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.

GSM PHYSICAL CHANNEL

GSM Bandwidth Allocation

GSM uses Frequency Division Duplexing(FDD) where the uplink and downlink of each channel operates on a different frequency . Therefore , two frequency band are allocated to GSM, separated by 20 MHz.

                 The following frequency band were initially allocated to GSM.

•    Uplink sub-band 890 MHz to 915 MHz
•    Downlink sub-band 935 MHz to 960 MHz

GSM FDMA Structure

Each band is divided into number of frequency channels, each channel carries having a 200 KHz bandwidth. Therefore 124 carriers have available within each of the up and down link bands, allowing for 2*100 khz guard bands. 

                                                    The up and downlink frequency channel pair allocation has been arranged such that the two frequencies comprising a channel pair are 45 MHz apart. Each of these frequency pairs are identified by an "Absolute Radio Frequency Carrier Number"(ARFCN) in the range 1-124 for GSM.   Up link and Downlink channel frequencies can be calculated as follows:

                   Up link frequencies : Fu(n)= 890+0.2n (1<=n<=124)

                   Downlink frequencies : Fd(n)=Fu(n)+45

GSM TDMA Structure

Each GSM carrier channel is subdivided by time into 8 timeslots. Timeslots are repeated in frames, each frame comprising 8 timeslots. The duration of single timeslot is 0.577ms. Therefore each TDMA frames repeat every 4.615ms. These timeslot are known as 'Physical channels'.     

Within each timeslot, a radio 'burst' is transmitted. This burst is comprises 8*156.25=1250 bit periods.If a frame is transmitted every 4.615ms , the row data rate over the carrier channel is (1\4.615ms)*1250=270kbps. The corresponding physical raw data rate is 270/8=33.75kbps.

Logical channels

The reason logical channels are used in because GSM has a number of control channels which do not require to operate at the full rate of a physical channel. Most of these control channels can therefore multiplexed onto single physical channel known as the 'control channel'. In a single carrier cell, this is always allocated to the physical channel 'timeslot 0'.

GSM Logical channel

• There  are two types of logical channel are defined: Traffic channel and Control channel.
• Each is further sub-divided as follows:

All the control and traffic channels are described as follow link GSM CHANNEL