Research On An Anti-multipath CDMA Baseband Receiver

With the improvement of the data rate of wireless communication and the require-ments of communication quality,the interfering of the multipath signal to system isbecoming more and more concerned,especially to the direct sequence spread-spectrum(DSSS) system. Due to the particularity of sea surface communication environment,there?ections of waveguide layers and sea surface make the multipath fading become themain interference sources. The study of CDMA wireless baseband receiver is based onthe communication environment mentioned above. Because the displacement betweenthe receiver and the transmitter lessens as the time passed by,the relative delay of themultipath signal varies from more than a chip width to less than a chip width. Whilethe traditional RAKE receiver can distinguish and use the power of the multipath signalwhose relative delay is more than a chip width to acquire the diversity gain; therefore theRAKE technology can suppress this kind of multipath interference. But when the rela-tive delay of multipath signal is less than a chip width,the synchronization and bit errorrate (BER) performance of the system deteriorates badly. This article aims to study sys-tematically on how the sub-chip multipath components affect the CDMA system and thereasons cause the above issues. More deeply,we propose a solution that can suppressthe interference of sub-chip components to the CDMA receiver practically.This paper mainly discusses the following two aspects:First,the in?uence of sub-chip multipath signal to CDMA baseband receiver isstudied. Due to the fewer amounts of literatures on how sub-chip signals affect the DSSSsystem,the subject focuses on the synchronization and BER performance of the receiverunder the sub-chip multipath fading channel. Through analyzing the in?uence of the sub-chip multipath components of different delay on the receiver,the reasons that deterioratethe synchronization and BER performance can be found.Next,an anti sub-chip multipath algorithm is proposed,which is applied to CDMAreceiver characterized by multipath interference of high amplitude ratio and low samplerate per chip. The anti-multipath algorithm of amplitude tracking lock loop is put forwardto compensate multipath interference both in synchronization acquisition loop and track-ing loop. The algorithms can solve the problem that multipath fading makes the systemhard to catch the phase of the received spread spectrum sequences. The system stimula- tion platform based on MATLAB is built and the algorithm is stimulated and analyzed onthis platform. In conclusion,with the algorithm mentioned above applied to the system,the performance of CDMA receiver under sub-chip multipath fading channels is improvedeffectively in a long distance communication.