Non-orthogonal multiple access (NOMA) has come to the fore as a spectral-efficient technique for fifth-generation and beyond communication networks. We consider the downlink of a NOMA system with untrusted users. In order to consider a more realistic scenario, imperfect successive interference cancellation is assumed at the receivers during the decoding process. Since pair outage probability (POP) ensures a minimum rate guarantee to each user, it behaves as a measure of the quality of service for the pair of users. With the objective of designing a reliable communication protocol, we derive the closed-form expression of POP. Further, we find the optimal power allocation that minimizes the POP. Lastly, numerical results have been presented which validate the exactness of the analysis, and reveal the effect of various key parameters on achieved pair outage performance. In addition, we benchmark optimal power allocation against equal and fixed power allocations with respect to POP. The results indicate that optimal power allocation results in improved communication reliability.

翻译：非正交多址接入作为第五代及未来通信网络中提高频谱效率的关键技术，已引起广泛关注。本文考虑下行链路中具有不可信用户的非正交多址系统。为更贴近实际场景，假设接收机在解码过程中存在不完美的串行干扰消除。由于配对中断概率确保每个用户获得最低速率保障，该指标可衡量用户对的服务质量。为设计可靠通信协议，我们推导了配对中断概率的闭式表达式，并进一步求得使配对中断概率最小化的最优功率分配方案。最后通过数值结果验证了理论分析的精确性，揭示了各关键参数对配对中断性能的影响规律。此外，我们以配对中断概率为基准，将最优功率分配与等功率分配及固定功率分配进行对比。结果表明，最优功率分配可显著提升通信可靠性。