Abstract: Non-orthogonal multiple access (NOMA) is one of the key candidate technologies of 5G network. The CR-NOMA system formed by the combination of cognitive radio (CR) technology can achieve higher spectrum efficiency, greater throughput and lower transmission delay. In this paper, coordinated direct and relay transmission (CDRT) technology is introduced into CR-NOMA system, where CDRT means that the secondary source (SS) communicates directly with near user and only with far user through relay. In the case of non-ideal successive interference cancellation (SIC) and full-duplex (FD) relay, the exact closed expression of NOMA user outage probability (OP) is derived, and the approximate expression of user OP is obtained when the ITC or the transmission power of the SS approaches infinity. In addition, based on user fairness and OP performance, a user power allocation factor optimization algorithm is proposed. Monte Carlo simulation verifies the consistency between theoretical analysis and experimental results. After the optimization of the proposed algorithm, compared with the fixed power allocation scheme, the fairness of user OP performance is enhanced and the system rate is significantly increased.

Key words: non-orthogonal multiple access, cognitive radio, coordinate direct and relay transmission, outage probability, power allocation