Non-orthogonal multiple access (NOMA) has been recognized as one of the key enabling technologies for future generation wireless networks. Sharing the same time-frequency resource among users imposes secrecy challenges in NOMA in the presence of untrusted users. This paper characterizes the impact of user-pair selection on the secrecy performance of an untrusted NOMA system. In this regard, an optimization problem is formulated to maximize the secrecy rate of the strong user while satisfying the quality of service (QoS) demands of the user with poorer channel conditions. To solve this problem, we first obtain optimal power allocation in a two-user NOMA system, and then investigate the user-pair selection problem in a more generalized four user NOMA system. Extensive performance evaluations are conducted to validate the accuracy of the proposed results and present valuable insights on the impact of various system parameters on the secrecy performance of the NOMA communication system.

翻译：非正交多址接入（NOMA）已被公认为未来无线网络的关键使能技术之一。在非可信用户存在的场景下，共享同一时频资源给NOMA带来了保密性挑战。本文刻画了用户对选择对非可信NOMA系统保密性能的影响。为此，我们构建了一个优化问题，在满足信道条件较差用户的服务质量（QoS）需求的同时，最大化强用户的保密速率。为解决该问题，我们首先推导了双用户NOMA系统的最优功率分配方案，进而研究了更普适的四用户NOMA系统中的用户对选择问题。通过广泛的性能评估，验证了所提出结果的准确性，并深入揭示了不同系统参数对NOMA通信系统保密性能的影响。