Performance enhancements to the UMTS (W-CDMA) initial cell search algorithm

The purpose of the cell search algorithm in UMTS is to estimate the spreading code of the serving base-station and its corresponding timing offset. The search procedure consists of 3 sequential and distinct stages: (1) slot-boundary synchronization, (2) frame-boundary synchronization with code-group identification, and (3) scrambling code identification. Algorithms that have appeared in literature for cell search have confined the “code-time” estimation in each stage to a single hypothesis. Furthermore, these algorithms have neither considered the benefits of oversampling, nor the detrimental effects of non-ideal sampling that may arise due to clock jitter and/or residual frequency offset. In this paper, we study the performance benefits of estimating multiple “code-time” hypotheses in each stage of the cell-search process. In addition, we also study the effect of oversampling and non-ideal sampling. Our results indicate that, in the presence of non-ideal sampling, performance improves significantly if the received signal is oversampled by a factor of 4 or more. We also show that estimating 4 “codetime” hypotheses instead of 1 in the cell-search stages reduces the search-time (i.e. the code-acquisition time) considerably, in particular at low SINR.