On the Achievable Capacity of Cooperative NOMA Networks: RIS or Relay?

In this letter, a novel reconfigurable intelligent surface (RIS)- and relay-assisted cooperative network with non-orthogonal multiple access (NOMA) is proposed, where the line-of-sight (LoS) and non-LoS (NLoS) scenarios are both considered for different locations of users. For the proposed cooperative NOMA systems, we first analyze the capacities of the RIS- and relay-assisted downlinks, respectively. Since it is difficult to obtain the closed-form expressions in terms of the achievable capacity, we apply the central limit theorem (CLT) and Jensen's inequality to determine a tight upper bound for the channel gain. Then, we focus on the solutions in relay and RIS providing more capacity advantages. Numerical and simulation results verify the correctness of the derived expressions and the superiority of our proposed model. Finally, we clarify that, with different conditions of the transmit scenarios, RIS- and relay-assisted cooperative networks show their various advantages and limitations.