User Grouping and Relay Selection for Improving Outage Performance of Downlink Wireless Systems

A two-user cooperative non-orthogonal multiple access (NOMA) network can be improved by exploiting the relay selection (RS) scheme. The best relay selected from the relay can forward signals to distant users relying on a fixed power allocation factor scheme and NOMA. We adopt a decode-and-forward (DF) relay and focus on performance analysis for the case of Rician channel fading. The user is assumed to send two-bit feedback to inform relays of its detection operation. We derive closed-form expressions of the outage probability for both users. The numerical results illustrate that our considered NOMA-RS system can be improved its outage probability at a higher number of relays and better channel conditions.