Relay-aided non-orthogonal multiple access with noisy network coding

We consider two-user relay-aided uplink nonorthogonal multiple access (NOMA) with noisy network coding (NNC) at the relay. The NNC-based cooperation strategy at the relay relies on noisy quantization followed by joint source-channel coding, while at the destination it utilizes joint decoding using the non-orthogonal signals received from the sources and the relay. After presenting the information theoretic achievable rates for the NNC-based cooperation scheme, we discuss how it outperforms conventional compress-and-forward (CF) cooperation even when the relay has access to global channel state information (CSI). For the NNC-based approach, we also derive closed-form expressions for probability of outage when the relay has access to local CSI, as well as when the relay has knowledge of the receive CSI only. These closed-form expressions help in appropriately choosing the quantizer so as to optimize the outage performance. Simulation indicate that NNC-based relaying outperforms CF by 2.2 dB when global CSI is available at the relay. Additionally, the availability of global CSI results in an improvement of 3.8 dB and 5.4 dB over the case when relay has access only to local CSI and the receive CSI, respectively.