Performance Analysis of Compact FD-MIMO Antenna Arrays in a Correlated Environment

Full dimension multiple-input-multiple-output (FD-MIMO) is one of the key technologies proposed in the third Generation Partnership Project (3GPP) for the fifth generation communication systems. The reason can be attributed to its ability to yield significant performance gains through the deployment of active antenna elements at the base station in the vertical as well as the conventional horizontal directions, enabling several elevation beamforming strategies. The resulting improvement in spectral efficiency largely depends on the orthogonality of the sub-channels constituting the FD-MIMO system. Accommodating a large number of antenna elements with sufficient spacing poses several constraints for practical implementation, making it imperative to consider compact antenna arrangements that minimize the overall channel correlation. Two such configurations considered in this paper are the uniform linear array (ULA) and the uniform circular array (UCA) of antenna ports, where each port is mapped to a group of physical antenna elements arranged in the vertical direction. The generalized analytical expression for the spatial correlation function (SCF) for the UCA is derived, exploiting results on spherical harmonics and Legendre polynomials. The mutual coupling between antenna dipoles is accounted for and the resulting SCF is also presented. The second part of this paper compares the spatial correlation and mutual information (MI) performance of the ULA and UCA configurations in the 3GPP 3-D urban-macro and urban-micro cell scenarios, utilizing results from random matrix theory on the deterministic equivalent of the MI for the Kronecker channel model. Simulation results study the performance patterns of the two arrays as a function of several channel and array parameters and identify applications and environments suitable for the deployment of each array.