Principles and Optimization of Reflective Intelligent Surface Assisted mmWave Systems

A conceptual example is first analyzed to show that efficient wireless communications is possible, when user equipment (UE) receiver, BS transmitter or/and the scatter (reflector) in wireless channels employ the required channel state information (CSI) to remove the randomness of signal phase. Then, the principles and optimization of three reflective intelligent surface (RIS) assisted mmWave (RIS-mmWave) models are introduced. The first model assumes one BS, one RIS and one UE; the second one assumes one BS, one RIS and multiple UEs; while the third RIS-mmWave model assumes one BS, multiple RISs and multiple UEs. Furthermore, the optimization of BS precoder and RIS phase-shifts is addressed in the context of the massive RIS-mmWave scenarios, where the number of BS antennas and that of RIS reflection elements are significantly larger than the number of supported UEs. The analyses demonstrate that, while the deployment of RISs with mmWave is capable of solving the blockage problem and has the potential to significantly improve efficiency, finding the near-optimum solutions for RIS phase-shifts is highly challenging in practice.