Coverage analysis and deployment design of RIS-assisted systems

Reconfigurable intelligent surface (RIS) is a novel technology that can help enhance the wireless propagation environment by altering the electromagnetic properties of its massive low-cost passive reflecting elements. This paper aims to explore the mechanism of how the deployment of RIS influences cell coverage from the system level and to obtain valuable guidance for actual RIS deployment design. We derive the coverage area gain (CAG) brought by deploying RIS to the whole system and analyze how the deployment parameters influence performance. The results reveal that it is more favorable to deploy the RIS close to the base station (BS) to gain more long-range coverage, and the coverage area gain can keep approximately constant as long as the ratio of the RIS element number to the deployment distance (between the BS and the RIS) remains unchanged. The scaling law between the CAG and the ratio is obtained. Furthermore, we extend the analysis to the scenario with two RISs deployed and respectively derive the coverage area gains brought by the two RISs when they are symmetrically and vertically deployed. We find that the aforementioned ratio rule still holds here, and the vertical deployment can achieve higher CAG than the symmetric deployment.