基于物理的6极化MIMO信道建模理论及信道特性研究

The multipolarized MIMO technology provides another dimensionality for wireless communications in addition to the time, frequency and spatial domains through the utilization of the polarization characteristics of electromagnetic waves. Furthermore, the multipolarized MIMO systems could be realized through the co-located antennas, which provide an architecture for the minimization of MIMO antenna arrays and also a kind of realization of MIMO system in the strong LOS (line-of-sight) environments.The theoretical research results show that the polarization degrees of freedom (DOF) obtained from the hexapolarized MIMO system is 2-6, which depends strongly on the scattering characteristics of the environment. However, up to now there is no accurate description of the inherent relationship between the scattering mechanism and the coupling effect among polarizations of the multipolarized systems all over the world, and unequal DOFs could be obtained through different channel modeling methods for the same type of communication environment.In this project, we will try to find the inherent connection between the characteristics of multipolarized MIMO channel and the scattering situations of the environment through the combination of electromagnetic wave propagation theory and the information theory. We will begin with the study of the coupling effect on the polarizations by the single scatter, and then utilize a physics-based channel modeling which combines the full-wave electromagnetic analysis and the statistical descriptions of the distribution of scatterers. Moreover, we will test the accuracy of the channel models through several kinds of experiments, including the numerical computation, chamber test and field measurement. Through the implementation of this project, the understanding of the physical properties of vector electromagnetic channels will be advanced and the potential of using multipolarized antennas in some typical wireless systems will be revealed, such as the cellular wireless networks, the wireless LANs and the terrestrial multimedia broadcasting networks .

多极化MIMO 技术通过对电磁波极化特性的利用使无线通信系统的容量在时、频和空域之外获得了另一维度，并可利用空间共点的天线来实现，为MIMO 天线阵的小型化和强直射信道中MIMO技术的实现提供了一种可能。理论研究表明6 极化MIMO 系统可获得的自由度为2-6，强烈依赖于环境的散射特性。但目前世界范围内还缺乏对多极化MIMO信道散射机理与极化耦合作用间的内在联系的准确描述。本项目将通过电波传播理论与信息理论的结合揭示多极化MIMO信道特性与环境散射状况间的本质联系。准备从单个散射体的极化耦合作用研究入手，采用一种结合全波电磁场分析和散射体分布的统计描述的基于物理的多极化MIMO 信道建模方法，并通过数值计算、混波室实验和现场测量等多种角度对信道模型的准确性进行验证。通过本项目将推进对矢量电磁信道物理特性的理解，揭示此技术在蜂窝网，无线局域网和地面多媒体广播网等一些典型无线系统中的应用潜力。

由于设计6极化天线难度很大，因此世界范围内对多极化MIMO信道特性的研究多限于理论层面。在相关研究中，多针对理想点源天线采用基于远场平面波的统计模型或模式理论，得到的结论是在多径丰富的信道中6极化MIMO 系统的自由度最大值为6。但在基于远场平面波分解的信道模型中，只考虑了电磁波的传播成分，而忽略了其消散成分，严重影响了多极化MIMO信道建模的准确性。本项目提出了基于电磁场物理特性的MIMO信道建模理论，首先利用理想电、磁偶子天线的Maxwell方程的精确解获得了自由空间中6极化MIMO信道矩阵，在此基础上提出了一种基于虚源方法的信道建模理论，获得了存在多个反射面的信道中精确的、同时适用于远场和近场的电磁场解析表达式，从而可以准确地体现天线的辐射和信道的散射特性对多极化MIMO信道特性的共同影响。M. Andrews等发表于nature上的文章中指出，在自由空间中，即使使用6极化天线，所获得的自由度也是2，但这是基于远场平面波的假设。而我们发现，利用Maxwell方程精确解所得到的自用空间中的6极化MIMO信道矩阵也存在6个非零特征值，只是其中4个在远场非常小。随着多径散射的增加，这些小的特征值在远场中获得明显增加。当多径分布从1维增加到2维和3维时，6极化信道容量增益的平均值从4.25增加到4.70 和4.93，最大值从5.50增加到5.67和5.76。此外还通过全波电磁场数值计算对理论结果进行了验证，两者吻合较好。以往的多极化天线设计中，多考虑电偶极子天线，而低耦合的共点多极化磁偶极子天线由于设计难度较大，还很少见到。本项目采用两种基于不同零相移传输线的环天线，得到了一种共点正交、低耦合的三极化环天线，并设计了一种三极化电偶极子天线，对它们的MIMO信道特性在无损混波室、有损混波室、走廊和办公室四种环境下分别进行了广泛的测量和比较。实验结果发现，在这四种多径分布明显不同的环境下，对应以上两种三极化天线的MIMO信道矩阵的条件数都具有相似的、接近独立同分布复高斯信道的概率密度。首次证明了利用三个共点正交三极化环天线在现实的通信环境下与三个正交电偶极子天线类似，也可获得3路独立信号。通过本项目，推进了对多极化MIMO信道特性的认识和理解，对多极化天线的设计和实现也提供了重要的参考价值和指导意义。成果已授权发明专利并在IEEE TWC, IEEE TAP等期刊发表。

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