A compound method for initial frequency acquisition in WCDMA systems

In this paper, we propose frequency offset estimation and compensation techniques for a mobile station (MS) operated in the wideband code division multiple access (WCDMA) system. The initial frequency offset between a MS and a base station (BS) due to the inaccuracy of the crystal oscillator can be as large as ±20 kHz [Y.-P.E. Wang and T. Ottosson, Sept 1999]. The WCDMA system is an asynchronous CDMA scheme, which doesn´t require synchronization between the base stations as in the synchronous CDMA systems. The 3GPP group adopts the method of a three-step cell search scheme (Mar 2005) by which the mobile station can obtain initial code synchronization by resolving both the code and the timing uncertainty. The proposed scheme first utilizes the results of a differential detection method [J. Moon and Y.-H. Lee, Sept 2002], which is used in the first step in cell search procedure, to obtain an initial frequency estimation. Then the received signal is compensated by the estimated frequency offset. After that, common pilot channel (CPICH) symbols are acquired by descrambling and despreading the compensated signal, and the methods based on the fast Fourier transform (FFT) of CPICH symbols with polynomial interpolation in the frequency domain are adopted to achieve fine frequency estimation. Simulation results show that the proposed algorithm delivers considerable improvement in the frequency offset estimation performance when compared to a differential phase detection and a conventional FFT based algorithm.