User Scheduling and Passive Beamforming for FDMA OFDMA in Intelligent
Reflecting Surface
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.