Slow frequency hopping is a process of changing the radio frequencies of a communications on a regular basis (pattern). The duration of transmission on a single frequency is typically much longer than the amount of time it takes to send several bits of digital information. Slow frequency hopping is used to reduce the effects of radio signal fading and to minimize the effects of interference from radio channels that are operating on the same frequency.

Radio signal fading is often limited to a specific frequency range. Radio frequencies that are separated by more than 1 MHz may do not fade simultaneously [iii]. If successive time slot bursts are transmitted on different frequencies, if a radio signal fade occurs, it will not likely occur on consecutive bursts.

The effects of radio signal interference that is received from nearby cell sites that operate on the same frequency can be reduced by using slow frequency hopping. Interfering radio signals may only affect particular time slots. Because frequency hopping is combined with error protection that is distributed over multiple time slots (which the GSM system does), a signal fade will produce a lower number of bit errors

The hopping sequence pattern is created by the radio system by assigning a hopping sequence number (HSN) and a mobile allocation index offset (MAIO). The combination of these variables selects hopping pattern and where the mobile device should be operating within the hopping pattern.

Figure 1.20 shows a simplified diagram of how a slow...