Double-IRS Assisted Multi-User MIMO: Cooperative Passive Beamforming Design (2008.13701v5)

Abstract: Intelligent reflecting surface (IRS) has emerged as an enabling technology to achieve smart and reconfigurable wireless communication environment cost-effectively. Prior works on IRS mainly consider its passive beamforming design and performance optimization without the inter-IRS signal reflection, which thus do not unveil the full potential of multi-IRS assisted wireless networks. In this paper, we study a double-IRS assisted multi-user communication system with the \emph{cooperative} passive beamforming design that captures the multiplicative beamforming gain from the inter-IRS channel. Under the general channel setup with the co-existence of both double- and single-reflection links, we jointly optimize the (active) receive beamforming at the base station (BS) and the cooperative (passive) reflect beamforming at the two distributed IRSs (deployed near the BS and users, respectively) to maximize the minimum signal-to-interference-plus-noise ratio (SINR) of all users. Moreover, for the single-user and multi-user setups, we analytically show the superior performance of the double-IRS cooperative system over the conventional single-IRS system in terms of the maximum signal-to-noise ratio (SNR) and multi-user effective channel rank, respectively. Simulation results validate our analytical results and show the practical advantages of the proposed double-IRS system with cooperative passive beamforming designs.

• The paper introduces a cooperative dual-IRS beamforming design that jointly optimizes active and passive schemes to maximize SINR.
• It demonstrates that a double-IRS setup enhances signal quality and increases effective channel rank for multi-user MIMO systems.
• Simulation results confirm significant improvements in data rates and reliability over conventional single-IRS configurations.

Overview of "Double-IRS Assisted Multi-User MIMO: Cooperative Passive Beamforming Design"

The paper explores the integration of double Intelligent Reflecting Surfaces (IRS) to facilitate cooperative passive beamforming in multi-user Multiple-Input Multiple-Output (MIMO) systems. The authors propose a system model where two distinct IRSs are positioned strategically to enhance the wireless communication path between a base station (BS) and multiple users. This strategic positioning allows the system to harness the multiplicative beamforming gain potential via double reflection channels, something existing works with single IRS setups overlook. The paper rigorously analyzes and optimizes the relative efficacy of cooperative passive beamforming in comparison to conventional single-IRS systems.

System Model and Optimization Approach

The proposed system consists of two IRSs: IRS1, placed proximate to the users, and IRS2, positioned near the BS. This double-IRS configuration exposes both double- and single-reflection paths, which necessitates a coordinated passive beamforming strategy. The authors focus on maximizing the minimum signal-to-interference-plus-noise ratio (SINR) across all users by jointly optimizing the active beamforming at the BS and the passive beamforming managed by the IRSs. The formulation leverages alternating optimization (AO), semidefinite relaxation (SDR), and bisection methods to iteratively solve the beamforming issues for both single-user and multi-user setups.

Key Findings and Results

1. Single-User SNR Enhancement: In the single-user scenario, the research finds that the maximum signal-to-noise ratio (SNR) with the dual IRS setup is consistently no less than that of a single IRS system. The cooperative strategy efficiently balances the reflective gains from both single and double path reflections, providing superior performance without additional infrastructure costs.
2. Multi-User Channel Rank: For multi-user contexts, it is established that the effective channel rank is generally higher in a double-IRS configuration. This results in augmented spatial multiplexing gains, allowing more efficient use of the wireless medium to support simultaneous users. Consequently, the max-min SINR performance notably surpasses that of a single-IRS design, especially in scenarios with significant user interference.
3. Simulation Validation: The authors provide simulation results that validate their theoretical analysis. The double-IRS system consistently demonstrates significant improvements in data rates and signal reliability across varying transmission power levels and IRS sizes, confirming the practical value of the proposed beamforming strategies.

Implications and Future Directions

The practical implications of this research lie in the enhanced efficiency of deploying multiple IRSs in wireless networks to bolster both signal quality and network capacity. While focused on theoretical gains and complex algorithmic optimization, the paper suggests substantial potential for real-world application in scenarios like urban environments or indoor settings where line-of-sight communication is frequently compromised.

The authors hint at several future research trajectories, such as more sophisticated CSI acquisition methods tailored to distributed IRS setups and exploring the interplay between IRS element distribution and the system's spatial multiplexing capabilities. There is fertile ground for further experimental validation and exploration of algorithmic efficiency improvements, especially considering deployment in heterogeneous network environments.

In conclusion, this paper serves as a detailed exploration into transforming and optimizing passive wireless environments with multiple IRSs, contributing significant theoretical insights and practical guidance for evolving 5G and future communication networks.