Performance Analysis of Downlink Cooperative User Relaying FD/HD NOMA Over Nakagami-$m$ Fading Channels

This article investigates a cooperative user relaying non-orthogonal multiple access (NOMA), in which the near user (NU) works as a relay for the far user (FU) to convey data from the base station (BS). Two situations are explored: (1) No direct link (NDL) between the BS and the FU, and (2) Direct link (DL) between the BS and the FU. The NU communicates in half-duplex (HD) or full-duplex (FD) mode using a decode-and-forward mechanism. Channel coefficients are obtained using Nakagami-
$m$
distribution, whereas self-loop interference at the NU in FD mode is modelled using Rayleigh distribution. Three performance metrics are analyzed: outage probability, ergodic rate, and energy efficiency. For these parameters, closed-form expressions are derived considering different possible scenarios, or asymptotic expressions are provided. Simulation results show an error floor in the outage probability plots in the FD NOMA case. The system throughput for two NOMA users is investigated in delay-limited and delay-tolerant transmission modes. Using the DL helps to improve the diversity of the FU in both the HD and FD modes. A throughput ceiling exists for the NU in the FD NOMA case and the FU in both the HD and FD modes. It is clear from the simulation results that FD NOMA outperforms HD NOMA in terms of the considered performance metrics in the low and mid-SNR regions. Numerical studies explain how diverse system factors, including self-loop interference, variable node distances, source transmit power, and power allocation coefficients, affect various performance indicators.