Performance Evaluation of a Multi Carrier CDMA System for LEO, MEO Mobile Satellite Communications

摘要

Recently there has been an increasing interest in the application of the direct sequence Code Division Multiple Access (CDMA) technique as a radio interface to provide multimedia services for mobile radio communications. To improve the capacity of an asynchronous CDMA system specialized signal processing techniques based on multiuser detection, could be employed. Another alternative is based on the attempting to synchronize transmissions at the chip level and to use orthogonal spreading codes, such as Walsh Hadamard codes. This approach is commonly used in the forward link channel. However its implementation in the uplink channel may be difficult, especially for CDMA systems with small chip periods, since the synchronization offset among the users should be maintained under a small fraction of the chip period. Increasing the chip period would make easier the synchronization of the signals. For such purpose a multi carrier scheme can be used. A lot of hybrid schemes named Multi Carrier CDMA (MC-CDMA) have been presented in literature. They are based on a code division and orthogonal frequency division multiplexing (OFDM) like techniques. In this paper the performance of a quasi-synchronous MC-CDMA technique is evaluated through simulation in a realistic mobile satellite communication scenario on the uplink communication channel. Nongeostationary satellite constellations are considered. The effects of frequency errors, due to the mobility of the users and of the satellites and/or to the instabilities of the oscillators, are now included. A semianalytic approach is used to perform the calculation of the performance parameters. Results are compared with those for single carrier DS-CDMA system.