On the use of CDMA for mobile satellite communication

Summary form only given, as follows. The performance of a satellite mobile system employing direct sequence (DS) CDMA is presented. One cannot expect CDMA performance on satellite links comparable to what one can achieve using CDMA on terrestrial links. It is shown that because of the length of the round trip propagation delay on a satellite channel, closed-loop power control is not effective. Because most of the proposed systems operate in a frequency division duplex (FDD) mode, open loop power control cannot track out variations due to multipath fading (although it can track variations in received power due to both distance/shadowing attenuation). The situation is exacerbated by the fact that the coherence bandwidth of many satellite links is on the order of 10 MHz. This implies that in order for a DS system to allow performance enhancement by virtue of RAKE reception, the spread bandwidth must exceed 10 MHz. Any signal using a smaller spread bandwidth will experience flat fading. This combination of flat fading and the lack of effectiveness of closed-loop power control causes a satellite-based CDMA system to perform significantly worse than a terrestrial one. The results emphasize the effects of power control error on the performance of the system. These results also include the use of dual satellite diversity to help alleviate the flat fading degradation referred to above. System capacity is examined from the perspectives of both average probability of error and outage probability. Further, capacity considerations are examined when multiple satellite systems attempt to share common spectrum.