Holographic multiple-input multiple-output (MIMO) communications are widely recognized as a promising candidate for the next-generation air interface. With holographic MIMO surface, the number of the spatial degrees-of-freedom (DoFs) considerably increases and also significantly varies as the user moves. To fully employ the large and varying number of spatial DoFs, the number of equipped RF chains has to be larger than or equal to the largest number of spatial DoFs. However, this causes much waste as radio frequency (RF) chains (especially the transmit RF chains) are costly and power-hungry. To avoid the heavy burden, this paper investigates green holographic MIMO communications with a few transmit RF chains under an electromagnetic-based communication model. We not only look at the fundamental capacity limits but also propose an effective transmission, namely non-uniform holographic pattern modulation (NUHPM), to achieve the capacity limit in the high signal-to-noise (SNR) regime. The analytical result sheds light on the green evaluation of MIMO communications, which can be realized by increasing the size of the antenna aperture without increasing the number of transmit RF chains. Numerical results are provided to verify our analysis and to show the great performance gain by employing the additional spatial DoFs as modulation resources.

翻译：全息多输入多输出（MIMO）通信被广泛认为是下一代空中接口的重要候选技术。在全息MIMO表面中，空间自由度数量显著增加，并随用户移动而大幅变化。为充分利用这种大规模且动态变化的空间自由度，射频链路数量需始终不小于空间自由度的最大值。然而，这种配置方法会导致巨大浪费，因为射频链路（尤其是发射链路）成本高昂且功耗巨大。为避免上述重负，本文基于电磁通信模型，研究了采用少量发射射频链路的绿色全息MIMO通信。我们不仅分析了基础容量极限，还提出了一种高效传输方案——非均匀全息图案调制（NUHPM），以实现高信噪比条件下的容量极限。分析结果表明，通过增大天线孔径尺寸（而非增加发射射频链路数量）即可实现MIMO通信的绿色化演进。数值结果验证了理论分析，并展示了将额外空间自由度作为调制资源带来的显著性能增益。