To accommodate the stringent requirements of enhanced coverage quality and improved spectral efficiency, simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-aided communication has been perceived as an interesting research topic. This paper investigates a downlink STAR-RIS-aided non-orthogonal multiple access (NOMA) system, where a STAR-RIS is deployed to enhance the transmission qualities between users and a multiple-antenna base station (BS). The considered STAR-RIS utilizes the energy splitting (ES) protocol to serve a pair of NOMA users located at both sides of STAR-RIS. Based on the ES protocol, each reconfigurable element can operate in the modes of transmission and reflection simultaneously. In an effort to characterize the secrecy performance, we first derive the closed-form expressions of secrecy outage probability (SOP) for STAR-RIS-aided NOMA system. Then, the asymptotic performance of the derived SOP is analyzed. For gleaning further insights, secrecy diversity order (SDO) is derived according to the asymptotic approximation in the high signal-to-noise ratio and main-to-eavesdropper ratio regimes. As a further advance, the system parameters are optimized to minimize the SOP of the system. Our analytical results demonstrate that the multiple-antenna BS has almost no impact on SDO for STAR-RIS-aided NOMA system. In simulations, it is demonstrated that the theoretical results match with the simulation results very well and the SOP of STAR-RIS-aided NOMA is less than that of conventional orthogonal multiple access (OMA) system obviously.
Simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) is capable of achieving full coverage area compared with conventional reflection-only reconfigurable intelligent surface (RIS). Motivated by this observation, the secrecy performance of STAR-RIS-assisted non-orthogonal multiple access (NOMA) systems over Rayleigh fading channels is investigated, where the incident signals sent by base station are reflected and transmitted to a pair of NOMA users in the presence of two eavesdroppers. To evaluate the secrecy performance of STAR-RIS-NOMA systems, the exact and asymptotic expressions of secrecy outage probability (SOP) and secrecy diversity order are derived, in which the correlation of the channels between the elements of each column on STAR-RIS is taken into consideration. As a benchmark, the secrecy performance of the STAR-RIS strategy for orthogonal multiple access (OMA) system is also analysed. Numerical results are provided to verify the effectiveness of the analytical results and demonstrate that: The SOP of STAR-RIS-NOMA outperforms that of STAR-RIS-assisted OMA and conventional cooperative communication systems. Furthermore, the scheme proposed here can obtain full order of secrecy diversity.
Secure transmission is essential for future non-orthogonal multiple access (NOMA) system. This paper investigates relay-antenna selection (RAS) to enhance physical-layer security (PLS) of cooperative NOMA system in the presence of an eavesdropper, where multiple antennas are deployed at the relays, the users, and the eavesdropper. In order to reduce expense on radio frequency (RF) chains, selection combining (SC) is employed at both the relays and the users, whilst the eavesdropper employs either maximal-ratio combining (MRC) or selection combining (SC) to process the received signals. Under the condition that the channel state information (CSI) of the eavesdropping channel is available or unavailable, two effective relay-antenna selection schemes are proposed. Additionally, the closed-form expressions of secrecy outage probability (SOP) are derived for the proposed relay-antenna selection schemes. In order to gain more deep insights on the derived results, the asymptotic performance of the derived SOP is analyzed. In simulations, it is demonstrated that the theoretical results match well with the simulation results and the SOP of the proposed schemes is less than that of the conventional orthogonal multiple access (OMA) scheme obviously.
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