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Performance of Max-Min User Relay Cooperative NOMA Systems with Imperfect CSI

In this paper, we derive the effect of channel errors on the performance of user relaying cooperative NOMA system in closed-form. The max-min selection strategy is adapted to the selection of a relay among near users (NUs) and selection combining (SC) is applied to a far user (FU) for the performance improvements. As the previous studies found, the max-min relay selection without channel errors is effective to improve the outage performance. However, we demonstrate that the outage performance is highly sensitive to the channel error. It is also found that the outage probability of the relay user is more susceptible to the channel errors. As the channel correlation coefficient approaches to 0, the performance gains are banishing and the outage probability with the max-min relay selection becomes identical to that with the random relay selection. NOMA system has an optimal power allocation coefficient to minimize the outage performance, but as the correlation coefficient decrease, the effect of power allocation to the performance becomes lessen. The provided numerical results confirm that the derived expressions match well with Monte Carlo simulations.