Sparse array of sub-surface aided blockage-free multi-user mmWave communication systems

Recently, Reconfigurable Intelligent Surfaces (RISs) have drawn intensive attention in the realization of the smart radio environment. However, existing works mainly consider the RIS as a whole uniform plane, which may be unrealistic to be installed on the facade of buildings when the RIS is extremely large. In contrast, this paper investigates a practical Sparse Array of Sub-surface (SAoS) deployment of the RIS for uplink multi-user millimeter Wave (mmWave) communication systems, in which the Mobile Stations (MSs) are distributed in the blind coverage area due to the blockage. In order to exploit the benefits of the sparse deployment, the correlation of the effective channel is firstly investigated. Then the approximation and lower bounds of the ergodic spectral efficiency are derived under frequency and spatial multiplexing scenarios, respectively. Based on the autocorrelation of the effective channel, we obtain an optimal reflect coefficient design as well as the deployment guidelines of RIS tiles. Moreover, the RIS tile scheduling algorithms are also proposed. Numerical results show that the ergodic spectral efficiency approximation matches well with the Monte Carlo result under frequency multiplexing scenarios, and the lower bound is tight under spatial multiplexing scenarios only when the effective channel is strongly correlated. On the basis of the RIS tile scheduling algorithm and the reflect coefficient design, the system performance can be significantly improved under frequency multiplexing scenarios. On the other hand, by deploying more sparse RIS tiles, we can increase the multiplexing gain under spatial multiplexing scenarios.