Advanced communications technology satellite Ka-band propagation research in a spatially diversified network with two ultrasmall-aperture terminals

Congestion in the radio spectrum below 18GHz is stimulating greater interest in the Ka (20/30GHz) frequency band. Transmission at these shorter wavelengths is greatly influenced by rain resulting in signal attenuation and decreased link availability. The size and cost of Ultra Small Aperture Terminals (USATs) make site diversity attractive for rain fade compensation. Separation distances between terminals must be small to be of interest commercially. This study measured diversity gain at a separation distance <5km and investigated utilization of S-band weather radar reflectivity in predicting diversity gain.;Two USAT ground stations, separated by 2.43km for spatial diversity, received a continuous Ka-band tone sent from NASA Lewis Research Center via the Advanced Communications Technology Satellite (ACTS) steerable beam. Received signal power and rainfall were simultaneously measured using rain gauges for rainfall at the test sites and weather radar for precipitation along the USAT-to-ACTS slant paths. Signal attenuation was compared for the two sites, and diversity pin was calculated for fades measured on eleven days. Calculated diversity gain exceeded values predicted using the ITU-R diversity gain model.;Correlation of S-band radar reflectivity with measured Ka-band attenuation consisted of locating radar volume elements along each slant path, converting reflectivity to Ka-band attenuation, with rain rate calculation as an intermediate step. Specific attenuation for each associated path segment was summed, resulting in total attenuation along the slant path. Derived Ka-band attenuation did not correlate closely with empirical data (r = 0.239), but a measured signal fade could be matched with an increase in radar reflectivity in all fade events. Applying a lowpass filter to radar reflectivity prior to deriving Ka-band attenuation improved the correlation between measured and derived signal attenuation (r = 0.733).;Results indicate that site diversity at small separation distances is a viable means of rain fade compensation, and that existing models underestimate diversity gain for this geometry in Florida. Also, Ka-band attenuation can be predicted from radar reflectivity after applying a lowpass filter.