Non-binary low-density parity-check (NB-LDPC) codes can be directly constructed by using algebraic methods, or indirectly constructed by mapping well-designed binary parity-check matrices to non-binary parity-check matrices. Given the Tanner graph (TG) of a NB-LDPC code, the selection of edge weights in the TG significantly affects the performance of the NB-LDPC code. The authors introduce an edge weight distribution (EWD) parameter for the TG of NB-LDPC codes. By utilising particle swarm optimisation (PSO), the EWD is optimised and it has been demonstrated that the optimal EWD approaches a two-element distribution for large field size and high average variable-node degree. With the optimised EWD, the authors construct a class of field-compatible LDPC (FC-LDPC) codes over GF(q) whose parity-check matrices only include elements 0, 1 and 2, and can be encoded and decoded over different field sizes. The simulations demonstrate that the performance of the proposed FC-LDPC codes improves monotonically with increasing field size, and significantly outperforms that of the corresponding algebraic NB-LDPC codes or NB-LDPC codes generated with uniform distribution of non-zero elements over GF(q).

Orthogonal frequency division multiplexing (OFDM) is considered as a key technology for future wireless communication systems. One of the main disadvantages of OFDM systems is the high peak-to-average power ratio (PAPR) of the transmitted signals. When a non-linear power amplifier (PA) is present, the high PAPR may cause the introduction of non-linear distortions. On the other hand, cooperative communications have emerged as a promising technology for wireless networks. A theoretical analysis of an amplify-and-forward OFDM system taking into consideration the non-linear distortions introduced by a PA is done. More specifically, approximate analytical expressions for the outage probability and average symbol error rate (SER) are derived and studied under several special situations. The performance analysis sheds light on the influence of PA non-linearity on the outage probability and the average SER of an OFDM system. Particularly, it is shown that the non-linear PA has a more significant effect over the average SER for medium and high signal-to-noise ratio (SNR) values and for small-order constellations. The analytical results are validated by means of computer simulations for different system configurations and SNR levels.

In this work, the authors consider an amplify-and-forward (AF)-based three-phase two-way system with interference at the relay. An optimum scheme to minimise the outage probability is formulated, and a closed form expression for the ‘channel-dependent’ combining coefficient ‘α’ at the relay is derived. A novel approach is presented to derive an expression for the overall outage probability of the proposed three-phase outage-optimal two-way relaying (3P-OTWR) system. The authors also perform finite signal-to-noise (SNR) diversity-multiplexing tradeoff and throughput analysis of the system. Furthermore, the problem of outage-optimum power allocation is investigated. Using the asymptotic expression for the outage probability, it is shown that there is loss of diversity owing to interference at the relay. The authors perform Monte Carlo simulations to demonstrate that the 3P-OTWR system outperforms a conventional three-phase two-way scheme. With outage-optimum power allocation, the proposed scheme achieves a gain of approximately 3 dB in the medium-to-high SNR regime over a conventional scheme with α = 0.5 and equal power at terminals. The proposed three-phase scheme in presence of co-channel interference can provide a better throughput for higher transmission rates as compared with a two-phase scheme when operated in the high SNR regime. Numerical results demonstrate accuracy of the derived expressions.

Cooperative communication can improve the performance of communication systems under multi-path fading conditions through relaying. Increasing the number of relays will also boost the performance of the system, but since relays transmit through orthogonal channels, this improvement is obtained at the cost of bandwidth or rate of the system. Best relay selection is a well-known technique to achieve full order of diversity and use bandwidth efficiently in a cooperative two-hop multi-relay network. A new beamforming at the relay is proposed that selects the best two among several relays and then the selected relays forward their detected symbols in a common channel to the destination. For achieving full diversity order at the destination, a modified maximal ratio combining detector is proposed. In the proposed beamforming scheme, the phase of the selected relays is adjusted in such a way to compensate the channel between the relays and the destination and achieve acceptable performance at the receiving end. The selected relays require two-bit quantised phase information of the channel gain between the relay and the destination, which can be sent to the relay by the destination. It is shown that the proposed method achieves full diversity order and has considerable performance gain in comparison with the best relay selection method.

Two-way amplify-and-forward (AF) relaying networks have been widely studied because of their simplicity and high spectral efficiency characteristics. Power allocation (PA) and relay selection (RS) for two-way AF relaying systems are of particular interest. However, most of the previous studies only consider the PA and RS for these systems with symmetric traffic requirements. In this study, the authors’ focus on the minimum transmit power (MTP) under/considering RS for asymmetric two-way relaying networks. The authors’ derive the MTP required for two-way AF relaying transmission with asymmetric traffic requirements, and propose an optimal RS criterion based on the derived MTP. First, the outage probability (OP) of two-way relaying transmission based on statistical channel state information (CSI) is derived. Then, by converting a geometric programming to a convex optimisation problem, the closed-form solution of the PA aiming at minimising total transmit power under the OP constraints is obtained. Lastly, a distributed RS criterion based on MTP is proposed. In addition, the proposed RS scheme only needs local statistical CSI rather than instantaneous CSI, which makes it simple in implementation. Numerical results demonstrate that the proposed scheme can significantly decrease the power consumption under the predefined OP constraints.

Energy harvesting (EH) has emerged as a promising technique for Green Communications and it is a novel technique to prolong the lifetime of the wireless networks with replenishable nodes. In this study, the authors consider the energy shortage analysis of fixed rate transmission in communication systems with EH nodes. First, the authors study the finite-horizon transmission and provide the general formula for the energy shortage probability (ESP). The authors also give some examples as benchmarks. Then, the authors continue to derive a closed-form expression for infinite-horizon transmission, which is a lower bound for the ESP of any finite-horizon transmission. These results are proposed for both Additive White Gaussian Noise (AWGN) and fading channels. Moreover, the authors show that even under random energy arrival, one can transmit at a fixed rate equal to capacity in the AWGN channels with negligible aggregate shortage time. The authors achieve this result using our practical transmission schemes, proposed for finite-horizon. Also, comprehensive numerical simulations are performed in AWGN and fading channels with no Channel State Information (CSI) available at the transmitter, which corroborate our theoretical findings. Furthermore, the authors improve the performance of our transmission schemes in the fading channel with no CSI at the transmitter by optimising the transmission initiation threshold.

A new method for wideband spectrum sensing in cognitive radio networks is proposed. Since the problem of estimating the number of occupied channels can be considered as the problem of estimating the number of signal sources in array signal processing, so the model used for direction of arrival (DOA) estimation is utilised here for spectrum-sensing modelling. In the proposed algorithm, wideband spectrum is divided into subchannels, where each subchannel resembles a sensor in array processing for DOA estimation. Furthermore, the detection problem in practical situations is complicated because noise is most likely non-Gaussian and non-stationary unlike the assumption of previously presented algorithms. Therefore, in the proposed algorithm, a generalised autoregressive conditional heteroscedasticity model is used to model the additive noise. The number and locations of the occupied subchannels will be jointly estimated using the maximum likelihood approach. The introduced method is blind as there is no need for initial information about the primary users’ signals and noise variance. The results indicate the efficiency of the proposed method.

This study proposes a soft ordered successive interference cancellation (OSIC) receiver accompanied by a simplified log-likelihood ratio calculation method for a dual-polarised multiple-input multiple-output (MIMO) digital video broadcasting-second generation terrestrial system. The future terrestrial transmission systems will be required to provide high data rates for ultra-high definition television service. For this purpose, dual-polarised MIMO techniques with a little cross-antenna interference and high-order modulation schemes have been considered. As the cross-antenna interference needs to be removed, a soft OSIC receiver can be used effectively. However, the receiver has to endure high computational complexity for calculating soft values. To reduce it, the authors simplified the calculation based on the decision threshold (DT) method. The main contribution of this study is to extend the conventional DT method, which is restricted to only low-order modulations, to an algorithmic process applicable to any modulation order. The proposed soft OSIC receiver has about 1 dB degradation from the soft OSIC receiver with the maximum-likelihood calculation of soft values. The degradation can be fully rewarded by the significantly reduced computational complexity. Furthermore, this study presents an additional possibility of improving the decoding performance by modelling the OSIC equaliser as a biased estimator.

In this study, two relay selection methods in the half-duplex decode-and-forward relaying for cooperative multiple-input multiple-output-orthogonal frequency division multiplexing (MIMO-OFDM) systems have been introduced. The relay selections are implemented using the symbol-based and subcarrier-based methods. By considering different number of antennas for source, each relay, and destination, the authors have derived closed-form outage expressions for these proposed methods. The MIMO channel model for each subcarrier, between source–relay and relay–destination is considered as a MIMO Rician fading channel with independent but not necessary identically distributed. Moreover, the outage expressions for MIMO independent and identically distributed Rician and Rayleigh fading channels are derived as special cases. Furthermore, the correlation among the OFDM subchannels is analysed. In addition, the implementation issues of both relay selection methods are also investigated. Monte Carlo simulations are performed to validate the outage analysis where the excellent agreement between the analytical and simulation results is observed.

In this study, the authors apply a generalised quantise-and-forward (GQF) relaying scheme for multiple access relay channel (MARC). The authors find achievable rate regions of GQF on MARC for both discrete memoryless case and additive white Gaussian channel. They assume that the relay has no access to channel state information (CSI) of the relay–destination link. Through numerical examples, they show that in the absence of CSI at the relay, the GQF scheme outperforms other relaying schemes. They also show that the outage probability of the individual users is lower than that of classic compress-and-forward scheme.

The detection of signals and the estimation of signal bandwidth is a perpetual topic in radio communication systems. Both issues are extremely challenging, since the wireless channel is unreliable in nature. A radio monitoring system faces the most difficult conditions in this task; it normally scans a wide frequency range of several hundred MHz and has to detect a multitude of different signals. Owing to the computational costs, the radio monitoring systems use nowadays mainly energy detectors based on fast Fourier transform spectrum analysers and a static threshold, defined by a previous noise estimation. A refined algorithm based on the self-splitting competitive learning (SSCL) clustering is presented that quantises the power spectral density (PSD) according to the present signal power levels. The quantisation of the PSD results in a promising channel segmentation. In contrast to the traditional threshold evaluation, this approach is independent of a previously assumed noise estimation and therefore more robust against noise level and noise distribution changes. The presented definition of the essential cluster validity criterion is key for a successful channel segmentation. Furthermore, the novel postprocessing of the clustering result introduced in this study evaluates the progression of the PSD data and significantly improves the channel segmentation.

Cognitive radios emerged as a solution to spectrum scarcity problem. The integration of cognitive radios and wireless sensor networks enables a new paradigm of communication, in which the sensor nodes can avoid heavily-crowded transmission bands by tuning their transmission parameters to less-crowded bands. The authors consider the problem of spectrum assignment for cognitive radio sensor network (CRSN) under coverage, interference, minimum data rate and power budget constraints. A mixed-integer non-linear programming problem formulation that addresses optimal power allocation, channel selection and node scheduling is presented. Following a practical assumption, that any CRSN node can only access one channel for its transmission with the CRSN base station, the problem is transformed to a binary linear programming (BLP) problem. Using the relaxation techniques, the problem is transformed to a linear programming problem that is solvable in polynomial time, and has the same optimal solution of the BLP problem. Hence, the minimum power algorithm that achieves the optimal solution of our problem is proposed. To further reduce the complexity of the solution, three heuristic lower-complexity algorithms are proposed to solve the problem: random, greedy and two-stage (decoupled) algorithms.

Multiuser multiple-input–multiple-output (MIMO) has great potential to substantially improve throughput of wireless networks. Unfortunately, it requires a significant amount of feed back for user selection, which prevents practical implementation. The authors propose a feed back reduced downlink multiuser MIMO system, where a signal-to-interference-plus-noise ratio (SINR) threshold is carefully designed. Only the users whose SINRs are above the threshold feed back their SINRs and channel direction information (CDI) to the base station. They establish a monotonic relation between the threshold and the average feed back overhead. Based on the established relation, they design the threshold that balances the tradeoff between the feed back load and the sum rate. Simulation results show that the proposed approach can achieve the same performance as the conventional full-feed back scheme (where every user feeds back its SINR and CDI), while the proposed approach is able to reduce the system overhead by more than 40%.

In cognitive radio networks, conventional power control algorithms (PCAs) based on instantaneous perfect channel gain may lead to performance degradation in practical systems, since channel uncertainties are inevitable because of quantisation errors and estimation errors. As a result, robustness of the algorithms becomes an important issue. However, traditional robust PCAs with probabilistic models require to perfectly know the distribution information of the estimation error (e.g. Gaussian distribution) which is difficult to obtain. Moreover, the distribution function of the actual error may not be Gaussian distribution. In this study, instead of using deterministic distribution model, a robust PCA based on a distribution-free method is designed to minimise total transmit power of secondary users subject to probabilistic interference and signal to interference plus noise ratio constraints. Based on the minimax probability machine, the original problem is reformulated as a second order cone programming problem solved by interior-point method. An adaptive estimation scheme is proposed to estimate the actual mean and covariance matrix of uncertain parameters. Simulation results demonstrate the effectiveness and robustness of the proposed algorithm by comparing with the robust algorithms under worst-case constraints and probabilistic constraints, respectively.

List-sphere detection (LSD) is a sub-optimal multiple-input multiple-output (MIMO) detection scheme which searches candidate symbol vectors that lie within a sphere of a given radius. This study presents an efficient LSD based method for a joint iterative MIMO detection scheme. The proposed method utilises a channel condition in order to define the list size. During the search process, the radius is adaptively updated to reduce the computational complexity. Owing to the list size and corresponding radius are adaptively determined by the channel condition, the authors can operate the detector at the most appropriate complexity to produce the required performance. Simulation results show that the proposed methods provide substantial complexity reduction without bit error rate performance degradation.

This study investigates the performance of two-way decode-and-forward (DF) relaying networks, considering transmissions over independent but not necessarily identically distributed (i.n.i.d.) Rayleigh fading channels, in the presence of multiple co-channel interferers at both the relay and end-source nodes. Both asymmetrical and symmetrical cases, of whether the channels from source terminals to the relay are identically distributed or not, are considered. Specifically, closed-form expressions for the cumulative distribution function of the equivalent signal-to-interference-plus-noise ratio (SINR) in different cases are derived, based on which the exact symbol error probability (SEP) and the systems’ achievable rate are derived and analysed. Based on the analytic results, the authors study the impacts of system parameters, such as interference power and number of interferers on the performance of the system. Furthermore, the system behavior at high signal-to-noise ratio (SNR) values is studied via deriving the asymptotic SEP. The results of this study are attested through Monte Carlo simulations.

The authors report an experimental investigation on the measurement of the available bandwidth for the users in gigabit passive optical networks (GPON) and the limitations caused by the Internet protocols, and transfer control protocol (TCP) in particular. We point out that the huge capacity offered by the GPON highlights the enormous differences that can be showed among the available and actually exploitable bandwidth. In fact, while the physical layer capacity can reach value of 100 Mb/s and more, the bandwidth at disposal of the user (i.e. either throughput at transport layer or goodput at application layer) can be much lower when applications and services based on TCP protocol are considered. In the context of service level agreements (SLA) verification, we show how to simultaneously measure throughput and line capacity by offering a method to verify multilayer SLA. We also show how it is possible to better exploit the physical layer capacity by adopting multiple TCP connections avoiding the bottleneck of a single connection.

A new method to improve the bit error rate (BER) performance by optimising the bit energy is proposed in this study. This method is mainly based on a formula derived from the union bound that can estimate the BER for every bit in the codeword sequence. Then by mathematical optimisation and assisted with a BER distribution, the energy for every bit can be optimised so that the average BER achieves the minimum value. Compared with other methods, this method considering all the codewords with minimum Hamming weights and their multiplicity to each bit of the codewords. The simulation examples show that the average BER is improved not only at high signal-to-noise ratio (SNR), but also at low and moderate SNRs.

The power consumption of mobile devices (MDs) has been increasing dramatically since more functions and bigger screens are adopted in the latest MDs. To extend the usage time of MDs and improve customer satisfaction levels, this research study, for the first time, studies the influence of different deployment strategies of a few low-power base stations (BSs) to reduce the power consumption of a large number of MDs. For both microcell and picocell scenarios, the authors’ analyse the power reductions at the MD side with the deployment of a low-power BS. The optimum low-power cell radius is derived for achieving the maximum MDs power reduction. The tradeoff relationships between MD power reduction and the key parameters of low-power BSs are investigated and discussed. It is shown that authors’ proposed method can achieve about 18% power reduction for microcell MDs.

Multi-user multiple-input–multiple-output (MU-MIMO) systems are a promising technology for increasing spectral efficiency and diversity gain in the up-link channel. Space-time block coding (STBC) or the spatial-multiplexing (SM) scheme may selectively be used by each user. The simple techniques, zero-forcing (ZF), STBC and successive interference cancellation are jointly applied to separate and detect all streams at the ZF-STBC MIMO receiver. Unfortunately the channel estimation error (CEE) and error propagation (EP) are usually presented in practical multi-layers detection. In this study, the author investigates the effect of CEE and EP on the bit error rate (BER) performance of the ZF-STBC MIMO receiver over Rayleigh flat fading channels. They derive a nearly exact closed-form BER for general phase shift keying/quadrature-amplitude modulation with CEE and EP for arbitrary number of users. Finally, simulation results demonstrate the validity of the author's analysis.

In the research of decode-and-forward (DF) cooperative communications, it is often supposed that the included relay only transmits correctly decoded signals. This simplifies the analysis of DF cooperation. However, in practical circumstances, erroneous messages may be forwarded which makes the decision at the destination even worst. In this study, the authors formulate a special detection way, named ‘detection with erasure’, to make certain that the relay-forwarded signals are mostly correct. Specifically, whenever the decoded result at the relay falls into some preset ‘erasure regions’, the relay will not process and transmit it anymore. This effectively prevents the forwarding of erroneous messages. With general pulse amplitude-modulated or squared quadrature amplitude-modulated signals, the authors demonstrate that these erasure regions can be adjusted to minimise the end-to-end symbol error rate (SER). This detection scheme can be extended to be used in the DF cooperation employing multiple relays as well. Simulations reveal that in terms of the end-to-end SER performance, the proposed detection with erasure can outperform some signal-to-noise ratio threshold-based relaying approaches, and can perform quite similarly as compared with those decoding/combining methods realised at the destination with the advantage of less channel state information exchange.

Time-domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) enjoys the higher spectrum efficiency and faster synchronisation than the classical cyclic prefix OFDM (CP-OFDM). However, TDS-OFDM suffers from performance degradation especially under severely fading channels with long delays. To solve these problems, the authors propose an energy-efficient OFDM scheme called time–frequency-training orthogonal frequency division multiplexing (TFT-OFDM) based on the time–frequency joint channel estimation under the framework of compressive sensing (CS). The power of the guard interval (GI) in the proposed scheme can be reduced to achieve higher energy efficiency, which is infeasible for CP-OFDM. This method first utilises the time-domain pseudo noise sequence to acquire partial support information of the channel, and then some frequency-domain pilots are used for the exact channel estimation. Simulation results show that TFT-OFDM with CS can achieve much higher energy efficiency than the classical CP-OFDM, and outperforms the conventional OFDM schemes in both static and mobile environments. Moreover, for the channel with long delay spread, the TFT-OFDM scheme with CS can demonstrate robustness and much better performance than the conventional OFDM schemes. In this way, the TFT-OFDM scheme can use the same GI length for larger broadcasting coverage and hence further achieve higher energy efficiency.

In this study, the authors focus on the tradeoff between spectrum sensing and energy efficiency of cognitive radio networks (CRN). Considering two interference-avoidance schemes, that is, channel handoff and stop-and-wait, the authors, respectively, model the energy efficiency (EE) of CRN as the mean of the throughput-to-power ratio. Research shows that stop-and-wait scheme is a special case of channel handoff from an EE perspective. Based on the proposed EE model, the authors build up an EE optimisation problem under the constraint of the sensing quality, and formulate the sensing-energy efficiency tradeoff (SET) for CRN. The similarity and difference between the SET and the sensing-throughput tradeoff are also investigated. Simulation analysis confirms that there is indeed an optimal sensing time to make the EE maximum, and it is larger than that for maximum throughput. Another interesting result is that the EE and the throughput of CRN can be enhanced together by optimising MAC frame structure in combination with sensing bandwidth adjustment and power control. Our work provides some insights for green CRN in view of MAC frame optimisation.

The problem of the system throughput maximisation and the fairness issue among users is considered for the downlink multiuser multiple input multiple output-orthogonal frequency division multiplexing system with MMSE precoding. Since finding the optimal solution by using the exhaustive search has very high computational burden, the authors propose a low-complexity user selection strategy plus the optimal power allocation in this study. With resorting to the mathematical simplification process, the proposed user selection scheme is designed to compare the related value of SINR. The power allocation is performed after the user selection. However, the power allocation is not in interference-free environment but in the presence of crosstalk, which is a nonlinear non-convex optimisation problem. In this study, the authors use an optimisation technique to obtain the optimal solution of the constrained non-convex nonlinear problem by replacing the objective function with a difference of two convex functions. A bisection-based strategy to solve this problem with interference-awareness to find local optimal solutions is presented in this study. Simulation results show that the performance of the proposed algorithm is close to that of the optimal solution. The trade-off between the system throughput and the fairness among users is also considered.

Strategic bargaining cooperative games have found extensive applications to resource management in wireless networks. In this survey, basics of a strategic bargaining game and solution concepts are firstly presented. Geometrical interpretations are introduced to better understand real meanings of them. Then, the authors survey the applications of various strategic bargaining games for the emerging wireless networks, where the authors concentrate on several interesting problems based on their previous systematic studies: (i) distributed resource management design for cognitive radio networks based on geometrical interpretation of the cooperative solution; (ii) asymmetric bargaining modelling for green communications; (iii) a unified utility tradeoff design between spectral and energy efficiency in heterogeneous cellular networks; (iv) the cooperative rate splitting game for Long Term Evolution-coordinated multi-point system; and (v) a general bargaining formulation with different tradeoffs between efficiency and fairness. In addition, the authors survey the applications of strategic bargaining games to cooperation incentive mechanism, bargaining game on capacity region of interference channel and multiuser and multimedia applications. Finally, challenges and potential research direction are summarised in this work.

Femtocell networks are expected to offer significant performance improvement with low cost. However, adaptive transmission based on maximising the sum rate of macro and femto networks requires perfect channel quality information (CQI) of the macro and femto links. This assumption requires an infinite resolution feedback link, which is not always practical since it requires an excessive amount of bandwidth. This study considers the problem of maximising average sum rate under the constraint of average maximum power transmission at macro and femto users. A suboptimal iterative algorithm is developed for finding the optimal CQI quantisers as well as the discrete power and rate at macro and femto transmitter for each quantised CQI level so as to maximise the average sum rate of the system. The author's numerical results give the number of bits required to sufficiently represent the CQI to achieve almost the maximum sum rate attained using full knowledge of the CQI.