I. Introduction
Multiple-input multiple-output (MIMO) technologies have drawn attention for their ability to increase spectral efficiency and system capacity significantly [1]. Meanwhile, despite difficulty and high price in realization, the three dimensional (3D) antenna array is a promising solution to improve directional performance of MIMO systems. However, insufficient antenna spacing or lack of scattering would reduce these benefits due to increased spatial correlation (SC). Therefore, the exploitation of SC is vital for design, optimization, and performance evaluation of wireless MIMO communication systems. Especially, the closed-form expressions of spatial correlation functions (SCFs) are essential to derive the theoretical results of system performance [2], [3], i.e., capacity, energy efficiency, and bit error rate (BER).