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.

翻译：先前关于智能反射面（IRS）的研究大多仅考虑点对点通信（包括单用户场景）以简化分析。然而，实际无线系统需同时支持多用户。由于缺乏频率选择性反射能力（即相移集无法在不同频率子信道间差异化），IRS的集成对频分多址（FDMA）及广泛采用的正交FDMA（OFDMA）等频分复用方法构成了根本性挑战。这促使我们研究基于（O）FDMA的多用户IRS通信，以阐明在此非频率选择性反射环境下何种用户调度与无源波束赋形方案更优。理论分析与数值评估表明：当用户数量较少时，（O）FDMA无需用户调度；当用户数量增大时，无论用户调度还是IRS反射优化均无必要。这些发现有助于大幅简化基于（O）FDMA的IRS通信系统设计。