A systematic method based on the Theory of Characteristic Modes (TCM) for finding orthogonal radiation patterns of any electronically steerable parasitic array radiator (ESPAR) is described. This method can be useful for designing single-RF front-end multiple-input multiple-output (MIMO) systems in which orthogonal patterns can be utilized as a basis set for space modulation (SM) or full multiplexed MIMO systems. The method is based on the $N$ -port formulation of TCM and is not restricted to antenna type or array configuration. It can also provide closed formed solutions for the antenna element currents required to generate the orthogonal patterns. In addition an approach to finding the required load reactances in ESPAR for generating the orthogonal patterns is provided and is based on the quasi-Newton method utilizing a closed form expression for the initial approximate solution. Approximate estimation of the effective aerial degrees of freedom of the ESPARs is also discussed. Two simulation examples of ESPARs, a 4-element linear dipole array and an 8-element rectangular planar inverted-F antenna array, using SM as well as full multiplexed MIMO are provided, demonstrating the effectiveness of the proposed method.

中文翻译：

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单射频MIMO系统的ESPAR特征模式分析

描述了一种基于特征模式理论（TCM）的系统方法，用于找到任何电子可控寄生阵列辐射器（ESPAR）的正交辐射图。此方法对于设计单RF前端多输入多输出（MIMO）系统可能有用，在该系统中，可以将正交模式用作空间调制（SM）或全复用MIMO系统的基础集。该方法基于 $ N $ TCM的端口设计，并且不限于天线类型或阵列配置。它还可以为生成正交图所需的天线元件电流提供封闭的成形解决方案。此外，提供了一种在ESPAR中查找所需负载电抗以生成正交模式的方法，该方法基于准牛顿法，该方法将闭合形式的表达式用于初始近似解。还讨论了ESPAR有效空中自由度的近似估计。提供了两个ESPAR仿真示例，分别使用SM和全复用MIMO的4元素线性偶极子阵列和8元素矩形平面倒F天线阵列，证明了该方法的有效性。