FREQUENCY HOPPING

FREQUENCY HOPPING:- Changing of frequency according to some algorithm in dedicated mode is called frequency hopping.

• In one sec 1/256 times frequency hopping happens.
• Multi wave propagation varies the signal strength which causes fading dips. This is known as Rayleigh fading which is frequency dependent.
• Frequency hopping changes the frequency with time to eliminated frequency dependent Rayleigh fading.
• Frequency hopping reduces the effects of interference and gives a great support to cater the capacity upgrading as well as.

TO USE OF FREQUENCY HOPPING:- To maximize network capacity and minimize the interference and improve the network quality.

There are two types of frequency hopping.
Base band hopping(BBH)

• In this types of hopping, only frequencies used by the TRXs can be allocated to the frequency hopping sequences.
• The BCCH supporting time slots cannot hop. TS0 of the BCCH TRX is always transmitting on the BCCH frequency, other timeslots can use other frequencies unless the BCCH frequency is transmitted by any other TRX at the same time.
• The major advantage of the BBH is the time TS1 to TS7  of the BCCH frequency containing TRX is allowed to perform hopping. But frequency hopping performs best with at least 4 TRx and it is the disadvantage of this system.

Synthesized frequency hopping(SFH)

• In synthesized frequency hopping each frame unit is connected to one carrier unit.
• Hopping is performed by changing the carrier unit by changing a synthesizer.
• As the communication is not hopping between the carrier units but the carrier unit frequency itself is hopping . so many frequencies can be used as hopping frequencies.
• In SFH , TRX returns in every time slot to a different frequency. Thus TCH remains on the same TRX  but the frequencies of that TCH hops.
• In GSM basically used synthesized frequency hopping.