Full-Duplex Decode-and-Forward Cooperative Non-Orthogonal Multiple Access

Non-orthogonal multiple access (NOMA) scheme is considered as a promising technology for 5G networks due to its ability to increase the spectral efficiency. In this paper, by combining the principle of NOMA and full-duplex (FD) transmission, a downlink FD cooperative NOMA (C-NOMA) scheme is proposed to enhance the system performance. We consider a practical approach where the weak user's signal is only forwarded in the second timeslot after it is decoded, while at the same time the base station (BS) retransmits the weak user's signal again and new information to the strong user. The analytical expressions for the sum rate using maximal ratio combining (MRC) and maximum ratio transmission (MRT) are formulated to investigate the performance of the proposed schemes. Also, in contrary to most existing C-NOMA methods, we assume that the signal from the BS and from the strong user cannot be separated for diversity combining. Hence, accurate analytical sum rates for half-duplex (HD) C-NOMA with MRC and MRT are derived. Numerical simulation results demonstrate that the proposed FD C-NOMA system using MRT offers better sum rate performance than FD C-NOMA using MRC and the HD C-NOMA schemes. Furthermore, the proposed scheme attains a close sum rate performance compared to NOMA and a superior fairness for the weak user.