This letter considers covert communication in downlink NOMA systems, where a public action (communication from Alice to the NOMA-weak user U1) is used to provide a cover for a secret action (communication from Alice to the NOMA-strong user U2). To aid system design, we first derive the detection error probability (DEP) at Willie and the connect outage probabilities (COPs) at U1 and U2. Then, considering the covertness and outage constraints, the optimization of power allocation factor ratio between U1 and U2 is tackled in order to maximize the effective covert rate. The results demonstrate that the minimum DEP is the power allocation factor for U1. In addition, for a given covert constraint, relaxing the outage constraint will effectively increase the maximum effective covert rate within a certain range of the maximum transmit power of U1.

中文翻译：

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具有随机发射功率的下行链路 NOMA 系统中的隐蔽通信

这封信考虑了下行链路 NOMA 系统中的隐蔽通信，其中公共动作（从 Alice 到 NOMA 弱用户 U1 的通信）用于为秘密动作（从 Alice 到 NOMA 强用户 U2 的通信）提供掩护。为了帮助系统设计，我们首先推导出 Willie 的检测错误概率 (DEP) 和 U1 和 U2 的连接中断概率 (COP)。然后，考虑隐蔽性和中断约束，优化U1和U2之间的功率分配因子比以最大化有效隐蔽率。结果表明，最小 DEP 是 U1 的功率分配因子。此外，对于给定的隐蔽约束，放宽中断约束将在U1的最大发射功率一定范围内有效提高最大有效隐蔽速率。