SIC-Based Secrecy Performance in Uplink NOMA Multi-Eavesdropper Wiretap Channels

Abstract

A new paradigm of uplink non-orthogonal multiple access along with multiple eavesdroppers to achieve secrecy transmission is studied in this paper. We investigate the secrecy performance of a single transmitter in both non-colluding- and colluding-eavesdropper scenarios on basis of two decoding methods at the legitimate receiver, zero-forcing (ZF), and minimum mean-square error (MMSE), jointly with successive interference cancellation (SIC). We first evaluate the secrecy performance in three metrics: secrecy outage probability, effective secrecy throughput (EST), and positive secrecy capacity probability. Analytical results show: 1) the collusion of eavesdroppers deteriorates the secrecy performance, which is an affine combination of those regarding each single eavesdropper and 2) MMSE-SIC outperforms ZF-SIC, while the performance gap can be overcome via reducing the number of interferers, or increasing signal-to-noise ratio (SNR), or enhancing the spatial diversity gain. We then analyze the asymptotic behaviors of the secrecy performance, revealing the high-SNR secrecy performance for both ZF-SIC and MMSE-SIC approaches to the same result which is location-dependent only. Furthermore, we study the problem of optimal power allocation to each transmitter subject to limited total transmit power. An interesting solution to this problem is demonstrated with the aid of numerical results. Finally, we propose an SIC order scheduling scheme which is conjectured to be optimal in achieving total maximum EST in a high SNR regime.