6.1 Cellular Networks Synchronization Requirements
6.1.1 Frequency Accuracy
In cellular networks the frequency accuracy requirement is clearly higher than needed for eliminating interference between neighbouring radio bands. The user equipment (UE) measures the reception of neighbouring base stations in preparation of a possible handover from one base station to another. For this purpose, the user equipment has to tune to another radio frequency for a very short while to avoid interrupting voice and data streams. There is no time for fine tuning so the frequency references used by the base stations have to be almost exactly aligned. In the case of soft handover in WCDMA, UE receives signals from neighbouring base stations simultaneously at the same frequency, but with different codes. Consequently, the frequencies of the base stations need to be almost the same. The probability of receiving an undistorted signal from both base stations decreases as the frequency difference increases, dropping signal-to-noise ratio. When the difference grows further, the handovers fail completely.
When UEs move, the frequencies of the base stations seen by the UE shift due to the Doppler Effect. When approaching or receding from a base station, the frequency increases or decreases, correspondingly. In the present GSM systems the UE has to work with Doppler shift up to speeds of 250 km/h at 900 MHz, and 130 km/h at 1800 MHz. This corresponds to a frequency offset of around 250 Hz in both cases . ...