Cross-RIS-Aware Phase Design for Multi-Cell Multi-RIS Networks

Reconfigurable intelligence surfaces (RISs) have emerged as an efficient technology to enhance the spectral and energy efficiency of wireless networks. Most existing RIS studies have adopted an ideal phase-shift model under a narrow-band assumption and designed the RIS phases selfishly to maximize the rate of each cell, ignoring reflections paths via neighboringcell RISs that operate on different frequencies. In this paper, we studied the RIS-aided multi-cell wireless system. Specifically, for a two-cell downlink at two different center-frequencies, we consider a practical (frequency-selective, i.e, wide-band) model and explicitly account for other-cell RIS, i.e., cross-RIS, reflections. We formulate a new optimization problem of designing a common RIS phase shift, jointly considering the two BS signals at the RIS to maximize the average rate of two users. We also evaluate sensitivity to the codebook size for quantized phases and compared against a self-RIS baseline. The proposed design yields higher average rate across the two users, offering insights for the use of neighboring-cell RIS in next-generation networks.