FREQUENCY HOPPING
The frequency hopping function permits the dynamic switching of radio links from one carrier frequency to another. Frequency hopping changes the frequency used by a radio link every new TDMA frame in a regular pattern. Frequency hopping is a GSM feature which can be enabled or disabled on a per cell basis.
Reason for Frequency Hopping
The reasons of using frequency hopping are:
Decreasing the probability of interference
Frequency hopping will spread the annoyance of interference over different mobile stations in a particular cell.
Suppressing the effect of Rayleigh fading
Rayleigh fading or multipath fading is caused by different paths followed by the radio signal. Rayleigh fading can cause coverage holes.
Rayleigh fading is location and frequency dependent. When the mobile station is stationary or moves at a slow speed, Frequency hopping will significantly improve the level of the air-interface performance. However, when the mobile station moves at a high speed, Frequency hopping does not harm, but does not help much either. The more frequencies are used in a particular cell, the more frequency can gain in suppressing the effect of rayleigh fading.
Frequency hopping will spread the annoyance of interference over different mobile stations in a particular cell.
Suppressing the effect of Rayleigh fading
Rayleigh fading or multipath fading is caused by different paths followed by the radio signal. Rayleigh fading can cause coverage holes.
Rayleigh fading is location and frequency dependent. When the mobile station is stationary or moves at a slow speed, Frequency hopping will significantly improve the level of the air-interface performance. However, when the mobile station moves at a high speed, Frequency hopping does not harm, but does not help much either. The more frequencies are used in a particular cell, the more frequency can gain in suppressing the effect of rayleigh fading.
Frequency hopping process
The regular pattern by which a radio link changes frequency is described by the hopping sequence. The hopping sequence can have a cyclic pattern or a pseudo-random pattern.
In order to calculate the hopping sequence, a function is used which maps a particular TDMA frame to a radio frequency within the set of frequencies, using parameters such as TDMA frame number and number of frequencies in the set of frequencies. Both the uplink and the downlink use the same hopping sequence. For this purpose the parameter used to calculate the hopping sequence are also transferred from the BTS to the mobile station.
There is an exception for the frequency sequence. This exception is applicable to timeslots bearing the common channels ( BCCH, FCCH, SCH, PAGCH and RACH ) in timeslots number 0 of the lowest frequency. To reduce complexity of the GSM system, the common channels do not hop ( they are fixed).
The frequency hopping are two type. Which are
Baseband hopping
Synthesizer hopping
Baseband hopping
Synthesizer hopping
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