Cell search in W-CDMA

摘要

In a CDMA cellular system, the process of the mobile stationnsearching for a cell and achieving code and time synchronization to itsndownlink scrambling code is referred to as cell search. Cell search isnperformed in three scenarios: initial cell search when a mobile stationnis switched on, idle mode search when inactive, and active mode searchnduring a call. The latter two are also called target cell search. Thisnpaper presents algorithms and results for both initial and target cellnsearch scenarios for the wideband CDMA (W-CDMA) standard. In W-CDMA, thencell search itself is divided into five acquisition stages: slotnsynchronization, frame synchronization and scrambling code groupnidentification, scrambling code identification, frequency acquisition,nand cell identification. Initial cell search needs all five stages,nwhile target cell search in general does not need the last two stages. Anpipelined process of the first three stages that minimizes the averagencode and time acquisition time, while keeping the complexity at anreasonable level, is considered. The frequency error in initial cellnsearch, which may be as large as 20 kHz, is taken care of by partialnsymbol despreading and noncoherent combining. Optimization of key systemnparameters such as the loading factors for primary synchronizationnchannel, synchronization channel, and common pilot channel for achievingnthe smallest average code and time acquisition time is studied. Afterncode and time synchronization (the first three stages), a maximumnlikelihood (ML)-based frequency acquisition method is used to bring downnthe frequency error to about 200 Hz. The gain of this method is morenthan 10 dB compared to an alternative scheme that obtains a frequencynerror estimate using differential detection