LAC Planning

Location Area Planning

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Copyright © 2009 Huawei Technologies Co., Ltd. All rights reserved.

Definition of Location Area 

GSM mobile communication network is divided into multiple service areas according to the codes of location areas.



Location area is the basic unit of paging areas in GSM system. The paging message of a subscriber is sent in all cells of a location area.

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A location area contains one or more BSCs, but it belongs to one MSC only.

Copyright © 2009 Huawei Technologies Co., Ltd. All rights reserved. Page2

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Definition of Location Area


Location Area CodeLAC 

Definition 

To locate the location of MS, the whole area covered by each GSM PLMN is divided into different location areas. LAC is used to identify different location areas.



Format 

LAI contains LAC, which is composed of two bytes. LAC adopts hexadecimal coding. The available range is from 0001H to FFFEH. The code 0000H and FFFFH cannot be used (please refer to specification GSM0303, 0408, and 1111). One location area can contain one or more cells.


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Location Update 

Purpose of location update: 

To inform the system the LA in which the MS is to facilitate paging



When need the MS perform location update 

Getting into a new LA



Periodical location update


Size of location areas 

The size of a location area, plays a key role in a GSM system.

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If the size of location area is too big, the paging traffic will be higher, perhaps PCH will be congestion.

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If the size of location area is too small, the MS will occur more location update process. The signaling traffic is higher, SDCCH channel perhaps be congestion.


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Paging VS Location update Traffic signaling traffic

Paging

Location update optimum number of cells in Loc. area

# of cells in Loc. area

minimize signaling traffic optimum varies with network evolution


Division of location areas 

If two or more location areas are present in a big city of great traffic, the landforms, such as mountains and rivers within this city can be used as edges of the location areas.

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If no such landforms available within this city, the areas (such as streets and shopping centers) with great traffic cannot be used as edges of the location areas.

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In the intersected areas of urban areas and suburban areas, to avoid frequent location update, should design the edges of location areas near the outer base stations.


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Location Area Design For Dual-band 

If 1800 MHz cells and 900 MHz cells are under the control of two MSCs respectively, their location areas are different.

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Related parameters should be set to maintain the mobile stations stay in the 1800 MHz cells where the traffic is absorbed. In this case, the times for the mobile station to handover between the two bands and reselect cells will decrease.

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Designing signaling channels, fully consider the load resulted from location update.



If 1800 MHz cells and 900 MHz cells share a MSC, at the early network construction stage, they are suggested to use the same location area without affecting the network capacity.

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If the restriction on paging capacity is present, two location areas must be divided for them either in terms of band or geographic location, as shown in Figure .


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Location Area Design For Dual-band


Location Area Design For Dual-band


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If the location is divided in terms of geographic location, the frequent location updates resulted from inter-band handover and cell reselection can be avoided.



However, need to modify the related data of the original 900 MHz network. In addition, at the edges of the location areas, because the location updates caused by intra-band and inter-band handover and cell reselection is present simultaneously, the signaling flow is huge at these edges. As a result, carefully design the edges of the location areas.


Physical Combination of Logical Channel 26-frame multi-frame TCH/F+SACCH/F (full-rate TCH) TCH/H+SACCH/H (half-rate TCH)

51-frames multi-frame FCCH+SCH+BCCH+CCCH (main BCCH) FCCH+SCH+BCCH+CCCH+SDCCH/4+SACCH/4 (combined BCCH) BCCH+CCCH (extended BCCH) SDCCH/8+SACCH/8 (main SDCCH)


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Structure of Main BCCH 1 multi-frame (51TDMA Frames) 235.38ms Downlink Group3,4

Group

Group1

Group2

Grpup5

(same as Group2)

Channel Frame Number

F

S

BX4

CX4

F

S

CX4

CX4

……

F

S

CX4

CX4

I

0

1

2-5

6-9

10

11

12-15

16-19

20-39

40

41

42-45

46-49

50

1 multi-frame (51TDMA Frames) 235.38ms Uplink Channel

R

R

R

R

R

R

R

R

R

R

R

R

R

R……R

R

R

R

R

0

1

2

3

4

5

6

7

8

9

10

11

12

13-46

47

48

49

50

Frame Number

F:FCCH; S:SCH; B:BCCH; C:CCCH; I:IDLE; R:RACH Copyright © 2009 Huawei Technologies Co., Ltd. All rights reserved. Page15

Structure of Combined BCCH 1 multi-frame (51TDMA Frames) 235.38ms Downlink Group

Group1

Channel

F

S

Channel

F

S

0

1

Frame Number

B 

4

B 

4

2-5

Group2 C 4



C 4



6-9

C

F

S

F

S

10

11

4



C 4

Group3 C 4



C 4





12-

16-

15

19

F

S

F

S

20

21

D0 4



D0 4

Group4 D1 4



D1 4





22-

26-

25

29

F

S

F

S

30

31

1 multi-frame (51TDMA Frames) 235.38ms

Grpup5

D2 4



D2 4

D3 4



D3 4





32-

36-

35

39

F

S

F

S

40

41

A0 4



A2 4

A1 4



A3 4





42-

46-

45

49

I

I

50

Uplink

Channel

D34

R

R

A24

A34

R……R

D04

D14

R

R

D24

Channel

D04

R

R

A04

A14

R……R

D04

D14

R

R

D24

Frame Number

0-3

4

5

6-9

10-13

14-36

37-40

41-44

45

46

47-50

F:FCCH; S:SCH; B:BCCH; C:CCCH; D:SDCCH ;A:SACCH; I:IDLE; R:RACH Copyright © 2009 Huawei Technologies Co., Ltd. All rights reserved. Page16

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Structure of Logical Channel Combination Frame-Main SDCCH 1 multi-frame (51TDMA Frames) 235.38ms Downlink Channel

Channel Frame Number

D0

D1

D2

D3

D4

D5

D6

D7

A0

A1

A2

A3

4

4

4

4

4

4

4

4

4

4

4

4

D0

D1

D2

D3

D4

D5

D6

D7

A4

A5

A6

A7

4

4

4

4

4

4

4

4

4

4

4

4

0-3

4-7

8-11

12-

16-

20-

24-

28-

32-

36-

40-

44-

15

19

23

27

31

35

39

43

47

I

I

I

I

I

I

48

49

50

1 multi-frame (51TDMA Frames) 235.38ms Uplink Channel Channel Frame Number



A5 4

A6 4

A7 4

I

I

I

A1 4

A2 4

A3 4

I

I

I

0-3

4-7

8-11

12

13

14





D0 4



D1 4

D2 4



D3 4

D4 4

D5 4

D6 4

D7 4

A0 4



D0 4



D1 4

D2 4



D3 4

D4 4

D5 4

D6 4

D7 4

A4 4

1518

1922

2326

2730

3134

3538

3942

4346

4750

D:SDCCH; A:SACCH; I:IDLE Copyright © 2009 Huawei Technologies Co., Ltd. All rights reserved. Page17

ATT 

Value range: Yes, No



ContentIt is used to inform MS whether IMSI attach-detach is allowed in this cell. If it is set to “Yes”, the network will not process the connection to the called mobile subscriber when MS is power-off. Thus network processing time and radio resources are saved. Otherwise the network will process the connection even though the MS has been powered off.



RecommendationYes


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CCCH_CONF 

Value range: 1 Non-Compounding CCCH, 1 Compounding CCCH, 2 Non-Compounding CCCHs, 3 Non-compounding CCCHs, 4 Noncompounding CCCHs.



Content: It is “Common Control Channel Configuration”. CCCH configuration determines the capacity of PCH, AGCH and RACH. This parameter can be automatically configured by the BSC Data Auto Configuration System according to the TRX channel configuration.

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Recommendation: When there is one TRX in the cell, one combined CCCH is recommended (in a system with few paging messages in location area). For others, it is configured according to the number of TRX in the cell.


BS_AG_BLKS_RES 

Value range: 0~2 (1 combined CCCH), 0~7 (others)

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Unit: Block

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Content: It is also called Access Granted Blocks Reserved. It is the number of CCCH channel message blocks that are reserved in one multi-frame for access granted channels (AGCH).



Recommendation: 2 (non-combined CCCH)


10

BS_PA_MFRAMES 

Value range: 2 9



Unit: Multi-frame period (51 frames)



Content: It is Paging Channel Multi-frames. It defines the number of multi-frames used as a cycle of paging subchannels.



Recommendation: 2


T3212 

Value range: 0~255



Unit: 6 minutes



Content: It is the Periodic Location Update Timer. It defines the interval of periodic location update.



Recommendation: 30 (for urban area), 20 (for suburban area)


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CRH 

Value Range: 0~14, the step size is 2dB



Unit: dB



Default Value: 6



Description: Cell Reselection Hysteresis. It is the parameter used when cell reselection happens between two location areas.



Location: Cell Attributes/Idle Mode/Basic Idle Parameters


TCH Immediate Assignment 

Value Range: Yes, No



Default Value : No



Description: Option "Yes" means that TCH channel can be immediately assigned as signaling and traffic channel when SDCCH has no available resource. The option "No" means that only SDCCH can be assigned.



Location: Cell Attributes


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Calculating For Location Area 

Generally, the capacity of a location area is calculated as follows: The number of paging blocks sent in each second



the

number of paging messages sent in each paging block = the maximum paging times in each second.


Calculating For Location Area The number of paging blocks sent in each second 1 frame = 4.61ms, 1 multiframe = 51 frames = 0.2354s;


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suppose the number of access grant blocks is AGB, the number of blocks, the number of paging blocks sent in each second is calculated by the following formulas: 

For non-combined BCCH, the number of paging blocks sent in each second = (9 – AGB)/0.2345 (paging block/second).



For non-combined BCCH, the AGB is 2 according to Huawei BSC. Therefore, the number of paging blocks sent in each second is 29.7 (paging block/second).



suppose the number of access grant blocks is AGB, the number of blocks, the number of paging blocks sent in each second is calculated by the following formulas: 

For combined BCCH, the number of paging blocks sent in each second = (3 – AGB)/0.2345 (paging block/second).



For combined-BCCH, the AGB is 1, so the number of paging blocks sent in each second is 8.5 (paging blocks/second).


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The number of paging messages sent in each paging block (X) 

According to protocols, each paging block has 23 bytes, and can send 2 IMSI pages, or 2 TMSI and 1 IMSI pages, or 4 TMSI pages.



According to the paging strategies of Huawei MSC, if the IMSI paging mechanism is adopted, the number of paging messages sent in each paging blocks is 2 (paging times/paging block); if the TMSI paging mechanism is adopted, it is 4 (paging times/paging block)



If the IMSI paging mechanism is adopted, for non-combined BCCH when AGB = 2, P = 59.47 (paging times/second);



For combined-BCCH when AGB = 1, P = 16.99 (paging times/second);


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LAC Planning

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