  In this paper, we completely eliminate all numerical integrations needed to
compute the far-field envelope cross-correlation (ECC) in
multiple-input-multiple-output (MIMO) systems by deriving accurate and
efficient analytical expressions for the frequency-domain cross-correlation
Green's functions (CGF), the most fundamental electromagnetic kernel needed for
understanding and estimating spatial correlation metrics in multiple-antenna
configurations. The analytical CGF is derived for the most general
three-dimensional case, which can be used for fast CGF-based correlation matrix
calculations in MIMO systems valid for arbitrary locations and relative
polarizations of the constituent elements.
