Antenna spacing requirement for a mobile Radio base-station diversity

The minimum required antenna spacing between two base-station antennas in order to take advantage of spatial diversity technique was investigated. The measurements were made for two cases: (i) the incoming radio signal was perpendicular to the axis of two base-station antennas (the broadside case), and (ii) the signal was in-line with the axis of two base-station antennas (the in-line case). The correlation of signals received from two separated antennas at the base station was found to be much higher for the in-line case than for the broadside case with any given antenna spacing. For correlation up to 0.7, from which most of the advantage of two-branch diversity can still be obtained, we found the minimum required antenna spacing is around 70λ–80λ for the in-line case and 15λ–20λ for the broadside case. In order to achieve a correlation always less than 0.7 between two base-station signals regardless of the arrival direction of the incoming signal, a triangular configuration with a three-antenna array used with a three-branch diversity receiver is proposed, requiring less antenna spacing in the array than for a two-antenna setup.