Massive multiple-input multiple-output (MIMO) is esteemed as a critical technology in 6G communications, providing large degrees of freedom (DoF) to improve multiplexing gain. This paper introduces characteristic mode analysis (CMA) to derive the achievable DoF. Unlike existing works primarily focusing on the DoF of the wireless channel,the excitation and radiation properties of antennas are also involved in our DoF analysis, which influences the number of independent data streams for communication of a MIMO system. Specifically, we model the excitation and radiation properties of transceiver antennas using CMA to analyze the excitation and radiation properties of antennas. The CMA-based DoF analysis framework is established and the achievable DoF is derived. A characteristic mode optimization problem of antennas is then formulated to maximize the achievable DoF. A case study where the reconfigurable holographic surface (RHS) antennas are deployed at the transceiver is investigated, and a CMA-based genetic algorithm is later proposed to solve the above problem. By changing the characteristic modes electric field and surface current distribution of RHS, the achievable DoF is enhanced. Full-wave simulation verifies the theoretical analysis on the the achievable DoF and shows that, via the reconfiguration of RHS based on the proposed algorithm, the achievable DoF is improved.

翻译：大规模多输入多输出（MIMO）被视为6G通信中的关键技术，其通过提供大量自由度（DoF）以提升复用增益。本文引入特征模分析（CMA）来推导可达自由度。与现有工作主要关注无线信道自由度不同，我们的自由度分析同时纳入了天线的激励与辐射特性，这些特性影响着MIMO系统通信中独立数据流的数量。具体而言，我们利用CMA对收发天线的激励与辐射特性进行建模，以分析天线的激励与辐射行为。基于此，建立了基于CMA的自由度分析框架，并推导出可达自由度。随后，构建了以最大化可达自由度为目标的特征模天线优化问题。研究了一个在收发端部署可重构全息表面（RHS）天线的案例，并进一步提出了一种基于CMA的遗传算法来解决上述优化问题。通过改变RHS的特征模电场与表面电流分布，可达自由度得以提升。全波仿真验证了关于可达自由度的理论分析，并表明基于所提算法对RHS进行重构后，可达自由度获得了改善。