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The Interference Broadcast Channel with Reconfigurable Intelligent Surfaces: A Cooperative Sum-Rate Maximization Approach

Konstantinos D. Katsanos, Paolo Di Lorenzo, George C. Alexandropoulos

TL;DR

This paper addresses joint, distributed optimization of BS precoding and beyond-diagonal RIS phase configurations in a wideband interference broadcast channel with multiple BD RISs. It develops a cooperative alternating-optimization framework that constructs local surrogates for the non-convex sum-rate objective and solves tractable subproblems for precoding, RIS capacitance settings, and switch configurations, with an updating rule that guarantees convergence. Key contributions include a frequency-selective BD RIS model, closed-form or efficiently solvable subproblems, and a decomposition strategy that enables per-BS locality while accounting for inter-BS coupling. The results show that cooperative BD RISs yield meaningful sum-rate gains over diagonal RISs and non-cooperative setups, especially at higher transmit powers, highlighting the practical potential of distributed, BD RIS-enabled beamforming in wideband networks.

Abstract

This paper studies the interference broadcast channel comprising multiple multi-antenna Base Stations (BSs), each controlling a beyond diagonal Reconfigurable Intelligent Surface (RIS) and serving multiple single-antenna users. Wideband transmissions are considered with the objective to jointly design the BS linear precoding vectors and the phase configurations at the RISs in a distributed manner. We take into account the frequency selectivity behavior of each RIS's tunable meta-element, and focusing on the sum rate as the system's performance criterion, we present a distributed optimization approach that enables cooperation between the RIS control units and their respective BSs. According to the proposed scheme, each design variable can be efficiently obtained in an iterative parallel way with guaranteed convergence properties. Our simulation results demonstrate the validity of the presented distributed algorithm and showcase its superiority over a non-cooperative scheme as well as over the special case where the RISs have a conventional diagonal structure.

The Interference Broadcast Channel with Reconfigurable Intelligent Surfaces: A Cooperative Sum-Rate Maximization Approach

TL;DR

This paper addresses joint, distributed optimization of BS precoding and beyond-diagonal RIS phase configurations in a wideband interference broadcast channel with multiple BD RISs. It develops a cooperative alternating-optimization framework that constructs local surrogates for the non-convex sum-rate objective and solves tractable subproblems for precoding, RIS capacitance settings, and switch configurations, with an updating rule that guarantees convergence. Key contributions include a frequency-selective BD RIS model, closed-form or efficiently solvable subproblems, and a decomposition strategy that enables per-BS locality while accounting for inter-BS coupling. The results show that cooperative BD RISs yield meaningful sum-rate gains over diagonal RISs and non-cooperative setups, especially at higher transmit powers, highlighting the practical potential of distributed, BD RIS-enabled beamforming in wideband networks.

Abstract

This paper studies the interference broadcast channel comprising multiple multi-antenna Base Stations (BSs), each controlling a beyond diagonal Reconfigurable Intelligent Surface (RIS) and serving multiple single-antenna users. Wideband transmissions are considered with the objective to jointly design the BS linear precoding vectors and the phase configurations at the RISs in a distributed manner. We take into account the frequency selectivity behavior of each RIS's tunable meta-element, and focusing on the sum rate as the system's performance criterion, we present a distributed optimization approach that enables cooperation between the RIS control units and their respective BSs. According to the proposed scheme, each design variable can be efficiently obtained in an iterative parallel way with guaranteed convergence properties. Our simulation results demonstrate the validity of the presented distributed algorithm and showcase its superiority over a non-cooperative scheme as well as over the special case where the RISs have a conventional diagonal structure.
Paper Structure (13 sections, 5 theorems, 28 equations, 1 figure)

This paper contains 13 sections, 5 theorems, 28 equations, 1 figure.

Table of Contents

  1. Introduction
  2. System Model and Problem Formulation
  3. System Model
  4. BD RIS Structure and Element Response
  5. Received Signal Model
  6. Problem Formulation
  7. Distributed Sum-Rate Maximization
  8. Local Linear Precoding Optimization
  9. Local RIS Reflection Configuration Optimization
  10. Local RIS Switch Selection Matrix Optimization
  11. Updating Solution Step
  12. Numerical Results
  13. Conclusion

Key Result

Proposition 1

The frequency response of each $m$-th unit element of each $q$-th RIS can be reformulated as follows: where $\mathcal{N}_{mq}(f,C_{mq})$ and $\mathcal{D}_{mq}(f,C_{mq})$ are defined as follows:

Figures (1)

  • Figure 1: Achievable sum-rate performance for $Q=4$ BSs and RISs, each with $N\!=\!4$ antennas and $M\!=\!100$ unit elements, respectively.

Theorems & Definitions (5)

  • Proposition 1
  • Lemma 1
  • Theorem 1
  • Corollary 1
  • Corollary 2