Antenna diversity in outdoor microcells

For a diversity scheme to be effective decorrelated channels of nominally equal mean levels are required. For macrocellular base stations two vertically polarised antennas might be used separated in either the horizontal or vertical plane. Correlation coefficients less than or equal to 0.7 result in significant diversity gain and several studies have been made to determine the minimum antenna separation required to achieve this for macrocellular applications. Typically spacings of 10-20 wavelengths are required. This large separation is due to the narrow angular spread of the multipath signals at the base station. For microcells the base station antenna height is much lower and hence the angular spread will be much larger. Consequently, the antenna separation required for high diversity gains will be much smaller. This paper presents the results of some measurements made at 948 MHz (CT2+) to demonstrate this in some real outdoor microcell environments and to suggest suitable antenna separations.