Fast cell search algorithm in DS-CDMA mobile radio using long spreading codes

In DS-CDMA cellular mobile radio systems, asynchronous cell site operation, assigning different long spreading codes to each cell yields the advantage of flexible system deployment from outdoors to indoors. However, in general, much longer cell search time is required in asynchronous cell site operation than in synchronous operation. This paper proposes a fast cell search algorithm based on the periodic masking of long spreading codes. The control channel (CCH) is spread by a combination of cell site-unique long code and a short code common to all cell sites (this short code is called CSC here). The mobile station (MS) exploits the fact that the CSC periodically appears in the signals transmitted from all cell sites and can detect the long code timing (or more precisely, masking timing) by using a matched filter. To avoid searching all long codes used in the system, each cell transmits a long code group identification code (GIC) together with the CSC. This greatly reduces the cell search time. Simulation results demonstrate that cell search can be accomplished in less than about 500 ms at 90% of the locations when the number of long spreading codes (having a repetition period of 10 ms) used in the system is 512 and the number of GICs is 16.