Secrecy Performance Analysis for SWIPT Near Users with Untrusted Far Users in NOMA Networks

Simultaneous wireless information and power transfer (SWIPT) can let energy-limited users harvest energy and receive information using the same radio frequency signals, while non-orthogonal multiple access (NOMA) can improve network throughput by allowing multiple users to access the same spectrum band at the same time. In this paper, the physical layer security in SWIPT NOMA networks with untrusted users is investigated. Specifically, a NOMA network with a near user and a far user is considered, where the near user is assumed to adopt SWIPT for energy harvesting and information decoding, and the far user is assumed to be a potential eavesdropper for the near user. Two secrecy performance metrics for the near user, namely non-zero secrecy rate probability and ergodic secrecy rate are theoretically analyzed. It is proved by theoretical analysis that adopting SWIPT at the near user degrades its secrecy performance in terms of non-zero secrecy rate probability and ergodic secrecy rate. It is also proved by theoretical analysis that the NOMA power allocation coefficient has no impact on the near user non-zero secrecy rate probability, while allocating more power to the near user can improve the ergodic secrecy rate. The theoretical results are corroborated by conducting numerical simulations.