To address the demands of improved uplink (UL) coverage and low latency, an enhanced duplex scheme, termed sub-band full-duplex (SBFD), has been proposed by the 3rd Generation Partnership Project. By dividing the conventional time-division duplex (TDD) band into non-overlapping UL and downlink (DL) subbands, SBFD combines the advantages of both TDD and frequency-division duplex. However, system performance is significantly affected by self-interference (SI) in SBFD. In this paper, the SI is modeled as a combination of transmitter (Tx) leakage and receiver (Rx) selectivity to better characterize its spatial properties. Based on this model, we propose a joint Tx and Rx precoding design for self-interference cancellation (SIC) in frequency-selective SBFD multiple-input multiple-output systems. The proposed joint design performs SIC based on multiple subbands. Simulation results demonstrate that the proposed algorithm significantly reduces SI, enhances UL performance to near interference-free ideal scenarios, and minimizes DL performance degradation.

Integrated sensing and communication (ISAC) is a key technology for next-generation communication systems. Accurate sensing relies on estimating the channel’s delay and Doppler shift using reference signals (RSs), enabling the determination of reflector distances and velocities. In this paper, we propose a multi-antenna RS design for orthogonal frequency division multiplexing (OFDM)-based ISAC systems with a single receiving antenna. We exploit the two-dimensional (2D) convolutional characteristics of the delay-Doppler domain channel and the quasi-orthogonality of zero correlation zone (ZCZ) sequences to efficiently separate channel responses from multiple antennas with low complexity. The proposed parameter estimation algorithm is compatible with conventional 2D-fast Fourier transform algorithms. Numerical simulations demonstrate that in single-path scenarios, the estimation performance reaches the Cramér-Rao lower bound (CRLB). However, in multipath scenarios, performance diverges from the CRLB at high signal-to-noise ratios due to interference between paths.

Low-power wake-up signal (LP-WUS) serves as a critical technology addressing the energy efficiency requirements of Internet of things (IoT) devices. LP-WUS adopts the on-off keying (OOK) modulation scheme to implement device wake-up functionality, which achieves substantial energy consumption reduction. Furthermore, 3GPP has introduced a novel low-power synchronization signal (LP-SS) which ensures that wake-up receiver (WUR) can determine the start timing of LP-WUS coded bits. This periodic reference signal is also applicable for signal measurement to detect channel quality. The measurement method current employs the ON-OFF characteristics of sequences to estimate signal power and noise power. Under fading channels, the measurement performance of LP-SS exhibits a limitation, which has nothing to do with the measurement methods. Meanwhile, timing offset in non-coherent detection receivers also affects measurement performance. In this paper we analyze the impacts of fading and timing offset on measurement performance and derive the theoretical results of measurement errors. Meanwhile we conduct simulation analysis to verify the deviation between theoretical values and practical results. It demonstrates that timing offset causes measurement range shift rather than overall expansion, and that increasing average samples provides additional gains. These findings establish theoretical foundations for enhancing the performance of measurement methodologies.

In order to suppress the inter-cell interference, the interference rejection combining (IRC) receiver is considered an effective scheme. The performance of the IRC receiver mainly depends on the accuracy of the covariance matrix of the received interference signals and noise. The basic covariance matrix estimation is inaccurate when the samples used in averaging operation is limited. In this paper, two IRC receivers based on eigenvalue filtering are presented to acquire a high accuracy covariance matrix. One of the proposed IRC receivers is subband based, filtering the eigenvalues of the covariance matrix on each subband. The other one is full frequency band based, acquiring the covariance matrix by the common channel characteristics on the entire bandwidth. Simulation results show that both of the proposed IRC receivers are robust and have a significant gain over the basic IRC receiver in various scenarios. The subband based IRC receiver can achieve a performance close to the ideal IRC receiver with a higher complexity. The full frequency band based IRC receiver has a lower complexity and outperforms the subband based one when the interfering channel has a dominant propagation path.

Millimeter wave communication systems offer a new perspective for addressing bandwidth congestion, but their high hardware cost and power consumption limit their application in Internet of Things (IoT) devices. A wireless beam modulation (WBM) system has been proposed as a cost-effective alternative. The patch antenna array on the IoT nodes formulates two transmit beams with different directions. Information bits are transmitted through the energy difference of aligned beams by WBM. However, challenges arise from the random emission of beams and the avoidance of complex data processing, resulting in signal attenuation and reduced communication quality. In this paper, the optimal precoding scheme is first derived, and then we design a manifold optimization-based precoding scheme that directly handles the constant modulus constraint of the precoding vectors. Simulation results show that the proposed precoding scheme achieves a significant performance improvement over existing ones and performs close to the theoretical upper bound when the gains of the different paths in the channel are fairly consistent.

Physics-informed neural networks (PINNs) have recently been utilized to tackle wave equation-based forward and inverse problems. However, they encounter challenges in accurately predicting the high-frequency wavefields, known as the spectral bias problem. Based on the previously used frequency upscaling (FU) and neuron splitting (NS) concepts to help with the high frequency wavefields, we present a sequence of strategies [i.e., multiscale Fourier feature mapping (MFFM), frequency transferring (FT), a revised NS (RNS), and denser sampling (DS)] to overcome the spectral bias challenge of PINN in solving the frequency-domain acoustic wave equation. MFFM projects PINN inputs onto sinusoids with off-axis frequency distributions, characterizing the PINN wavefield by a Fourier decomposition. FT initializes the NN for higher frequency (e.g., 16 Hz) using the pretrained NN for the nearest lower frequency (e.g., 15 Hz), which is refined from FU (where the 16 Hz NN is initialized by that of 8 Hz). Thus, we show that FT is more effective than FU. We also introduce RNS, which is a modified strategy derived from NS by adding a small amount of random noise, drawn from a uniform distribution, to the replicated weights/biases to break the symmetry after replication, improving the NN’s representational capacity. Because MFFM, FT, and RNS alone are inadequate for generating high-frequency wavefields, we further develop DS to use a smaller spatial sampling interval within the same computational domain (the wavefield is more continuous and smoother). DS helps PINN simulate high-frequency components easier. The experiments on typical models validate the effectiveness of the introduced strategies. We share the codes and data through a public repository.

Future networks will need to support more usage scenarios, including higher speed scenarios and ultra-reliability scenarios for which the current waveforms are not suitable. Recently, the proposed orthogonal time frequency space (OTFS) modulation proves to be robust to Doppler spead, but it uses a conventional linear receiver which is susceptible to multipath channel gains and has high complexity. In this paper, we propose a sequence spread modulation scheme to improve system reliability. The proposed scheme spreads symbols to the Doppler domain before OTFS modulation. At the receiver, we develop a Rake receiver over the Doppler domain. Simulation results show that the performance of the proposed sequence modulation scheme is almost independent of the channel multipath gains compared to a conventional linear receiver based on OTFS, which means the performance of the proposed receiver is always close to the best performance of a linear receiver.

Coverage is crucial to the commercial success of New Radio, directly affecting the quality of service and network construction cost. Physical Uplink Control Channel (PUCCH) is a bottleneck channel, and its coverage enhancement technology needs to be studied as a priority. For scenarios requiring coverage enhancement, the signal-to-noise ratios of the receivers are relatively low, and channel estimation is the bottleneck in terms of link-level performance. Therefore, it is necessary to consider sequence-based PUCCH transmission schemes, which can provide less needed demodulation signal-to-noise ratio. In this paper, a serial cascade splicing sequence scheme is proposed and discussed. In this scheme, the receiver first performs sequence detection on the first half of the sequence to initially estimate the range of sequence numbers. Then the sequence detection is performed on the second half of the sequence. Finally, the detected sequence number is converted into bit information. This scheme can reduce the number of sequence detections and, thus, the complexity of the receiver. The simulation results show that the method can effectively improve transmission reliability.

Orthogonal Time Frequency Space (OTFS) is a two-dimensional modulation technique designed in the delay-Doppler (DD) domain. Each symbol experiences a sparse channel with low variability in the DD domain rather than the time-varying multi-path channel in conventional orthogonal frequency division multiplexing systems. In practical applications, fractional Doppler shifts is unavoidable due to the limited frame duration. Channel dispersion caused by fractional Doppler shifts makes the channel estimation more difficult and causes data aliasing. In this paper, we propose an adaptive scheme to reduce fractional Doppler shifts. Doppler shift compensation is performed on time-domain signals and the number of time-domain symbols is changed to approximate the quantized Doppler shifts integers. With the proposed adaptive scheme, the overhead of the reference signal can be significantly reduced with the fractional Doppler shifts eliminated. Simulation results show that block error rate performance is not degraded and system throughput is improved with the proposed scheme.

Orthogonal time frequency space (OTFS) is a multi-carrier technique where a time-varying channel can be converted into a time-invariant channel in the delay-Doppler (DD) domain through a two-dimensional transformation. In practical wireless systems, due to the non-biorthogonal waveforms, the inter-carrier interference (ICI) will bring up phase shifts related to Doppler shifts, so that the channel in the DD domain is no longer invariant. Moreover, the limited frame durations will lead to channel dispersion, which makes it more difficult to estimate Doppler shifts. However, existing channel estimation schemes for OTFS are unable to accurately estimate the Doppler shifts. In this paper, we propose a novel channel estimation scheme. Firstly, we estimate Doppler shifts by linear fitting based on the channel dispersion in the DD domain. Then we calculate the power of dispersion paths as combining coefficients to obtain channel fading. Simulation results show that the proposed scheme can significantly reduce the normalized mean square error compared with conventional schemes, especially if Doppler shifts are large.

In this study, secure communications in a two‐way relaying wiretap system with a silent eavesdropper are studied. Under the assumption that the channel state information relating to the eavesdropper is unknown at legitimate nodes, the precoding vectors at the two source nodes are jointly designed to decrease the wiretap capacity at the eavesdropper. The derived relationship between the two precoding vectors is partially similar in nature to the linear minimum mean squared error (MMSE) equaliser. An iterative process is proposed to jointly achieve two precoding vectors, and its convergence is proved. Simulation results are presented to demonstrate that the proposed MMSE‐based precoding algorithm can be used for source nodes with any number of antennas, and outperforms its conventional zero‐forcing‐ and matched‐filter‐based counterparts. Signal leakage from the relay node to the eavesdropper is nearly completely suppressed by the proposed precoding design.

Fifth generation wireless networks are expected to support a variety of new application scenarios, such as internet-of-things (IoT) and machine-type communications (MTC), etc. These scenarios have a large number of terminal accesses and occupy a lot of spectrum resources. The existing orthogonal frequency division multiple access (OFDMA) technology cannot support massive connectivity, the severe out-of-band (OOB) power leakage not only waste spectrum resources but also hinders the application of OFDM in the fragmented spectrum. Therefore, in this paper, we combine non-orthogonal multiple access (i.e., SCMA) with new multi-carriers (i.e., GFDM and FBMC) to address the problem of massive connections and spectrum resource shortages in the IoT. Meanwhile, in order to further reduce the interference to the primary system in the fragmented spectrum, an active interference cancellation (AIC)-based OOB power leakage suppression scheme is designed for GFDM. Simulation results show that SCMA-GFDM and SCMA-FBMC/QAM have much better BER performance than SCMA-OFDM in asynchronous uplink communications, and AIC technique can obviously suppress the OOB power leakage of GFDM.

In fifth generation (5G) wireless network, it is well known that coverage enhancement is one of the most important aspects for massive machine type communication (mMTC) and Internet of things (IoT), etc. These application scenarios also introduce repeated transmission mechanism to enhance the coverage. Therefore, in this paper, we propose a new complementary generalized frequency division multiplexing (GFDM) repeated transmission scheme. We first analyze the interference model of conventional GFDM and then propose the conditions to eliminate inter-carrier interference (ICI) according to the interference model. Based on this condition, we have designed a pulse shaping filter corresponding to the conventional GFDM to combat ICI. Next, we analyzed the effect of this method on the signal-to-interference ratio (SIR). Simulation results show that this repeated transmission scheme has better BER performance than the conventional GFDM scheme.

In conventional schedule‐based wireless transmission system, frequent interactions between devices and access networks lead to significant signalling overhead, and thus limit the development of low‐latency applications. Contention‐based data transmission (CBDT) is widely used as a promising solution to address the above issues. In this study, the authors propose a detection‐based retransmission scheme for CBDT to further reduce the delay‐outage probability which cannot be avoided due to collision. In this scheme, each device simultaneously monitors the occupied resource blocks during data transmission. When a collision event is detected, the retransmission for this packet is determined without waiting for the acknowledgment message from the receiver. The reliability performance is significantly improved for low‐latency applications, since the possible retransmission times for each packet is increased within the limited transmission period. In addition, the impacts of imperfect detection on extra transmission load and packet loss are analysed in detail. The simulation and analytical results demonstrate that the detection‐based scheme significantly improves the reliability of CBDT.

The latency and reliability of single‐relay and multi‐relay systems are investigated in this study. In a single‐relay system with the amplify‐and‐forward protocol, a new approximation analytical method is proposed for the outage probability, which can also be extended to be used in the multi‐relay system. The latency‐reliability trade‐off degree (LRTD) of single‐ and multi‐relay systems, i.e. the slope of the latency‐outage probability curve with logarithmic scales, is analysed and proved to be the same as the number of relay nodes. In the multi‐relay system, the relationship between the system LRTD and the channel state information overhead is investigated. The optimum number of relay nodes can be derived according to the approximate expression of the outage probability. A simple iterative method is proposed to obtain the optimum number of relay nodes. The convergence of the iterative method is also proved.

The control–data separation architecture (CDSA) has been regarded as a promising solution to effectively manage the heterogeneous network (HetNet) in the fifth‐generation mobile communication systems. Thus, this study investigates the resource allocation problems in CDSA‐based HetNet. To meet various requirements of data transmission, the authors first present a new framework with flexible resource management and efficient traffic control. In this framework, the control–data transmission supported by macro cell evolved nodeB is time‐sensitive and continuous with high reliability. On the basis of it, a traffic‐aware resource allocation approach is proposed to improve the resource efficiency. Once the traffic‐congestion requirements of the pattern are satisfied with the minimum spectrum, the throughput of small cells is simultaneously maximised due to orthogonal spectrum between macro and small cells by using this approach. Moreover, the traffic‐congestion analytical result is elaborately derived in terms of current network state. Three adaptive resource allocation schemes for implementing this approach are also designed with different complexities. Numerical results validate the accuracy of the proposed analytical method, and also indicate that the traffic‐aware resource allocation approach is more efficient than the static one.

This paper studies secrecy communications in a multi-antenna two-way relaying wiretap system. Each source node transmits user and jamming signals with distinct precoding vectors. Based on the assumption that the channel state information relating to the eavesdropper is unknown by the legitimate nodes, new precoding design algorithms are proposed. The jamming signals are designed to ensure that minimum interference is perceived by the other source node. The precoding vectors of the user signals are designed to form two parallel vectors in the equivalent wiretap channel at the eavesdropper. Two algorithms are proposed based upon the zero-forcing and matched-filter schemes, which are dubbed the selective zero-forcing-based precoding (SZFP) and matched-filter-based precoding schemes. Analytical and simulation results are presented to demonstrate that the SZFP algorithm is more applicable when the eavesdropper is equipped with a large number of antennas or the eavesdropper's channels are in good condition.

Downlink non-orthogonal multiple access (NOMA), where users are paired as user set and multiplexed in the power domain, is a promising technology for fifth generation (5G) communication system. This paper studies joint optimization of user pairing and power allocation to maximize generalized proportional fair metric subject to transmit power constraints. This problem can be divided into two parts: user pairing and power allocation. In order to reduce the computation burden, we present a pre-defined multi-user pairing criterion to exclude user sets, which are unsuitable for multiplexing. Furthermore, for a given user set, a low complexity multi-user power allocation scheme is proposed by exploiting the convexity of the optimization problem. Simulation results show that the proposed user pairing and power allocation scheme can significantly enhance the downlink system performance and reduce complexity compared to the existing schemes in NOMA systems.

In the recent fifth generation (5G) research, filter bank multi-carrier (FBMC) technique is one of the competitive substitutes to orthogonal frequency division multiplexing (OFDM). The conventional FBMC schemes, such as FBMC staggered-modulated multitone (FBMC-SMT) and frequency spreading FBMC (FS-FBMC), have the intrinsic time-domain interference which is caused by the overlap of symbols and depend on offset quadrature amplitude modulation (OQAM). In order to overcome the two drawbacks, a non-overlapping QAM-FBMC scheme is proposed based on the FBMC-SMT and analyzed in this paper. Moreover, the non-overlapping QAM-FBMC scheme not only discards cyclic prefix (CP) as other FBMC schemes, but also shortens the duration length of symbol waveforms compared with that of FS-FBMC. Therefore, its spectrum efficiency (SE) outperforms other multi-carrier systems. Simulation results indicate that non-overlapping QAM-FBMC scheme indeed improves the SE and obtains almost the same bit error rate performance as that of the FBMC-SMT as well as OFDM in multi-path fading channel.

Non-orthogonal multiple access (NOMA) is a promising technology for the fifth generation. However, the increasing number of multiplexed users brings two challenges. One is the computational complexity of existing grouping schemes increases rapidly. The other is the performance of multiplexed users deteriorates sharply. To deal with these challenges, we propose a novel multi-user grouping scheme with low-complexity to select several UEs for multiplexing. Moreover, a corresponding power allocation scheme is proposed to guarantee the performance of the multiplexed users. Simulation results show that the novel multi-user grouping scheme can reduce the computational complexity at the cost of user fairness in contrast to the full search method. Besides, the proposed power allocation scheme can improve the spectrum efficiency compared with the existing power allocation schemes in NOMA systems.

In this study, physical layer security techniques are studied for multi‐user peer‐to‐peer relay networks using amplify‐and‐forward protocol in the presence of an eavesdropper. Two system designs are considered: the minimum achievable secrecy rate maximisation and the total transmit power minimisation. First, considering user fairness, the authors study the max–min achievable secrecy rate problem subject to a total transmit power constraint. Mathematically, the optimisation problem is non‐linear and non‐convex. They propose a polynomial time algorithm based on the bisection technique to solve it. Second, they study the total transmit power minimisation problem which aims at minimising the total transmit power at relays such that individual secrecy rate for each user is satisfied. The total power optimisation problem is also non‐convex, so they use the relaxation technique to transform it into a convex problem which can be efficiently solved by the gradient‐based algorithms. Numerical evaluation of the obtained secrecy rate and transmit power results shows that the proposed designs can significantly improve the performance of secure wireless communications.

In this paper, the resource management problem in geographically distributed cloud systems is considered. The Follow Me Cloud concept which enables service migration across federated data centers (DCs) is adopted. Therefore, there are two types of service requests to the DC, i.e., new requests (NRs) initiated in the local service area and migration requests (MRs) generated when mobile users move across service areas. A novel resource management scheme is proposed to help the resource manager decide whether to accept the service requests (NRs or MRs) or not and determine how much resources should be allocated to each service (if accepted). The optimization objective is to maximize the average system reward and keep the rejection probability of service requests under a certain threshold. Numerical results indicate that the proposed scheme can significantly improve the overall system utility as well as the user experience compared with other resource management schemes.

Nowadays, it has become feasible to use mobile nodes as contributing entities in computing systems. In this paper, we consider a computational grid in which the mobile devices can share their idle resources to realize parallel processing. The overall computing task can be arbitrarily partitioned into multiple subtasks to be distributed to mobile resource providers (RPs). In this process, the computation load scheduling problem is highlighted. Based on the optimization objective, i.e., minimizing the task makespan, a buyer-seller model in which the task sponsor can inspire the SPs to share their computing resources by paying certain profits, is proposed. The Stackelberg Pricing Game (SPG) is employed to obtain the optimal price and shared resource amount of each SP. Finally, we evaluate the performance of the proposed algorithm by system simulation and the results indicate that the SPG-based load scheduling algorithm can significantly improve the time gain in mobile grid systems.

The TD-LTE system is envisaged to adopt dynamic time division duplexing (TDD) transmissions for small cells to adapt their communication service to the fast variation of downlink (DL) and uplink (UL) traffic demands. However, different DL/UL directions for the same subframe in adjacent cells will result in new destructive interference components, i.e., eNB-to-eNB and UE-to-UE, with levels that can significantly differ from one subframe to another. In this paper, a feasible UL power control mechanism is proposed to manage eNB-to-eNB interference, where different UL power control parameters are set based on different interference level. We consider the geometric location information and the subframe set selection process about adjacent eNBs when the interference level is estimated. The performance of the proposed scheme is evaluated through system level simulations and it is shown that the scheme can achieve preferable improvement in terms of UL average and 5%-ile packet throughputs compared with the original scheme without power control. Also, the UE-to-UE interference is not worse when the UE transmit power become higher.

With the increase of wireless communication demands, licensed spectrum for long term evolution (LTE) is no longer enough. The research effort has focused on implementing LTE to unlicensed frequency bands in recent years, which unavoidably brings the problem of how to make LTE co-exist with other existing systems on the same band. This paper proposes an adaptive co- existence mechanism for LTE and wireless local area networks (WLAN) to enable a significant system performance of WLAN while LTE does not lose much as well. LTE realizes the co-existence by allocating time resources dynamically according to the traffic load of WLAN system.

Cloud-based mobile services are interactive in nature. Due to the strong correlation between uplink (UL) requests and downlink (DL) responses, it is irrational to optimize the UL and DL resource control problems separately and independently. In this paper, a novel traffic model for interactive mobile cloud services (IMCSs) is constructed, based on which a joint UL-DL scheduling and power allocation strategy is developed. The objective is to minimize the roundtrip delay of causally related UL and DL data groups subject to the average UL transmission power constraint. The algorithm can operate without knowledge of the traffic arrival rates and be implemented in a distributed way so that the transmission overheads between the mobile terminals and the cloud server is reduced.

This study investigates the security of the two‐way relaying system with untrusty relay nodes. Cooperative jamming schemes are considered for bi‐directional secrecy communications. The transmit power of each source node is divided into two parts corresponding to the user and jamming signals, respectively. Two different assumptions of the jamming signals are considered. When the jamming signals are a priori known at the two source nodes, closed‐form power allocation expressions at two source nodes are derived. Under the assumption of unknown jamming signals, it is proven that the cooperative jamming is useless for the system secrecy capacity, because that all the power should be allocated to the user signals at each source node. Relay selection is also investigated based on the analysis of cooperative jamming. Simulation results are presented to compare the system secrecy capacities under the two jamming signal assumptions.

This study comprehensively investigates the performance of the downlink massive multiple‐input multiple‐output (MIMO) system with channel state information (CSI) delay, when the base station serves multiple user terminals (UTs) taking advantage of maximum ratio transmission precoding. Through the characteristic analysis of the received signal‐to‐interference‐plus‐noise ratio at UT, the tight lower bound of average area spectrum efficiency (SE) and its asymptotic bound with the infinite number of UTs are deduced, respectively. Based on the asymptotic bound and the realistic power consumption model consisting of both radiated power and circuit power, the trade‐off between SE and energy efficiency (EE) is developed, and the optimal EE with respect to SE is attained concisely. Moreover, the approximate expressions of both outage probability and bit error ratio (BER) are derived when the massive MIMO system works at its interference limited region. All theoretical results coincide to numerical results, and they show that deploying more transmission antennas will be helpful to improve SE, EE and reliability simultaneously; SE will be enhanced; however, the reliability, including both outage probability and BER, will be worsened by multiplexing more UTs; enlarging CSI delay can extensively deteriorate SE, EE and reliability.

The security of the two-way relaying system with an eavesdropper is investigated in this paper. A cooperative jamming and power allocation scheme is proposed to enhance the system secrecy capacity. Both user and pre-defined jamming signals are transmitted by each source node simultaneously. The optimum power allocation between the user and jamming signals at each source node is derived. Our analytical results suggest that the proposed cooperative jamming scheme improves on the system secrecy capacity, especially when the channel gains of the two source-relay links are of large difference. Simulation results in close agreement with analytical results clearly demonstrate the advantage of the proposed cooperative jamming scheme.

The security of the three-phase two-way relaying system with an eavesdropper is investigated in this paper. A cooperative jamming and power allocation scheme is proposed to enhance the system secrecy capacity. When a source node transmits user signals to the relay node, the other source node interferes the relay node with pre-defined jamming signals simultaneously. Optimum power allocation between the user and jamming signals at each source node is analyzed. Our analytical results suggest that the proposed cooperative jamming scheme improves on the system secrecy capacity, especially when the channel gains of the two source-relay links are of large difference. Simulation results in close agreement with analytical results clearly demonstrate the advantage of the proposed cooperative jamming scheme.

The secrecy capacity of relay communications is investigated in this paper, where an eavesdropper is present. Distributed precoding through multiple relay nodes can be used to enhance the received signal power at the destination node and to mitigate the signal leakage to the eavesdropper, simultaneously. Due to individual power constraints at relay nodes, the distributed precoding scalar at each relay node is equivalent to a distributed precoding angle. An iterative algorithm is proposed to find the suboptimum distributed precoding angles at all the relay nodes, where the channel state information (CSI) sharing overhead among the relay nodes can also be substantially reduced. Each relay node receives the equivalent CSI from its preceding relay node, computes its distributed precoding angle, and updates the equivalent CSI for the next relay node. Compared with the simple decode-and-forward relaying protocol with random distributed precoding, the proposed iterative distributed precoding algorithm is able to further improve on the secrecy capacity of the multirelay wiretap system with an acceptable CSI sharing overhead.

The two-way multiple-input and multiple-output relay system is investigated in this paper, where bi-directional communications between two source nodes are supported by a relay node with multiple antennas. The achievable diversity order of the three-phase two-way relaying protocol based on zero-forcing is derived. It is found in this paper that the upper bound of the diversity order is related to the number of antennas at the relay node and the number of data streams. Numerical and simulation results are presented to validate theoretical analysis.

Recently, distributed resource allocation for relay enhanced cellular networks has gained increasing attention. However, it poses a new challenge for the backhaul link scheduling. In this paper, we propose a queue-status-based resource allocation scheme for the backhaul link in a distributed manner in relay enhanced networks. By applying this scheme, the evolved Node B (eNB) adjusts the data rate for each user equipment (UE) in the backhaul link according to its queue status at the relay node (RN). It can guarantee the rate match between the backhaul and access links while minimizing feedback signaling. Simulation results show that the proposed scheme can achieve a good tradeoff between the spectral efficiency and the signaling overhead.

In 3GPP LTE-Advanced networks deployed with type I relay nodes (RNs), resource partition is required to support in-band relaying. This paper focuses on how to partition system resources in order to attain improved fairness and efficiency. We first formulate the generalized proportional fair (GPF) resource allocation problem to provide fairness for all users served by the evolved node B (eNB) and its subordinate RNs. Assuming traditional proportional fair scheduling is executed independently at the eNB and each RN to achieve local fairness, we propose the proportional fair resource partition algorithm to tackle the GPF problem and ensure global fairness. Through system level simulations, the proposed algorithm is evaluated and compared with both non-relaying and relaying systems with the fixed resource partition approach. Simulation results demonstrate that the proposed algorithm can achieve a good trade-off between system throughput and fairness performance.

In this paper, we investigate a cooperative system consisting of one source-destination pair equipped with single antenna and L relays equipped with M antennas as wireless access points. We propose a new relay selection scheme for this system based on partial relay selection, where the best relay is selected according to the instantaneous and one-hop channel state information (CSI). We use maximal ratio combining (MRC) and maximal ratio transmitting (MRT) at relays to obtain the maximal achievable received signal-to-noise ratio (SNR). Close-form expressions for the outage probability and symbol error rate (SER) are derived. High SNR analysis is tightly approximated to explicitly characterize the diversity order and array gain. Monte Carlo simulations verify our analytical results. Results show that the diversity order increases with the antenna numbers at the selected relay according to M. For partial relay selection, two available relays are enough to get the array gain, and impacts of relay locations are also discussed in this paper.

In this paper, a novel non-saturation analytical model for enhanced distributed channel access (EDCA) in IEEE 802.11e wireless local area network (WLAN) is proposed. Unlike previous work focused on saturation conditions, this model uses Markov and M/M/1/K theories to predict MAC delay, service time, throughput and loss. Moreover, the EDCA technique resets the default contention window (CW) of stations statically after each successful transmission. The static behavior which can not adapt to the network state results in bad performance. Consequently, we introduce an enhancement scheme based on quality of experience (QoE) perceived by users to adjust CW adaptively. Instead of solely relying on technical parameters such as loss and delay, our scheme is based on QoE which contains the parameters inherently. Simulations validate this model and demonstrate better performance of our scheme compared with traditional EDCA regarding QoS parameters and user satisfaction.

This paper focuses on how to associate users with serving nodes in order to achieve improved fairness and efficiency for the cellular networks enhanced with multiple relay nodes (RNs). We first formulate the generalized proportional fair (GPF) problem with the aim of providing fairness for all users served by the evolved node B (eNB) and its subordinate RNs. Routing is incorporated into the GPF problem as part of the resource allocation strategy. Assuming traditional proportional fair (PF) scheduling algorithm is executed independently at the eNB and each RN, we propose efficient PF-based routing algorithm aiming at optimizing the GPF objective, which also takes into account the impact of relay link resource allocation. Through system-level simulations, the proposed algorithm is evaluated and compared with both non-relaying and relaying systems with benchmark routing algorithms. Simulation results demonstrate that the proposed algorithm can achieve better system throughput and fairness performance.

In multi-input-multi-output relay systems with amplify-and-forward protocols, the source-relay and the relay-destination links can be treated as two groups of spatial channels. In this letter, an adaptive spatial channel mapping matrix is presented to be used between the two groups. A unified framework of the spatial channel mapping matrix design is presented, which includes various relay protocols. The spatial channel matrix is designed to reduce the relaying noise power or the local noise in the destination node. Simulation results demonstrate the advantage of the adaptive spatial channel mapping on the system reliability and capacity with different relay protocols.

In wireless sensor networks, several relay nodes can be used to support multiple transmission links between source and destination nodes simultaneously to achieve higher spectral efficiency. A channel state information sharing scheme among multiple relay nodes is presented in this letter, where only one angle per source-destination pair is sent from one relay node to another. Then, to enhance received signal powers, a distributed multiuser precoding scheme is presented to be used at each relay node with the angles sent by another relay node. Simulation results demonstrate that the proposed distributed precoding scheme enhances the capacity performance with slight overhead.

QR decomposition (QRD) based relay protocols can achieve both the distributed array gain and the intra-node array gain in multi-relay systems. Because the QRD is not unique if column permutation is utilized, the traditional QRD based relay protocol can be further improved. Row permutation of the forward channel and column permutation of the backward channel are proposed in this paper. Exhaustive and iterative search algorithms are presented to solve the permutation optimization problem. The iterative search algorithm with modified QRD is a good trade-off between capacity performance and complexity.

Due to similarity between cooperative systems and traditional multi-input-multi-output (MIMO) systems, precoding techniques can be introduced into MIMO relay systems. In this paper, a practical distributed precoding scheme with limited feedback scheme is proposed in the MIMO relay system, where the zero-forcing (ZF) relaying protocol is proposed to be used in the relay node so that the information of relaying channel and noise can be compressed into two positive real coefficients. Precoding is proposed to be operated through two continuous transmitted vectors of the source node, which is termed as distributed spatial-temporal precoding (DSTP). Power allocation is combined with the proposed DSTP for higher spectrum efficiency. DSTP outperforms the direct transmission and the simple ZF relaying protocol. Analysis and numerical results demonstrate that DSTP with suitable receiver performs very close to the capacity low bound of the ZF relaying system.

The incremental relaying (IR) protocol outperforms simple amplify-and-forward and decode-and-forward protocols due to usage of feedback. Based on the IR protocol, a novel protocol termed as fractional incremental relaying (FIR) is firstly presented in this paper to achieve higher spectral efficiency. When a packet of data needs to be transmitted by the relay node, symbols are transmitted a few at a time as needed instead of the entire packet. Then, analysis result shows that the proposed protocol makes more efficient use of channel freedom degrees than the traditional IR protocol does. Finally, numerical results under independent Rayleigh fading demonstrate the advantage of the proposed FIR protocol.

In this paper, influence of spatial correlation on codebook-based precoding technology is analyzed. Optimum pre- coder distribution under spatial correlated channel is achieved at first, which is no longer isotropical on the unitary matrix (vector) set. Selection probability of the matrix (vector) in codebook is not uniform too. Then, a novel codebook design is presented for spatial correlated channel, which is termed as nonuniform codebook. Codebook is designed as function of spatial correlation characteristic of channel and presents a nonuniform quantization of unitary vector set. Smaller quantization error and higher spectral efficiency can be achieved by proposed codebook design than that of traditional fixed uniform codebook design, with the cost of slight feedback overhead increasing.

In current wireless system performance evaluation, separate link and system level simulators are needed due to less computation complexity. The link-to-system (L2S) interface method to interconnect two simulators is essential and need careful definition. Exponential effective signal-to-interference-and-noise ratio mapping (EESM) is a typical L2S interface method suitable for evaluation of orthogonal frequency division multiplexing systems. When itpsilas used in simulation, all the sub-carriers for one user have to use the same modulation and coding scheme. In this paper, two modified L2S interface EESM methods are presented based on the idea that a high-level modulation symbol could be equivalently converted to several low-level modulation symbols with different reliability. As a result, usage of EESM can be extended to the situation that sub-carriers for one user use different modulation schemes.

Hybrid automatic-repeat-request(HARQ) scheme can be incorporated with the linear precoder in multi-input multi- output(MIMO) transmission to ensure highly reliable communications. To fully utilize type-I HARQ diversity gain especially in the slow fading channels, we propose the optimal design principle of linear precoders, whose column vectors are orthogonal to each other correspondingly. Simulation results demonstrate the effectiveness of these precoders in reducing the detection bit-error rate and increasing the unified throughput.

In this paper, tabu search (TS) is applied to the detection algorithm of MIMO system, which is originally designed to find the approximate solution of combinatorial optimization problems. Resorting to an original two embedded cubes stereograph, we give a simple illustration of TS process. As a simple add-on to traditional suboptimal detectors, TS provides the potential of moving toward the best ML solution. The method of skipping out of the search under a certain condition in advance is proposed, leading to the flexible tradeoff between the performance and the complexity. Simulation results show that the advantage of TS detection on performance comparing with the traditional detection is great, especially when the system has more transmit-antennas.

This paper proposes a promising detection that applies PSO (particle swarm optimization) as the selection of surviving paths in QRM detection for the coded MIMO system, named as QRM-PSO. Because only a few children nodes's metric are needed to calculate in the PSO search process, QRM-PSO can cut down the complexity further, while keeping the same performance as QRM due to the quasi-random movement of the particles. Simulation results demonstrated that the advantage of QRM-PSO in complexity is obvious especially when antenna number and modulation level are high. The required real multiplication number of QRM-PSO is only 0.4% of that of QRM when transmit antenna number is 20 and 16QAM is employed.