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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This paper quantifies the impact of correlated fading on secure communication of multiple amplify-and-forward (AF) relaying networks. In such a network, the base station (BS) is equipped with multiple antennas and communicates with the destination through multiple AF relays, while the message from the relays can be overheard by an eavesdropper. We focus on the practical communication scenario, where the main and eavesdropper's channels are correlated. In order to enhance the transmission security, transmit antenna selection (TAS) is performed at the BS, and the best relay is chosen according to the full or partial relay selection criterion, which relies on the dualhop relay channels or the second-hop relay channels, respectively. For these criteria, we study the impact of correlated fading on the network secrecy performance, by deriving an analytical approximation for the secrecy outage probability (SOP) and an asymptotic expression for the high main-to-eavesdropper ratio (MER). From these results, it is concluded that the channel correlation is always beneficial to the secrecy performance of full relay selection. However, it deteriorates the secrecy performance if partial relay selection is used, when the number of antennas at the BS is less than the number of relays.

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.

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Abstract-We present two physical layer secure transmission schemes for multi-user multi-relay networks, where the communication from M users to the base station is assisted by direct links and by N decode-and-forward relays. In this network, we consider that a passive eavesdropper exists to overhear the transmitted information, which entails exploiting the advantages of both direct and relay links for physical layer security enhancement. To fulfill this requirement, we investigate two criteria for user and relay selection and examine the achievable secrecy performance. Criterion I performs a joint user and relay selection, while Criterion II performs separate user and relay selections, with a lower implementation complexity. We derive a tight lower bound on the secrecy outage probability for Criterion I and an accurate analytical expression for the secrecy outage probability for Criterion II. We further derive the asymptotic secrecy outage probabilities at high transmit signal-to-noise ratios and high main-to-eavesdropper ratios for both criteria. We demonstrate that the secrecy diversity order is min (M N, M + N ) for Criterion I, and N for Criterion II. Finally, we present numerical and simulation results to validate the proposed analysis, and show the occurrence condition of the secrecy outage probability floor.Index Terms-Physical layer security, multi-user multi-relay networks, direct links, secrecy outage probability, secrecy diversity order.

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