Performance Improvement of GNSS Receiver by Mitigation of Multipath Effects

Abstract

The Rake Receiver is excessively used in modern CDMA communication but for navigation application it is not self-sufficient to provide satisfactory performance. To overcome the shortcomings of Rake Receiver for navigation purpose one needs to introduce some differential rake architecture which will mitigate the multipath corresponding to the incoming data after it has been processed and demodulated. In this manner a particular will be able to cancel out the multipath and at the end will have only the strongest multipath from which decision can be made to recover the data back. The other very critical module is Delay Locked Loop (DLL) which will align the code at the receiver end so as to minimize the pseudo range error. The DLL will try to lock the incoming signal with the local code and in order to do so it will consider 3 different locally generated codes Early, Prompt and Late. According to the parameter defined, it may accuracy up to one tenth of a chip. The DLL will use the code-phase provided by the previous blocks and try to find the local code which will give us the minimum pseudo range error. If the multipath signals are delayed by more than 1.5 chips then matched filter algorithm will detect all three signals by processing on auto-correlation function. But when delay is less than 1.5 chips then NLMS algorithm is used for multipath detection. This two algorithm is incorporated in this design. Whenever incoming signal is received it will first try to find out multipath components within 1.5 chips. After that it will go to the next step in order to find multipath components outside 1.5 chips. In this project the above mentioned approaches are combined so as to get a system which will give the optimum performance in terms of the SNR and the pseudo range.