Joint Beamforming Optimization and Mode Selection for RDARS-Aided MIMO Systems

2024-01-20

IEEE Transactions on Wireless Communications

AI Summary

To enable green 6G communications, this work addresses the joint optimization of receive beamforming, reconfigurable intelligent surface (RIS) phase shifts, and distributed antenna system (DAS) unit association in uplink multi-user MIMO, aiming to minimize the total mean-square error (MSE). Method: We propose a reconfigurable distributed antenna and reflecting surface (RDARS) architecture—first unifying DAS spatial diversity gain and RIS passive beamforming gain—and introduce dynamic mode switching to exploit selection gain. We formulate a novel joint optimization framework incorporating mode selection and develop two algorithms: (i) a convergent penalty dual decomposition (PDD)-based inexact block coordinate descent (BCD) method, and (ii) a low-complexity greedy alternating optimization (AO) algorithm yielding semi-closed-form solutions. Results: Simulations demonstrate that RDARS significantly reduces MSE compared to conventional fully passive RIS or standalone DAS, achieving superior trade-offs among energy efficiency, reliability, and hardware cost, while providing practical deployment guidelines.

Technology Category

• Search and Optimization: Mixed Discrete/Continuous Search
• Planning, Routing, and Scheduling: Optimization of Spatio-temporal Systems
• Multiagent Systems: Agent Communication

Application Category

• Systems and Infrastructure for Web, Mobile and WoT: Energy management for devices in mobile Web and WoT environments
• Search and Retrieval-Augmented AI: Retrieval-Augmented Generation (RAG) and multi-modal RAG
• User Modeling, Personalization and Recommendation: On-Device user modeling, personalization, and recommendation

Abstract

Reconfigurable intelligent surface (RIS) has emerged as a cost-effective solution for green communications in 6G. However, its further extensive use has been greatly limited due to its fully passive characteristics. Considering the appealing distribution gains of distributed antenna systems (DAS), a flexible reconfigurable architecture called reconfigurable distributed antenna and reflecting surface (RDARS) is proposed. RDARS encompasses DAS and RIS as two special cases and maintains the advantages of distributed antennas while reducing the hardware cost by replacing some active antennas with low-cost passive reflecting surfaces. In this paper, we present a RDARS-aided uplink multi-user communication system and investigate the system transmission reliability with the newly proposed architecture. Specifically, in addition to the distribution gain and the reflection gain provided by the connection and reflection modes, respectively, we also consider the dynamic mode switching of each element which introduces an additional degree of freedom (DoF) and thus results in a selection gain. As such, we aim to minimize the total sum mean-square-error (MSE) of all data streams by jointly optimizing the receive beamforming matrix, the reflection phase shifts and the channel-aware placement of elements in the connection mode. To tackle this nonconvex problem with intractable binary and cardinality constraints, we propose an inexact block coordinate descent (BCD) based penalty dual decomposition (PDD) algorithm with the guaranteed convergence. Since the PDD algorithm usually suffers from high computational complexity, a low-complexity greedy-search-based alternating optimization (AO) algorithm is developed to yield a semi-closed-form solution with acceptable performance. Numerical results demonstrate the superiority of the proposed architecture compared to the conventional fully passive RIS or DAS. Furthermore, some insights about the practical implementation of RDARS are provided.

Problem

• RDARS-assisted MIMO optimization
• Intelligent Reflecting Surface (RIS)
• Distributed Antenna System (DAS)

Innovation

• RDARS
• Distributed Antenna System (DAS)
• Reconfigurable Intelligent Surface (RIS)