Ergodic Capacity Performance of NOMA-SWIPT Aided IoT Relay Systems with Direct Link

For the delay-tolerant transmission mode, ergodic capacity (EC) is an appropriate measure for the performance analysis of the system. In this paper, we investigate the EC and ergodic sum capacity (ESC) performance of cooperative non-orthogonal multiple access (NOMA) simultaneous wireless information and power transfer (SWIPT) aided Internet of Things (IoT) relay systems with direct links over the Rayleigh fading channels. To the best of our knowledge, there is no published literature that investigates the EC and ESC of the NOMA-SWIPT aided IoT relay systems with the direct link, in which one source or base station (BS) transmits symbol to two destination nodes through the direct link and with the help of EH based relay node. Specifically, we study the time switching (TS), and power splitting (PS) relaying architecture for increasing the spectral and energy-efficiency of the considered system. Analytical expressions for the EC and the ESC are mathematically derived and validated by the simulation results. Our results not only provide a thorough comparison of the TS and PS relaying EH architecture for the considered system model, but it also demonstrates that the ESC performance could be significantly improved through the optimal choice of the power splitting $\epsilon$ factor for PS relaying with NOMA compared to TS relaying with NOMA.