1

Capacity planning exercise

M.Sc. Mika Husso

9.2.2007

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Traffic reviewed …

• The unit of traffic is E [erlang]– Single line or sever can handle up to 1 E traffic.

• Offered Traffic (total traffic created by subcribers)– A= h = call intensity * mean service time

• Carried Traffic (or served traffic) = Total amount of traffic the network is able to serve

• Lost Traffic (or rejected traffic)= Offered Traffic – Carried Traffic

• Potential traffic: Offered traffic if there would be no restrictions on the use of the service.

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… Traffic reviewed

• In practice, it is not feasible for mobile network to have the capacity to handle any possible load at all times.

• Fortunately, not all subscribers place calls at the same time and so it is reasonable to size the network to be able to handle some expected level of load.– > planner has to design the network to meet a

predefined blocking probability (e.g. 2 %), which depends on the desired GoS

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Capacity planning process (TDMA/FDMA)

1. Considering the available resources (number of carriers etc.) and the GoS requirements (blocking prop., )

2. Estimating the amount of Offered Traffic on each area

3. Estimating how many cells (BSs) and how many traffic channels per cell are needed to serve the offered traffic on the area with the given blocking propability (e.g. 2 %)

-> Determining the CAPACITY based cell area (and radius)

4. Checking if also COVERAGE can be granted for the capacity based cell (i.e. can the signal reach the user/BS without attenuating too much?)

-> if not the cell radius is decreased so that COVERAGE can be granted

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1. Considering the resources

• There are available resources– Number of carriers (channels)– Multiplexing (TDMA, FDMA, CDMA)– Duplexing (TDD, FDD)– …

• There are also requirements for GoS– Blocking propability– Call dropping propability– …

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2. Estimating the traffic …

• Offered voice traffic from a user group can be predicted as follows

, where• N is the number of persons• C denotes the penetration• T is the average traffic generated by one user• F denotes the area coverage probability (users not on

the network coverage area can’t offer traffic)

* * *tot userT N C T F

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… Estimating the traffic

• The traffic offered by each user is:A = μH Erlangs, whereH is the average holding time of a callμ is the average number of calls requested/time unit by the user

For exampleH = 2 minutes and μ = 0.8 calls / hour

-> A = 2 * (0.8/60) ≈ 26.7 mErlang

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Traffic estimate• Penetration CP = 0.25

• Offered traffic per user: TO,1=TO,2=20 mErlang

• Coverage probability: F1=0.8 (Pedestrian), F2=0.95 (Vehicular)

• Distance between pedestrians S1=4 m

• Distance between vehicles S2=25 m

• Number of people in a car 2=1 (1=1)

• Number of pedestrians

N1 = 1 Ls/S1 = 1•10000/4=2500

• Number of cars

N1 = 2 Ls/S2 = 1•10000/25=400

• Traffic offered by pedestrian users

T1=F1CP TO,1 N1 = 0.8•0.25•0,02•2500=10 Erlang

• Traffic offered by vehicular users

T2=F2CP TO,2 N2 = 0.95•0.25•0,02•400=1.9 Erlang

• Offered traffic A= T1+T2 = 11.9 Erlang

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3. Estimating the number of BSs needed

• Assuming we have estimated a total traffic of 20 Erlang on a given area, how many BSs do we need if the smax number of tranceivers (channels) on a BS is 5 and the desired blocking level is 3 %?– Using Erlangs B formula (or table)

We need 27 channels, which can serve a total of 20.31 Erlang (from the Erlang B table)

We need 27 / 5 = 5.4 -> 6 Base Stations (using 5 transceivers in a BS)

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4. Can coverage be granted?

• Will be dealt with in the coverage planning exercise

• Based on the calculation of a link budget– Can the signal be received with adequate power?

• If the signal attenuates too much, the maximum distance between BS and user (i.e. cell radius) must be reduced

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Example of capacity planning• System parameters

– Penetration (all user groups): 25 %– Offered traffic/user (all user groups): 20 mErlang– Coverage probability target: vehicular users 95%, 1 user/car,

pedestrian users 80%– Multiple access method: FDMA, 28 TRX/cell – Blocking probability target: 2 %– Service area divided into 4 homogenous Regions with spatially

uniformly distributed users– In Region A the vehicular generated traffic is handled by

macrocells and pedestrian generated traffic by microcells, in other Regions all traffic is handled by macrocells

• Approach: Minimum excess capacity, starting from Region with highest traffic density, cells possibly overlapping to adjacent Regions will reduce the area in these to be covered correspondingly

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Geometry of the service area

A

B

C

D

Region types:A: dense cityB: cityC: suburbanD: rural

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Parameters of the Regions in the Service Area

Region

Size, L1L2-area of

o3ther Regions

Block size,

LB1LB2

Vehicle

spacing, Sv

Pedestrian spacing,

Sp

A 55 km2 = 25 km2 0.20.2 km2 25 m 4 m

B

1515 25 km2 =

200 km2 0.250.25 km2 50 m 10 m

C

4040 225 km2 =

1375 km2

0.1250.25

km2 200 m 125 m

D

120120 1600

km2 = 12800 km2 22 km2 1000 m 550 mBasic assumption: Vehicles, pedestrians, and traffic are assumed to be spatially uniformly distributed in each Region.

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Choosing the cell structure

hex_quad_cells

2

36

2 21

0.173

R R

FROLR

An ideal cell would have a circular shape.To get complete coverage a certain overlapping must be allowed. Minimum overlapping with hexagonal structure, which is the most common in theoretical investigations Another possible cell structure giving complete coverage is the quadratic cell structure In this example the quadratic cell structure gives easier calculations and will be used FROL = Fractional Overlapping

hex_quad_cells

2

22

1

0.363

RFROL

R

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Estimating the population in the regionsEstimation of population in the Regions

Region

Length of street/road network

1 2 2 1

1 2

2

B B B

L L L L AL

L L L

Number of cars N1= L/S

Number of pedestrians N2= L/S

A L km

2 25

0 2250

. N1

250

0 02510000

. N2

250

0 00462500

.

B L km

2 200

0 251600

. N1

1600

0 05032000

. N2

1600

0 010160000

.

C L

km

FH

IK

1375

0 125

1375

0 25

16500

. .

N116500

0 282500

.

N216500

0 125132000

.

D L

km

2 12800

212800

N112800

112800 2

1280023273

0.55N

Hint. Don't forget to subtract the area of inner Regions

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Estimating the offered trafficEstimation of offered traffic in the Regions

Region

Vehicle originated traffic T F C T N

N

N

p o1 1 1 1

1

1

0 95 0 25 0 02

0 00475

. . .

.

Pedestrian originat-ed traffic T F C T N

N

N

p o2 2 2 2

2

2

0 80 0 25 0 02

0 004

. . .

.

Total traffic and traffic density

T T T 1 2/Erlang

T

A

T

A

T

A1 2 /Erl/km2

A T1 0 00475 10000

47 5

.

. Erlang T2 0 004 62500

250 0

.

. Erlang T

47 5 250 0 297 5

1 90 10 00 11 90

. . .

. . .

B T1 0 00475 32000

152 0

.

. Erlang T1 0 004 160000

640 0

.

. Erlang T

152 0 640 0 792 0

0 76 3 20 3 96

. . .

. . .

C T1 0 00475 82500

391 9

.

. Erlang T1 0 004 132000

528 0

.

. Erlang T

391 9 528 0 919 9

0 285 0 384 0 669

. . .

. . .

D 1 0.00475 12800

60.8 Erlang

T

1 0.004 23273

93.1 Erlang

T

60.8 93.1 153.9

0.00475 0.00727

0.0120

T

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Estimating the amount of traffic channels and BSs to be used

Minimum number of cells with different number of TCs/BS.

Traffic channels/BS and maximum traffic/BS Region Offered Traffic

Offered traffic density 4 8 12 16

20 24 28

Erlang Erl/km2 1.09 3.63 6.61 9.83 13.18 16.63 20.15 B Amacro C D Amicro

792.0 47.5 919.9 153.9

250.0

3.96 1.90 0.669 0.012

10.00

80.57 60.09 47.62 2.86 55.32 9.25

15.03

39.31 2.36 45.65 7.64

12.41

The Region with the highest traffic density is treated first, as this approach would minimise equipment in most cases. In this case Region B is investigated first, because the traffic density is highest there (In Region A only car generated traffic is served by macrocells, while pedestrian generated traffic is served by microcells)

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Dimensioning cells …Cell dimensioning in Region B, 1/3 Region B cell size of cell completely inside the region:

x

kmkm

x km R kmB

22

2200

39 315 09

2 26 2 26 2 1 59

Region B total area

minimum cell number ..

. . .

Offered traffic in edge cells partly covering Region C (overlapping area from geometrical considerations): Cells B1,B2,B3,B4,B5,B6,B14,B21,B28,B35,B40,B45:

T S SB B C C

3 96 2 26 2 26 0 82 0 669 2 26 0 82 14 13. . ( . . ) . . . . Erlang

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… Dimensioning cells …Cell dimensioning in Region B, 2/3

From the Erlang B-table: 22 TRXs in the FDMA-system giving a capacity of 14.90 Erlang/cell Corner cell B7 partly covering Region C: T S SB B C C

3 96 2 26 0 82 0 669 2 26 2 26 0 82 10 242 2 2. ( . . ) . . ( . . ) . Erlang

17 TRXs in the FDMA-system giving a capacity of 10.66 Erlang Offered traffic in edge cells B29,B30,B36,B41 partly covering Region A:

T S SB B A A

3 96 2 26 2 26 0 48 1 90 2 26 0 48 17 99. . ( . . ) . . . . Erlang

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… Dimensioning cells …Cell dimensioning in Region B, 3/3

26 TRXs in the FDMA-system giving a capacity of 18.38 Erlang/cell Corner cell B31 partly covering Region B:

T S SB B A A

3 96 2 26 0 48 1 90 0 48 19 752 2 2. . . . . .e j Erlang

28 TRXs in the FDMA-system giving a capacity of 20.15 Erlang In all other 27 Region B cells completely inside the region:

T SB B 3 96 2 26 20 152. . . Erlang 28 TRXs in the FDMA-system giving a capacity of 20.15 Erlang

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… Dimensioning cells

• When region B is dimensioned, it usually partly overlaps regions A, C and D and therefore also serves some of their offered traffic– > When dimensioning regions A, C and D the traffic

already served by region B should not be served again (equipment should be minimized)

• Otherwise the dimensioning process is done exactly as for region B

• When the next region is dimensioned (in this case D), the traffic served by it in other regions should also not be served again

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Macrocell layout (capacity planning)

D1 D2 D3

D4 D5 D6

D7 D8

C1 C6 C7 C8

C9 C10 C11 C12 C13 C14 C15

C2 C3 C4 C5

C16

C17 C18 C19 C20 C21 C22 C23 C24

C25 C26 C27 C28 C29 C30 C31 C32

C33 C34 C35 C36 C37 C38 C39 C40

C41 C42 C43 C44 C45

C46 C47 C48 C49 C50

C51 C52 C53 C54 C55

B1 B2 B3 B4 B5 B6 B7

B8 B9 B10 B11 B12 B13 B14

B15 B16 B17 B18 B19 B20 B21

B22 B23 B24 B25 B27 B28B26

B29 B30 B31 B32 B33 B34 B35

B41 B42 B43 B44 B45

A1A2

B36 B37 B38 B39 B40

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Dimensioning microcells

• The procedure is similar, but– In the city area, buildings cause significant attenuation

to the signal– To minimize equipment, the BSs should generally be

located at the street crossings– Usually pedestrian originated traffic on the area is

served using microcells and vehicular originated using macrocells

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Microcell layout (capacity planning)

Coverage area (path loss <150 dB) calculated with COST231 Walfisch Ikegami model

f=960 MHz

hbs=15 mhms=1.6 m

hroof=30 mw=25 mb=100 m

WI_

cove

rage

.dsf

1.5

0

10

20

30

40

50

60708090100110

120

130

140

150

160

170

180

190

200

210

220

230240

250260 270 280 290

300

310

320

330

340

350

1.4 km

0.2

0.4

0.6

0.8

1.0

1.2

Am1 Am2 Am3

Am4 Am5 Am6

Am7 Am8 Am9

Am10 Am11 Am12

Am13 Am14 Am15

Am16 Am17 Am18