Downlink Multi-RIS Aided Transmission in Backhaul Limited Networks

This letter proposes an analytical framework for multi-reconfigurable intelligent surface (RIS) aided networks with limited backhaul capacity, where a simple symmetric structure is considered to explore the characteristics of multi-RIS systems with coherent transmissions. By using the Gamma distribution to model the composite RIS-aided channels, closed-form expressions in terms of the outage probability and the ergodic rate are derived. The theoretical results provide two design guidelines: 1) the minimum outage can be achieved by deploying the assisted RISs at certain locations; 2) the rate performance is bounded, and an optimal number of assisted RISs exists for rate maximization. Simulations verify our analysis and demonstrate the backhaul capacity is the bottleneck for multi-RIS systems. Compared with single-RIS systems, multi-RIS systems require a minimal backhaul capacity to ensure the performance gain.