User Scheduling and Passive Beamforming for FDMA/OFDMA in Intelligent Reflection Surface

TL;DR

This paper investigates user scheduling and passive beamforming in IRS-assisted FDMA/OFDMA systems, revealing that under certain conditions, complex scheduling and optimization can be avoided, simplifying system design.

Contribution

It provides the first analysis of multi-user IRS communication with non-frequency-selective reflection, showing when user scheduling and IRS optimization are necessary or can be skipped.

Findings

FDMA/OFDMA does not require user scheduling with few users.

With many users, neither user scheduling nor IRS reflection optimization is needed.

Results simplify the design of IRS-assisted multi-user communication systems.

Abstract

Most prior works on intelligent reflecting surface (IRS) merely consider point-to-point communications, including a single user, for ease of analysis. Nevertheless, a practical wireless system needs to accommodate multiple users simultaneously. Due to the lack of frequency-selective reflection, namely the set of phase shifts cannot be different across frequency subchannels, the integration of IRS imposes a fundamental challenge to frequency-multiplexing approaches such as frequency-division multiple access (FDMA) and the widely adopted technique called orthogonal FDMA (OFDMA). It motivates us to study (O)FDMA-based multi-user IRS communications to clarify which user scheduling and passive beamforming are favorable under this non-frequency-selective reflection environment. Theoretical analysis and numerical evaluation reveal that (O)FDMA does not need user scheduling when there are a few users. If the number of users becomes large, neither user scheduling nor IRS reflection optimization is necessary. These findings help substantially simplify the design of (O)FDMA-based IRS communications.