Comparative evaluation of multiple access and narrowband interference mitigation techniques for CDMA systems.

摘要

In this thesis the practical implementation of a multi-user CDMA system is considered. Comparative studies are made between different types of receivers with different types and lengths of signature code sequences. Multiple Access Interference (MAI) and Inter-Symbol Interference (ISI) are studied for Walsh and Gold signature codes. A unique approach of understanding these interferences is developed. Many practical systems have high BER requirements, and in order to achieve this BER, different lengths of Walsh and Gold codes are studied. It is shown that Gold-code sequences have better performance than Walsh-code sequences. The expense of increasing the transmission bandwidth with the increase of processing gain is studied for a different number of users.;The near-far effect, which is an issue in CDMA, is also considered. It is shown that adding the near-far effect does not significantly impact the BER for a lower number of users in a system. Thus, the processing gain takes care of some of the near-far effect when there are a lesser number of users in the system. Since most of the interferences are caused by the multipath nature of the channel, two different approaches to multiuser detection in fading multipath channels are studied. These are precombining and postcombining interference suppression receivers. The postcombining receiver depends on the instantaneous channel values and thus it is not recommended in a fast fading environment. Hence, an adaptive precombining rake receiver is studied, which does not depend on the channel values. The performance of the adaptive precombining receiver is compared with that of the conventional rake receiver. Processing cost vs. performance is studied for a practical implementation of this adaptive precombining receiver. Finally, the effect of narrowband interference is studied on the multi-user system. The location of the interference in the spread spectrum is studied and showed that regardless of the location of the interference, which corrupts the spread spectrum, it will degrade the performance of the system.