Future wireless systems are envisioned to create an endogenously holography-capable, intelligent, and programmable radio propagation environment, that will offer unprecedented capabilities for high spectral and energy efficiency, low latency, and massive connectivity. A potential and promising technology for supporting the expected extreme requirements of the sixth-generation (6G) communication systems is the concept of the holographic multiple-input multiple-output (HMIMO), which will actualize holographic radios with reasonable power consumption and fabrication cost. The HMIMO is facilitated by ultra-thin, extremely large, and nearly continuous surfaces that incorporate reconfigurable and sub-wavelength-spaced antennas and/or metamaterials. Such surfaces comprising dense electromagnetic (EM) excited elements are capable of recording and manipulating impinging fields with utmost flexibility and precision, as well as with reduced cost and power consumption, thereby shaping arbitrary-intended EM waves with high energy efficiency. The powerful EM processing capability of HMIMO opens up the possibility of wireless communications of holographic imaging level, paving the way for signal processing techniques realized in the EM-domain, possibly in conjunction with their digital-domain counterparts. However, in spite of the significant potential, the studies on HMIMO communications are still at an initial stage, its fundamental limits remain to be unveiled, and a certain number of critical technical challenges need to be addressed. In this survey, we present a comprehensive overview of the latest advances in the HMIMO communications paradigm, with a special focus on their physical aspects, their theoretical foundations, as well as the enabling technologies for HMIMO systems. We also compare the HMIMO with existing multi-antenna technologies, especially the massive MIMO, present various...

翻译：未来无线系统被设想为能够创建一个具有全息能力、智能且可编程的无线电传播环境，从而为高频谱效率与能量效率、低延迟以及大规模连接提供前所未有的能力。支持第六代（6G）通信系统预期极端需求的一项潜在且有前景的技术是全息多输入多输出（HMIMO）概念，它将以合理的功耗和制造成本实现全息无线电。HMIMO由超薄、极大且近乎连续的表面构成，这些表面集成了可重构且亚波长间距的天线和/或超材料。包含密集电磁激励元件的此类表面能够以最大灵活性和精度记录并操控入射场，同时降低成本和功耗，从而以高能效塑造任意目标的电磁波。HMIMO强大的电磁处理能力开启了达到全息成像级无线通信的可能性，为在电磁域（可能与其数字域对应技术相结合）实现的信号处理技术铺平了道路。然而，尽管潜力巨大，HMIMO通信的研究仍处于初始阶段，其基本极限尚待揭示，且存在一定数量的关键性技术挑战需要解决。在本综述中，我们全面概述了HMIMO通信范式的最新进展，特别关注其物理方面、理论基础以及HMIMO系统的使能技术。我们还对HMIMO与现有多天线技术（尤其是大规模MIMO）进行了比较，并介绍了多种……