Intelligent Reflecting Surface for Spectral Efficiency Maximization in the Multi-User MISO Communication Systems

This paper proposes an intelligent reflective surface (IRS) design scheme to improve the spectral efficiency (SE) for downlink multi-user (MU) multiple-input-single-output (MISO) system. IRS composed of low-cost reflecting elements is expected to be used in various scenarios in future wireless communication systems. It can be used even when the direct path is blocked by obstacles. Since the reflecting elements can only adjust the phase, the design of IRS matrix causes a non-convex problem. In this paper, the proposed schemes perform optimization by transforming a non-convex problem into a solvable convex function. Typically, base station-IRS (BS-IRS) channel can be assumed as the line-of-sight (LOS) channel environment. In this case, multi-user system suffers from performance degradation due to the LOS channel rank problem. To alleviate this problem, this paper considers deterministic scattering and sufficient spacing between the reflecting elements of the IRS. The simulation results show that the proposed schemes achieve better SE performance than the randomly generated IRS scheme. In addition, the proposed minimum mean square error (MMSE)-based scheme can achieve high performance compared to other schemes even in a low-rank channel environment.