Physical layer security has been recognised as a promising technique to realise secure wireless communications. A novel interference masking approach is proposed for secure broadcast scenarios. Specifically, when there is an external jamming node, precoding matrices at the source and jammer are carefully designed to ensure that each destination can detect only its own message, and not the information intended for other destinations which is masked with artificial noise. When there is no external jamming node, interference masking is implemented through user cooperation, in which destinations switch roles from jammer to relay nodes during different time slots. Both analytical and numerical results are provided to demonstrate the performance of the proposed approaches.

The authors present a beamforming scheme with opportunistic relaying (namely opportunistic beamforming) for wireless security under amplify-and-forward (AF) and decode-and-forward (DF) strategies. The relay selection is performed using the method of distributed timers, where all the relays use their timers to estimate own instantaneous channel gains and compete to access the wireless medium according to their own channel conditions. The performance of opportunistic beamforming scheme in terms of secure outage probability is analysed and the beamforming problem as a semidefinite programming problem with its optimisation framework is formulated. The results show that, in an opportunistic beamforming scheme, competition among cooperative relays offers diversity benefits in the direction of destination that enhances secrecy rate (i.e. minimises secure outage probability) and adhere to the ‘opportunistic’ cooperation rule giving priority to the ‘best’ available relay even when they are not chosen to transmit but rather chosen to cooperatively listen. Opportunistic beamforming scheme with DF strategy enhances secrecy rate than the beamforming with multiple DF relays while in the case of AF strategy, it provides significant improvement in the secrecy rate compared to the opportunistic relaying, the equal-power multiple-relay transmission and the single-relay transmission schemes.

In this study, the authors propose an equivocation scheme for wiretap channels, which is composed of bit-extension mapping, coset coding and hybrid automatic repeat request (HARQ). The inner bit-extension code and outer coset code are used for equivocation of a wiretapper channel, whereas the HARQ scheme is to mitigate noisy errors in a main legitimate channel. These concatenated codes and HARQ are effective and practical for various channel conditions. The average equivocation and the probability of causing imperfect secrecy are analysed for finite codeword lengths. As a function of channel conditions, they investigate the block error rate at the legitimate receiver and the information leakage to the wiretapper. From simulation results, they further determine the minimum requirements of code design for some target values of the ‘residual’ block error rate and information leakage at maximum retransmission.

The use of multiple-antenna arrays has attracted much attention for the physical layer security of wireless systems, where the so-called artificial-noise solution can be applied to enhance the communication confidentiality. In the first part of this study, the authors consider secure communication over a multiple-input single-output Rayleigh-fading channel in the presence of a multiple-antenna eavesdropper – referred to as a multiple-input single-output multiple-eavesdropper (MISOME) wiretap channel. Specifically, secure beamforming with artificial noise is treated when the transmitter has access to full channel state information (CSI) of a legitimate channel but only partial CSI of an eavesdropper channel. First, the optimal power allocation between the information-bearing signal and artificial noise (or simulated interference) is derived to maximise the achievable secrecy rate in the presence of a weak or strong eavesdropper. Then, the first-order optimal power allocation strategy is developed in a switched fashion by selecting the best of weak- and strong-eavesdropper solutions for a general eavesdropping attack, and a closed-form expression is derived for the ergodic secrecy rate achieved by secure beamforming with this switched power allocation in the MISOME wiretap channel. The numerical results show that the switched power allocation is a simple but effective approach that nearly achieves the optimal secrecy rate.

The authors consider secure beamforming with artificial noise in a multiple-input single-output multiple-eavesdropper (MISOME) wiretap channel, where a transmitter has access to full channel state information (CSI) of a legitimate channel but only partial CSI of eavesdropper channels. In the second part of this study, the authors first put forth a new notion of symbol error probability (SEP) for confidential information – called the ‘δ-secrecy SEP’ – to connect the reliability and confidentiality of the legitimate communication in MISOME wiretap channels. For single-antenna colluding and non-colluding eavesdroppers, the authors then quantify the diversity impact of secure beamforming with artificial noise on the δ-secrecy SEP and show that the artificial -noise strategy with n _t transmit antennas preserves the secrecy diversity of order n _t − n _e for n _e colluding eavesdroppers and n _t − 1 for n _e non-colluding eavesdroppers, respectively. In addition, the authors determine the optimal power allocation between the information-bearing signal and artificial noise to minimise the δ-secrecy SEP in the presence of weak or strong eavesdroppers, and further develop the switched power allocation for general eavesdropping attacks.

The authors consider the secure transmission of information over power line communication (PLC) networks. The focus is on the secrecy guaranteed at the physical layer, named physical layer security (PLS). Although PLS has been deeply investigated for the wireless case, it is not the same for the PLC environment. Thus, starting from the knowledge in the wireless context, the authors extend the results to typical PLC scenarios. In particular, the PLC channel statistics is evaluated and a performance comparison among PLC and wireless channels is performed, in terms of secrecy rate distribution. For the PLC case, the secrecy rate distribution, under a total power constraint, is evaluated for both optimal and uniform power distributions in broadband channels. To provide experimental evidence, the authors consider channel measures obtained in an in-home measurement campaign. The underlying network presents a tree topology, which introduces frequency and spatial correlation among channels, and suffers from the keyhole effect, generated by branches that depart from the same node. As shown by the numerical results, these effects can reduce the secrecy rate. Finally, the authors evaluate the secrecy rate region for the multi-user broadcast channel considering both simulated channel realisations and experimental channel measures.

The authors address the physical layer security in multiple-input-single-output communication systems. This study introduces a robust strategy to cope with the channel state information errors in the main link to convey confidential information towards a legitimate receiver while artificial noise is broadcast to confuse an unknown eavesdropper. The authors study how an eavesdropper physically located in the vicinity of the transmitter can put at risk the network's security, and hence, as a countermeasure, a ‘protected zone’ was deployed to prevent the close-quarters eavesdropping attacks. The authors determine the size of the protected zone and transmission covariance matrices of the steering information and the artificial noise to maximise the worst-case secrecy rate in a resource-constrained system and to minimise the use of resources to ensure an average secrecy rate. The proposed robust masked beamforming scheme offers a secure performance even with erroneous estimates of the main channel showing that a protected zone not only enhances the transmission security but it allows us to make an efficient use of energy by prioritising the available resources.

Cooperation between wireless network users is a promising technique for improving the physical layer (PHY) security of wireless transmission in the presence of eavesdroppers. In practice, users may not always act as reliable cooperators. The misbehaviour of cooperative users may change the original security transmission condition. In this study, an algorithm that user splits or merges from coalition is proposed based on coalitional game with non-transferable utility. It obtains the maximum PHY security capacity of users in wireless networks even when there are users with dynamic behaviour. Simulation results show that a certain degree of misbehaviour can be tolerated by coalition. Analyses also show that the sensitivity of misbehaviour of the coalition is related to the position of users.

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

Recently, radio-frequency (RF) fingerprinting has been proposed for mitigating primary user emulation (PUE) attacks in cognitive radio networks (CRNs). The widespread practical implementation of cognitive radio (CR) is likely to utilise software-defined radios with a low-end (i.e. low-cost) receiver built with inexpensive analogue components. This study experimentally analyses the feasibility of RF fingerprinting for mitigating PUE attacks using low-end software-defined CRs. Seven universal software radio peripherals are used as low-end CR receivers and their resulting performance is analysed for ad hoc and infrastructure CRN scenarios. The performance analysis is performed for the largest known data set of its kind, which consists of 490 000 measurements from seven identical transmitters across eight receivers. It is found that impairments in the front-end of a low-end receiver affects the accuracy of transmitter classification and this accuracy varies across receivers. The results suggest that RF fingerprinting can be effectively used for mitigating PUE attacks in an ad hoc CRN at high receiver signal-to-noise ratio whereas RF fingerprinting is not a practical solution for mitigating PUE attacks in an infrastructure CRN.

In a multi-user wireless network, physical layer security is an important issue, especially when some of the served users are malicious. As these users actively participate in the network, their channel state information can be estimated or feedback, although not perfectly known. With this channel uncertainty, the worst-case signal leakage to eavesdropping users’ needs to be managed. The authors propose a robust instantaneous relay design that improves the worst-case secrecy rate performance, and decompose the problem into two sub-problems: First, the achievable signal leakage or leakage-to-noise ratio region is studied. Second, for fixed signal leakage constraints, the achievable secrecy rate region is computed. To approach the non-convex optimisation problems, the authors propose to iteratively solve for the phases and magnitudes of the equivalent channel matrix, a function of the relay matrix. The numerical simulations illustrate that the proposed alternating optimisation algorithm achieves higher secrecy rates than those in a system without an instantaneous relay.

The authors study a two-user state-dependent generalised multiple-access channel (GMAC) with correlated states. It is assumed that each encoder has ‘non-causal’ access to channel state information (CSI). They develop an achievable rate region by employing rate-splitting, block Markov encoding, Gelfand–Pinsker multicoding, superposition coding and joint typicality decoding. In the proposed scheme, the encoders use a partial decoding strategy to collaborate in the next block, and the receiver uses a backward decoding strategy with joint unique decoding at each stage. The author's achievable rate region includes several previously known regions proposed in the literature for different scenarios of multiple-access and relay channels. Then, they consider two Gaussian GMACs with additive interference. In the first model, they assume that the interference is known non-causally at both of the encoders and construct a multi-layer Costa precoding scheme that removes ‘completely’ the effect of the interference. In the second model, they consider a doubly dirty Gaussian GMAC in which each of interferences is known non-causally only at one encoder. They derive an inner bound and analyse the achievable rate region for the latter model and interestingly prove that if one of the encoders knows the full CSI, there exists an achievable rate region which is ‘independent’ of the power of interference.

In this study, the authors propose a blind carrier frequency offset (CFO) estimation method for the tile structure orthogonal frequency division multiple access (OFDMA) uplink with multi-antenna receiver. They employ an iterative approach to extract the signal component for each user gradually and propose a carefully designed CFO estimator to update the CFO estimate during the iterative procedure. The key ingredient of the proposed method is using few subcarriers on both sides within each tile as the ‘guard subcarriers’, which can greatly mitigate the effect of multi-user interference. The proposed method supports not only fully loaded transmissions, but also the generalised assignment scheme that provides the flexibility for dynamical resource allocation. The numerical results are provided, which indicate that the proposed method can almost converge to the analytical lower bound within a few iterative cycles. It is seen that the proposed method also outperforms the existing competitor with multi-antenna receiver in terms of estimation performance, especially with few adopted blocks.

The maximum a posterior (MAP) decoding scheme is presented here for cooperative communication networks that adopt the parity forwarding as a cooperation protocol. The MAP decoder is optimal in the sense that it minimises the error probability. The authors consider a wireless network that is composed of two sources: two relays and a single destination. A closed-form expression is derived for upper bound on the bit error probability. The complexity of derivation comes from the fact that although the source generates data with equal probability, the data received at the destination does not have the same a priori probability. That is because of the error that occurs in the source-to-relay link. Therefore, the MAP decoding rule cannot be simplified to the maximum likelihood decoding rule. The results show that the analytical upper bound is very tight and almost coincides with the exact error probability obtained from simulations at higher values of the signal-to-noise ratio. Accordingly, the closed-form expression of the upper bound can be used to fully study and understand the diversity performance of the system.

The authors consider applying the systematic low-density parity-check codes with the parity based approach to the lossless (or near lossless) distributed joint source channel coding (DJSCC) with the decoder side information for the non-uniform sources over the asymmetric memoryless transmission channel. By using an equivalent channel coding model, which consists of two parallel subchannels: a correlation and a transmission sub-channel, respectively, they derive the codeword averaged density evolution (DE) for the DJSCC with the decoder side information for the asymmetrically correlated non-uniform sources over the asymmetric memoryless transmission channel. A new code ensemble definition of the irregular codes is introduced to distinguish between the source and the parity variable nodes, respectively. Extensive simulations demonstrate the effectiveness of the codeword averaged DE.

Codes over Gaussian integers have been proposed for coding over two-dimensional (2D) signal spaces, for example, using quadrature amplitude modulation. Here, it is demonstrated that the concept of set partitioning can be applied to Gaussian integer constellations that are isomorphic to 2D modules over rings of integers modulo p. This enables multilevel code constructions over Gaussian integers. The authors derive upper bounds on the achievable minimum distance in the subsets and present a construction for the set partitioning. This construction achieves optimal or close to optimal minimum distances. Furthermore, it is demonstrated that this set partitioning can be applied to an interleaving technique for correcting 2D cyclic clusters of errors. The authors propose a novel combination of generalised concatenated codes with 2D interleaving to correct 2D error clusters and independent errors.

A new near-optimum detection scheme for asynchronous wireless relay networks is proposed to cancel the interference component at the destination node caused by timing misalignment from the relay nodes. The detection complexity at the destination node as compared with a previous sub-optimum detection scheme is reduced. Closed-loop extended orthogonal space time block coding and outer convolutive coding are utilised to maximise end-to-end performance. A relay selection technique is also proposed in this study for two dual-antenna relay nodes to enhance the system performance by selecting the best links and the smallest time delay error among the relay nodes. Simulation results confirm that the proposed near-optimum detection scheme with relay selection is very effective at removing intersymbol interference at the destination node and achieving full cooperative diversity with unity data transmission rate between the relay nodes and the destination node.

In this study, a two-stage decode-and-forward cooperative network is investigated consisting of a source, a corresponding destination, an eavesdropper and several intermediate nodes. In this study, the authors present an analysis of overall secrecy rate considering both distributed relay selection and secure beamforming problems. The achievable rate constraint is newly included into the system model. This modelling framework offers a more reliable approximation of the practical wireless channel. The relay selection is formulated as a coalitional game with transferable utility, which decreases the computation complexity in solving the distributed relay selection problem. A new ‘Max–Pareto order’ is constructed, which not only considers the player value, but also reflects the dominated weight of coalition value. Moreover, a distributed merge-and-split coalition formation algorithm is presented in this study. This algorithm achieves the system performance close to the theoretical upper limit, but it requires much less computation consumption.

Microring resonators (MRRs) can be used to generate optical millimetre-wave solitons with a broadband frequency of 40–60 GHz. Non-linear light behaviours within MRRs, such as chaotic signals, can be used to generate logic codes (digital codes). The soliton signals can be multiplexed and modulated with the logic codes using an orthogonal frequency division multiplexing (OFDM) technique to transmit the data via a network system. OFDM uses overlapping subcarriers without causing inter-carrier interference. It provides both a high data rate and symbol duration using frequency division multiplexing over multiple subcarriers within one channel. The results show that MRRs support both single-carrier and multi-carrier optical soliton pulses, which can be used in an OFDM based on whether fast Fourier transform or discrete wavelet transform transmission/receiver system. Localised ultra-short soliton pulses within frequencies of 50 and 52 GHz can be seen at the throughput port of the panda system with respect to full-width at half-maximum (FWHM) and free spectrum range of 5 MHz and 2 GHz, respectively. The soliton pulses with FWHMs of 10 MHz could be generated at the drop port. Therefore, transmission of data information can be performed via a communication network using soliton pulse carriers and an OFDM technique.

In this paper, the authors investigate the distributed power allocation for the multicell orthogonal frequency division multiple access networks by taking both the energy efficiency and the intercell interference (ICI) mitigation into account. A performance metric termed as throughput contribution is exploited to measure how the ICI is effectively coordinated. To achieve a distributed power allocation scheme for each base station (BS), the throughput contribution of each BS to the network is first given based on a pricing mechanism. Different from the existing works, a biobjective problem is formulated based on the multiobjective optimisation theory, which aims at maximising the throughput contribution of the BS to the network and minimising its total power consumption at the same time. By using the method of the Pascoletti and Serafini scalarisation, the relationship between the varying parameters and the minimal solutions is revealed. Furthermore, to exploit the relationship an algorithm is proposed based on which all the solutions on the boundary of the efficient set can be achieved by adaptively adjusting the involved parameters. With the obtained solution set, the decision maker has more choices in the power allocation schemes in terms of both the energy consumption and the throughput. Finally, the performance of the algorithm is assessed by the simulation results.

In this study, a joint design of precoder and equaliser of a linear transceiver for multiple-input–multiple-output system with orthogonal frequency division multiplexing in the presence of intercarrier interference (ICI) is presented. The matrix structures of the precoder and equaliser are banded for the sake of reducing the computational complexity and feedback overhead from the receiver to the transmitter. The design criterion is to minimise the mean-squared error subject to a total transmitted power constraint of which the power is allocated over space and frequency domains in the precoder. The authors use the Karush–Kuhn–Tucker conditions to derive an iterative procedure to obtain a convergent solution and a closed-form procedure for the optimal full transceiver. Numerical results show that the banded precoder is an efficient scheme to improve the bit error rate (BER) of the transceiver by simply increasing its band size and can provide better BER performance than that of the existing jointly designed full transceiver in the presence of ICI. With small band sizes, the proposed transceiver can give performance close to that of the jointly designed full transceiver but with lower implementation complexity and feedback overhead to the transmitter.

In this paper, the authors study the system performance of adaptive transmission techniques in the presence of adjacent channel interference (ACI). The authors obtain closed-form expressions for system capacity when employing different adaptive transmission policies with and without diversity schemes. The simulation results of spectrum efficiency in a WiMAX network employing 256 orthogonal frequency division multiplexing subcarriers are obtained for the analytical expressions derived for selection combining diversity under various adaptation policies. Analytical results show that the optimal power and rate adaptation policy provides the highest spectrum efficiency. Both analytical and simulation results are found to be in close agreement with each other, which shows that the obtained analytical results works well with the scenario of ACI. Also, maximal ratio combining scheme yields the highest spectrum efficiency when subjected to ACI.

In this study, the design of improved Luby transform codes with decreasing ripple size (LTC-DRS) with feedback is presented. Under the proposed design, a new degree distribution algorithm named generalised degree distribution algorithm (GDDA) is proposed, which can achieve arbitrary ripple size revolution accurately. On the basis of GDDA, an accurate ripple size revolution based on binomial fitting is proposed, which can keep the ripple size to a suitable value throughout the decoding process. Furthermore, the authors introduce the feedback and propose a shifted ripple size revolution to diversify the degree values. The improved LTC-DRS with feedback is evaluated and compared with the existing schemes. The simulation results demonstrate that it outperforms other existing schemes in terms of average overhead, average degree of encoded symbols, memory usage and energy consumption.

Mobile devices are becoming the primary platforms for many users who always roam around when accessing the cloud computing services. From this, the cloud computing is integrated into the mobile environment by introducing a new paradigm, mobile cloud computing. In the context of mobile computing, the battery life of mobile device is limited, and it is important to balance the mobility performance and energy consumption. Fortunately, cloud services provide both opportunities and challenges for mobility management. Taking the activities of cloud services accessing into consideration, the authors propose a service-aware location update mechanism, which can detect the presence and location of the mobile device without traditional periodic registration update. Analytic model and simulation are developed to investigate the new mechanism. The results demonstrate that the service-aware location update management can reduce the location update times and handoff signalling, which can efficiently save power consumption for mobile devices.

The conventional cyclic-prefix code division multiple access (CP-CDMA) system usually requires a chip-rate frequency-domain equaliser (FDE) at the cost of a tremendous discrete Fourier transform (DFT) size. To reduce the required DFT size in the CP-CDMA uplink receiver, this work studies a new symbol-rate equaliser moving the code despreader ahead of the FDE. The authors formulate the composite effect of the chip-rate channel impulse response (CIR) and code despreading as an equivalent symbol-rate channel model together with a small additive noise. Then, an iterative two-stage multiuser interference (MUI) cancellation algorithm can be employed with the decision feedback equalisation method. In addition, a novel compressive sensing (CS)-based interpolation method is proposed for compressible CIR estimation to be used in the MUI cancellation algorithm. With the CS method, the DFT size is dramatically reduced for channel estimation and no high-resolution interpolation kernels are required. The numerical results show that the new receiver is effective on MUI cancellation and only a couple of iterations are required to achieve a performance similar to the chip-rate equaliser.