Joint Optimization of IRS Beamforming and Transmit Power for MIMO-NOMA Downlink With Statistical Channel Information

Abstract

We consider non-orthogonal multiple access (NOMA) downlink with a multiple-antenna base station (BS) and single-antenna mobile users. Cell-edge users that are blocked from the BS are assisted by intelligent reflective surface (IRS) for data reception. Cell-center and IRS-assist users are paired to share the same frequency-time resource block and zeroforcing transmit beamforming from the BS. We propose to optimize IRS-element coefficients and transmit power to minimize the total power for a given rate threshold. Differing from the existing work, the proposed joint optimization is based on channel covariance instead of fast-changing channel coefficients. Our scheme can reduce larger total power when the number of IRS elements increases. The performance of IRS-assist users decreases significantly if the rank of BS-IRS channel matrix is low.