RIS-Assisted Short-Packet NOMA Systems with Non-Optimal Continuous Phase Shifts and Hardware Impairments

Reconfigurable intelligent surface (RIS) and nonorthogonal multiple access (NOMA) are two promising technologies for future wireless communication networks. This paper proposes a new transmission design for short-packet communications (SPCs) in a downlink RIS-assisted NOMA system, considering hardware impairments at the transceiver nodes, non-optimal continuous phase shifts (CPSs) with phase estimation errors at the RIS, and imperfect successive interference cancellation (SIC) for a pair of NOMA users. Using the method of moment matching, we approximate the end-to-end channel gain with a Gamma distribution, and derive the closed-form approximate expressions for the average block error rates (BLERs) for NOMA users. Moreover, the system throughput is obtained to characterize the transmission efficiency of the network. Analytical results of the proposed scheme indicate that the phase estimation errors can seriously affect the system performance. Therefore, when conducting theoretical analysis, the non-optimal CPSs should be considered for RIS deployment. Numerical results validate our theoretical analysis, and confirm the performance improvement of transmission reliability and effectiveness of the proposed scheme, as compared to existing transmission schemes.