24/04/2013 FREQUENCY HOPPING GSM Telecom Traffictelecomtraffic.com/topic68-gsm-frequency-hopping.html 1/1Multi wave propagation varies the signal strength which causes fading dips. This is known as Rayleigh fading which isfrequency dependent. Frequency Hopping changes the frequency with time to eliminate frequency dependentRayleigh fading. Frequency Hopping reduces the effects of interferences and gives a great support to cater thecapacity upgrading as well.There are 2 types of frequency hopping, Base Band Hopping (BBH) and Synthesized Frequency Hopping (SFH)1 - Base band hoppingIn this type of hopping, only frequencies used by the TRXs can be allocated to the FHS (Frequency HoppingSequence).The BCCH supporting Time Slot cannot hop. TS 0 of the BCCH TRX is always transmitting on the BCCHfrequency. Other timeslots can use other frequencies unless the BCCH frequency is transmitted by any other TRX atthe same time.The major advantage of the BBH is the time TS 1 to TS 7 of the BCCH frequency containing TRX is allowed to performhopping. But Frequency Hopping performs best with at least 4 hopping frequencies. So cells need at least 4 TRXsand it is the disadvantage of this system.2 - Synthesized Frequency HoppingIn SFH each frame unit is connected to one carrier unit. Hopping is performed by changing the carrier unit bychanging a synthesizer. As the communication is not hopping between the carrier units but the carrier unit frequencyitself is hopping. So many frequencies can be used as hopping frequencies. In SFH, TRX returns in every time slot toa different frequency. Thus the TCH remains on the same TRX but the frequencies of that TCH hops.