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构建一种内禀全息能力、智能化且可编程的无线电传播环境，这将为高频谱效率、高能量效率、低延迟及大规模连接提供前所未有的能力。为支撑第六代（6G）通信系统的预期极端需求，一种潜在且具有前景的技术是全息多输入多输出（HMIMO）概念，该概念将实现具有合理功耗和制造成本的全息无线电。HMIMO依赖于超薄、极大且近乎连续的表面，这些表面集成了可重构且亚波长间距的天线和/或超材料。由密集电磁（EM）激发单元构成的此类表面能够以极高的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全息成像级无线通信的可能性，为在电磁域实现的信号处理技术（可能与其数字域对应技术协同）铺平道路。然而，尽管潜力巨大，HMIMO通信的研究仍处于初期阶段，其基本极限尚待揭示，并且存在一定数量的关键性技术挑战亟待解决。本综述全面概述了HMIMO通信范式的最新进展，特别关注其物理层面、理论基础及HMIMO系统的使能技术。我们还将HMIMO与现有多天线技术（尤其是大规模MIMO）进行比较，并呈现多种...