Exploiting Opportunistic Scheduling Schemes to Improve Physical-Layer Security in MU-MISO NOMA Systems

This paper studies opportunistic scheduling schemes to enhance the secrecy performance in multi-user multiple-input single-output (MU-MISO) non-orthogonal multiple access (NOMA) systems, in which a multiple antenna base station (BS) serves multiple single-antenna cell-center and cell-edge users in the presence of multiple single-antenna eavesdroppers. In order to improve the secrecy performance, we propose an opportunistic scheduling scheme, called the proposed antenna/users selection (PAUS) scheme. Additionally, we consider two practical eavesdropping scenarios, namely colluding and non-colluding eavesdroppers. We derive the exact closed-form expression for secrecy outage probability (SOP) and asymptotic SOP of the PAUS scheme under different eavesdropping scenarios. We provide the 2D golden section search-based algorithm to find the optimal values of the transmit power and power allocation coefficient that minimize the system secrecy outage performance. The developed analyses are verified by Monte Carlo simulations. The numerical results show that the PAUS scheme improves secrecy performances compared to the benchmark random antenna/users selection (RAUS) scheme. To provide further insights into the system secure performance, the effects of transmit signal-to-noise ratio (SNR), power allocation coefficient, the number of transmit antennas at the BS, number of cell-center and cell-edge users, and the eavesdropper placements are extensively evaluated and discussed.