  Deploying reconfigurable intelligent surface (RIS) to enhance wireless
transmission is a promising approach. In this paper, we investigate large-scale
multi-RIS-assisted multi-cell systems, where multiple RISs are deployed in each
cell. Different from the full-buffer scenario, the mutual interference in our
system is not known a priori, and for this reason we apply the load coupling
model to analyze this system. The objective is to minimize the total resource
consumption subject to user demand requirement by optimizing the reflection
coefficients in the cells. The cells are highly coupled and the overall problem
is non-convex. To tackle this, we first investigate the single-cell case with
given interference, and propose a low-complexity algorithm based on the
Majorization-Minimization method to obtain a locally optimal solution. Then, we
embed this algorithm into an algorithmic framework for the overall multi-cell
problem, and prove its feasibility and convergence to a solution that is at
least locally optimal. Simulation results demonstrate the benefit of RIS in
time-frequency resource utilization in the multi-cell system.
