NOMA-Based User Cooperation with Incremental Hybrid Forwarding Protocols

In this paper, we investigate the performance of a non-orthogonal multiple access (NOMA) wireless system, in which signal transmission of a far user is assisted by a near user. In particular, we examine the outage performance of the underlying scenario with two cooperative forwarding protocols incorporated, namely, the hybrid decode-amplify-forward (HDAF) protocol and the incremental hybrid decode-amplify-forward (IHDAF) protocol. HDAF combines the advantages of decode-and-forward (DF) and amplify-and-forward (AF) and overcomes their limitations. On the other hand, the IHDAF protocol gives priority to successful decoding from the direct link before enabling HDAF. Specifically, we first derive outage probability expressions for the two users under the HDAF protocol. Furthermore, a closed-form outage expression of the weak user is derived considering the IHDAF protocol, which is envisioned to further boost the outage performance. Also, we present a comprehensive mathematical framework in order to evaluate the system throughput performance of the underlying system model, for the HDAF and IHDAF protocols. Analytical and simulation results reveal that the outage performance of the weak user under IHDAF outperforms that of HDAF.