I. Introduction
Signal correlation is a critical metric in evaluating the performance of multiple antenna systems, where multiple antennas are employed at both base stations and terminals to improve the performance of wireless communications [1]. Conventionally, for the reference multipath environment of uniform 3-D angular power spectrum (APS), signal correlation between two antennas can be calculated from the full spherical radiation patterns of the antennas, which are obtained with both phase and polarization information [2]. Unfortunately, it is both time-consuming and expensive to measure antenna patterns, due to the use of specialized equipment and facilities.