Experimental evaluation of the impact of network frequency synchronization on GSM quality of service during handover

Poor accuracy of frequency references used in GSM base transceiver stations (BTS) can lead to dropped calls, slow handover between cells and even co-channel interference. In this paper, we report some results of an experimental trial carried out in the Omnitel-Vodafone test plant in Milano (Italy). In authors´ knowledge, this is the first paper confirming with experimental data that lack of BTS synchronization affects GSM handover performance, leading to some degradation of quality of service. The speech quality of calls between two GSM handsets, undergoing handover between two BTSs synchronized with variable frequency offset, was measured in terms of mean opinion score (MOS) and speech clipping. For values of fractional frequency offset Δf/f_n=0 and 10^-7, all completed calls featured optimal quality in spite of handover. On the contrary, for Δf/f_n=10^-6 and 10^-5, performance degradation was experienced: about 40% of completed calls exhibited unpleasant MOS values and high speech-clipping percentages. Such experimental results confirm the importance of GSM network synchronization and warn GSM operators about leaving BTS clocks to operate in free-run: frequency accuracy in the order of 10^-6 to 10^-5 is typical for quartz oscillators commonly used in this equipment.