Dynamic Decode-and-Forward Based Cooperative NOMA With Spatially Random Users

Non-orthogonal multiple access (NOMA) is a promising spectrally-efficient multiple access technique for the fifth generation (5G) wireless networks. In this paper, we propose a dynamic decode-and-forward (DDF) based cooperative NOMA scheme for downlink transmission with spatially random users. In DDF-based cooperative NOMA, the base station transmits the superposition of the signals intended for the paired NOMA users. The user closer to the base station forwards the signal intended for the far user as soon as it can successfully decode its own signal and the signal intended for the far user. We consider two user pairing strategies, namely random and distance-based user pairing, which require one-bit feedback and the users' distance information, respectively. For each user pairing strategy, we derive the outage probability of the proposed NOMA scheme by using tools from stochastic geometry. Furthermore, based on the obtained outage probability, we derive the diversity order and the sum rate of the paired NOMA users. Simulation results validate the analytical results and demonstrate that the proposed DDF-based cooperative NOMA scheme achieves a lower outage probability and a higher sum rate than orthogonal multiple access, conventional NOMA, and cooperative NOMA.