Josiane Nzouonta CSE 5631
Report 1: Cell Searching in WCDMA
S. K. Bahl, IEEE potentials, April/May 2003. pp16-19
Third generation (3G) systems were developed with the aim to improve the data speed rate of 2G systems and the global roaming problem across 3G. The technologies more likely to provide 3G services include CDMA2000 and WCDMA (Wideband Code Division Multiple Access).
Spread spectrum CDMA
In a spread spectrum CDMA system, the transmitted signal is spread over a wide frequency band. The band is wider than the minimum bandwidth required to transmit the information being sent. Each user has a unique scrambling code. The signal received by the mobile station from the transmitting base station is correlated with the user’s scrambling code. This dispreads only that particular user signal while the other signals remain spread. Spreading consists of multiplying the input data by a scrambling code sequence whose bit rate is much higher than the data bit rate.
CDMA2000 supports synchronized base stations (use of an external time reference at all the base stations. E.g. Global Position Systems – GPS-) whereas WCDMA supports asynchronous base stations (each base station has an independent time reference).
Adjacent base stations can only be identified by distinct scrambling codes in WCDMA and therefore cell search takes longer. The cell search is performed when a mobile is switched on (initial cell search) and during active or idle modes (target cell search). It involves the process of achieving code, time, and frequency synchronization of the mobile station with the base station.
Cell search is also complicated in WCDMA because of interference from the other mobile station present in a cell. It is therefore important to develop algorithms and hardware implementations to perform cell search using lower acquisition time and minimum hardware resources for asynchronous CDMA systems.
The process of achieving synchronization of the mobile station with the base station is divided into five stages: - slot synchronization; - frame synchronization and code group identification; - scrambling code identification; - cell identification.
To reduce the complexity of the cell search in WCDMA, the scrambling codes are grouped into code groups. Combining frame synchronization and code group identification reduces the complexity of the search algorithm.
The last two stages of the cell search algorithm only need to be performed during the initial cell search (not in the target cell search).
Efficient hardware design to perform cell searches faster need to be studied especially for fading environments. The reduced cell search time will minimize the number of calls getting dropped during the synchronization process and also lower the power consumption.