Holographic MIMO refers to an array (possibly large) with a massive number of antennas that are individually controlled and densely deployed. The aim of this paper is to provide further insights into the advantages (if any) of having closely spaced antennas in the uplink and downlink of a multi-user Holographic MIMO system. To this end, we make use of the multiport communication theory, which ensures physically consistent uplink and downlink models. We first consider a simple uplink scenario with two side-by-side half-wavelength dipoles, two users and single path line-of-sight propagation, and show both analytically and numerically that the channel gain and average spectral efficiency depend strongly on the directions from which the signals are received and on the array matching network used. Numerical results are then used to extend the analysis to more practical scenarios with a larger number of dipoles and users. The case in which the antennas are densely packed in a space-constrained factor form is also considered. It turns out that the spectral efficiency increases as the antenna distance reduces thanks to the larger number of antennas that allow to collect more energy, not because of the mutual coupling.

翻译：全息MIMO指一种天线阵列（可能规模较大），其中包含大量独立控制且密集部署的天线。本文旨在深入探讨多用户全息MIMO系统上行链路和下行链路中密集天线间距的优势（若有）。为此，我们利用多端口通信理论，该理论确保了上行和下行链路模型的物理一致性。首先考虑一个简单的上行场景：两个并排的半波长偶极子、两个用户及单路径视距传播。解析与数值分析均表明，信道增益和平均频谱效率高度依赖于信号接收方向以及所使用的阵列匹配网络。随后，通过数值结果将分析扩展至包含更多偶极子和用户的更实际场景。此外，还考虑了天线在空间受限因子形式下密集排列的情况。结果表明，频谱效率随天线间距减小而提升，其原因是更多天线可收集更多能量，而非互耦效应所致。