When Future Communications Shift Toward Narrow Beams: A Forward-Looking Survey on Pointing Errors and Alignment Limits

Directional links in free-space optical (FSO), millimeter-wave (mmWave), and terahertz (THz) systems are a cornerstone of emerging 6G networks, yet their reliability is fundamentally limited by pointing errors and misalignment. Existing studies address this impairment using technology-specific definitions, models, and mitigation approaches, which hinders cross-domain comparison and transferable design insight. This survey provides a unified treatment of pointing errors across optical and high-frequency wireless communications. We establish consistent terminology and a cross-technology taxonomy of pointing errors, review angular misalignment and statistical distribution models, and analyze their impact on system performance. Mitigation techniques are systematically surveyed with emphasis on optical systems and their connection to underlying pointing-error models. The survey further provides a detailed examination of pointing-error effects in orbital angular momentum (OAM) links and quantum optical communications, and surveys the corresponding mitigation approaches tailored to mode-dependent impairments and quantum measurement constraints. The survey also outlines open challenges and future research directions. By consolidating fragmented literature into a coherent framework, this work supports consistent analysis and robust design of next-generation directional communication systems.