Intelligent Reflecting Surface-Aided Multiuser Communication: Co-design of Transmit Diversity and Active/Passive Precoding

TL;DR

This work addresses reliable multiuser downlink transmission in delay-sensitive scenarios by introducing an IRS-integrated base station that jointly leverages transmit diversity for high-mobility users (no CSI) and active/passive precoding for low-mobility users (CSI). The core method combines a per-symbol common IRS phase-shift to realize transmit diversity with a traditional active/passive IRS precoding framework, and solves the resulting nonconvex optimization problem via alternating optimization. The proposed algorithm couples an MMSE/ZF transmit-precoding design with SDR-based reflect-precoding, guaranteeing convergence while reducing BS transmit power under per-user SINR constraints. Simulation results show substantial power savings and SER gains from the co-design, highlighting the practical impact for future dense IRS deployments in heterogeneous mobility environments.

Abstract

Intelligent reflecting surface (IRS) has become a cost-effective solution for constructing a smart and adaptive radio environment. Most previous works on IRS have jointly designed the active and passive precoding based on perfectly or partially known channel state information (CSI). However, in delay-sensitive or high-mobility communications, it is imperative to explore more effective methods for leveraging IRS to enhance communication reliability without the need for any CSI. In this paper, we investigate an innovative IRS-aided multiuser communication system, which integrates an IRS with its aided multi-antenna base station (BS) to simultaneously serve multiple high-mobility users through transmit diversity and multiple low-mobility users through active/passive precoding. In specific, we first reveal that when dynamically tuning the IRS's common phase-shift shared with all reflecting elements, its passive precoding gain to any low-mobility user remains unchanged. Inspired by this property, we utilize the design of common phase-shift at the IRS for achieving transmit diversity to serve high-mobility users, yet without requiring any CSI at the BS. Meanwhile, the active/passive precoding design is incorporated into the IRS-integrated BS to serve low-mobility users (assuming the CSI is known). Then, taking into account the interference among different users, we formulate and solve a joint optimization problem of the IRS's reflect precoding and the BS's transmit precoding, with the aim of minimizing the total transmit power at the BS.

Figures (9)

• Figure 1: IRS-aided multiuser downlink transmission for simultaneously serving high-mobility and low-mobility users over one given frequency band.
• Figure 2: Communication protocol for IRS-aided transmit diversity and active/passive precoding.
• Figure 3: Simulation setup for IRS-aided multiuser downlink communication.
• Figure 4: Convergence behavior of Algorithm 1, with $N=400$, $\gamma=8$, and $\gamma_{L+1}=1$.
• Figure 5: BS transmit power versus BS/IRS-user distance, with $N=400$, $\gamma=10$, and $\gamma_{L+1}=1$.