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This paper presents the secrecy performance analysis of an amplify-and-forward relay-assisted cooperative communication system in the presence of a passive external eavesdropper. In contrast to existing works that assume high signal-to-noise ratio (SNR) approximations, we have investigated exact and secrecy outage probabilities. Furthermore, we consider a more challenging scenario where the source may not be reachable to the intended user directly. But the eavesdropper can tap both the source link and the relay link. First of all, the outage probability is analyzed at the intended user as well as the eavesdropper. Next, defining the secrecy rate for the amplify-and-forward (AF) relaying system, the expression of the secrecy outage probability (SOP) and the secrecy intercept probability (SIP) have been derived, respectively. Noticing the complexity involved in the integration of SOP and SIP expressions, the closed-form expressions have been derived for asymptotic cases. Finally, the exact and asymptotic analysis has been verified by performing Monte-Carlo simulations. It is observed that the relay position should be closer to the source compared to the eavesdropper to achieve improved SOP. [ABSTRACT FROM AUTHOR]

In this letter, the ergodic performance of a non-orthogonal multiple access (NOMA) system with untrusted users and imperfect successive interference cancellation based decoding is manifested. Closed-form expressions of the ergodic rate and the ergodic secrecy rate of each user have been derived for all possible decoding orders. In addition, Taylor series expansion based approximation has been proposed to gain analytical insights and obtain optimal decoding order. Numerical simulations endorse the analytical findings, and highlight impact of various key parameters on the ergodic performance of the system. [ABSTRACT FROM AUTHOR]

Observing the significance of spectrally-efficient secure non-orthogonal multiple access (NOMA), this paper proposes a novel quality of service (QoS) aware secure NOMA protocol that maximizes secrecy fairness among untrusted users. Considering a base station (BS) and two users, a novel decoding order is designed that provides security to both users. With the objective of ensuring secrecy fairness between users, while satisfying their QoS requirements under BS transmit power budget constraint, we explore the problem of minimizing the maximum secrecy outage probability (SOP). Closed-form expression of pair outage probability (POP) and optimal power allocation (PA) minimizing POP are obtained. To analyze secrecy performance, analytical expressions of SOP for both users are derived, and individual SOP minimization problems are solved using concept of generalized-convexity. High signal-to-noise ratio approximation of SOP and asymptotically optimized solution minimizing this approximation is also found. Furthermore, global-optimal solution from secrecy fairness standpoint is obtained at low computational complexity, and tight approximation is derived to get analytical insights. Numerical results present useful insights on globally optimized PA which ensure secrecy fairness and provide performance gain of about 55.12%, 69.30%, and 19.11%, respectively, compared to fixed PA and individual users’ optimal PAs. Finally, a tradeoff between secrecy fairness performance and QoS demands is presented. [ABSTRACT FROM AUTHOR]

This letter investigates the utility of a trusted decode-and-forward relay in OFDMA-based secure communication system with untrusted users. For deciding on the use of relay, we first present optimal subcarrier allocation policies for direct communication (DC) and relayed communication (RC). Next, we identify exclusive RC mode, exclusive DC mode, and mixed (RDC) mode subcarriers which can support both the modes. In RDC mode, we present optimal mode selection policy and a suboptimal strategy independent of power allocation which is asymptotically optimal at both low and high SNRs. Via numerical results we present insights on relay utility regions. [ABSTRACT FROM PUBLISHER]