In this study, a concatenated coding scheme based on Reed–Muller (RM) codes and bit-extension codes is proposed for equivocation of a wiretap channel. RM codes and their cosets are adopted for message encoding, and bit-extension codes are used to enhance the equivocation capability for a wiretapper's channel. The average equivocation is discussed when only RM codes are used in the system, and the probability causing imperfect secrecy is also determined. Analytical results show that the proposed code can be used for the equivocation capability of wiretap channels and suggest a proper management over a wiretap channel.

In this study, the authors examine resource allocation in an orthogonal frequency-division multiple-access-based cognitive radio (CR) network which dynamically senses primary users (PUs) spectrum and opportunistically uses available channels. The aim is resource allocation such that the CR network throughput is maximised under the PUs maximum interference constraint and cognitive users (CUs) transmission power budget. This problem is formulated as a mixed-integer non-linear programming problem which is 𝒩𝒫-hard in general and infeasible to solve in real-time. To reduce the computational complexity, the authors decouple the problem into two separate steps. After initial power allocation, in the first step, an adaptive algorithm is employed to assign subcarriers to the CUs toward throughput maximisation by using these initial powers. In the second step, power is allocated optimally to the assigned subcarriers. Simulation results show that the proposed method nearly achieves the optimal solution in a small number of iterations meaning significant reduction in the computational complexity.

The authors present a novel frequency division multiplexing scheme which can generate a signal whose worst-case peak-to-average power ratio (PAPR) is tunable by an input parameter; they call this scheme tunable PAPR frequency division multiplexing (TP-FDM). Special cases of TP-FDM are orthogonal FDM (OFDM) and single carrier (SC) modulation. Unlike several other proposed PAPR reduction schemes, TP-FDM requires no per-symbol side information to be sent from transmitter to receiver. TP-FDM has overall computational requirements almost the same as OFDM. The author's study of the symbol error rate (SER) of TP-FDM in a Rayleigh fading channel shows that higher PAPR signals give a better SER performance. Thus, instead of choosing either of the extremes, OFDM and SC modulation, as is currently done in some cellular systems, one can tune the PAPR with TP-FDM to the maximum suitable for the transmitter and receiver device electronics, so as to minimise SER. They compare and contrast TP-FDM with transform precoded OFDM methods in terms of SER and PAPR. They also extend TP-FDM to a multi-user scenario.

The authors present an improved achievable rate region for two-pair bidirectional Gaussian relay networks based on successive compute-and-forward method. In these networks, one relay helps in the communication between two pairs of users. In their proposed scheme, the authors use nested lattice codes for encoding the messages at the users, and Gaussian random codes for the encoding at the relay. They use the successive compute-and-forward strategy to decode two integer linear combinations of the lattice codewords in the uplink, and successive interference cancellation for decoding the Gaussian codewords in the downlink. The downlink channel can be considered as a broadcast channel with two receiver groups, but within each group, a pair of users is considered as an additive white Gaussian noise channel (instead of a broadcast channel) because each node knows its own transmitted message. It is shown that for all channel gains of downlink channels and all channel gains of symmetric uplink channel pairs, the strategy achieves rates to within constant gaps of 1/2 and 3/4 bit/s/Hz per user of the cut-set upper bound for restricted and non-restricted models, respectively. These gaps are tighter than those previously obtained for this network, which have not exploited the successive compute-and-forward method.

This study designs and analyses a new phase-coded spread-spectrum communication system where both phase-coded carrier and spreading factor are varied based on a chaotic behaviour in the communication process. This design aims to reduce the probability of interception of the considered system. Discrete values generated by a chaotic map are exploited to create a non-return-to-zero (NRZ)-chaos sequence and simultaneously make bit duration variable. The NRZ-chaos sequence is then modulated by binary phase-shift keying technique to produce the phased-coded carrier. Owing to chip duration being constant, the variation of bit duration also leads to the variation of spreading factor. Spectrum spreading in the transmitter is performed by multiplying directly the variable-duration bits with the phase-coded carrier. A coherent receiver relying on a direct correlator is used for recovering the data. Design of the transmitter and receiver as well as analysis of bit error probability for the proposed system in cases of single-user and multi-user under additive white Gaussian noise channel is presented. Simulation results are shown to confirm the operation of the designed structures and the obtained analytical performance.

Assuming a land-mobile satellite link with negligible multipath and shadowing effects, the corresponding channel model is adequately described by a correlated Rician distribution. In this context, the estimation of carrier and channel parameters has been investigated in some earlier work by means of an algorithmic approach primarily based on heuristic considerations. In the current study, the authors will review the results obtained so far from a maximum-likelihood (ML) point of view. A major output of the analysis is that the simple ad-hoc framework achieved previously is indeed an ML solution, when the impairments introduced by Rician fading are spectrally flat and strictly band limited.

This study investigates energy efficient allocation of radio resource blocks in orthogonal frequency division multiple access (OFDMA) systems while considering the status of the users’ data buffers to reduce packet dropping rate by the downlink scheduler. The proposed scheme exploits the fluctuations of traffic load to efficiently schedule users’ data packets by reducing the overall energy consumption of the system whenever the status of data buffers permits. From information theory point of view, the proposed scheme exploits the fundamental trade-off between energy efficiency and spectral efficiency to perform scheduling. First, the problem is formulated as an optimisation problem and then a novel solution, based on dynamic programming, is applied. By comparing the analytical solution with the ones obtained by exhaustive search, it is demonstrated that the proposed scheme is close to the optimal solution, with low computational complexity. In addition, comprehensive simulations are conducted to evaluate the performance of the suggested algorithm. Both analytical and simulation results demonstrate the superiority of the proposed algorithm compared with the well-known benchmark schemes in terms of energy efficiency and packet dropping rate.

In this study, the authors investigate eavesdropping mitigation for wireless communications with physical-layer security techniques. They consider a multicasting scenario with one transmitter, multiple trusted receivers and multiple eavesdroppers, each equipped with one antenna. Aiming at protecting trusted communications from data interception, they develop two novel waveform designs for eavesdropping reduction under two scenarios: (i) when the eavesdroppers’ channel state information (CSI) is known, they design the waveform and transmit energy which can minimise the maximal signal-to-interference-plus-noise ratios (SINRs) at the eavesdroppers while achieving guaranteed SINRs at the trusted receivers and (ii) when the eavesdroppers’ CSIs are not available, they adopt ‘artificial noise’ (AN) strategy aiming at generating as much disturbance to eavesdroppers as possible. A novel waveform and transmit power design is proposed which can maximise the AN's energy with assured quality-of-service at trusted receivers. Extensive simulation studies illustrate the efficiency of the proposed designs for eavesdropping mitigation in wireless communications.

Over the past few decades, frequency hopping (FH) techniques have attracted considerable interest in both military and commercial communications. However, fading and interference can significantly degrade the performance of an FH system. One method commonly employed to overcome the degradation in performance because of the fading is the use of diversity. Furthermore, fast FH (FFH) systems can enjoy both time and frequency diversity and hence FFH systems employed in conjunction with diversity-combining techniques are effective for combating fading and interference effects. In this study, the authors provide a comprehensive survey on the past and current research work on diversity-combining techniques for suppressing different types of interference in FFH M-ary frequency-shift-keying systems over various fading channels. Some commonly used theoretical approaches for analysing performance of FFH systems are also presented. In addition to the survey, they will also discuss some interesting areas of future work in this research topic.

Cognitive radio and cooperative networks are increasingly regarded as revolutionary technologies that utilise the radio spectrum efficiently. In this study, spectrally encoded (spread-time) code division multiple access (SE-CDMA) scheme is proposed as a flexible and adaptable technique for physical layer of cognitive relay networks. All primary and secondary users transmit their signals to a common base station (BS) and then the BS selects one of the cognitive secondary users based on feedback information to relay the primary user's (PU's) data. To study the efficiency of the system, two scenarios are considered. In the first scenario, the secondary users transmit their information using the SE-CDMA technique and the PU is considered narrowband. In the second scenario, both primary and secondary users utilise the SE-CDMA technique. The performance of the system is evaluated in terms of the error probability of the PU, the outage probability of the primary and secondary users and comparing theoretical results with simulations. Moreover, the performance of the proposed method is compared with orthogonal frequency division multiple access-based cognitive relaying scheme. The results indicate the efficiency of the SE-CDMA technique.

In this study, the authors investigate energy efficiency in antenna selection multi-input multi-output automatic repeat request (MIMO ARQ) wireless systems. The authors first derive an approximate expression for the average frame-error rate (FER) in antenna selection MIMO systems over quasi-static Nakagami-m fading channels. The FER approximation is then used to obtain an analytical expression of an energy-efficiency metric that is defined as the total energy required to successfully deliver one information bit. The authors prove that this energy-efficiency metric is a quasi-convex function with respect to the average signal-to-noise ratio value. Based on this analysis, the authors obtain the optimal value of the average energy per transmitted data symbol such that the total energy consumption in the system is minimised. The results show that the energy efficiency in antenna selection MIMO ARQ systems is improved when the number of equipped antennas is increased. Simulation results are provided to validate the analysis.

The performance of modern communications transmitters utilising multi-carrier modulation techniques is highly sensitive to non-linear distortions arising mainly from the high-power amplifier. The increase in symbol duration makes the system more sensitive to time-varying channels. In addition, the wideband characteristics of multi-carrier signals result in frequency-dependent distortions, typically known as memory effects. This study presents analytical expressions that can be used to evaluate the impact of the distortions induced by time-varying non-linearities with memory on the bit error rate performance of a multi-carrier signal such as the orthogonal frequency division multiplexing. The time-varying non-linearity with memory is modelled by a time-varying Volterra series. The resulting mathematical expressions are applied to specific situations, allowing for a quantitative evaluation of the system performance degradation.

To enable uplink inter-cell interference coordination in cellular networks, estimates of the powers generated by the own serving cell, the neighbour cells and the thermal noise processes need to be made available. An algorithm for such high-bandwidth interference power splitting is derived in the study. The algorithm does not require inter-base station communication. The resulting estimates support self-organising network interference management and coordinated scheduling, both technologies being important in new heterogeneous networks. A new method for G -matrix tracking is also outlined, based on the estimates obtained from the proposed algorithm. The performance evaluation, performed for a 3G wideband code division multiple access setup of the algorithm, shows that the estimation inaccuracy's of the proposed algorithm is below 10–15% in the relevant operating region.

This study deals with an interweave cognitive-radio (CR) system, where an uplink orthogonal frequency-division multiple access system is considered for the primary users (PUs). Our purpose is to estimate the PU carrier frequency offsets (PU-CFOs) as well as the channels to estimate the transmitted symbols. However, in wideband wireless communications, the PU received-signal spectrum usually exhibits a localised fading because of the multipath propagation channel. When the PU faded frequencies are used by the CR system, the PU system is contaminated by a CR narrowband interference (CR-NBI). In that case, if a Kalman filter (KF)-based approach is used for the estimations of the CFOs and the channels, the state-space representation does not necessarily take into account the CR-NBI; this hence has a negative impact on the algorithm performance. Therefore the authors propose to make this solution more robust to the CR-NBI by detecting when it appears and disappears to avoid using the data disturbed by the CR-NBI. The authors’ contribution is to assume that the CR-NBI is because of the transmission of contiguous blocks and to propose various criteria based on the covariance matrix of the KF innovation.

In this paper, a new secure channel coding scheme is presented which randomly inserts and deletes bits in a codeword of a quasi-cyclic-low-density parity check (QC-LDPC) code. It is shown that the key size is smaller than other code-based cryptosystems based on permutation and scrambling matrices. The positions of the inserted and deleted bits are determined using a secret key. It is shown that the error performance of the resulting code after the insertions and deletions is better than a random low-density parity check code with similar parameters. An important advantage of this cryptosystem is that even if the QC-LDPC code is revealed, the system remains secure. Furthermore, the proposed approach using insertions and deletions can be employed with other classes of error correcting codes.