An approximate closed-form expression for 3-D channel spatial correlation (SC) is derived, given cross-polarized rectangular arrays and standards-based antenna radiation patterns. Furthermore, we evaluate the impact of 3-D SC on system performance and derive the capacity relationship between 2-D and 3-D channels. As expected, the 3-D channel is characterized by lower SC and higher channel capacity than its 2-D counterpart by taking the advantage of the additional degrees of freedom in the elevation domain, where the 3-D antenna arrays and the 3-D channel propagation are considered.