Analysis of diversity effects for satellite communication systems

The transmission quality of LEO (low Earth orbit) satellite communication systems suffers from shadowed links and the power limitation of the satellites. In order to combat these two impacts, the use of diversity techniques is normally a good solution. Because of the non-frequency selective behavior of the satellite channel, frequency diversity cannot be exploited. Furthermore, time diversity is limited due to the short delay times allowed for speech transmission. Under these circumstances the application of space diversity can improve the transmission quality enormously. A basic model of a LEO satellite concept is the Globalstar system. This system utilizes DS-CDMA technology (direct sequence-code division multiple access), in which space diversity is implemented by combining the convolutionally encoded signals from different satellites with the help of a RAKE receiver. The performance of this concept is compared to a new approach using turbo codes. Regarding the transmission via two satellites, the coded bits of the two coders are split between the two satellite channels. At the receiver iterative decoding can be implemented. This concept needs less processing time and it performs better than the Globalstar system in terms of the bit error rate