Baseband frequency hopping vs synthesized frequency hopping GSM

This page mentions gsm frequency hopping types. It compares Baseband frequency hopping vs synthesized frequency hopping and mentions difference between Baseband frequency hopping and synthesized frequency hopping.

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The aim of frequency hopping is to provide secured communication between GSM Mobile Station and Base Station. Following are the two main types of frequency hopping employed in GSM for the same.

GSM frequency vs time frame
Fig:1 GSM TDMA Frame

In GSM, frequency band having 25 MHz bandwidth (from 890MHz to 915MHz in the uplink and from 935MHz to 960MHz in the downlink) is divided into 124 carriers having 200 KHz bandwidth each. Each of these carriers will have eight time slots to carry the bursts.

Eight time slots comprised of 1 burst having duration of 4.615ms. One burst in GSM carry 156.25 bits.

What is frequency hopping: The change of RF carrier frequency during the transmission of information is known as frequency hopping. In GSM RF carrier is changed every 4.615ms. This gives 217 hopes per second. This means in GSM, RF carrier frequency is changed in each of the bursts transmission. Hence all the bits of each bursts are transmitted on same frequency.

The frequency hopping helps to improve interference diversity and frequency diversity. There are two types of frequency hopping based on use of RF carrier frequencies for the RF transceivers for burst transmission in GSM. They are synthesized frequency hopping and baseband frequency hopping.

GSM baseband frequency hopping

gsm baseband frequency hopping
Fig:2 Baseband hopping

• In baseband frequency hopping, each transceiver is assigned one dedicated RF carrier frequency. Here number of hopping frequencies are equal to number of transceivers used in a single GSM cell.
• The bursts from transceiver controllers are routed to different transmitters by bus interface as shown in figure-2.
• Narrow band filter combiner is used here which can connect upto 16 RF transceivers. It can incur no more than 3 dB of loss.
• It is not possible to hop on frequencies more than the number of transceivers deployed.

Handover of baseband signal is done in baseband frequency hopping type. As one RF transceiver is being used during the entire duration of call for all the burst transmissions, if the transceiver fails, it will affect all the consecutive calls as well as current call.

GSM synthesized frequency hopping

synthesized frequency hopping transmitter
Fig:3 FHSS (Frequency Hopping Spread Spectrum) Transmitter

Number of hopping frequencies do not depend on number of RF transceivers. Due to this, more than one frequencies are used for each of the transceivers for consecutive transmission of bursts using them. For each burst there will be new RF carrier for transmission through single RF transceiver. As there are multiple bursts to be transmitted depending upon information (data/voice call) size. Hence multiple RF carriers are needed. Here number of hopping frequencies are greater than number of transceivers in a cell.

Here frequency of RF synthesizer is changed as per hopping sequence and hence it is known as synthesized frequency hopping. The figure-3 depicts FHSS transmitter. Based on PN sequence randomly channel table is being prepared which is used to change the synthesizer output frequency. The different synthesizer output frequencies are used to generate different RF carrier frequencies using RF mixer as shown.

gsm synthesizer frequency hopping
Fig:4 GSM synthesizer hopping

As shown in the figure-4. RF transmitters change the frequency for each and every burst. It is possible to hop on multiple frequencies even greater than number of transceivers available in the system. As hybrid combiners are used for connecting transmitters, loss will be higher compare to baseband hopping type.

gsm synthesized frequency hopping
Fig:5 GSM burst wise frequency hopping

For each burst new set of synthesizer frequency is used to generate new RF carrier frequency. The PN sequence is known to both the transmitter and receiver and hence easy to recover the data transmitted using frequency hopping sequence.

Each burst is transmitted on different unique frequencies in gsm synthesized FH type. This is shown in the figure-5. Both baseband frequency hopping and synthesized frequency hopping falls under slow frequency hopping>> type.

GSM frequency hopping parameters

GSM HSN base sequences
Fig:6 Use of HSN in GSM

Following are the frequency hopping (FH) parameters used in GSM.
MA: It stands for Mobile Allocation. MA list are the RF frequencies which are given to one GSM sector. These channels are used for CS call and PS call in GSM. There are total 63 frequencies in MA list.
HSN: It stands for hopping sequence number. This is the number which specifies FH algorithm used in GSM. It varies from 0 to 63. Hence there are total 64 hopping algorithms. When HSN is equal to 0, no hopping and sequence is cyclic. When HSN is between 1 and 63, hopping is applied and frequencies are choosen randomly from the list for transmission.
MAIO: It stands for Mobile Allocation Index Offset. It sets initial frequencies in the MA list. It ranges from 0 to (N-1), where N is number of total frequencies in MA list. MAIO value of 0 for the RF transceiver refers to first RF carrier (i.e. f1) to be used by it.


GSM protocol stack which covers GSM layer 1, layer 2 and layer 3 basics.Read More
GSM Physical layer or GSM layer 1 modules in depth.READ MORE
GSM Mobile originated call flow,READ MORE
GSM Mobile Terminated call flow,READ MORE
GSM downlink packet switched data call, READ MORE
GSM uplink packet switched data call, READ MORE
Circuit Switching(CS) versus Packet Switching(PS), READ MORE

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