Fast cell search algorithm in inter-cell asynchronous DS-CDMA mobile radio

Inter-cell asynchronous DS-CDMA cellular mobile radio allows continuous system deployment from out-doors to indoors since no outer timing source is required. All the forward link channels (control and traffic channels) of each cell site are first spread by orthogonal short spreading codes and then randomized by a long random code uniquely assigned to each cell site. However, inter-cell asynchronous systems generally require much longer cell search time than inter-cell synchronous systems. This paper proposes a fast cell search algorithm based on the periodic masking of the long random code when transmitting the control channel (CCH) signal. The same short spreading code is used for the CCHs of all cell sites. The same short spreading code periodically appears in the signals transmitted from all cell sites so the mobile station can detect the long random code timing (or more precisely the masking timing) by using a matched filter. By grouping the long random codes used in the system and transmitting a group identification (GI) code from each cell site during the masking period, we can avoid searching all long random codes. This significantly reduces the cell search time. Simulation results demonstrate that cell search can be accomplished in less than 500 ms at 90% of the locations when the number of long random codes (having a repetition period of 10 ms) is 512 and the number of those per group is 32.