Multiple RISs-Aided Networks: Performance Analysis and Optimization

This paper analyzes the performance of multiple reconfigurable intelligent surfaces (RISs)-aided networks with opportunistic RIS scheduling. The paper also provides some optimization results on the number of reflecting elements on RISs and the optimal placement of RISs. We first derive accurate closed-form approximations for RIS channels' distributions assuming independent non-identically distributed (i.ni.d.) Nakagami-m fading environment. Then, the approximate expressions for outage probability (OP) and average symbol error probability (ASEP) are derived in closed-form. Furthermore, to get more insights into the system performance, we derive the asymptotic OP at the high signal-to-noise ratio (SNR) regime and provide closed-form expressions for the system diversity order and coding gain. Finally, the accuracy of our theoretical analysis is validated through Monte-Carlo simulations. The obtained results show that the considered RIS scenario can provide a diversity order of a/2K, where a is a function of the Nakagami fading parameter m and the number of meta-surface elements N, and K is the number of RISs.