Coordinated and Non-Coordinated Direct-and-Relay Transmission: A Signal Space Diversity Approach

In this work, two signal-space diversity (SSD) based schemes are proposed for a system where one user is directly in contact with both the base station and the relay, while the other user is only in contact with the relay. The first scheme combines the coordinated direct and relay transmission (CDRT) technique with SSD to improve the achievable sum-rate of the system and is called the SSD-based CDRT (SSD-CDRT) scheme. On the other hand, the second scheme exploits only the SSD technique to improve the diversity order of one of the users and is termed SSD based non-CDRT (SSD-nCDRT). Various performance metrics such as outage probability, system outage throughput, diversity order, and achievable sum-rate are evaluated in closed-form for both schemes to compared them with their counterparts non-orthogonal multiple access (NOMA) based schemes, i.e., NOMA-CDRT and NOMA-nCDRT schemes. The comparative study confirms that the proposed SSD-CDRT and SSD-nCDRT are superior to NOMA-CDRT and NOMA-nCDRT, respectively, in terms of all the considered performance metrics.