This paper presents the fast cell search time performance based on laboratory and field experiments of a 2-step cell search algorithm that uses scrambling code masking for inter-cell asynchronous wideband DS-CDMA (W-CDMA) mobile radio. The scrambling code is masked at different time positions during each scrambling period on the forward-link common control channel (CCH) to detect the scrambling code timing at the mobile receiver. Experiments were conducted using the CCH-to-dedicated traffic channel (DTCH) power ratio, R of 3 dB, 10 DTCHs, and 16 scrambling codes in a single-cell and two-cell models. The field experimental results show that the cell search time of about 600 msec was achieved in vehicular environments at the detection probability of 90% and the average received Eb/N0 (N0 is the background noise without interference) of 13-15 dB for DTCH, even in the worst case scenario when the received signal power ratios of the CCH from two cell sites were 0 dB. The cell search time that was achieved with the 3-step cell search algorithm previously proposed by the authors is estimated from the experimental results; the cell search can be accomplished within about 720 msec at a probability of 96% for 512 scrambling codes and 16 scrambling code groups.

This paper proposes a fast target cell search algorithm used during intermittent reception in the idle mode of a mobile station (MS) for inter-cell asynchronous W-CDMA mobile radio. In the proposed scheme, since the base station (BS) informs a MS of the relative average received timing differences between the scrambling code of its BS and those of the surrounding BSs in addition to the scrambling codes, the MS only has to search over the restricted timing duration for the informed scrambling codes. Therefore, the target cell search (i.e., in which the number of candidate cells is limited) can be achieved as fast as in inter-cell synchronous systems. A computer simulation demonstrates that the target cell search time per one super frame (= 720 msec) at the cell detection probability of 95% is accomplished within 5.9 msec (this corresponds to the intermittent time ratio required for the target cell search to become 0.82%), when the transmit power ratios of the common pilot channel (CPICH) and common control physical channel (CCPCH) required for cell search to a dedicated traffic channel (DTCH) are 3 and 6 dB, respectively. In this simulation, the average power delay profile was generated by averaging the instantaneous ones (it was coherently accumulated pilot signal over a 512-chip duration (= 125 µsec) using 4 correlators) over a period of three super frames for 19 target cell-site candidates using the search window with a 10-chip duration (= 2.4 µsec).

Inter-cell asynchronous DS-CDMA cellular mobile radio allows continuous system deployment from outdoors 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.