Ultra-wideband direction-of-arrival considerations for antenna arrays in the presence of mutual coupling

This paper discusses the theoretical considerations for direction-of-arrival (DOA) estimation using antenna arrays in the presence of mutual coupling. In arrays, the relative proximity of antenna elements results in some manner of near-field mutual coupling that can negatively impact the array performance. In particular, mutual coupling can degrade the quality of DOA estimations and reduce the ability of the array to perform high-quality correlation processing and direction finding. The expected variance of an array performing DOA estimation is inversely related to the Fisher information matrix of the system. Theoretical radiated fields of a linear antenna array were compared to more realistic behavior of the equivalent architectures produced in electromagnetics simulation software. The mutual coupling between all the elements in an array can be a difficult phenomenon to precisely define, as it is an iterative process with many higher-order effects. To circumvent this, a mutual coupling matrix is defined as the relation between the theoretical radiation characteristic of an array and its simulated counterpart. An inverse solution method was used to solve for the mutual coupling interactions. The expected impact of mutual coupling in a DOA estimation context was then mathematically established by propagating the mutual coupling matrix through calculation of the Fisher information matrix and compared to the case of no mutual coupling. It was found that taking mutual coupling into consideration yields a higher Cramer-Rao Lower Bound and as a result a greater RMS angle error in a DOA estimation context. Mutual coupling was also found to have a somewhat greater impact on the Cramer-Rao Bound at S-band than at X-band.