Non-orthogonal multiple access (NOMA) is an essential enabling technology for the fifth generation (5G) wireless networks to meet the heterogeneous demands on low latency, high reliability, massive connectivity, improved fairness, and high throughput. The key idea behind NOMA is to serve multiple users in the same resource block, such as a time slot, subcarrier, or spreading code. The NOMA principle is a general framework, and several recently proposed 5G multiple access schemes can be viewed as special cases. This survey provides an overview of the latest NOMA research and innovations as well as their applications. Thereby, the papers published in this special issue are put into the content of the existing literature. Future research challenges regarding NOMA in 5G and beyond are also discussed.Z. Ding is also with the School of Computing and Communications, Lancaster University, Lancaster, UK (email: z.ding@lancaster.ac.uk). X. Lei is with the Institute

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Abstract-This paper proposes relay selection to increase the physical layer security in multiuser cooperative relay networks with multiple amplify-and-forward relays, in the presence of multiple eavesdroppers. To strengthen the network security against eavesdropping attack, we present three criteria to select the best relay and user pair. Specifically, criteria I and II study the received signal-to-noise ratio (SNR) at the receivers, and perform the selection by maximizing the SNR ratio of the user to the eavesdroppers. To this end, criterion I relies on both the main and eavesdropper links, while criterion II relies on the main links only. Criterion III is the standard max-min selection criterion, which maximizes the minimum of the dual-hop channel gains of main links. For the three selection criteria, we examine the system secrecy performance by deriving the analytical expressions for the secrecy outage probability. We also derive the asymptotic analysis for the secrecy outage probability with high main-to-eavesdropper ratio. From the asymptotic analysis, an interesting observation is reached: for each criterion, the system diversity order is equivalent to the number of relays regardless of the number of users and eavesdroppers.Index Terms-Multiuser communications, multi-relay cooperative networks, multiple eavesdroppers, physical layer security, secrecy outage probability.

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In this paper, we study the impact of correlated fading channels on multiple secure decode-and-forward (DF) relaying with outdated relay selection, where the information transmission assisted by the N DF relays from the source to the destination can be overheard by the eavesdropper in the network. The eavesdropping channels are correlated with the main channels, which affects the network security. To enhance the network security, one best relay is chosen to assist the secure transmission, which is however maybe outdated in time-varying channel environments. The impact of both channel correlation and outdated relay selection on the secrecy performance is studied by deriving the analytical expression of the secrecy outage probability (SOP). The asymptotic SOP is also provided with high main-to-eavesdropper ratio (MER). From the asymptotic SOP, we find that only the outdated degree of relay selection affects the network secrecy diversity order, but the channel correlation does not. Moreover, it is interesting to find that the channel correlation is beneficial to the transmission security in the high MER regime.

Abstract-The performance of two-way amplify-and-forward (AF) relaying networks over independently but not necessarily identically distributed (i.n.i.d.) Nakagami-m fading channels, with integer and integer plus one-half values of fading parameter m, is studied. Closed-form expressions for the cumulative distribution function (CDF), probability density function (PDF), and moment generating function (MGF) of the end-to-end signalto-noise ratio (SNR) are presented. Utilizing these results, we analyze the performance of two-way AF relaying system in terms of outage probability, average symbol error rate (SER), and average sum-rate. Simulations are performed to verify the correctness of our theoretical analysis.Index Terms-Two-way relaying network, outage probability, average symbol error rate (SER), average sum-rate, Nakagami-m fading.