Multipath diversity combining and synchronization for the IS-95 CDMA system
by Wong Fei Wah
M.Phil. Electrical and Electronic Engineering
xi, 68 leaves : ill. ; 30 cm
Code division multiple access (CDMA) has become more and more popular in recent years. Rake Receiver and synchronization are two essential components in a CDMA receiver....[ Read more ]
Code division multiple access (CDMA) has become more and more popular in recent years. Rake Receiver and synchronization are two essential components in a CDMA receiver.
A Rake Receiver, which consists of multiple Rake Fingers, can provide multipath diversity. The diversity gain can be maximized by increasing the number of Raite Fingers, or by improving the combining technique adopted in the Rake Receiver. The former method works only when the number of multiple paths is larger than the number of Rake Fingers. As, in most practical situations, there are typically only a few multiple paths, increasing diversity gain by the former method is not always possible.
In this thesis, the effect of using different combining techniques in the Rake Receiver was analyzed, and a new and simple combining scheme for the Rake Receiver was proposed based on the analysis. Results show that by using the new combining scheme, the output signal-to-noise-interference ratio (SNIR) is improved, and is close to that which would be obtained through the use of optimal combining.
In a CDMA system, a fast and simple synchronization algorithm which would allow the PN code phase of the received signal to be found quickly is desirable. In this thesis, the biased Sequential Probability Ratio Test (SPRT) synchronization algorithm was extensively investigated and compared with the Fixed Sample Size (FSS) synchronization algorithm, through simulations. An optimal set of parameters (the bias and the thresholds) which is independent of the instantaneous SNIR, is found. It was also found that the biased SPRT is almost twice as fast, gives similar performance, and is less complex when compared to the FSS scheme.
Crystals are imperfect and the potential discrepancy between the crystals used in the transmitter and the receiver will manifest as frequency offset in the received signal. Hence, at start up, the receiver has to obtain the PN code phase and the frequency offset in the received signal. The total time required to search through all the PN code phases and the frequency offsets is critical. In this thesis, an optimal set of parameters (the number of frequency bins and the length of pilot averaging) which minimizes the total search time was determined. Finally, practical synchronization implementation issues were addressed at the end of this thesis.