Security Transmission Scheme of NOMA Systems With an Untrusted Near User

Aiming at the serious threat of untrusted users to the system security in the non-orthogonal multiple access (NOMA) downlink system, where an untrusted near user assists a far user to receive data, a cooperative transmission scheme is proposed to improve the physical layer security (PLS) for our presented wiretap NOMA system model. The cooperative transmission scheme is implemented in two communication phases. In the first phase, the reverse power allocation method is used, in which the higher power is allocated to the data transmitted to the near user, and the far user is introduced as a cooperative jammer to send the artificial noise (AN), so as to improve the security when the base station sends data. Considering the situation that the near user decodes its data, two information forwarding processes are designed in the second phase. If the near user can decode its data, it will employ the amplify-and-forward (AF) protocol to forward the remaining signal after removing its own data. Otherwise, two relay selection (RS) strategies are explored to improve reliability of the considered system. In this case, the selected relay adopts the physical-layer network coding (PNC) technique, and the AN sent by the far user is regarded as the key to participating in the bit-level exclusive-or (XOR) operation, which ensures the absolute security of the data transmitted to the far user. The exact closed-form expressions of the outage probability (OP) and intercept probability (IP) are derived respectively, and the approximate results are also given under high signal-to-noise ratio (SNR). Furthermore, the influence of relevant system parameters is discussed concretely. The numerical simulation results indicate that substantial performance gain is achieved over orthogonal multiple access (OMA) and other schemes, and the two RS strategies involved in this paper have the same reliability.