Performance of a New Framework for Coordinated Direct AF Relay-Aided Downlink NOMA

This paper investigates the performance of a new coordinated direct and relay transmission (CDRT) NOMA framework in which a base station (BS) communicates directly to a near user (NU), and through an amplify-and-forward (AF) relay to a far user (FU). In the first signaling phase, the BS transmits superposed symbols to both NU and relay (R), whereas in the second phase R amplifies and re-transmits the signal from the BS. In this framework, the NU optimally combines the signals from BS and R, which gives it a significant performance advantage, and allows it to harness a diversity of two. This serves as an incentive to NU to participate in NOMA signalling to assist the FU. We derive closed form expressions for the outage probability and throughput of NU and FU. We elaborate on how the power allocation and target rates can be optimally chosen so as to maximize the NU throughput while ensuring a desired FU throughput. Computer simulation results validate the improved energy efficiency of the scheme and accuracy of the derived expressions.