Abstract
This paper studies physical layer security for two-way relay non-orthogonal multiple access systems. Specifically, by employing a decode-and-forward relay and considering both the maximum-ratio combining and selection combining schemes at the eavesdropper, instantaneous end-to-end signal-to-interference-plus-noise ratios are first formulated and approximated for the interference-limited scenario. Then analytical expressions for the secrecy outage probability and the intercept probability are obtained. Moreover, we analyze the effective secrecy diversity order and demonstrate that the eavesdropper severely degrades the secrecy performance, and even reduces the diversity order to zero. Numerical and simulation results verify our derivations and reveal that, due to information leakage in the first time phase, there exists a secrecy performance floor. It is also demonstrated that the secrecy outage probability performance degrades when the eavesdropper is closer to either of the users. Between the two combining schemes, the results demonstrate that the secrecy outage probability performance is better when the selection combining scheme is employed by the eavesdropper.
Highlights
With the increasing demand for wireless communications services and applications, non-orthogonal multiple access (NOMA) has attracted significant attention in 5G research community owing to its superior spectrum efficiency [1]–[3]
Closed-form expressions of the secrecy outage probability (SOP) are derived for both cases of having maximum ratio combining (MRC) and selection combining (SC) schemes employed at the eavesdropper
The intercept probability is investigated as a special case of the SOP
Summary
With the increasing demand for wireless communications services and applications, non-orthogonal multiple access (NOMA) has attracted significant attention in 5G research community owing to its superior spectrum efficiency [1]–[3]. In contrast to the traditional orthogonal multiple access, NOMA exploits the power domain in addition to the time or frequency domain [4] to manage signal transmissions by multiple users. In NOMA, users are allocated different powers depending on the channel conditions, and the transmitter employs superposition coding to transmit the combined signal to users [5]. NOMA has been introduced in cooperative relaying which offers enhanced coverage, throughput, and reliability [8], [9], especially for the users with bad channel conditions [11]–[21]. To achieve the spatial diversity, communication between
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