When the voice signal is transmitted after being processed and modulated, the frequency hopping technique will be used too, i.e. the transmission carrier varies constantly at different timeslots (of course, the variation should comply with the frequency planning principles).
The following two factors are considered in introduction of the frequency hopping technology:
1. For the fading process is related to the frequency band, the application of the frequency hopping in the system may reduce the effects of the rayleigh fading.
2. Due to the interference diversity, in areas with dense traffic, the cell capacity is restricted by the interference caused by the frequency multiplexing. In addition, the system is designed to meet the demands of subscribers, the maximum capacity of the system is calculated on the assumption that the quality of a certain number of calls is reduced distinctly due to interference. The lower the diversity measured around the specified C/I value, the larger the system capacity. The interference on a call is the average value of the interference level caused by many other calls. Thus, for a specified interference intensity, the more the interference sources, the better the system performance.
The radio interface of the GSM system is designed with the slow frequency hopping (SFH) technique. The difference between SFH and the fast frequency hopping (FFH) is that the frequency change of the latter is faster than the modulating frequency. During the whole burst sequence transmission period of the GSM system, the transmitting frequency remains unchanged. Therefore, it belongs to slow frequency hopping, as shown in the above diagram.
The GSM system allows 64 types of different frequency hopping sequences. There are mainly two parameters involved in description of them: mobile allocation index offset (MAIO) and hopping sequence number (HSN). The values for MAIO can be as many as the frequencies in a group; and there are 64 different values available for HSN.