Physical layer security based on NOMA and AJ for MISOSE channels with an untrusted relay

Abstract

Physical layer security based on non-orthogonal multiple access (NOMA) and artificial jamming (AJ) is considered in a one-way amplify-and-forward (AF) relay network with an untrusted relay. In the first phase of the two-phase operation of the untrusted AF relay network, a source node transmits the first information and AJ symbols, and the destination decodes the AJ symbol first and waits for the relayed replica of the first information symbol, which is forwarded by the untrusted relay. In the second phase, the source sends the second information symbol and the relay amplifies and forwards the received signal in the first phase with a fixed gain. In such an operation scenario, an untrusted relay tries to eavesdrop on the information signal while relaying the received signal properly. To decode the information symbol, the destination cancels out the AJ signal and then decodes the first information symbol by combining the signals received during the two phases. Finally, the second information symbol is decoded with the second-phase signal after cancelling out all the other symbols, i.e., the first information and AJ symbols. The eavesdropper and relay try to eavesdrop on the information signal with all possible orders of interference cancellation, i.e., NOMA receive processing. In this study, an ergodic secrecy rate is derived according to the operation scenario during the two phases. The behaviours of the ergodic secrecy rate with respect to power allocation to two information symbols as well as an AJ symbol are investigated. It is shown that the expected secrecy rate can be maximized by selecting the transmit power and rates for the three symbols properly. It is also demonstrated that the proposed physical layer security scheme achieves a higher secrecy rate than a secure beamformed transmission based on NOMA and artificial noise.