A method of estimating the difference in frequency between base station transmissions received over a radio channel and a locally generated carrier frequency in a mobile receiver. The differential phase shifts imparted to different parts of a received synchronization code because of a frequency offset of the local reference oscillator are detected in the receiver. A series of partial correlations of the received synchronization code over a single transmission slot allows detection of the differential phase shifts. Signal to noise ratios may be improved by means of a series of overlapping partial correlations. The period over which the correlations are performed is much less than the coherence time of the radio channel.