Study of Mutual Coupling in Finite Antenna Arrays for Massive MIMO Applications

Licentiate thesis, 2020

This thesis focuses on the study of mutual coupling (MC) in finite antenna arrays for base station antennas (BSAs) for Massive multiple-input multiple-output (MIMO) applications, with an emphasis on the development of a computationally-efficient modeling technique for the analysis of MC which can be readily applied in the design or synthesis schemes for BSAs. Traditionally, the effects of MC have been ignored or underestimated in the analyses performed within the information-theoretic-based communities by assuming idealized antenna elements with no MC between them or by considering the fictitious isotropic radiator models. In contrast, this thesis demonstrates the essentialness of proper modeling and inclusion of the physical antenna effects in the models used to predict the performance of a Massive MIMO system, as evidenced through the performed sum-rate analysis of a downlink line-of-sight (LoS) MIMO system in the presence of MC.

The developed model for the analysis of MC is inspired by the concept of multiple scattering by which the overall effect of the antenna array MC can be determined by cascading the scattering responses of all array elements. Such an approach requires the full-wave characterization of only a single element in isolation, while the mutual interactions between different elements are modeled by approximating the incident field as a single plane wave with mutually-orthogonal polarization taken from the spherical wave expansion (SWE) of the field scattered from any other array element. This process is described mathematically through the iterative scheme based on the classical Jacobi and Gauss-Seidel iterative methods.

Additionally, a sum-rate model of a downlink LoS multi-user MIMO system including the MC, has been developed. Herein, the effects of MC are accounted through the S-matrix of the BSA and the embedded element patterns (EEPs) of all BSA elements, which are used to approximate the channel matrix in a LoS environment. The S-matrix and the EEPs obtained by using the Jacobi-based MC model have been incorporated into the MIMO system model, showing good agreement in terms of the achievable sum rate compared to the reference result which uses the MoM-based simulation data. The accuracy and run-time benefits of the Jacobi-based model make it a possibly promising candidate for use in BSA design and synthesis applications, particularly when large array configurations need to be (repeatedly) analyzed.