Passive Beamforming and Information Transfer Design for Reconfigurable Intelligent Surfaces Aided Multiuser MIMO Systems (1912.10209v2)

Abstract: This paper investigates the passive beamforming and information transfer (PBIT) technique for the multiuser multiple-input multiple-output (MIMO) systems with the aid of a reconfigurable intelligent surface (RIS), where the RIS enhances the primary communication via passive beamforming and at the same time delivers additional information by the spatial modulation (which adjusts the on-off states of the reflecting elements). For the passive beamforming design, we propose to maximize the sum channel capacity of the RIS-aided multiuser MIMO channel and formulate the problem as a two-step stochastic program. A sample average approximation (SAA) based iterative algorithm is developed for the efficient passive beamforming design of the considered scheme. To strike a balance between complexity and performance, we then propose a simplified beamforming algorithm by approximating the stochastic program as a deterministic alternating optimization problem. For the receiver design, the signal detection at the receiver is a bilinear estimation problem since the RIS information is multiplicatively modulated onto the reflected signals of the reflecting elements. To solve this bilinear estimation problem, we develop a turbo message passing (TMP) algorithm in which the factor graph associated with the problem is divided into two modules: one for the estimation of the user signals and the other for the estimation of the RIS's on-off states. The two modules are executed iteratively to yield a near-optimal low-complexity solution. Furthermore, we extend the design of the multiuser MIMO PBIT scheme from single-RIS to multi-RIS, by leveraging the similarity between the single-RIS and multi-RIS system models. Extensive simulation results are provided to demonstrate the advantages of our passive beamforming and receiver designs.

• The paper proposes a novel RIS-aided scheme for simultaneous passive beamforming optimization and information transfer via RIS toggling in multiuser MIMO systems.
• It details a two-stage stochastic programming approach for optimizing passive beamforming and a turbo message passing algorithm for efficient receiver signal detection and RIS state estimation.
• Numerical results demonstrate substantial gains in achievable rates with this RIS configuration, highlighting its potential to enhance spectral efficiency and redefine future wireless network architectures.

Passive Beamforming and Information Transfer in RIS-Aided Multiuser MIMO Systems: An Overview

The paper addresses the design challenges of passive beamforming and information transfer (PBIT) in multiuser multiple-input multiple-output (MIMO) systems enhanced by reconfigurable intelligent surfaces (RIS). A novel RIS-aided communication scheme is proposed to simultaneously optimize passive beamforming for primary communication and to implement information delivery through spatial modulation by toggling the reflecting elements.

Key Contributions

1. Passive Beamforming Design: The paper emphasizes a two-stage stochastic programming approach to maximizing the sum channel capacity in multiuser MIMO systems. Utilizing a sample average approximation (SAA), the authors supply an iterative algorithm to efficiently design passive beamforming. With large RIS deployments, a simplified beamforming design is also extended as an alternating optimization problem to enhance efficiency.
2. Receiver Design: The task of detecting signals with embedded RIS information is posed as a bilinear estimation problem. A turbo message passing algorithm divides the intricate factor graph into distinct modules for estimating user signals and RIS states iteratively. This approach ensures a robust, low-complexity receiver design enabling efficient signal recovery.
3. Multi-RIS Extension: Recognizing that multiple RIS configurations bring additional gains, the paper expands its theoretical framework to accommodate multi-RIS system architectures. This theoretic extension leverages the similarities with single-RIS models and prepares the foundation for substantial improvements in practical deployments.

Numerical Results

The simulations demonstrate substantial advantages of the proposed RIS configurations, ensuring improved achievable rates. Specifically, optimized beamforming provides notable gains across increasing RIS sizes and multi-RIS setups with complex spatial modulation schemas. The intricate analysis of RIS-aided channels, including both direct and reflecting links, also highlights the considerable impacts of carefully tuned phase shifts and RIS layouts on communication efficacy.

Implications and Future Directions

The research offers compelling insights into the integration of RIS technology in modern wireless infrastructure. As RIS elements manage to enhance spectral efficiency while retaining energy conservation, their application could redefine the architectural paradigms of future wireless networks. The detailed exploration of channel characteristics and optimization frameworks supports further explorations into machine-learning-enhanced beamforming solutions and adaptive algorithms to tackle real-time network variations.

The extensions of current work should include adaptive algorithms for rapid real-time applications and investigations into the applications of RIS technology in dense urban environments. Considering the blend of passive and active communications, translating these theoretical findings into practical, deployable systems remains an area ripe for exploration, particularly as multi-user environments become more complex.

Conclusively, integrating RIS technologies stands to revolutionize traditional wireless systems by facilitating efficient signal routing and distribution. The PBIT design frameworks proposed provide a foundational step forward in understanding and harnessing the full potential of these intelligent surfaces in multiuser settings. The implications affirm RIS technology as a pivotal advancement poised to undergird the next generation of wireless communication systems.