Site diversity for high-altitude platforms: a method for the prediction of joint site attenuation statistics

The technique of site diversity for satellite-Earth radio links exploits the fact that high rain rates are associated with relatively small rain cells, of the order of a few kilometres diameter. Interest in high-altitude platforms (HAPs) raises the possibility of using site diversity on HAP-Earth links. However, while an ITU model exists for the prediction of site diversity gain (and hence joint site attenuation statistics) on satellite-Earth links, the method is entirely empirical and is not directly applicable to HAPs. The paper combines various models in the literature to provide a method for predicting the joint site attenuation statistics for a HAP linked to two Earth stations. The results show that there is an optimum site separation in the range 0-20 km, depending on the frequency and availability level required. For example, for the licensed HAP frequencies of 28 and 48 GHz, the optimum site separation for 99% availability lies in the range 5-10 km. This distance increases to 14-16 km for an availability of 99.9%. Although to date there are no experimental HAP-Earth data available, the method is readily adapted to the satellite-Earth scenario, where experimental data are available. The method is found to predict joint site attenuation statistics at 12 GHz with an overall RMS error of less than 0.4 dB, considering a limited amount of experimental data. The RMS error increases from 0.11 dB at 90% availability to 0.57 dB at 99.99% availability