplexing (NOFDM) transmission, where subcarrier spacing is set significantly lower than that of OFDM. Eigenvalue-decomposition-aided precoding is conceived for eliminating the detrimental effects of NOFDM-specific inter-carrier interference and correlated noises. The classic capacity formula is extended to that supporting our precoded NOFDM scheme. Optimal power allocation (PA) is developed for maximizing the derived capacity of the proposed scheme. In analytical and numerical results, we demonstrate that the proposed NOFDM scheme with optimal PA outperforms the conventional NOFDM and the classic OFDM schemes.

This paper proposes a novel ultra-dense non-orthogonal frequency-division multiplexing (NOFDM) transmission, where subcarrier spacing is set significantly lower than that of orthogonal frequency-division multiplexing (OFDM). In order to eliminate the detrimental effects of NOFDM-specific inter-carrier interference and correlated additive noises, eigenvalue-decomposition-aided precoding is conceived. The classical capacity formula is extended to one supporting our precoded NOFDM scheme. Optimal power allocation (PA) is developed in order to maximize the derived capacity of the proposed scheme. Furthermore, truncated PA is developed to combat the limitations due to significantly low eigenvalues, which are specifically imposed by the proposed ultra-dense NOFDM. Through analytical and numerical results, we demonstrate that the proposed NOFDM schemes with optimal and truncated PA outperform the conventional NOFDM and the classical OFDM schemes.

This paper proposes a precoded faster-than-Nyquist (FTN) signaling scheme based on singular-value decomposition (SVD) with optimal power allocation. An information-theoretic analysis is conducted on the proposed SVD-precoded FTN signaling architecture. Analytical performance results demonstrate that the proposed optimal scheme outperforms its conventional Nyquist-criterion- based counterpart and the conventional SVD-precoded FTN signaling scheme, which does not use power allocation. In order to overcome the limitations associated with significantly low singular values, suboptimal truncated power allocation is incorporated.

This paper proposes a virtual full-duplex relaying protocol with the aid of multiple half-duplex relay nodes equipped with buffers and multiple antenna elements. The protocol increases the spectral efficiency, in comparison to the conventional half-duplex counterpart. More specifically, multiple relay nodes simultaneously receive a source packet, while a single relay node retransmits a buffered packet to the destination node at the same time slot. In order to eliminate the detrimental effects of inter-relay interference, multiple antenna elements are used at relay nodes. Our performance results show that the proposed virtual full-duplex protocol is capable of achieving a lower delay profile than that of the conventional virtual full-duplex max- max relay selection, while combating limitations imposed by inter-relay interference.

Two decades of full-diversity high-rate MIMO research has created perfect Space-Time Block Codes (STBCs), including the Golden code. However, the major stumbling block of their wide-spread employment is their limited energy-efficiency. On one hand, the superposition of their signals results in a high Peak-to-Average Power Ratio (PAPR). On the other hand, the total number of equivalent Inter-Antenna Interference (IAI) contributions that the receiver has to deal with is increased to IAI = M2upon using M Transmit Antennas (TAs), which is a substantial extra price compared to the IAI = M of V-BLAST. Against this background, we propose a new family of Finite-Cardinality Generalized Space-Time Shift Keying (FC-GSTSK). More explicitly, the proposed FC-GSTSK is capable of outperforming both V-BLAST and STBC, which is the ultimate objective of full-diversity high-rate MIMO design. Furthermore, following the index modulation philosophy, the proposed FC-GSTSK replaces the signal-additions by the data-carrying signal-selection process. As a benefit, the FC-GSTSK substantially reduces the PAPR of signal transmission. As a further advantage, the equivalent IAI imposed on signal detection is reduced back to the same level as that of the V-BLAST. Moreover, the proposed FC-GSTSK is even capable of consistently outperforming the perfect STBCs in terms of its Peak Signal to Noise-power Ratio (PSNR) that takes into account the power consumption at the transmitter. As a further advance, the reduced-RF-chain based version of FC-GSTSK is also capable of outperforming both Generalized Spatial Modulation (GSM) and Space-Time Block Coded Spatial Modulation (STBC-SM) without increasing the PAPR and the equivalent IAI.

This letter proposes a differentially modulated non-orthogonal spectrally efficient frequency-division multiplexing (D-SEFDM) architecture, which allows us to dispense with any pilot overhead needed for channel estimation at the receiver, while increasing the bandwidth efficiency in comparison to the orthogonal frequency-division multiplexing counterpart. While it is a challenging task to carry out differential-modulation-assisted non-coherent detection for multi-carrier transmissions in the presence of a severe inter-carrier interference (ICI), in the proposed scheme ICI elimination and symbol demodulation are implemented in a non-coherent manner, by exploiting the fact that ICI imposed by D-SEFDM is deterministically given, once we have a compression factor of non-orthogonal subcarriers. It is verified in our simulations that the proposed D-SEFDM exhibits a higher performance than the conventional coherent SEFDM scheme, especially for the scenarios of a high Doppler frequency.

In this letter, motivated by the recent differential faster-than-Nyquist (DFTN) signaling concept, we propose an improved 16-point double-ring star quadrature amplitude modulation (QAM)-aided DFTN signaling transmission, which allows us to attain a higher bandwidth efficiency as well as a simple receiver based on noncoherent detection. We derive an analytical error-rate bound for the proposed star-QAM DFTN signaling in an uncoded scenario. The derived bound is used for optimizing the star-QAM constellation for our DFTN signaling in terms of error-rate performance in an uncoded scenario. Our simulation results demonstrate that the proposed star-QAM DFTN scheme outperforms its conventional phase-shift-keying-based DFTN counterpart.

The matrix-based differential encoding invoked by Differential Space-Time Modulation (DSTM) typically results in an infinite-cardinality of arbitrary signals, despite the fact that the transmit antennas (TAs) can only radiate a limited number of patterns. As a remedy, the recently developed differential spatial modulation (DSM) is capable of avoiding this problem by conceiving a beneficial sparse signal matrix design, which also facilitates low-complexity single-RF signal transmission. Inspired by this development, the differential space-time block code using index shift keying (DSTBC-ISK) further introduces a beneficial diversity gain without compromising the DSM's appealingly low transceiver complexity. However, the DSTBC-ISK's performance advantage tends to diminish as the throughput increases, especially when an increased number of Receive Antennas (RAs) is used. By contrast, the classic Differential Group Code (DGC) that actively maximizes its diversity gain for different multiple-input-multiple-output (MIMO) system setups is capable of achieving a superior performance, but its detection complexity grows exponentially with the throughput. Against this background, we propose the differential space-time shift keying using Diagonal Algebraic Space-Time scheme, which is the first DSTM that is capable of achieving the DGC's superior diversity gain at high throughputs without compromising the DSM's low transceiver complexity. As a further advance, we also conceive a new differential space-time shift keying using Threaded Algebraic Space-Time arrangement, which is capable of achieving an even further improved diversity gain at a substantially reduced signal detection complexity compared to the best DGCs. Furthermore, in order to strike a practical tradeoff, we develop a generic multi-element and multi-level-ring Amplitude Phase Shift Keying design, and we also arrange for multiple reduced-size DSTM sub-blocks to be transmitted in a permuted manner, which exhibits an improved diversity-throughput tradeoff.

In this paper, we propose a novel secure buffer-aided decode-and-forward relay selection that amalgamates the benefits of the buffer-state-based relay selection, the max-ratio criterion, the simultaneous activation of multiple source-to-relay links, and the cooperative beamforming in dual-hop networks. More specifically, the proposed scheme is designed for selecting a single or multiple relay nodes for packet reception or transmission based on the buffer states of relay nodes, while avoiding the detrimental effects of both an empty buffer state and a buffer overflow. Analytical bounds on the secrecy outage probability and the average delay are derived for our proposed scheme, based on a Markov chain process, in order to verify the system model of our proposed scheme. Furthermore, we introduce the concept of cooperative jamming into the proposed scheme, in order to interfere with an eavesdropper's reception, while dispensing with the full channel state information associated with an eavesdropper at a central coordinator. Our simulation results demonstrate that the proposed schemes outperform the existing buffer-based secure relay selection schemes, in terms of both the secrecy outage probability and the average delay, as the explicit benefits of our novel introduced concepts.

This study proposes a precoded faster-than-Nyquist (FTN) signaling scheme based on singular-value decomposition (SVD) with optimal power allocation. An information-theoretic analysis is conducted on the conventional and proposed SVD-precoded FTN signaling architectures. The associated information rate bound is derived in a closed-form by extending the classic Shannon capacity to support the SVD-precoded FTN signaling schemes. Our analytical performance results demonstrate that the proposed scheme outperforms its conventional Nyquist-criterion-based counterpart and the conventional SVD-precoded FTN signaling scheme, which does not use power allocation. Several important implementation issues, such as inter-block interference, the use of a frequency-selective (dispersive) channel, truncation, and sampling rate, are also considered. Finally, a suboptimal power allocation scheme that overcomes the truncation issue is presented.

In this paper, we propose a novel differential faster-than-Nyquist (DFTN) signaling scheme, which allows us to dispense with any channel estimation at the receiver while benefiting from the rate boost of faster-than-Nyquist (FTN) signaling. At the transmitter, differentially modulated binary phase-shift keying (DBPSK) symbols are transmitted with the symbol interval that is smaller than that defined by the Nyquist criterion. The receiver noncoherently estimates the DBPSK symbols, suffering from the effects of FTN-specific inter-symbol interference (ISI), based on frequency-domain equalization. This is enabled, because FTN-specific lSI is deterministic, by assuming that the FTN's symbol packing ratio and the roll-off factor of a shaping filter are known in advance at the receiver.

In this paper, we propose a simple yet powerful mapping scheme that converts any conventional square-matrix-based differential space-time coding (DSTC) into a nonsquare-matrix-based DSTC. This allows DSTC schemes to be used practically in open-loop large-scale multiple-input multiple-output scenarios. Our proposed scheme may be viewed as the differential counterpart of coherent spatial modulation (SM), of the generalized SM, of Bell Laboratories layered space-time architecture, and of subcarrier-index modulation. The fundamental impediment of the existing DSTC schemes is the excessive complexity imposed by the unitary constraint. Specifically, the transmission rate of conventional DSTC schemes decays as the number of transmit antennas increases. Our proposed scheme eliminates this impediment and thus achieves a significantly higher transmission rate. We introduce four novel construction methods for the nonsquare codewords, some of which include an arbitrary number of nonzero elements in each codeword column. Our analysis shows that the proposed encoding technique reduces the complexity of both the inverse Fourier transform and the detection processes. Our proposed scheme is shown to approach the performance of its coherent counterpart for low-mobility scenarios, where the number of transmit antennas is increased up to 256.

In this paper, we propose a novel full-duplex relaying protocol that relies on half-duplex relay nodes with data buffers. The proposed protocol doubles the transmission rate compared to the conventional half- duplex relaying protocol. In the proposed scheme, multiple relay nodes receive a source packet, while a single relay node transmits a buffered packet to the destination node at the same time slot. Specifically, our protocol exploits the multiple source-to-relay (SR) broadcast links rather than the single SR unicast link of the conventional buffer-aided full-duplex protocol, and hence provides an additional design degree of freedom. We also derive the theoretical bounds of the outage probability and average packet delay based on the Markov chain model. Our numerical results demonstrate that the proposed full-duplex protocol is capable of attaining a lower outage probability than that of the previous full-duplex max-max relay selection scheme, and the lower outage probability is achieved without imposing any additional cost.

In this paper, we propose a generalized buffer- state-based relaying protocol in the context of finite buffer-aided cooperative systems. More specifically, the proposed relaying scheme relies on two concepts: the simultaneous activation of multiple source-to-relay links; and buffer-state- based relay selection for packet transmission and reception. In order to avoid buffer overflow as well as an empty buffer at the relay nodes, we introduce a novel thresholding scheme that gives priority to link selection. As a result, the proposed scheme is capable of achieving a lower outage probability and a lower end-to-end packet delay than existing buffer-aided relaying protocols. Our simulation results demonstrate the explicit performance advantage of the proposed scheme over conventional schemes, especially for a scenario of a high number of relay nodes.

In this paper, we propose a novel dual-mode time- domain single-carrier index modulation (DM-SCIM) scheme, where two constellation modes are used for carrying index information further to modulated symbols, hence increasing the transmission rate. Accordingly, the proposed scheme is capable of attaining a higher transmission rate than the conventional SC and SCIM schemes, without imposing any additional performance penalty. Furthermore, in order to achieve decoding complexity tractable at the receiver, the successive detection algorithm comprising of frequency-domain equalization (FDE) and exhaustive symbol-vector search is employed. We also derive the error-rate bound of the proposed scheme. Our simulation results demonstrate the explicit performance advantage of the proposed DM- SCIM scheme over the existing SC schemes.

We show that certain signal constellations invoked for classic differential encoding result in a phenomenon we term as the unbounded differential constellation size (UDCS). Various existing differential transmission schemes that suffer from this issue are identified. Then, we propose an enhanced algebraic field extension-based differential spatial modulation (AFE-DSM) scheme and its enhanced counterpart that strikes a diversityrate tradeoff (AFE-DSM-DR), both of which overcome the UDCS issue without compromising its full transmit diversity advantage. Furthermore, the proposed schemes are extended to incorporate amplitude and phase shift keying (APSK) in order to exploit all the available degrees of freedom. Additionally, we propose a pair of detection schemes specially designed for APSK-aided differential transmission schemes. Explicitly, we conceive the buffered minimum mean squared error (B-MMSE) detector and buffered maximum likelihood (B-ML) detector, which exploit the knowledge of previously detected symbols in order to further improve the detection performance. Our simulation results have shown that the proposed detectors are capable of bridging the performance gap between the conventional differential detector (CDD) and the coherent detector that has full channel state information. Specifically, when employing the proposed APSKaided AFE-DSM scheme operating at a rate of 2 b per channel use, the B-MMSE and B-ML detectors are observed to give about 3and 3.5-dB signal-to-noise ratio gain with respect to their CDD counterpart at a bit error ratio of 10-5.

In this paper, we propose semi-blind iterative frequency domain joint channel estimation (CE) and data detection (DD) of faster-than-Nyquist signaling (FTNS). The proposed scheme achieves low-complexity operation, while maintaining a performance close to that of the perfect channel state information scenario. More specifically, we derive low-complexity frequency-domain CE for the faster-than-Nyquist pilot (FTNP) transmission scenario, where the symbol duration of a pilot block is lower than the Nyquist-criterion-based pilot transmission. Moreover, we propose a serially concatenated channel-encoded FTNS transceiver that takes into account FTNS-specific colored noise effects. The proposed low-complexity receiver carries out soft-decision frequency-domain equalization with the aid of the minimum-mean square error criterion while whitening the colored noise. As explicit benefits of the proposed frequency-domain CE for the FTNP, the demodulated FTNS data block can also be used as a long pilot block, so the iterative joint CE and DD becomes realistic. Simulation results demonstrate that the proposed two-stage-concatenated FTNS system relying on binary phase-shift keying-based FTNP and FTNS-data symbols achieves a better error-ratio performance than previous systems that do not consider colored noise effects in the high-symbol-packing FTNS regime.

In this paper, a novel differential space-time coding scheme is conceived for open-loop noncoherent multiple-input multiple-output (MIMO) downlink scenarios, where the transmission rate increases logarithmically in a scalable manner upon increasing the number of transmit antennas. More specifically, the proposed scheme relies on the projection of a differentially encoded square matrix to its rectangular counterpart and so is capable of reducing the number of symbol intervals needed for block transmission. This is especially beneficial for massive MIMO scenarios, in which the number of transmit antennas is very high. Another advantage exclusive to the presented scheme is that no channel state information (CSI) is required at either the transmitter or the receiver, which eliminates pilot overhead, CSI estimation, CSI feedback, and time-division duplex reciprocity. Furthermore, the rectangular transmission matrix of the proposed scheme contains only a single non-zero element per column, and hence, the transmitter may rely on only a single RF chain, similar to the conventional coherent spatial modulation scheme.

In this paper, we propose a serially concatenated turbo-encoded faster-than-Nyquist signaling (FTNS) transceiver that takes into account FTNS-specific colored noise effects. The proposed low-complexity receiver carries out soft-decision frequency-domain equalization with the aid of the minimum-mean square error criterion while whitening the colored noise. Simulation results demonstrate that the proposed multi-stage-concatenated FTNS system achieves a better error-ratio performance than previous systems that do not consider colored noise effects in the high-symbol-packing FTNS regime. Furthermore, as an explicit benefit of the proposed iterative decoder, near-capacity performance is achieved with practical decoding complexity.

Motivated by the recent buffer-aided cooperative protocols that select the single best link at each time slot, in this paper we introduce additional design degree of freedom into them, by simultaneously exploiting broadcast channels between the source node and the multiple buffer- aided relay nodes. More specifically, our proposed scheme is designed to allow multiple relay nodes to receive a specific source packet through source-to-relay broadcast channels, hence resulting in multiple copies of the source packet, which are stored in buffers of the relay nodes. As the results, our proposed protocol is capable of reducing the overhead required for monitoring the available links and the end-to-end packet delay, in comparison to the conventional buffer-aided cooperative protocols, while attaining a high diversity order comparable to that of the max-link protocol.

In this paper, we propose a near-capacity irregular precoded power-imbalanced (PI) optical spatial modulation (OSM) scheme. The PI-OSM scheme was developed for negating the effects of high channel correlation, which is often observed in optical wireless communications. In our proposed scheme, multiple PI-OSM subcodes are used to create a novel irregular precoded PI-OSM architecture. This allows us to match the extrinsic information transfer curves of the inner and outer codes in a flexible manner, hence reducing the signal-to-noise ratio required for achieving an infinitesimally low bit error rate (BER). Our numerical simulation results demonstrate that the irregular precoded PI-OSM scheme achieves a high performance, particularly in noise-dominant channels, and it outperforms the repetition-coded benchmark scheme with pulse-amplitude modulation.

Motivated by the recent generalized spatial modulation (GSM)-multiple input multiple-output (MIMO) concept, in this paper we conceive a non- coherently detected GSM architecture, which is achieved with the aid of Cayley unitary differential encoding. More specifically, the proposed differentially-encoded transmitter structure enables us to eliminate any limitations imposed by pilot insertions and channel state information (CSI) estimation, while attaining the GSM scheme's increased transmission rate as well as reduced number of RF chains at the transmitter. Furthermore, we extend the proposed non-coherent GSM scheme to operate in a single-antenna transmitter structure. Our performance results reveal that a high transmission rate is achieved, while attaining reduced-RF branches and dispensing with the channel estimation at the receiver.

A key challenge of future mobile communication research is to strike an attractive compromise between wireless network's area spectral efficiency and energy efficiency. This necessitates a clean-slate approach to wireless system design, embracing the rich body of existing knowledge, especially on multiple-input-multiple-ouput (MIMO) technologies. This motivates the proposal of an emerging wireless communications concept conceived for single-radio-frequency (RF) large-scale MIMO communications, which is termed as SM. The concept of SM has established itself as a beneficial transmission paradigm, subsuming numerous members of the MIMO system family. The research of SM has reached sufficient maturity to motivate its comparison to state-of-the-art MIMO communications, as well as to inspire its application to other emerging wireless systems such as relay-aided, cooperative, small-cell, optical wireless, and power-efficient communications. Furthermore, it has received sufficient research attention to be implemented in testbeds, and it holds the promise of stimulating further vigorous interdisciplinary research in the years to come. This tutorial paper is intended to offer a comprehensive state-of-the-art survey on SM-MIMO research, to provide a critical appraisal of its potential advantages, and to promote the discussion of its beneficial application areas and their research challenges leading to the analysis of the technological issues associated with the implementation of SM-MIMO. The paper is concluded with the description of the world's first experimental activities in this vibrant research field.

In this letter, motivated by the recent universal encoding concept of space-time shift keying (STSK), we propose a novel single radio-frequency (RF) based virtual multiple-input multiple-output (VMIMO) architecture, employing a pattern reconfigurable antenna at a transmitter. More specifically, the proposed scheme is based on the concept of pattern-time dispersion matrix activation, which is capable of striking a flexible balance between the transmission rate and the diversity gain, while enabling symbol-based low-complexity detection. Furthermore, we conceive its non-coherently detected counterpart in order to dispense with any channel estimation as well as additional pilot overhead, hence having the potential of outperforming the conventional coherently-detected VMIMO schemes in many practical scenarios.

In this paper, we propose multi-carrier code division multiple access (MC-CDMA)-aided space-time shift keying (STSK) for mitigating the performance erosion of the classic STSK scheme in dispersive channels, while supporting multiple users. The codewords generated by the STSK scheme are appropriately spread in frequency-domain (FD) and transmitted over a number of parallel frequency-flat subchannels. We propose a new receiver architecture amalgamating the single-stream maximum-likelihood (ML) detector of the STSK system and the multiuser detector (MUD) of the MC-CDMA system. The performance of the proposed scheme is evaluated for transmission over frequency-selective channels in both uncoded and channel-coded scenarios. The results of our simulations demonstrate that the proposed scheme overcomes the channel impairments imposed by wideband channels and exhibits near-capacity performance in a channel-coded scenario.

In this paper, we propose to reduce the complexity of both the Approx-Log-MAP algorithm as well as of the Max-Log-MAP algorithm conceived for generalized PSK/QAM detection, where only a reduced-size subset of the PSK/QAM constellation points is taken into account for producing a single soft-bit output. Although the detectors of Gray-labelled low-order PSK/QAM schemes generally produce near-horizontal EXIT curves, our proposed detectors exploit the a priori LLRs gleaned from a channel decoder in order to retain the optimum detection capability for all PSK/QAM constellations. Furthermore, we demonstrate in this paper that the widely applied MIMO schemes including V-BLAST and STBC, which invoke the proposed soft PSK/QAM detectors may also benefit from our reduced-complexity design. Our simulation results confirm that a near-capacity performance may be achieved by the proposed detectors at a substantially reduced detection complexity.

In this paper, we propose a comprehensive reduced-complexity detector both for hard-decision-aided as well as for the soft-decision-assisted Spatial Modulation (SM)/Space-Time Shift Keying (STSK). More explicitly, the detection of the SM scheme, which activates a single one out of M antennas to transmit a single LPSK/QAM symbol, may be carried out by detecting the antenna activation index m and the LPSK/QAM symbol stseparately, so that the detection complexity may be reduced from the order of O(M · L) to the lower bound of O(M + log2L). However, the QAM aided STSK hard detection proposed in [1] results in a performance loss. Furthermore, the Max-Log-MAP algorithm proposed for soft STSK detection in [2] only takes into account the maximum a posteriori probabilities, which also imposed a performance degradation. Therefore, in this paper, we propose a novel solution for hard-decision-aided SM/STSK detection, which retains its optimal performance, despite its reduced detection complexity, when either LPSK or LQAM is employed. Furthermore, we propose the reduced-complexity Approx-Log-MAP algorithm conceived for the soft-decision-aided SM/STSK detector, in order to replace the suboptimal Max-Log-MAP algorithm.

Joint dispersion-matrix and constellation optimization algorithm is proposed, which is invoked for the recent space-time shift keying (STSK) scheme. More specifically, the theoretical gradients of the discrete-input continuous-output memoryless channel's (DCMC) capacity with respect to both a dispersion-matrix set and to the modem constellations are derived, which allows a substantial reduction in the computational complexity required for maximizing the system's capacity. Furthermore, we also conceive a near-capacity irregular-precoded STSK (IR-PSTSK) architecture, which is designed with the aid of extrinsic information transfer (EXIT) charts, while invoking STSK subcodes, which are optimized by using the proposed algorithm.

In this paper, we propose to reduce the complexity of both the Approx-Log-MAP algorithm as well as of the Max-Log-MAP algorithm, which were designed for soft-decision-aided Space-Time Block Code (STBC) detectors. First of all, we review the STBC design, which enables regular L-PSK/QAM detectors to be invoked in order to detect STBCs on a symbol-by-symbol basis. Secondly, we propose to operate the L-PSK/QAM aided STBC detection on a bit-by-bit basis, so that the complexity may be reduced from the order of O(L) to O(BPS = log2L). Our simulation results demonstrate that a near-capacity performance may be achieved by the proposed detectors at a substantially reduced detection complexity. For example, a factor six complexity reduction was achieved by the proposed algorithms, when they were invoked for detecting Alamouti's Square 16QAM aided G2 scheme.

Inspired by the concept of Space-Time Shift Keying (STSK), the further evolved philosophy of Space-Time-Frequency Shift Keying (STFSK) was proposed for Multiple-Input-Multiple-Output (MIMO) wireless communications, where a beneficial diversity gain may be gleaned from three different domains, namely the space-, time- and frequency-domain. In this paper we proposed soft-detected STFSK in order to conceive its iterative decoding aided version combined with channel codes. Our results showed that the STFSK soft demodulator, which iteratively exchanges extrinsic information with channel codes, may decrease the required transmit power by approximately 3 dB at the Eb/N0of 10-5, compared to hard-decision STFSK. Furthermore, the detection complexity of both the hard- and the soft-decision STFSK demodulator is quantified in terms of the number of multiplications and additions required for each detection iteration.

In this paper, we propose to reduce the complexity of both the Approx-Log-MAP algorithm as well as of the Max-Log-MAP algorithm, which were designed for soft-decision aided PSK detectors. First of all, we extend the shown a posteriori PSK symbol probability formula and streamline it by eliminating its unnecessary calculations in the context of the Approx-Log-MAP algorithm. Secondly, we reduce the complexity of the Max-Log-MAP algorithm, where the maximum a posteriori symbol probability may be obtained without evaluating and comparing all the candidate symbol probabilities. Furthermore, we apply our new soft detection arrangement to a variety of coded systems. Our simulation results demonstrate that a significant detection complexity reduction was achieved by our design without any performance loss. For example, a factor two complexity reduction was achieved by the proposed Max-Log-MAP algorithm, when it was invoked for detecting QPSK symbols, which is expected to be significantly higher, when invoked for 16QAM.

In this letter, we investigate the effects of training-based channel estimation on the achievable performance of the recent spatial modulation (SM) based multiple-input multiple-output (MIMO) scheme. This is motivated by the fact that the SM transmitter is constituted by a single radio-frequency (RF) branch and multiple antenna elements (AEs), hence simultaneous pilot transmissions from the AEs are impossible, unlike in the classic multiple-RF MIMO transmitters. Our simulation results demonstrate that the SM scheme's BER curve exhibits a performance penalty, while relying on realistic imperfect channel-estimation. In order to combat these limitations, we propose two single-RF arrangements, namely a reduced-complexity joint channel estimation and data detection aided SM scheme as well as a non-coherently detected single-RF space-time shift keying scheme dispensing with channel estimation.

In this letter, the concept of multiple serially-concatenated codes is invoked in the context of stochastic resonance (SR), where the achievable performance is improved by increasing noise power. More specifically, the receiver's iterative decoding process is characterized with the aid of extrinsic information transfer (EXIT) charts, such that the SR effect induced by a non-linear component is taken into account. Our simulation results demonstrate that although the SR demodulator's log-likelihood ratio (LLR) outputs are not Gaussian distributed, the corresponding EXIT trajectory matches the prediction from the associated inner- and outer-codes' EXIT curves, while reaching the perfect convergence point in terms of mutual information. Therefore, an infinitesimally low bit-error ratio (BER) is attained by appropriately designing the channel code's parameters based on EXIT charts.

A novel reduced-complexity near-optimal detection algorithm is proposed for enhancing the recent coherently-detected Space-Time Shift Keying (STSK) scheme employing arbitrary constellations, such as L-point Phase-Shift Keying (PSK) and Quadrature Amplitude Modulation (QAM). The proposed detector relies on a modified Matched Filter (MF) concept. More specifically, we exploit both the constellation diagram of the modulation scheme employed as well as the Inter-Element-Interference (IEI)-free STSK architecture. It is revealed that the proposed detector is capable of approaching the optimal Maximum Likelihood (ML) detector's performance, while avoiding the exhaustive ML search.

In this paper, we propose a Cooperative Differential Space-Time Spreading (CDSTS) scheme employing multiple relays, which eliminates the demanding requirement of channel estimation both at the relays and at the destination. More explicitly, the source node employs differential encoding during the first transmission interval, and the multiple relays perform Differential Space-Time Spreading (DSTS) based Amplify-and-Forward (AF) relaying in the second transmission interval. Eliminating high-complexity channel estimation is particularly important at the light-weight shirt-pocket-sized mobile handset based relays, since it is unrealistic to estimate the associated mobile-to-mobile channels, which would also pose a security threat and impose a Doppler-dependent pilot overhead. Finally, it would be prone to a channel-estimation-induced performance degradation, which might be close to the 3 dB non-coherent performance penalty. Loosely Synchronized (LS) CDMA spreading codes were adopted for the asynchronous CDMA uplink for the sake of achieving a near-single-user performance using a low-complexity single-user matched-filter detector. The potential performance degradation of the noncoherent receiver experienced in fast fading channels is mitigated by the proposed Multiple-Symbol Differential Sphere Decoding (MSDSD).

We develop a semi-blind adaptive space-time shift keying (STSK) based multiple-input multiple-output system using a low-complexity iterative channel estimation and data detection scheme. We first employ the minimum number of STSK training blocks, which is related to the number of transmitter antennas, to obtain a rough least square channel estimate (LSCE). Low-complexity single-stream maximum likelihood (ML) data detection is then carried out based on the initial LSCE and the detected data are utilised to refine the decision-directed LSCE. We show that a few iterations are sufficient to approach the optimal ML detection performance obtained with the aid of perfect channel state information.

Motivated by the recent development of Spatial Modulation (SM) and Differential Space-Time Shift Keying (DSTSK), we propose a reduced-complexity Conventional Differential Detector (CDD) as well as a reduced-complexity Multiple-Symbol Differential Sphere Detector (MSDSD) for DSTSK. Both schemes operate on a symbol-by-symbol basis in order to reduce the complexity of the classic block-by-block-based CDD and MSDSD, whilst still approaching the optimum performance of the full-search-based Maximum Likelihood (ML) detector.

Inspired by the concept of the Space-Time Shift Keying (STSK) modulation, in this paper we proposed the Space-Frequency Shift Keying (SFSK) modulation as well as the Space-Time-Frequency Shift Keying (STFSK) concept which spreads the transmit signal not only across the space and time domains, but also the frequency domain. The performance of STSK modulation is degraded by about 2 dB, when the channel changes from uncorrelated frequency-flat fading to the frequency-selective environment of the 6-tap COST207 model. By contrast, as a benefit of Frequency Shift keying, the SFSK and STFSK schemes are capable of maintaining their performance also in frequency-selective fading environments. Finally, we demonstrate that the STSK and SFSK schemes constitute special cases of the STFSK modulation.

Motivated by the recent development of spatial modulation (SM) and differential space-time shift keying (DSTSK), we propose a reduced-complexity conventional differential detector (CDD) as well as its reduced-complexity multiple-symbol differential sphere detector (MSDSD) counterpart for DSTSK. Both schemes operate on a symbol-by-symbol basis in order to reduce the complexity of the classic block-by-block-based CDD and MSDSD, whilst still attaining the optimum performance of the full-search-based ML detector.

In this letter, we propose to reduce the detection complexity of soft-decision aided Space-Time Shift Keying (STSK). More explicitly, we propose a vector-by-vector based STSK detector, which exhibits a lower complexity compared to the classic block-by-block based Space-Time Modulation (STM) detector. We further operate the STSK detector on a symbol-by-symbol basis, so that a near-capacity performance may be achieved with the aid of channel coding at a reduced complexity.

Motivated by the recent concept of Space-Time Shift Keying (STSK), we propose a novel cooperative STSK family, which is capable of achieving a flexible rate-diversity tradeoff, in the context of cooperative space-time transmissions. More specifically, we first propose a Coherent cooperative STSK (CSTSK) scheme, where each Relay Node (RN) activates Decode and-Forward (DF) transmissions, depending on the success or failure of Cyclic Redundancy Checking (CRC). We invoke a bit to-STSK mapping rule, where according to the input bits, one of the Q pre-assigned dispersion vectors is activated to implicitly convey log2Q bits, which are transmitted in combination with the classic log2L-bit modulated symbol. Additionally, we introduce a beneficial dispersion vector design, which enables us to dispense with symbol-level Inter-Relay Synchronization (IRS). Further more, the Destination Node (DN) is capable of jointly detecting the signals received from the source-destination and relay destination links, using a low-complexity single-stream-based Maximum Likelihood (ML) detector, which is an explicit benefit of our Inter-Element Interference (IEI)-free system model. More importantly, as a benefit of its design flexibility, our cooperative CSTSK arrangement enables us to adapt the number of the RNs, the transmission rate as well as the achievable diversity order. Moreover, we also propose a Differentially-encoded cooperative STSK (DSTSK) arrangement, which dispenses with CSI estimation at any of the nodes, while retaining the fundamental benefits of the cooperative CSTSK scheme.

A novel reduced-complexity near-optimal detection algorithm is proposed for enhancing the recent Coherently-detected Space-Time Shift Keying (CSTSK) scheme employing arbitrary constellations, such as {\cal L}-point Phase-Shift Keying (PSK) and Quadrature Amplitude Modulation (QAM). The proposed detector relies on a modified Matched Filter (MF) concept. More specifically, we exploit both the constellation diagram of the modulation scheme employed as well as the Inter-Element-Interference (IEI)-free STSK architecture. Furthermore, we generalize the Pulse Amplitude Modulation (PAM)- or PSK-aided Differentially-encoded STSK (DSTSK) concept and conceive its more bandwidth-efficient QAM-aided counterpart. Then, the proposed reduced-complexity CSTSK detector is applied to the QAM-aided DSTSK scheme, which enables us to carry out low-complexity non-coherent detection, while dispensing with channel estimation. It is revealed that the proposed detector is capable of approaching the optimal Maximum Likelihood (ML) detector's performance, while avoiding the exhaustive ML search. Interestingly, our simulation results also demonstrate that the reduced-complexity detector advocated may achieve the same performance as that of the optimal ML detector for the specific STSK scheme's parameters. Another novelty of this paper is that the star-QAM STSK scheme tends to outperform its square-QAM counterpart, especially for high number of dispersion matrices. Furthermore, we provided both the theoretical analysis and the simulations, in order to support this unexpected fact.

In this paper, motivated by the recent concept of Spatial Modulation (SM), we propose a novel Generalized Space-Time Shift Keying (G-STSK) architecture, which acts as a unified Multiple-Input Multiple-Output (MIMO) framework. More specifically, our G-STSK scheme is based on the rationale that P out of Q dispersion matrices are selected and linearly combined in conjunction with the classic PSK/QAM modulation, where activating P out of Q dispersion matrices provides an implicit means of conveying information bits in addition to the classic modem. Due to its substantial flexibility, our G-STSK framework includes diverse MIMO arrangements, such as SM, Space-Shift Keying (SSK), Linear Dispersion Codes (LDCs), Space-Time Block Codes (STBCs) and Bell Lab's Layered Space-Time (BLAST) scheme. Hence it has the potential of subsuming all of them, when flexibly adapting a set of system parameters. Moreover, we also derive the Discrete-input Continuous-output Memoryless Channel (DCMC) capacity for our G-STSK scheme, which serves as the unified capacity limit, hence quantifying the capacity of the class of MIMO arrangements. Furthermore, EXtrinsic Information Transfer (EXIT) chart analysis is used for designing our G-STSK scheme and for characterizing its iterative decoding convergence.

Motivated by the recent concept of Space-Time Shift Keying (STSK), we propose a novel cooperative STSK scheme, which is capable of achieving a flexible rate-diversity tradeoff, in the context of cooperative space-time transmissions. More specifically, in our cooperative STSK scheme each Relay Node (RN) activates Decode-and-Forward (DF) transmissions, depending on the success or failure of Cyclic Redundancy Checking (CRC). We propose a novel bit-to STSK mapping rule, where according to the input bits, one of the Q pre-assigned dispersion vectors is activated to implicitly convey log2 Q bits, which are transmitted in combination with the classic log2(L)-bit modulated symbol. Additionally, we introduce a beneficial dispersion vector design, which enables us to dispense with symbol-level Inter-Relay Synchronization (IRS). Furthermore, the Destination Node (DN) is capable of jointly detecting the signals received from the source-destination and relay-destination links, using a low-complexity single-stream-based Maximum Likelihood (ML) detector, which is an explicit benefit of our Inter-Element Interference (IEI)-free system model. More importantly, as a benefit of its design flexibility, our cooperative STSK arrangement enables us to adapt the number of the RNs, the transmission rate as well as the achievable diversity order.

In this paper, we propose a novel Space-Time Shift Keying (STSK) modulation scheme for MIMO communication systems, where the concept of spatial modulation is extended to include both the space and time dimensions, in order to provide a general shift-keying framework. More specifically, in the proposed STSK scheme one out of Q dispersion matrices is activated during each transmitted block, which enables us to strike a flexible diversity and multiplexing tradeoff. This is achieved by optimizing both the space-time block duration as well as the number of the dispersion matrices in addition to the number of transmit and receive antennas. We will demonstrate that the resultant equivalent system model does not impose any inter-channel interference, and hence the employment of single-stream maximum likelihood detection becomes realistic at a low-complexity. Furthermore, we propose a Differential STSK (DSTSK) scheme, assisted by the Cayley unitary transform, which does not require any Channel State Information (CSI) at the receiver. Naturally, dispensing with CSI is achieved at the cost of the usual error-doubling in comparison to Coherent STSK (CSTSK). Additionally, we introduce an enhanced CSTSK scheme, which avoids the requirement of inter-antenna synchronization between the RF chains associated with the transmit antenna elements by imposing a certain constraint on the dispersion matrix design.

In this paper, we propose a differential Space-Time Coding (STC) scheme designed for asynchronous cooperative networks, where neither channel estimation nor symbol-level synchronization is required at the cooperating nodes. More specifically, our system employs differential encoding during the broadcast phase and a Space-Time Spreading (STS)-based amplify-and-forward scheme during the cooperative phase in conjunction with interference rejection Direct Sequence (DS) spreading codes, namely Loosely Synchronized (LS) codes. The LS codes exhibit a so-called Interference Free Window (IFW), where both the autocorrelation and cross-correlation values of the codes become zero. The IFW allows us to eliminate both the Multi-User Interference (MUI) as well as the potential dispersion-induced orthogonality degradation of the cooperative space-time codeword and the interference imposed by the asynchronous transmissions of the relay nodes. Furthermore, the destination node can beneficially combine both the directly transmitted and the relayed symbols using low-complexity correlation operations combined with a hard-decision detector. Our simulation results demonstrate that the proposed Cooperative Differential STS (CDSTS) scheme is capable of combating the effects of asynchronous uplink transmissions without any Channel State Information (CSI), provided that the maximum synchronization delay of the relay nodes is within the width of IFW. It will be demonstrated that in the frequency-selective environment considered our CDSTS arrangement is capable of exploiting both space-time diversity and multi-path diversity with the aid of a RAKE combiner.

In this paper, motivated by the recent concept of Spatial Modulation (SM), we propose a novel Generalized Space-Time Shift Keying (G-STSK) architecture, which acts as a unified Multiple-Input Multiple-Output (MIMO) framework. More specifically, our G-STSK scheme is based on the rationale that P out of Q dispersion matrices are selected and linearly combined in conjunction with the classic PSK/QAM modulation, where activating P out of Q dispersion matrices provides an implicit means of conveying information bits in addition to the classic modem. Due to its substantial flexibility, our G-STSK framework includes diverse MIMO arrangements, such as SM, Space-Shift Keying (SSK), Linear Dispersion Codes (LDCs), Space-Time Block Codes (STBCs) and Bell Lab's Layered Space-Time (BLAST) scheme. Hence it has the potential of subsuming all of them, when flexibly adapting a set of system parameters. Moreover, we also derive the Discrete-input Continueousoutput Memoryless Channel (DCMC) capacity for our G-STSK scheme, which serves as the unified capacity limit, hence quantifying the capacity of diverse MIMO arrangements. Furthermore, EXtrinsic Information Transfer (EXIT) chart analysis is used for designing our G-STSK scheme and for characterizing its iterative decoding convergence.

This paper proposes a cooperative space-time coding (STC) protocol, amalgamating the concepts of asynchronous cooperation, non-coherent detection as well as Distributed Turbo Coding (DTC), where neither symbol-level time synchronization nor CSI estimation is required at any of the cooperating nodes, while attaining a high performance even at low SNRs. More specifically, a practical cooperative differential space-time spreading (CDSTS) scheme is designed with the aid of interference rejection spreading codes, in order to eliminate the effect of synchronization errors between the relay nodes without the assistance of channel estimation or equalization. Furthermore, a set of space-time codewords are constructed based on Differential Linear Dispersion Codes (DLDC), which allows our CDSTS system to support an arbitrary number of relay nodes operating at a high transmission rate due to its flexible design. Rather than using conventional single-relay-assisted DTCs, novel multi-relay-assisted DTCs and a three-stage iteratively-decoded destination receiver structure are developed. In our simulations the system parameters are designed with the aid of EXIT chart analysis, followed by the characterization of the achievable BER performance for various synchronization delay values as well as for various diversity-multiplexing relationships in frequency-selective fast and/or quasi-static Rayleigh fading environments.

A low-complexity semi-blind joint channel estimation and data detection scheme is proposed for space-time shift keying (STSK) based multiple-input multiple-output systems. The minimum number of STSK training blocks, which is related to the number of transmitter antennas, is first utilized to provide a rough initial least square channel estimate (LSCE). Then low-complexity single-stream maximum likelihood (ML) data detection is carried out based on the initial LSCE and the detected data are employed to refine the decision-directed LSCE. It is demonstrated that a few iterations are sufficient to approach the optimal ML detection performance obtained with the perfect channel state information.

This letter proposes a differential Space-Time Coding (STC) scheme designed for asynchronous cooperative networks, where neither channel estimation nor symbol-level synchronization is required at the cooperating nodes. More specifically, our system employs differential encoding during the broadcast phase and a Space-Time Spreading (STS)-based amplify-and-forward scheme during the cooperative phase in conjunction with interference rejection direct sequence spreading codes, namely Loosely Synchronized (LS) codes. Our simulation results demonstrate that the proposed Cooperative Differential STS (CDSTS) scheme is capable of combating the effects of asynchronous uplink transmissions without any channel state information.

Motivated by the recent concept of Spatial Modulation (SM), we propose a novel Space-Time Shift Keying (STSK) modulation scheme for Multiple-Input Multiple-Output (MIMO) communication systems, where the concept of SM is extended to include both the space and time dimensions, in order to provide a general shift-keying framework. More specifically, in the proposed STSK scheme one out of Q dispersion matrices is activated during each transmitted block, which enables us to strike a flexible diversity and multiplexing tradeoff. This is achieved by optimizing both the space-time block duration as well as the number of the dispersion matrices in addition to the number of transmit and receive antennas. We will demonstrate that the resultant equivalent system model does not impose any Inter-Channel Interference (ICI), and hence the employment of single-stream Maximum Likelihood (ML) detection becomes realistic at a low-complexity. Furthermore, we propose a Differential STSK (DSTSK) scheme, assisted by the Cayley unitary transform, which does not require any Channel State Information (CSI) at the receiver. {Here, the usual error-doubling, caused by the differential decoding, gives rise to 3-dB performance penalty in comparison to Coherent STSK (CSTSK).} Additionally, we introduce an enhanced CSTSK scheme, which avoids the requirement of Inter-Antenna Synchronization (IAS) between the RF chains associated with the transmit {Antenna Elements (AEs)} by imposing a certain constraint on the dispersion matrix design{, where each column of the dispersion matrices includes only a single non-zero component}. Moreover, according to the turbo-coding principle, the proposed CSTSK and DSTSK schemes are combined with multiple serially concatenated codes and an iterative bit-to-symbol soft-demapper{. More specifically,} the associated STSK parameters are optimized with the aid of EXtrinsic Information Transfer (EXIT) charts{, for the sake of achieving a near-capacity performance}.

In this paper, we introduce a novel Markov chain (MC) representation aided minimum bit error rate (MBER) detection method that is applicable to an M-QAM modulated SDM/SDMA uplink system. Compared to the conventional MBER scheme, the proposed MC-MBER scheme is capable of reducing the complexity imposed with the aid of its efficient detection candidate set generation assisted by the Markov chain process. Our performance results demonstrate that the MC-MBER multi-user detection (MUD) is capable of reducing the computational complexity by a factor of eight in comparison to the conventional MBER MUD in a rank-deficient system transmitting four 4-QAM uplink substream with the aid of two receive antennas at the base station (BS), while achieving a BER performance comparable to that of the MBER MUD.

In this paper, we propose a Differential Linear Dispersion Coded (DLDC) cooperative transmission scheme suitable for Vehicular Ad Hoc NETworks (VANETs), which facilitates striking a flexible diversity-multiplexing gain tradeoff without requiring channel estimation both at the cooperating nodes and at the destination node. More specifically, DLDC encoding is employed by the source transmitter for broadcasting the source information to the relays, while an LDC-based amplify-and-forward relaying scheme is used at the relays. The proposed technique allows each relay to decide, whether it joins the cooperating cluster without any negotiations with the other nodes. The destination's receiver detects the DLDC-encoded symbols using a low-complex detection algorithm, rather than an exhaustive Maximum Likelihood (ML) search. Based on the serial-concatenated turbo coding principle, the source node is constituted by the outer channel encoder, a Unity Rate Convolutional (URC) encoder and the DLDC-based mapper. The destination's receiver iteratively decodes the received signals, exchanging soft information between the DLDC demapper, the URC decoder and the channel decoder. We demonstrate that the proposed cooperative scheme attained a BER of 10 5 within 2.8 dB of the capacity-dependent maximum achievable rate and significantly outperformed the classic MIMOs suffering from the correlated shadow fading of the co-located elements imposed by large-bodied vehicles. We also investigate the effect of different source-relay channels, including perfect, Additive White Gaussian Noise (AWGN), Rician and Rayleigh channels. We conclude by demonstrating that the introduction of an appropriate source-relay SNR threshold for deciding as to whether to actively engage in cooperation improves the achievable performance.

In this paper, we present an iterative Markov chain Minimum Bit Error Rate (MC-MBER) detection aided steered Linear Dispersion Coded (LDC) structure, which amalgamates three different Multiple-Input Multiple-Output (MIMO) functions, namely Space-Time Coding (STC), Spatial Division Multiplexing (SDM) and beamforming. Furthermore, the concept of using a novel a priori Log Likelihood Ratio (LLR) threshold based technique is invoked for the sake of further reducing the computational complexity imposed. Both the EXtrinsic Information Transfer (EXIT) chart analysis and our BER performance results demonstrate that the achievable performance is substantially improved upon increasing the number of iterations I. At BER = 1times10-4, the required Eb/N0value is about 2.4 dB apart from that of the maximum achievable rate in conjunction with l = 10 in a rank-deficient system transmitting Q = 6 QPSK modulated streams in Q = 2 symbol durations with the aid of N = 2 receive antennas and M = 2 transmit antenna arrays, where each of the arrays is composed of L = 2 antenna elements. It is demonstrated that the computational complexity of the a priori LLR threshold assisted MC-MBER detector is reduced by 38% at BER = 1.4times10-5in comparison to that of the MC-MBER detector dispensing with the thresholding technique.

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 ¿.

A novel Markov chain (MC) representation aided minimum bit error rate (MBER) detection method is proposed for anM-QAM modulated SDM/SDMA uplink system. Compared to the conventional MBER scheme, the proposed MC-MBER scheme is capable of reducing the complexity imposed with the aid of its efficient detection candidate set generation assisted by the Markov chain process. Our performance results demonstrate that the MC-MBER multiuser detection (MUD) is capable of reducing the computational complexity by a factor of eight in comparison to the conventional MBER MUD in a rank-deficient system transmitting four 4-QAM substreams with the aid of two receive antennas, while achieving a BER performance comparable to that of the MBER MUD.