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In this paper, the secrecy performance in a MIMO cognitive radio (CR) relay network with energy harvesting (EH) and transmit antenna selection/maximal ratio combining (TAS/MRC) is invstigated, where the DF relaying protocol and multiple colluding passive eavesdroppers are considered. To improve the security of wireless transmission, two antenna selection schemes are proposed, namely, the optimal transmit antenna selection (OTAS) scheme and suboptimal transmit antenna selection (STAS) scheme. For the purpose of comparison, the space‐time transmission (STT) scheme is introduced as a baseline. The exact and asymptotic closed‐form secrecy outage probability (SOP) expressions for OTAS, STAS and STT schemes are derived over Rayleigh fading channels. An extension of the TAS framework to an artificial noise (AN) aided MIMO network is further presented and an AN aided transmit antenna selection (AN‐TAS) scheme is proposed, in which the unselected antennas at R are used to emit AN for interfering with Es. Numerical results show that the OTAS and STAS schemes perform better than STT scheme in terms of SOP. Meanwhile, the SOP of AN‐TAS scheme is much smaller than that of OTAS, STAS and STT schemes in the high SNR region, indicating the benefit of applying AN in MIMO network. [ABSTRACT FROM AUTHOR]

This paper investigates the relay selection problem and proposes a three‐stage relay selection strategy with power allocation (TRSPA) for a spectrum‐sensing‐based full‐duplex (FD) user relaying cooperative non‐orthogonal multiple access (CNOMA) scheme. Uniformly‐distributed strong user relays in the investigated scheme help a weak user communicates with the base station in an efficient and reliable way. The proposed TRSPA strategy maximizes the transmission data rate of the selected relay while ensuring successful transmissions for the weak user by precisely narrowing down relay candidates step‐by‐step and dynamically allocating optimal power coefficients. Exact and asymptotic outage probabilities and ergodic rates are worked out. Accordingly, diversity orders and spatial multiplexing gains are derived. We further exploit the impact of self‐interference (SI) on TRSPA for FD‐CNOMA and then compare its performance with TRSPA applied in other relaying modes, that is half‐duplex and orthogonal multiple access. Finally, simulation results reveal that: (i) theoretical derivation results are correct; (ii) TRSPA always outperforms other relay selection strategies in terms of outage probability and ergodic rate; and (iii) TRSPA for FD‐CNOMA in a real‐world scenario achieves better performance than other relaying modes in spite of the adverse effect of SI in FD mode. [ABSTRACT FROM AUTHOR]

In this study, the authors consider a two-hop network, where multiple source nodes (SNs) transmit to a destination node (DN) with the aid of a relay node (RN). The RN is equipped with a buffer, which is capable of storing multiple frames received from the SNs. During each time slot, the proposed protocol activates either the SN-RN hop or the RN-DN hop, depending on the channel quality of each hop and the buffer state at the RN. To optimise the hop activation for the network, they design a hop quality metric and propose a multi-user buffer-aided-relaying uplink (MUBR- UL) protocol, with the aid of the minimum signal-to-noise power ratio approximation. The benefits of the proposed protocol are analysed in terms of the end-to-end (e2e) outage probability and the e2e transmission delay. Then, the optimal power allocation is proposed for minimising the e2e outage probability under the total power constraint. The results indicate that the outage performance is significantly improved when the proposed power allocation is utilised in the MU-BR-UL protocol. [ABSTRACT FROM AUTHOR]

We have considered a dual-hop wireless network consisting of a decode-and-forward (DF) relay in presence of an eavesdropper. Relay can correctly decode and retransmit only if the received signal-to-noise ratio (SNR) meets a particular threshold. The eavesdropper can tap signals both from the relay and the source. The performance of the maximal ratio combining (MRC) and selection combining (SC) diversity at the eavesdropper is investigated. Whether knowledge of channel state information (CSI) at the transmitter is available or not is considered for the performance evaluation in secrecy outage probability and ergodic secrecy rate. Asymptotic analyses of performances are provided. We show that direct link has a significant impact on system secrecy. It is found that secrecy performances are the best if the relay is always able to decode correctly. We also found that knowledge of CSI helps to achieve better secrecy at lower rate. It is observed that unbalance created due to constraint on either of the source-relay link average SNR or relay-destination link average SNR does not yield similar secrecy outage performance but both can limit the secrecy outage performance. On the contrary, it is found that only source-relay link quality can limit the ergodic secrecy rate performance. [ABSTRACT FROM AUTHOR]

In this study, for a network-coded two-way relay channel (TWRC), the authors propose three different relaying schemes that satisfy certain quality of service requirements at both transceivers. In these schemes, the relay decides independently whether to help each transceiver or not based on the instantaneous signal-to-noise ratios received from the transceivers. Under the assumption of error propagation, they analyse the performance of the system in terms of the end-to-end bit error rate (BER), the outage probability, the average spectral efficiency (ASE) and the diversity order. All of these performance measures depend on two thresholds employed by the relay node. By optimising these thresholds under three different criteria, they achieve three different relaying schemes: (i) the BER optimal scheme that minimises the end-to-end BER; (ii) the outage-constrained scheme that minimises the end-to end BER under an outage probability constraint; and (iii) the BER-constrained scheme that maximises the ASE under an end-to-end BER constraint. The analysis of the system shows that the BER-optimal scheme guarantees the full diversity gain at both transceivers; the outage-constrained scheme results in a bidirectional relaying scheme with a constant outage probability; and the BER-constrained scheme makes use of spectrum as efficiently as possible while maintaining the target BER. [ABSTRACT FROM AUTHOR]

In this study, the authors consider two multiple-antenna transceivers exchanging information through a relayassisted network using a single-carrier communication scheme. The authors assume that the propagation delay in different relaying path is negligible and the relay nodes are synchronous. As a result, the end-to-end multipath channel is not frequency-selective (time-dispersive) and hence, the successive arriving signals at the transceivers do not interfere with each other. Otherwise, inter-symbol-interference (ISI) will be inevitable and cyclic insertion and removal matrices will be required to combat ISI. In such a two-way network, the relay nodes harvest energy from the surrounding environment and utilise this energy toforward their received messages using a harvest-then-forward protocol. For different receiver diversity combining techniques, the authors design an optimal relay beamforming to maximise the quality of the received signals at the transceivers subject to the energy casualty constraint at the relay nodes (the energy consumed for transmission of each block cannot exceed the accumulative harvested energy). For each diversity combining technique, a closed-form solution is obtained for the optimal signal-to-noise ratio (SNR) that shows how adjusting the data transmission rate of the transceivers and the amount of energy harvested at the relays affects the received SNR. [ABSTRACT FROM AUTHOR]

In this study a new approach tofilter-and-forward (FF) relay design in an overlay cognitive radio (CR) network with frequency selective channels is proposed. In this system, to implement two-way relaying scheme, a secondary multi-antenna node acts as a relay and helps two primary users (PUs) communicate with each other and in return the secondary node superposes its signal tofiltered signals of PUs. The authors intend to design secondary transceiver under two criteria, one from the viewpoint of improving the performance of secondary network and the other to guarantee quality of service (QoS) of primary network. The first criterion is to maximise the signal-to-interference-plusnoise ratio (SINR) of secondary receiver subject to constraints on QoS of PUs and the relay transmitted power. For the other one, the goal is to achieve max-min fairness among PUs through maximising the worse signal-to-interferenceplus- noise ratio (SINR) of PUs subject to constraints on QoS of secondary receiver and transmit power of relay. The authors employ an alternating optimisation algorithm to find the beamforming matrices and the allocated power to secondary receiver's message in these non-convex problems. Simulation results show that the proposed FF approach has better performance compared with simple AF relaying strategy. [ABSTRACT FROM AUTHOR]

In this study, the authors consider a cooperative communication framework based on one-way and two-way relays to provide secure communications for secondary users (SUs) within an orthogonal frequency-division multiple access-based underlay cognitive network. By proposing a radio resource allocation problem with the aim of maximising the secrecy sum-rate of SUs and solving it, they show that the deployment of relays is vital to achieve a non-zero secrecy sum-rate. Also, the impact of two-way relay in system performance improvement is clearly visible in comparison with one-way relay such that it can roughly double the resulting system secrecy sum-rate. The impact of different system parameters on the achievable secrecy sum-rate for both one-way and two-way relays is also investigated and compared through simulations. [ABSTRACT FROM AUTHOR]

The authors study robust beamforming and power allocation in cognitive relay networks using several relays in the secondary link. The channel state information is imperfect modelled by Gaussian random variables. First, minimisation of the total transmit power of relays and the secondary transmitter is considered with a constraint on the interference in the primary receiver. Also, a constraint is assumed on the received signal-to-interference plus noise ratio (SINR) in the secondary receiver. In the other scenario, maximisation of the SINR in the secondary receiver is investigated with a limit on the maximum transmit power of relays and the secondary transmitter. For each non-convex optimisation problem, an iterative and robust algorithm is derived. It is proved that the first proposed algorithm converges to the global optimum. Simulation results similarly show the convergence of both algorithms with a few number of iterations. The outperformance of the robust algorithms is shown compared with a non-robust design. [ABSTRACT FROM AUTHOR]