Cell search robust to initial frequency offset in WCDMA systems

The WCDMA system, an asynchronous CDMA scheme, uses different downlink scrambling codes for each base station, not requiring synchronization between the base stations as in the synchronous CDMA systems. However, the mobile station needs to obtain initial code synchronization by resolving both the code and timing uncertainty. The 3GPP Group adopts the use of a three-step cell search scheme. For ease of initial cell search, the base station transmits two synchronization codes through the synchronization channel. The mobile station searches the peak correlation value of the primary synchronization code to acquire the slot timing. For reliable detection, the receiver combines the correlation values from several slots. In this paper, we propose a new combining scheme that can reduce search time and also provide performance robust to a wide range of initial frequency offset. The proposed scheme utilizes a differential detection method, applicable to the transmit antenna diversity mode as well as the single antenna mode. The proposed scheme can also estimate the initial frequency offset for the automatic frequency control (AFC) performance, eliminating the need for an initial frequency acquisition circuit.