Downlink DS CDMA performance over a mobile LEOs channel

Code division multiple access (CDMA) has been proposed for personal communications networks for both terrestrial and satellite links. We analyze the performance of the down-link (i.e. the base-to-mobile link) of a low Earth orbiting (LEO) mobile satellite channel. An important characteristic of this fading channel is that the desired signal and the multiple access interference from the same base station fade simultaneously. As a consequence, the performance degradation due to fading will be smaller than for the mobile-to-base links, where fading of the desired signal and the interference are independent. In the limit, one can distinguish between two extreme scenarios. When the thermal noise is dominant relative to multiple access interference, the performance approaches that of a single user in fading channels. At the other extreme, when the energy-per-bit-to-noise power spectral density ratio is very high, the effect of fading on the desired signal becomes negligible, because the equivalent signal-to-noise ratio remains practically constant, in the absence of multiple access interference from other base stations. In practical situations, there may be multiple access interference from other satellites, and the ratio of energy-per-bit-to-thermal noise power spectral density has some finite valuer between the two extreme cases. We analyze the probability of error for the down-link of a mobile satellite channel when coherent detection, diversity and forward error correction are employed.<>