Physical-layer security for cellular multiuser two-way relaying networks with single and multiple decode-and-forward relays

We investigate the secrecy performance of cellular multiuser two-way relay networks, where the bidirectional information exchange between a multiantenna base station (BS), and several users can be accomplished with the assistance of decode-and-forward relay(s) in the presence of a multiantenna passive eavesdropper. We employ low-complexity antenna selection strategy at the BS coupled with user scheduling for maximizing the end-to-end signal-to-noise ratios (SNRs). To enhance the wiretap probability, the eavesdropper applies a maximal ratio combining technique to combine the received signals. Under such considerations, in the single-relay scenario, we deduce the exact closed-form secrecy outage probability (SOP) expression underRayleigh fading channels. For more insights into the system secrecy diversity order, we derive the closed-form asymptotic SOP expressions at high SNR based on the quality of the main and wiretap links. In the multirelay scenario, we formulate an opportunistic relay selection strategy, followed by its impact on the SOPperformance in the presence of Rayleigh channels. To show the impact of multiple relays on the secrecy diversity order, we derive the asymptotic SOP expressions in the high SNR regime depending on the quality of the main and wiretap links. It is shown from the asymptotic SOP expressions under both single-relay and multirelay scenarios that the secrecy diversity order reduces to zero in the high SNR regime, regardless of number of antennas, number of users, and number of relays. Our results also reveal that the SOP performance diminishes with an increased number of eavesdropper antennas. Numerical and simulation results verify our derived analytical findings.