8. Capacity Planning

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Capacity Planning


OBJECTIVESOBJECTIVES

•Method for calculating the system capacity

•Methods for site capacity planning


Network DimensioningTo estimate the amount of sites and cells.

• How many Subscribers are supported by one site?

• How many sites should be setup in the service area?

• How many TRX are supported by one site?

Capacity Planning


Some basic concept

Traffic （ Erlang ）Busy hour traffic per

subscriber （ Erlang ）BHCA(Busy Hour Call Attempt)Erlang B tableCall loss rate


Traffic

• Traffic refers to the numbers of subscribers the network can support and is usually described as follows:

A=n×T/3600

where,

n- calls made by a subscriber within an hour ；T- average duration of each call(in seconds)

A - Traffic, in Erlang

If one call is made by a subscriber within an hour and last 120 seconds, the traffic is calculated as: A=1×120/3600=33mErl 。For convenience of engineering calculation, the traffic is usually defined as 25mErl per subscriber.


GOS

• Grade of Service （ GOS）：GOS refers to the degree of network congestion or call loss rate. GOS=2% means that 98% of subscribers can make calls successfully and 2% of subscribers will end up with unsuccessfulness.

For network operators, 2-5% of GoS is adopted.


• BHCA: Busy hour calling amount or busy hour call attempt

• Erlang B table: define the relationship among call loss,channel numbers and traffic.


2% 5% 1 0. 020 0. 0532 0. 223 0. 3813 0. 602 0. 8994 1. 092 1. 5255 1. 657 2. 2186 2. 276 2. 9607 2. 935 3. 7388 3. 627 4. 5439 4. 345 5. 37010 5. 084 6. 21611 5. 842 7. 07612 6. 615 7. 95013 7. 402 8. 83514 8. 200 9. 73015 9. 010 10. 63316 9. 828 11. 54417 10. 656 12. 46118 11. 491 13. 33519 12. 333 14. 31520 13. 182 15. 24921 14. 036 16. 18922 14. 896 17. 13223 15. 761 18. 08024 16. 631 19. 03025 17. 505 19. 985

Erlang-B Table


Erlang-B Traffic TablesAbbreviated - For P.02 Grade of Service Only

#TrunksErlangs #TrunksErlangs #Trunks #TrunksErlangs #TrunksErlangs #TrunksErlangs #TrunksErlangs #TrunksErlangs

1 0.0204 26 18.4 51 41.2 76 64.9 100 88 150 136.8 200 186.2 250 235.82 0.223 27 19.3 52 42.1 77 65.8 102 89.9 152 138.8 202 188.1 300 285.73 0.602 28 20.2 53 43.1 78 66.8 104 91.9 154 140.7 204 190.1 350 335.74 1.09 29 21 54 44 79 67.7 106 93.8 156 142.7 206 192.1 400 385.95 1.66 30 21.9 55 44.9 80 68.7 108 95.7 158 144.7 208 194.1 450 436.16 2.28 31 22.8 56 45.9 81 69.6 110 97.7 160 146.6 210 196.1 500 486.47 2.94 32 23.7 57 46.8 82 70.6 112 99.6 162 148.6 212 198.1 600 587.28 3.63 33 24.6 58 47.8 83 71.6 114 101.6 164 150.6 214 200 700 688.29 4.34 34 25.5 59 48.7 84 72.5 116 103.5 166 152.6 216 202 800 789.310 5.08 35 26.4 60 49.6 85 73.5 118 105.5 168 154.5 218 204 900 890.611 5.84 36 27.3 61 50.6 86 74.5 120 107.4 170 156.5 220 206 1000 999.112 6.61 37 28.3 62 51.5 87 75.4 122 109.4 172 158.5 222 208 1100 109313 7.4 38 29.2 63 52.5 88 76.4 124 111.3 174 160.4 224 21014 8.2 39 30.1 64 53.4 89 77.3 126 113.3 176 162.4 226 21215 9.01 40 31 65 54.4 90 78.3 128 115.2 178 164.4 228 213.916 9.83 41 31.9 66 55.3 91 79.3 130 117.2 180 166.4 230 215.917 10.7 42 32.8 67 56.3 92 80.2 132 119.1 182 168.3 232 217.918 11.5 43 33.8 68 57.2 93 81.2 134 121.1 184 170.3 234 219.919 12.3 44 34.7 69 58.2 94 82.2 136 123.1 186 172.4 236 221.920 13.2 45 35.6 70 59.1 95 83.1 138 125 188 174.3 238 223.921 14 46 36.5 71 60.1 96 84.1 140 127 190 176.3 240 225.922 14.9 47 37.5 72 61 97 85.1 142 128.9 192 178.2 242 227.923 15.8 48 38.4 73 62 98 86 144 130.9 194 180.2 244 229.924 16.6 49 39.3 74 62.9 99 87 146 132.9 196 182.2 246 231.825 17.5 50 40.3 75 63.9 100 88 148 134.8 198 184.2 248 233.8

Erlangs


The number of control channels

Suppose SDCCH average process time is 3s ， Location updating process is 9s,BHCA=2

The traffic of SDCCH per subscriber is: (3×2 + 9) / 3600 = 0.0042 Erlang4SDCCH call loss=2% can support 1.092Erlang ， (1.092 / 0.0042 = 260sub) ×0.025 Erlang = 6.5Erlang look up in Erlang-B ， call loss=2% ， 6.5Erlang need 12TCH(2TRX)

8SDCCH call loss=2% can support 3.627Erlang(3.627 / 0.0042 = 863sub) ×0.025 Erlang = 21.6ErlangLook up in Erlang-B ， call loss=2% ， 21.6Erlang need 30 TCH(4TRX)


Allocation of TCH and CCH

Suppose: Busy hour traffic is 0.025Erl/per subscriber ， call loss rate is 2% 。

TRX CH CCH TCH TRAFFIC

1 8 1 7 2.94

2 16 2 14 8.20

3 24 2 22 14.90

4 32 2 30 22

5 40 3 37 28.25

6 48 3 45 35.61

7 56 4 52 42 .12

8 64 4 60 49.64


Steps

Subscriber prediction →Traffic density forecast →Site type and number

→Channel configuration

Based On

a 、 Traffic and traffic density

b 、 GOS

c 、 Available number of TCH/CCH

d 、 Available frequency band and frequency reuse pattern

Traffic forecast and capacity planning


Subscriber Forecast

Near term: 1-2years

Long term: 3-5years

• Growth trend forecast method• Population penetration method • Growth curve method• Conic method


Growth trend forecast

0

500

1000

1500

2000

2500

( )用户数万

( )用户数万 1. 534. 7517. 763. 8 157 363 685 13642496

1990

1991

1992

1993

1994

1995

1996

1997

1998 0

500

1000

1500

2000

2500

3000

( )用户数万 1. 5 4. 8 18 64 157 363 685 136 249

1990

1991

1992

1993

1994

1995

1996

1997

1998


Growth curve forecast

0

50

100

150

200

250

300

(%)年增长率 0 160 272 261 146 132 89 99 83

1990 1991 1992 1993 1994 1995 1996 1997 19980

50

100

150

200

250

300

(%)年增长率 160 272 261 146 132 89 99 83

1991 1992 1993 1994 1995 1996 1997 1998


Population penetration method

0

2000

4000

6000

8000

10000

用户数（万）

用户数（万）

363 685 1364

2496

3432

5053

7016

9219

1995

1996

1997

1998

1999

2000

2001

2002

0

1

2

3

4

5

6

7

8

9

%普及率 0.3 0.57 1.14 2.08 2.64 3.89 5.4 7.69

1995 19961997 1998 19992000 20012002


Traffic density forecast method

• Linear forecast

• Linear forecast combined with manual adjustment (population, income, vehicle)


Base station capacity planning

1. The number of BTS (Traffic limited)

i. Estimate the largest possible capacity (Erl) of each base station according to the frequency multiplex mode to be used.

ii. Minimum number of BTS =The total traffic / The largest capacity of each BTS.

2. The number of BTS (Coverage limited)

Maximum number of BTS=The area size / The (estimated) minimum coverage of a BTS.


Base station capacity planning

3 ． Estimate the BTS cell capacity

I. Estimate the cell coverage area. Multiply the BTS cell coverage by the traffic density concerned, then you can get the present traffic of the cell.

II. Query the ErlangB table according to the traffic and specified call loss index, you can get the number of voice channels needed by the BTS cell.

III. Add up the voice channels and the control channels that should be configured, then divide the number by 8, you can get the number of carrier frequencies of the BS and the cell.


LA PlanningPLMN

MSC

C E L LLA

C E L L

C E L L

C E L L

C E L LLA

C E L L

C E L L

C E L L

MSC

C E L LLA

C E L L

C E L L

C E L L

C E L LLA

C E L L

C E L L

C E L L

messages are paged based on LA.

One LA may contain one or more BSCs, but it belongs to only one MSC. One BSC or MSC can contain multiple LAs.

From the perspective of reducing frequent location update and saving channel resources, large LA size is recommended. The reason is that more location updates enlarges the SDCCH load, reduce channel resources, and add the load of MSC and HLR.


LA Planning

• Minimize the Location Update numbers on condition that there is no overload on PCH

• Paging load decide the maximum size of LA• Location Update Load decide the minimum size of

LA

In addition, a cell update takes about 10 s and during the update, an MS cannot make or receive calls. However, if the LA is so large that paging capacity cannot match, the paging signaling load is excessive, which causes missing paging messages and lowers the paging success rate. Also, low paging success rate make users call again and thus the paging load is increased and the success rate is further reduced. Therefore, the LA cannot be set to a too large value.


LA Planning

• LA should not be too big or too small

• LA:300-600TRX

• Plan LA on geographical situation and subscriber behaviors


Determining LA Edges

1. Determine the edges away from high traffic districts.

2. Set the LA edge to be athwart to the road

3. Avoid setting the edges of several location areas at the same small area.

4. When determining LA edges, consider the growth trend of traffic. Based on the design of paging capacity and traffic capacity, the expansion margin should also be considered to avoid frequent LA division and splitting.

Documents

8. Capacity Planning