Performance of a Cooperative NOMA/OMA Scheme with Energy Buffer-Aided Near-User

Cooperative downlink non-orthogonal multiple access (NOMA) is a spectrally-efficient signalling technique that can enhance throughput to an Internet of Things (IoT) distant from the access point, termed the far-user (FU) IoT, while simultaneously serving a closer IoT node, termed the near-user (NU) IoT. However, it requires relaying by the NU IoT, which can drain its battery energy. In this paper, we consider an energy-buffer equipped NU IoT that uses only harvested energy to relay to the FU IoT. To further enhance performance with the energy buffer, and allow it to accumulate energy, we switch between NOMA, cooperative NOMA and orthogonal multiple access (OMA) signalling modes. This hybrid scheme results in better FU IoT throughput performance as compared to individual modes, while simultaneously enhancing NU IoT performance, without requiring the latter to expend its battery energy. The energy buffers are modelled precisely using a continuous-state space Markov chain. The best-effort policy (BEP) energy management policy is considered. The limiting distribution of BEP policy is provided for stable buffers, and then use this to derive expression for throughput. Simulation results confirm accuracy of the derived analytical expressions.