Abstract
In this letter, we investigate the coordinated direct and user-assisted transmission in a non-orthogonal multiple access (NOMA) system. By integrating the coordinated direct transmission into user-cooperative NOMA, one far user (FU) is served along with two near users (NUs) that are opportunistically selected from a pool of K NUs. For the considered system, an NU scheduling strategy is proposed to minimize the outage probabilities of the NUs whilst ensuring the performance requirement of the FU. Exact and asymptotic outage probability of each served user is derived. Results demonstrate that the FU and one of the scheduled NUs can both achieve the full diversity order K, and the other scheduled NU achieves a diversity order equal to ${K}-1$ or K, depending on the transmit rate of the base station.
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