Multi-hop Multi-RIS Wireless Communication Systems: Multi-reflection Path Scheduling and Beamforming

TL;DR

This work tackles fairness-aware, multi-hop RIS-enabled wireless networks by formulating a max-min data-rate problem that jointly optimizes multi-reflection path selection, user activation grouping, BS beamforming, and time scheduling. A three-step framework is proposed: (1) graph-based path selection to maximize equivalent channel gain for each user, (2) MIS-driven activation grouping to prevent inter-path interference, and (3) convex-optimization-based joint beamforming and time allocation to maximize the minimum rate. The approach leverages known LoS channels, precomputed inter-RIS phase designs, and SDR-based beamforming with a bisection over target rates, achieving higher fairness and throughput than benchmarks, with insights on scalability, interference mitigation, and discrete-phase implementations.

Abstract

Reconfigurable intelligent surface (RIS) provides a promising way to proactively augment propagation environments for better transmission performance in wireless communications. Existing multi-RIS works mainly focus on link-level optimization with predetermined transmission paths, which cannot be directly extended to system-level management, since they neither consider the interference caused by undesired scattering of RISs, nor the performance balancing between different transmission paths. To address this, we study an innovative multi-hop multi-RIS communication system, where a base station (BS) transmits information to a set of distributed users over multi-RIS configuration space in a multi-hop manner. The signals for each user are subsequently reflected by the selected RISs via multi-reflection line-of-sight (LoS) links. To ensure that all users have fair access to the system to avoid excessive number of RISs serving one user, we aim to find the optimal beam reflecting path for each user, while judiciously determining the path scheduling strategies with the corresponding beamforming design to ensure the fairness. Due to the presence of interference caused by undesired scattering of RISs, it is highly challenging to solve the formulated multi-RIS multi-path beamforming optimization problem. To solve it, we first derive the optimal RISs' phase shifts and the corresponding reflecting path selection for each user based on its practical deployment location. With the optimized multi-reflection paths, we obtain a feasible user grouping pattern for effective interference mitigation by constructing the maximum independent sets (MISs). Finally, we propose a joint heuristic algorithm to iteratively update the beamforming vectors and the group scheduling policies to maximize the minimum equivalent data rate of all users.

Figures (9)

• Figure 1: The considered multi-hop multi-RIS communication system.
• Figure 2: Graph representation of the simulation setup.
• Figure 3: Designed multi-reflection paths for each user.
• Figure 4: The convergence property of the proposed cooperative multi-hop transmission algorithm. The number of antennas at the BS is $M_0=20$. There are $16$ RISs in the system and the number of reflecting elements at each RIS is $M_j=20, j=1,2,...,16$.
• Figure 5: The equivalent data rate of all users achieved by the proposed framework. The number of antennas at the BS is $M_0=20$. There are $16$ RISs in the system and the number of reflecting elements at each RIS is $20$. The transmit power at the BS is $P_T=10$ dB.