Comparison of cell search methods for asynchronous wideband CDMA cellular system

Abstract

Cell search within a CDMA system consists of the tasks carried out by a mobile station in order to find, identify, and synchronize to a new cell. We distinguish between initial cell search, where the mobile station after power on searches for the cell to which it has the lowest path loss, and cell search carried out for maintaining connection to the network in idle mode and to find handover candidates in active mode. In this paper, we focus on initial cell search. In cellular systems employing asynchronous base stations, no common time reference exists. Consequently, base stations cannot be identified by their relative time offset. One way to facilitate cell search in an asynchronous system is to have each cell transmitting a unique downlink scrambling code. The disadvantage is that the mobile station must now search for a large number of different scrambling codes, which leads to significantly higher complexity compared to the case of a synchronized system. One approach to cell search in an asynchronous system has been proposed for ETSI WCDMA. In this case, each cell uses a unique downlink scrambling code of length 10 ms. The set of scrambling codes is divided into groups with a limited number of codes in each group. In addition, each cell periodically transmits two special orthogonal code words, known as the primary and secondary synchronization codes, PSC and SSC respectively. In ETSI WCDMA, as well as in our work, orthogonal Gold codes are used for the PSC and SSC codes.