Cell Search in W-CDMA

In W-CDMA system, synchronization among base stations is not necessary. Each cell is identified by a unique primary scrambling code. Cell search is the process of the mobile station searching for a best cell and achieving time synchronization to its downlink primary scrambling code. Cell search is performed when 1) the mobile station is just switched on and tries to find a best serving cell (initial cell search) 2) when the mobile station has camped on a cell, and is looking for another that has a better transmission quality to it, in order to switch to (target cell search) or in idle mode. In addition, the low cost oscillator used in the mobile station usually incurs a large frequency offset when the mobile station is switched on. This practical defect further complicates the initial cell search.
In general, the cell search process is divided into five stage process:1) slot synchronization 2) frame synchronization and scrambling code group identification 3) scrambling code identification 4) frequency acquisition 5) cell identification . In this thesis, we investigated the first three stages of the initial cell search where large frequency offset and possible Doppler effect are the major obstacles. To avoid the match filtering performance degradation induced by the frequency offset, we considered using a partial correlating match filter which has a length smaller than the coherence time of the channel. Differential combining of the partial correlated segments is then applied to increase the stage 1 signal-to-noise ratio (SNR). The differential technique can also be used to roughly estimate the frequency offset which can be used at stage 2 and stage 3 to correct the phase rotation and improve the detection. In addition, a weighted combining at stage 1, and stage 2 was proposed to overcome the time-varying interference due to fast power control of the traffic and control channels. These proposed enhancements were firstly tested with a single-path Rayleigh fading channel, then applied to the multi-path fading channel models specified in the 3GPP standards. It was shown through simulation that the proposed enhancements improve the initial cell search performance significantly.