Low-complexity passive beamforming for reconfigurable intelligent surface aided mmWave communication systems

Millimeter wave (mmWave) communication technology has received wide attention in recent years due to its abundant frequency resources. However, severe propagation loss and the ease of being blocked by obstacles are not conducive to wide area coverage. Reconfigurable intelligent surface (RIS) is expected to solve the coverage problem of mmWave communication systems by reforming the wireless channel to some extent. In this work, we propose a low complexity scheme for the optimization of RIS phase shifts in the downlink of mmWave communication systems. Unlike previous works that usually jointly design the RIS phase shifts and precoding matrix with nonnegligible computational and implementation complexity, we propose to solely optimize the RIS phase shifts to maximize the overall throughput of the wireless channel and devise two low complexity RIS phase shift design schemes, i.e., TM UB-RPO and TM SCA-RPO. For TM UB-RPO, the suboptimal phase shifts are derived in closed form through theoretical analysis, while for TM SCA-RPO, the phase shifts are optimized using the successive convex approximation method, and the solution in each iteration is given in closed form. The proposed schemes can work with any conventional precoding scheme, such as zero-forcing precoding (ZF), thus can be easily deployed in existing schemes. Numerical results show that the proposed schemes outperform existing methods in terms of achievable rates while significantly reducing computational complexity.