8 OMO133060 BSC6900 GSM V9R11R12R13 Frequency Hopping Algorithm and Parameters ISSUE 1.02

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GSM Frequency Hopping Algorithm and Parameters



When using close frequency reuse technique, system interference is the most important

factor for frequency reuse ratio. Usually, power control and discontinuous transmission technologies are adopted to lower system interference. Frequency Hopping technology is

used in order to prevent interference and to improve system communication quality under the same interference condition.

Therefore, frequency hopping is a very important technology for reducing GSM system

interference and improving frequency reuse ratio. According to GSM standard recommendations, slow frequency hopping can be used in GSM communication system.

Frequency hopping refers to the regular hopping of carrier frequency within certain range. The frequency hopping function of the channel group in each cell can be enabled or

disabled separately. Since BCCH is the broadcast channel, it does not participate in

frequency hopping while TCH and SDCCH channel can utilize frequency hopping. There are two kinds of frequency hopping modes used by BTS: these are Baseband hopping and

Radio Frequency hopping. The mechanisms of their implementation are different.



Hopping can lower many kinds of interferences, such as co-channel interference, adjacent

channel interference, intermodulation interference, etc. Hopping turns the consecutive long time interference of mobile station into discontinuous interference of single burst,

and such interference can be greatly lowered through channel decoding, de-interleave and error correction technology. After hopping technology is adopted, GSM BTS and mobile

station can work in harsh radio wave environment.

In a word, hopping makes cell planning more flexible. The effect of interference equalization depends on the hopping mode. The best performance can be achieved if

interference signal and useful signal use different hopping sequences (e.g. irrelevant sequence). The less the relativity, the better the equalization effect. If both interference

signal and useful signal use the cycle hopping with the same hopping sequence, they may

remain on the same frequency with consistent pace, and the interference on useful signal will be no hopping is used. This is the same as the case in that two hopping sequences are

totally relative. Therefore, the interference diversity effect of random hopping will be better.

In a full load system, all interference signals are transmitted at the same time, resulting in a

continuous interference. Even so, random hopping sequence still can improve communication quality. However co-channel cells must use different hopping sequences.

The more frequencies involved in hopping, the better the performance.



Hopping can lower the influence of Rayleigh fading to signal strength. This influence

equals to frequency diversity. One voice code block of GSM is transmitted in 8 bursts separately after channel coding. Since the Rayleigh fading is based on frequency, in other

words, the valley point of signal fading appears in different locations at different frequencies. Therefore, after hopping is used, the modulation frequency of each burst is

different. If frequency selective fading occurs on certain frequency carrier, the period that

signal received by mobile phone located at valley point won't exceed one burst cycle, and only one burst is affected. The variation of signal strength is cut to a level small enough to

satisfy the requirement of channel coding and interleave error correction, thus the signal valley point of rayleigh fading can be omitted, and the valley point where rayleigh fading

signal level appears will become smooth. In this way, the slow moving mobile will be in an

environment with more even radio wave. For the mobile phone moving quickly, the influence from frequency diversity is the same regardless of its speed.



In GSM system, the interference strength and distribution on each frequency carriers in cell

are different, the interference on a communication is averaged due to carrier variation on the bursts. Therefore, the radio wave interference on communication is equalized. If no

hopping is used, the mobile station will always work on fixed frequency carrier, each burst during the entire communication may receive fixed strong interference. In others words,

hopping technology disperses the interference on different carriers with burst, this is called

"interference equalization" or "interference diversity". As shown in the figure, bursts B1, B2... are spread on frequencies f1, f2, f3... Co-channel interference exists in the cellular

network because of frequency reuse, hopping makes the signal interference discontinuous to improve radio wave environment. The interference varies from burst to burst, which

improves the communication quality. Otherwise, the entire communication will suffer from

strong interference. That is to say the interference is dispersed on different carriers with burst. Of course, the frequency conflict from hopping could be resulted in strong instant

interference, but it could be avoided by defining different MAIO. In frequency hopping, the more frequency carriers are used, the lesser likely the frequency conflict occurs, and

the higher the hopping gain. However, frequency availability will be lowered when there

are more hopping frequency carriers.





The BSC6900 supports three FH modes: baseband FH, radio frequency (RF) FH, and hybrid

FH. The parameter FHMODE specifies the FH mode to be used.



There is a limited number of relatively independent baseband processing units and carrier

processing units in the system. The working frequency carrier of each carrier processing unit is fixed, the service information of each communication path is processed by fixed

baseband unit, and then the processed information is transferred to carrier units of different frequency carriers for processing and transmitting through bus structure

according to time sequence and specific frequency hopping principles. This hopping mode

is called "baseband hopping". In baseband hopping, each transmitter works at a fixed frequency, the bursts of the same voice channel are sent into respective transmitters under

control for switching based on baseband signal. Since the frequency of each transceiver is fixed, no change is needed on the combiner, while both broadband combiner and cavity

combiner can be used. The maximum number of hopping frequencies are limited by the

number of TRX. In baseband hopping you have to consider that if a failure of one TRX board will occurs, then the corresponding code will be lost and the communication

performance will be affected.



The parameter BCCHTRXHP specifies whether the BCCH TRX participates in FH. (GBFD-

113702 BCCH Carrier Frequency Hopping)





In this mode, each route of service information is processed by fixed baseband unit and

and carrier frequency processing unit, while the working frequency carrier of carrier frequency processing unit is provided by frequency synthesizer. Under the control of the

control unit, frequency carrier can be changed according to certain rules. This mode is called "carrier frequency hopping" or "RF hopping". In RF hopping, one transmitter

processes the frequency carriers used by all bursts of one communication, which is realized

through the change of synthesizer frequency instead of the switching of baseband signal. The number of TRX depends on cell traffic instead of restricted by carrier. Because of the

change of synthesizer, the combiner should be changes, only broadband combiner can be used. This kind of synthesizer has a insertion loss of about 3db. The cascade connection of

multiple combiners has large insertion loss, so the actual application is restricted. But once

fault occurs to certain TRX, the troubleshooting function of the system will shut down this TRX.

GSM specifications do not specify that GSM BTS must use "baseband hopping" or “band hopping”, while the hopping mode used by BTS equipment is specified by the equipment

supplier. As to mobile terminals, since each terminal has only one carrier unit, it's bound to

use carrier frequency hopping.





In hybrid FH, some TRXs adopt RF FH and other TRXs adopt baseband FH in a cell. One

TRX, however, can adopt either RF FH or baseband FH.

For baseband FH, only the frequency assigned to each TRX participates in FH. For RF FH,

one TRX can use multiple frequencies. Generally, the number of hopping frequencies is greater than the number of TRXs to obtain sufficient FH gains. If the number of TRXs in a

cell is small but the frequencies are sufficient, RF FH can be adopted to improve the system

performance. If the number of TRXs in a cell is large but the frequencies are insufficient, baseband FH can be adopted to improve the system performance.

In general, hybrid FH is applicable to co-BCCH cells.

GSM900M/DCS1800M Co-BCCH cell refers to the existence of GSM900 and

DCS1800 carriers in the same cell. The hybrid cell must be configured as a

concentric cell

In the overlaid subcell of a co-BCCH cell, a tight frequency reuse pattern is adopted,

and thus the co-channel and adjacent-channel interference is strong. In addition, the overlaid subcell generally operates on the 1800/1900 MHz band, and the

frequencies are sufficient. The underlaid subcell of a co-BCCH cell generally

operates on the 900/850 MHz band, a loose frequency reuse pattern is adopted, and the frequencies are insufficient. In general, the BCCH is configured in the

underlaid subcell. To adapt to these characteristics of the co-BCCH cell, hybrid FH can be adopted. In the overlaid subcell, the TRXs use RF FH and more frequencies

are used to improve the anti-interference capability of the system. In the underlaid

subcell, the TRXs use baseband FH to improve the system interference, and no additional frequency is required.













Parameter Default

Value

GUI

Value

Range

Actual

Value

Range Unit MML Command Impact

HSN 0 0~63 0~63 None ADD GCELLMAGRP

(Optional) Cell



Parameter ID: HSN

Hopping sequence number (HSN), indicating 64 types of frequency hopping sequences. If this parameter is set to 0, the frequency hopping is performed in sequence.

If this parameter is set to a value from 1 to 63, the frequency hopping is performed in a pseudo-random manner, that is a disciplinary random manner.

GUI Value Range: 0~63 Actual Value Range: 0~63

Unit: None Default Value: 0

Data Configuration Related to FH

In FH, a set of frequencies used on a specific timeslot is called an MA list. A maximum of 64 MA lists are allowed in each cell, and one MA list contains a maximum of 64

frequencies. Different timeslots on the same transceiver (TRX) can use the same or different MA lists.

Different TRXs in the same cell can adopt the same or different MA lists to perform

frequency hopping. If different TRXs in the same cell use the same MA list, it is recommended that Channel MAIO of different TRXs should be different. Otherwise,

strong interference may occur. If the BSC supports Flex MAIO, it automatically calculates the optimum mobile allocation index offset (MAIO) according to the

hopping frequencies being used to obtain the best FH gains.

Generally, all the MA lists in a cell can be configured with the same or different HSNs. It is recommended that different cells use different MA lists. If two cells use the same

MA lists, it is recommended that different HSNs be configured.

TSC should be the same as the corresponding BCC. It should remain unchanged.

Parameter Default

Value

GUI

Value

Range

Actual

Value

Range Unit MML Command Impact

Channel

MAIO None 0~63 0~63 None SET GTRXCHANHOP

(Optional)

Physical

channel

TSC None 0~7 0~7 None

ADD GCELLMAGRP

(Optional) Cell

BCC None 0~7 0~7 None

ADD GCELLQUICKSETUP

(Optional) BTS



Procedure for Quick Activation

Run the BSC6900 MML command SET GCELLHOPQUICKSETUP to quickly set the frequency hopping of the cell. In this step, set Index Type to BYID(By Index),

and then set Cell Index. Set Frequency Hopping Mode, and then set Frequency Hopping Mode of BCCH TRX and TRXID LST.



http://127.0.0.1:52199/hedex/pages/31184422/02 (2010-06-21)/31184422/02 (2010-06-21)/resources/mbsc/m-mml/set-gcellhopquicksetup.html

Activation Procedure

Run the BSC6900 MML command DEA GCELL to deactivate the cell.

Run the BSC6900 MML command ADD GCELLMAGRP to add a FH group. In this

step, set Index Type to BYID(By Index), and then set Cell Index, Hop Index, Hop Mode, and Frequency.

NOTE:

Set Hop Mode to BaseBand_FH(Baseband FH) or RF_FH(RF FH) as

required.

If the BCCH participates in baseband FH, the cell must be configured with at least two FH groups.

Run the BSC6900 MML command SET GCELLHOPTP to set the FH type of the cell. In this step, set Index Type to BYID(By Index), and then set Cell Index and

Frequency Hopping Mode.

Run the BSC6900 MML command SET GTRXHOP to set the FH type of a TRX. In this step, set TRX ID and Hop Type.

Run the BSC6900 MML command SET GTRXCHANHOP. In this step, set Channel Hop Index and Channel MAIO.

NOTE:

When the same FH group is used for the TRXs, the MAIO of the same

channel of the TRXs must be different.

Run the BSC6900 MML command ACT GCELL to activate the cell.



http://127.0.0.1:52199/hedex/pages/31184422/02 (2010-06-21)/31184422/02 (2010-06-21)/resources/mbsc/m-mml/dea-gcell.html

http://127.0.0.1:52199/hedex/pages/31184422/02 (2010-06-21)/31184422/02 (2010-06-21)/resources/mbsc/m-mml/add-gcellmagrp.html

http://127.0.0.1:52199/hedex/pages/31184422/02 (2010-06-21)/31184422/02 (2010-06-21)/resources/mbsc/m-mml/set-gcellhoptp.html

http://127.0.0.1:52199/hedex/pages/31184422/02 (2010-06-21)/31184422/02 (2010-06-21)/resources/mbsc/m-mml/set-gtrxhop.html

http://127.0.0.1:52199/hedex/pages/31184422/02 (2010-06-21)/31184422/02 (2010-06-21)/resources/mbsc/m-mml/set-gtrxchanhop.html

http://127.0.0.1:52199/hedex/pages/31184422/02 (2010-06-21)/31184422/02 (2010-06-21)/resources/mbsc/m-mml/act-gcell.html















8 OMO133060 BSC6900 GSM V9R11R12R13 Frequency Hopping Algorithm and Parameters ISSUE 1.02

Documents

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