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In this study, a simple two-user multiple-input multiple-output (MIMO) relaying scenario is considered. To efficiently exploit the spectrum resources, a generalised singular value decomposition (GSVD) based precoding scheme with non-orthogonal multiple access (NOMA) is proposed, which converts the multiple channels into several parallel single channels. The performance of the proposed GSVD-MIMO relaying scheme is evaluated by using the distribution characteristics of squared generalised singular values of two users' channel matrices and the eigenvalues of the relay's channel matrix to obtain the expressions of the average data rate and outage probabilities. Both the asymptotic case and a practical scenario with nodes equipped with finite numbers of antennas are analysed because the distribution characteristics of squared generalised singular values in the two cases are different. Two special cases, single-input multiple-output and multiple-input single-output are considered. Simulation experiments are carried out to verify the effectiveness and accuracy of the obtained analytical results.

In this study, the authors consider the secrecy communication of one source-destination pair in a wireless multi-hop relay network, where each node is equipped with multiple antennas. They use the method of interference alignment for secrecy transmission in the presence of an eavesdropper. Firstly, they propose a decode-and-forward (DF) relaying protocol and give a new result on the joint probability density function of the first, second, horizontal ellipsis , kth ($k=1, 2,\ldots $k=1,2, horizontal ellipsis ) largest eigenvalues of the complex Wishart matrices $\boldsymbol{H} \boldsymbol{H}H$HHH, where $\boldsymbol{H}$H is an matrix whose elements are independent identically distributed complex Gaussian variables with zero mean and unit variance. They then utilise this result to characterise the legitimate outage probability and the diversity order of the proposed DF protocol. The case where the relay works in the amplify-and-forward (AF) mode is considered at last, and the diversity order of the proposed AF protocol is given.

Non-orthogonal multiple access (NOMA) is a promising multiple access scheme to enhance the spectrum efficiency of fifth generation networks. In this study, the authors study a downlink multiple-input single-output (MISO) system combined with NOMA, where a single beamforming (BF) vector is shared by a group of users. A joint BF and power allocation algorithm is proposed to maximise the sum rate of the users with better channel conditions while guaranteeing the quality of service at the user with poor channel conditions. The formulated problem for sum rate maximisation can be shown as non-deterministic Polynomial-time-hard, and therefore an effective solution based on branch and bound (BB) techniques is proposed. Numerical results are provided to verify that the proposed NOMA-BF system with the BB algorithm improves the sum capacity significantly compared with the NOMA-BF system with zero forcing.

This paper considers the secure communication issue in an amplify-and-forward relaying cooperative network, in which the energy harvesting relay is powered by radio-frequency signals from the source. Based on the two different energy harvesting strategies, that is, power splitting-based relaying protocols and time switching-based relaying protocols, the authors formulate the problem for maximising the secrecy rate subject to the transmitted power constraint by taking into account the power splitting coefficient and time switching coefficient. The authors solve these problems by applying the semi-definite programming relaxation approach and 1D optimisation over the coefficients of energy harvesting protocols. The simulation results are presented to verify the effectiveness of these solutions proposed in this study.

In the present study, the authors investigate robust secrecy rate optimisation problems for a multiple-inputsingle-output secrecy channel with multiple device-to-device (D2D) communications. The D2D communication nodes access this secrecy channel by sharing the same spectrum, and help to improve the secrecy communications by confusing the eavesdroppers. In return, the legitimate transmitter ensures that the D2D communication nodes achieve their required rates. In addition, it is assumed that the legitimate transmitter has imperfect channel state information of different nodes. For this secrecy network, the authors solve two robust secrecy rate optimisation problems: (a) robust power minimisation problem, subject to the probability based secrecy rate and the D2D transmission rate constraints; (b) robust secrecy rate maximisation problem with the transmit power, the probabilistic based secrecy rate and the D2D transmission rate constraints. Owing to the non-convexity of robust beamforming design based on two statistical channel uncertainty models, the authors present two conservative approximation approaches based on 'Bernsteintype' inequality and 'S-Procedure' to solve these robust optimisation problems. Simulation results are provided to validate the performance of these two conservative approximation methods, where it is shown that 'Bernstein-type' inequality based approach outperforms the 'S-Procedure' approach in terms of achievable secrecy rates.

This study focuses on the design of downlink transmission protocols in multi-cell multi-user mobile networks, where co-channel interference has been recognised as a challenging issue particularly for the users close to the boundary of cells. The key idea of this study is to jointly apply interference alignment (IA) and pre-cancellation to the addressed scenario, where the former technique can effectively increase the overall system throughput and the latter can significantly boost the diversity gains and reception reliability. To ensure the diversity gains can be achieved with zero-forcing IA, a precoder optimisation scheme is proposed based on the well-known iterative interference leakage minimisation scheme. Both analytic and numerical results have been developed to show the capacity and diversity gains obtained by using the proposed scheme. Besides, the computational complexity of the proposed scheme and the effects of imperfect channel state information on the performance are studied as well.

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.

Network coding can deliver multiple data streams simultaneously and make full use of broadcast nature of wireless channels. The authors propose two diversity-enabled network-coding (NC) schemes to optimise wireless uplink scheduling. The existing scheduling protocols normally have to allow the users with relatively low channel gains to transmit, and it can maintain fairness but reduce congregated throughput. The main idea of the proposed scheme is to always schedule users with the best channel condition, while the use of NC encourages the scheduled users to help others which have not been served previously. Delay and capacity performance for different network coded scheduling schemes are analysed. Round-robin and pure opportunistic scheduling are evaluated for performance comparison. In order to show the effectiveness of the proposed schemes, NC schedulers are applied to a time division duplexing code division multiple access wireless cellular networks. System-level simulation was carried out based on the third generation partnership project specifications. Per-sector average throughput and cumulative distribution function of user average throughput are adopted as the performance metrics. Analytical and simulation results show that the proposed NC schedulers can achieve a better tradeoff between fairness and throughput than those without NC.

This study deals with the localisation of all nodes in a network, also called as network localisation, based on pairwise distance measurements. The case of a fully connected network is considered, where ‘fully connected’ refers to that within the whole network every pair of nodes directly connect to each other, thus their pairwise distance can be measured and available. For the localisation of such a network, the multi-dimensional scaling (MDS) algorithm can provide a relative localisation solution, but only a coarse solution when there are measurement errors. To bridge the gap in the localisation performance between the MDS-based and optimum network localisation, the authors propose an efficient subsequent refinement, that is, the iterative least square (LS)/weighted least square (WLS) refinement for the widely existing independent zero-mean Gaussian measurement errors. Analysis and simulation study show that with sufficiently small measurement errors the proposed improved network localisation scheme can achieve, in very limited iterations, the LS/WLS solution, which exhibits the localisation performance the same as the Cramer–Rao lower bound. The authors also extend the proposed refinement to the absolute localisation case with sufficient position-known anchors that are fully and directly connected to all sensors of the network.

The authors consider a cooperative communication system where multiple sources transmit data to multiple destinations, being helped by multiple half-duplex relay nodes: relay nodes perform `amplify and forward` as their cooperative technique, with the use of complex weights for forwarding; complex forwarding weights are optimised, based on the obtained channel information, to improve data transmission performance, especially its corresponding diversity order. In particular, the authors investigate the constrained optimisation on the forwarding weights, under the two constraints of interference-free data reception at all destinations and total forwarding power at all relays, to (i) maximise the signal-to-noise ratio (SNR) of any given source`destination pair; (ii) optimise (here maximise the minimum among) the SNRs of all source`destination pairs. Performance (especially diversity order) analysis and Monte-Carlo simulations show the achieved diversity order and the consistence between analytical and numerical results.

This study investigates the superposition-based cooperative transmission system. In this system, a key point is for the relay node to detect data transmitted from the source node. This issued was less considered in the existing literature as the channel is usually assumed to be flat fading and a priori known. In practice, however, the channel is not only a priori unknown but subject to frequency selective fading. Channel estimation is thus necessary. Of particular interest is the channel estimation at the relay node which imposes extra requirement for the system resources. The authors propose a novel turbo least-square channel estimator by exploring the superposition structure of the transmission data. The proposed channel estimator not only requires no pilot symbols but also has significantly better performance than the classic approach. The soft-in-soft-out minimum mean square error (MMSE) equaliser is also re-derived to match the superimposed data structure. Finally computer simulation results are shown to verify the proposed algorithm.

A spectrally efficient cooperative protocol for uplink wireless transmission in a centralised communication system is proposed, where each of the N users play the relaying and source roles simultaneously by using superposition (SP) modulation. The probability density function of the mutual information between SP-modulated transmitted and received signals of the cooperative uplink channels is derived. Using the high-signal-to-noise ratio (SNR) approximation of this density function, the outage probability formula of the system as well as its easily computable tight upper and lower bounds are obtained and these formulas are evaluated numerically. Numerical results show that the proposed strategy can achieve around 3 dB performance gain over comparable schemes. Furthermore, the multiplexing and diversity trade-off formula is derived to illustrate the optimal performance of the proposed protocol, which also confirms that the SP relaying transmission does not cause any loss of data rate. Moreover, performance characterisation in terms of ergodic and outage capacities are studied and numerical results show that the proposed scheme can achieve significantly larger outage capacity than direct transmission, which is similar to other cooperative schemes. The superiority of the proposed strategy is demonstrated by the fact that it can maintain almost the same ergodic capacity as the direct transmission, whereas the ergodic capacity of other cooperative schemes would be much worse.

The authors propose a three-node full diversity cooperative protocol, which allows the retransmission of all symbols. By allowing multiple nodes to transmit simultaneously, relaying transmission only consumes limited bandwidth resource. To facilitate the performance analysis of the proposed cooperative protocol, the lower and upper bounds of the outage probability are first developed, and then the high signal-to-noise ratio behaviour is studied. Our analytical results show that the proposed strategy can achieve full diversity. To achieve the performance gain promised by the cooperative diversity, at the relays decode-and-forward strategy is adopted and an iterative soft-interference-cancellation minimum mean-squared error equaliser is developed. The simulation results compare the bit-error-rate performance of the proposed protocol with the non-cooperative scheme and the scheme presented by Azarian et al. (2005).