Double-RIS Communication with DF Relaying for Coverage Extension: Is One Relay Enough?

TL;DR

This paper explores the use of double RIS-assisted networks with DF relaying to enhance coverage, analyzing different relay architectures and phase optimization techniques to maximize achievable rates.

Contribution

It introduces novel relay architectures and phase optimization methods for double-RIS systems, providing closed-form solutions and performance comparisons.

Findings

Two relays near each RIS outperform single relay at low/medium SNRs.

Single relay can outperform two relays at high SNRs with high inter-relay interference.

Proposed phase optimization schemes improve achievable rates.

Abstract

In this work, we investigate the decode-and-forward (DF) relay-aided double reconfigurable intelligent surface (RIS)-assisted networks, where the signal is subject to reflections from two RISs before reaching the destination. Different relay-aided network architectures are considered for maximum achievable rate under a total power constraint. Phase optimization for the double-RIS channels is tackled via the alternating optimization and majorization-minimization (MM) schemes. Moreover, closed-form solutions are obtained for each case. Numerical results indicate that the deployment of two relays, one near each RIS, achieves higher rates at low and medium signal-to noise ratios (SNRs) compared to placing a single relay between the two RISs; while at high SNRs, the latter approach achieves higher rates only if the inter-relay interference for the former case is considerably high.