Outage Performance of Overlay Cognitive Satellite-Terrestrial Networks With Cooperative NOMA

In satellite-terrestrial systems, the joint utilization of nonorthogonal multiple access (NOMA) and cognitive radio can improve the spectrum efficiency. This article investigates an overlay cognitive satellite-terrestrial network (CSTN) with cooperative NOMA. Different from existing works where only the secondary transmitter (ST) cooperates with the primary user (PU), we propose to let both the ST and the full-duplex (FD) secondary receiver (SR) cooperate with the PU. Specifically, the ST can forward the primary message from the satellite source to the PU and transmit the secondary message to the SR simultaneously based on the NOMA protocol, while the SR can also cooperate to forward the primary message to the PU based on the FD protocol. An adaptive mode selection strategy is proposed to let the SR choose between the cooperation mode and noncooperation mode. We obtain the closed-form approximate expressions for the outage probabilities of the PU and the SR as well as the PU diversity order. It is shown that the PU diversity order can be improved by 1 compared to the case when the SR works only in the noncooperation mode when the residual loop-interference is very low. Monte Carlo simulations confirm the correctness of the theoretical analysis.