Crosscorrelation between the envelopes of 900 MHz signals received at a mobile radio base station site

An experimental investigation is reported of the crosscorrelation of 900 MHz signals received by two spatially separated antennas at a base station. The investigation embraced vertical, horizontal and combined horizontal and vertical separation of the antennas, for transmission from test routes 1.3 km from the base station. It was found that a crosscorrelation ≤0.7 (i.e. when diversity improvement becomes significant) can best be achieved using vertical separation of the antennas of between 11 λ and 13 λ for the 1.3 km cell radius. At 900 MHz such an antenna separation is easily obtained and, in addition, the roof space required is small. Moreover, the crosscorrelation using vertically spaced antennas is independent of the incoming arrival angle (unlike horizontally spaced antennas), and hence low correlation can be achieved while maintaining omnidirectional coverage.