Performance analysis of multiple RISs aided multi-user mmWave MIMO systems

Reconfigurable intelligent surface (RIS) is envisioned to be a promising solution to enhance coverage and capacity of future wireless networks. With the deployment of multiple RISs, the optimal placement and topology are crucial issues that need to be addressed, which are significantly different from that of active BSs/relays. This paper considers a mmWave MIMO system aided by multiple RISs in a circular layout to observe the effect of RIS placement and power scaling law. The active and passive beamforming are configured to combine the signals coherently to improve the spectral efficiency (SE). To this end, we investigate the effect of quantization error on the SE and derive an upper bound. It indicates that the SE loss is only affected by the number of quantization bits and numerical results show that 5-bit quantizer is sufficient to ensure an acceptable SE degradation. Furthermore, we evaluate the performance of the system by formulating the tight upper bounds on the average ergodic spectral efficiency and propose three effective schemes for RIS selection. Our simulation results verify the derivations and validate that the performance of deploying the RISs close to BS is significantly better than that of deploying the RISs randomly in the cell.