  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.
