Study On The Antifading Technologies Of The Mobile LEO Satellite Constellation CDMA System

The small LEO satellite communication is the trend of satellite communications. The mobile LEO small satellite CDMA communication is the joint result of satellite communications and land mobile communications.This dissertation has investigated the communication channel of the small LEO satellite CDMA system. The Loo model is the conventional channel model in satellite communication systems. The power spectrum of the Loo model is analyzed deeply and an important conclusion has been got, which is that the power spectrum of the Loo model is not the Jakes model.In the small LEO satellite CDMA system, there are always more than two satellites overhead at the some place of the land. Therefore this dissertation proposed a scheme of delay transmission diversity. Its performance of maximal ratio combining and selection diversity in the Loo model is analyzed. The comparison between the performance of maximal ratio combining and the performance of the selection diversity verifies that the system of maximal ratio combining is better than the system of selection diversity such as Globalstar in the performance and the capacity.The analyses of the multipath combining strategy, the system capacity, the satellite coverage, the communication link process and the diversity receiving illuminate father that the delay transmission diversity can be realized.A Rake receiver is used to get the diversity gain. According to the large delay inthe satellite system, this dissitation proposed that the multipath can be combined in the rage of a frame, and the Rake receiver structure can be bettered. Since the channel of the small LEO satellite CDMA system is time-varying, this dissertation proposes a scheme of the channel estimation, in which WMSA is used when the channel varies slow, and the method of interpolation on a second-order curve with the least square is used when the channel varies faster.As for communication system of the burst mode, if the synchronization time is shorter, the effectivity is higher. Under the condition of the large Doppler and the low SNR, the span of the frequency scanning shceme is bigger, or the acquisition time of the pseudo-noise code is shorter, the system synchronization time is shorter.In the small LEO satellite CDMA system, the large Doppler shift of thecarrier is the main factor, but when it is lower than — , the performanceATdegradation is negligible, so this dissertation proposed a scheme on carrier synchronization, in which the loop whose frequency estimion rank is up to — isused at first, then it is switched to another loop who can estimate the frequency bias more precisely and can track the variation of the frequency. In the small LEO satellite CDMA system, the multipath and shadowing makes the detect probability of the spread spectrum code drop largely. The multiple-dwell serial search spread-spectrum code acquisition is capable to improve the detect probability. This dissertation proposed a simplified multi-dwell acquisition model so that the hardware complexity is lower when the detect probability keeps a level. At last a simplified link is simulated and its result of the synchronization scheme is given.