Outage Probability and Capacity Analysis of the Collaborative NOMA Assisted Relaying System in 5G

It is challenging for the base station (BS) to serve multiple cell-edge users concurrently with data rate guarantee due to limited power and spectrum resource. Using the relay enhances cell-edge user received signal strength by short-distance communication in low transmit power. Non-orthogonal multiple access (NOMA), a promising spectral efficient technology for the 5-th generation network (5G), enables the relay to transmit multiple messages to cell-edge users concurrently. In this paper, a Collaborative NOMA Assisted Relaying (CNAR) system for 5G is proposed with the collaboration of the source-relay (S-R) and relay-destination (R-D) NOMA link. The relay decodes its own message from the S-R NOMA signal and transmits the remaining part with adjusted power to cell-edge users in the R-D link. Then the exact expression of system outage probability is derived by analyzing the outage behavior in S-R and R-D links separately. To guarantee the data rate, the optimal power allocation among NOMA users is provided by minimizing the outage probability. To further characterize the system performance, the ergodic sum capacity in high SNR regime is approximated from discussions on the interference at cell-edge users. Simulation results validate our mathematical analysis, and show that the relaying system assisted by NOMA achieves lower outage probability and higher sum capacity than orthogonal multiple access (OMA).