Opportunistic Non-Orthogonal Multiple Access Scheme with Unreliable Wireless Backhauls

The demand for increased connectivity and reliability of devices in the fifth generation (5G) of wireless communications requires new technology for ensuring massive connectivity and high spectral efficiency. In addition, wireless backhauls with guaranteed reliability are being considered to improve the overall system performance. In this paper, we investigate an opportunistic non-orthogonal multiple access (NOMA) system with unreliable wireless backhauls. In particular, we develop two opportunistic selection rules which allow the selection of the best among either near or far-away group transmitters, considering both the unreliability of wireless backhauls and fading effects of fronthauls. In order to analyze the performance, new exact and approximated closed-form expressions for the outage probabilities of the grouped receivers are derived, thus providing an insight into the impact of unreliable random backhauls and opportunistic NOMA. We show that the proposed scheme gives an outage performance gain of more than 3dB gains to a dominant receiver in the selection rules and improvement in receiver fairness when compared to the orthogonal multiple access (OMA) with an unreliable wireless backhaul. In addition, our results clearly reveal that unreliability levels of wireless backhaul links are responsible for the outage floors.