Study For Performance Of Ka-band Digital Satellite Communications System In Rainfall Enviroment

In this paper, firstly,the significance of Ka-band satellite communication systems and the development of the status at home and abroad are introduced. The influence factors of atmosphere on Ka-band satellite communication signal are discussed. In Ka-band, the absord of the oxygen and water vapor and the attenuation of the cloud are studied. On the Earth-space path, the atmospheric attenuation characteristic is researched in the Xi'an area. Based on the mechanism of rain attenuation, the special attenuation by rain is calculated by the equivalent dielectric permittivity model. According to ITU-R rain attenuation prediction model on the Earth-space path and the equivalent dielectric permittivity model, the rain attenuation in Xi'an and Guangzhou are calculated in Ka-band basing on 92°E orbital position of the geostationary satellite. Comparing to the results of calculating multi-year monthly average 5-minute rain rate, in low-frequency, the equivalent dielectric permittivity model is accuracy in Xi'an area. In high-frequency, ITU model is high accuracy in Guangzhou. Based on rain depolarization mechanism, combinating with the cross-polarization (XPD) ITU model, the cross-polarization over time probability distribution of Xi'an is forecasted. In the rainfall circumstance the probability distribution of satellite channel capacity is analyzed in QPSK modulation and 4QAM modulation. The BER characteristics in different link margin for up-link and down-link in Ka-band are analyzed at different modulations, respectively. Based on the RS codes, the improvements in system performance are given for different coding efficiency. The results show that in the case of the smaller link margin, the outage probability caused by the rainfall has little effect on the system in the Xi'an area. In the guangzhou area in order to make the outage probability caused by the rainfall meet the system, the larger link margin and RS codes is required. Finally, the effect of the rainfall depolarization on the dual-polarization frequency reuse system is analyzed. It has actual significance for the development of the satellite digital communications system.