Non-orthogonal multiple access (NOMA) has come to the fore as a spectrally efficient technique for fifth-generation networks and beyond. At the same time, NOMA faces severe security issues in the presence of untrusted users due to successive interference cancellation (SIC)-based decoding at receivers. In this paper, to make the system model more realistic, we consider the impact of imperfect SIC during the decoding process. Assuming the downlink mode, we focus on designing a secure NOMA communication protocol for the considered system model with two untrusted users. In this regard, we obtain the power allocation bounds to achieve a positive secrecy rate for both near and far users. Analytical expressions of secrecy outage probability (SOP) for both users are derived to analyze secrecy performance. Closed-form approximations of SOPs are also provided to gain analytical insights. Lastly, numerical results have been presented, which validate the exactness of the analysis and reveal the effect of various key parameters on achieved secrecy performance.

翻译：非正交多址接入（NOMA）作为第五代及后续网络中一种频谱高效的技术已脱颖而出。然而，由于接收端基于连续干扰消除（SIC）的解码过程，NOMA在面临非信任用户时存在严重的安全问题。为使系统模型更贴近实际，本文考虑了解码过程中不完美SIC的影响。基于下行链路模式，我们致力于为所考虑的双非信任用户系统模型设计一种安全的NOMA通信协议。为此，我们推导了功率分配界限，以确保近端和远端用户均能达到正保密速率。针对两个用户，分别推导了保密中断概率（SOP）的解析表达式，以分析其保密性能。同时提供了SOP的闭合近似表达式，以获取分析洞察。最后，通过数值结果验证了分析的准确性，并揭示了各关键参数对保密性能的影响。