加权分数傅里叶辅助的上行NOMA隐蔽通信

In this paper, we investigate the covert communications in uplink nonorthogonal multiple access (NOMA) systems with the random power allocation and weighted fractional Fourier transform(WFRFT) scheme to confront a two-phase detector. In the NOMA system, a covert user(CU) and a reliable user(RU) transmit messages to Bob in the presence of a warden (Willie) who tries to detect the CU's transmission behavior. A two-phase detector, i.e., energy detection and similarity detection phases, is designed to improve the detection performance. The similarity detection phase provides a priori probability for the energy detection phase and reduces the detection error probability. In addition, corresponding to the RU and CU, a random power allocation and WFRFT are proposed to cover the CU's transmissions. The expected minimum detection error probability (EMDEP) and connection outage probabilities (COPs) at the RU and CU are derived in closed-form expressions for the proposed scheme. To optimize the power allocation of the RU, the maximum expected covert rate(ECR) is analyzed under covertness and reliability constraints. Numerical results show that the proposed two-phase detector has a lower EMDEP, and that the random power allocation and WFRFT scheme improve the covertness performance.