3gsm key technologies

8/9/2019 3GSM Key Technologies

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Key technologies of

GSM System




Overview

• Time Advance• DTX• Frequency Hopping


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TA

Signals sent by a base station on the broadcastingchannel help mobile stations to synchronize with the basestation. After connection is set up between a mobilestation and the base station, the base station will makecontinuous tests, and provide the time advance (TA) on

the SACCH channel to all mobile stations twice everysecond according to the BS-MS-BS broadcasting delay.Mobile stations will make self-adaptive frame adjustment

according to time advance so that the time of mobilestation transmission to the base station matches that ofbase station reception.


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Two aims can be achieved by adopting DTX mode. One is tolower the total interference level in the air, and the other is to

save transmitter power. The DTX mode and the normal modeare optional, since the former will slightly lower the transmissionquality.

DTX



Frequency Hopping

• To enhance system anti-interference ability, the GSMsystem adopts the frequency hopping technology.

Frequency hopping refers to the hopping of carrier wavefrequencies according to a certain sequence in a verywide frequency range. Data of control information are

converted into base band signals after modulation, whichare then sent into carrier wave modulation. Afterwards,the carrier frequency changes under the control ofpseudo-random codes, the sequence of which isfrequency-hopping sequence. Finally, when filtered byradio filter, the carrier is transmitted to and radiated byantenna.



• Frequency hopping is to avoid external interference.In other words, it is to prevent or greatly reduce co-channel interference and frequency selective fadingeffect by converting frequencies to an extent thatinterference cannot catch up with.

• There are two frequency hopping modes: base bandfrequency hopping and radio frequency hopping.

Frequency Hopping



Baseband frequency hopping

Frequency Hopping



Radio frequency hopping

Frequency Hopping




CCCH_CONF

The CCCH can be one or more physical channels. TheCCCH and SDCCH can share the same physical channel.The combination mode of the common control channel in acell is determined by the CCCH_CONF.

CCCH_CONF Coding Meanings CCCH message blocks in oneBCCH

000 CCCH use one basic physical channel, not shared with SDCCH 9001 CCCH use one basic physical channel, shares with SDCCH 3010 CCCH use two basic physical channels, not shared with SDCCH 18100 CCCH use three basic physical channels, not shared with SDCCH 27

110 CCCH use 4 basic physical channels, not shared with SDCCH 36Others Reserved




Setting and Impact of CCCH_CONF

The CCCH_CONF is determined by the telecomoperation department according to the traffic model of acell. It is often decided in the system design period.According to the ordinary experience, if a cell has 1 or 2TRX, we recommend that the CCCH uses one basicphysical channel and shares it with the SDCCH; if a cellhas 3 or 4 TRX, we recommend that the CCCH uses onebasic physical channel but does not share it with theSDCCH.




AGBLK

Since the CCCH consists of the access grant channel(AGCH) and paging channel (PCH), it is necessary toset how many blocks of the CCCH information blocksare reserved and dedicated to the AGCH. To let the MSknow about the configuration information, the systemmessage of every cell contains a configuration

parameter, that is, the access grant reserve blocks(AGBLK).AGBLK is represented in decimal numerals, and its

value range is:• CCCH is not combined with SDCCH: 0 7.

• CCCH is combined with SDCCH: 0 2.




Setting and Impact of AGBLK

The AGBLK setting principle is: given that the AGCH isnot overloaded, try to reduce the parameter as much aspossible to shorten the time when the MS responds to thepaging and improve the quality of service of the system.

The recommended value of AGBLK is usually 1 (when theCCCH is combined with the SDCCH), 2 or 3 (when theCCCH is not combined with the SDCCH).




BS-PA-MFRMS

According to the GSM specifications, every mobile subscriberbelongs to a paging group. In every cell, every paging groupcorresponds to a paging subchannel, the MS calculates the

paging group to which it belongs by basing on its own IMSI, thencalculates the location of the paging subchannel belonging tothe paging group. In an actual network, the MS only"receives“ the contents in the paging subchannel to which itbelongs but ignores the contents in other paging subchannels,or even turn off the power supply of some hardware equipmentof the MS for other paging subchannels to save the MS power

overhead (i.e. DRX source). The BS-PA-MFRMS refers to howmany multi-frames are used as a cycle of a paging subchannel.This parameter in fact determines how many paging sub-channels are to be divided from the paging channels of a cell.




BS-PA-MFRMS

BS-PA-MFRMS Multiframes of the same paging groupthat cycle on the paging channel

2 2

3 3

4 4

5 5

6 6

7 7

8 8

9 9

is represented in decimal numerals and its value range is 2 9,its unit is multiframe 51 frames , its default value is 2




Periodic Updating Timer (T3212

The reasons for the location update in the GSM system areof two types: one is that the MS finds its location areachanged (with a different LAC), the other one is that thenetwork requires the MS to periodically update its location.The frequency of periodic location update is controlled viathe network and the period length is determined by the

parameter T3212.In general, in areas with larger traffic and signaling flow,you should select the larger T3212 e.g. 16 hours, 20hours or even 25 hours , but in areas with smaller trafficand lower signaling flow, you can set the smallerT3212 (3hours or 6 hours, etc.). In areas where traffic is far beyondthe system capacity, we recommend that you set T3212 as

0.

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