Capacity comparison of WCDMA and GSM

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• GSM: Capacity is hard blocked, limited by the amount of hardware. TRX number

• WCDMA: Capacity is Soft Capacity, it is a interference limited system, we can’t get

the capacity by the hardware quantity.

The traffic will be affected by following factor• 1 DL Power • 2 Signal quality (Ec/Io, RX level)• 3 Iub Transmission • 4 (OVSF)Code Resource • 5 CE Resource

Different Capacity Calculation of WCDMA and GSM

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GSM Capacity Calculation Flow

TRX TRX

1 TRX =8 time solts

Erlang-B table Budget Erlang

The utilization

can be higher than

100%

Case study

The Traffic is base on TRX time slot amount, there is no other factor affect the traffic, if the time slot is fixed, the Traffic is

fixed

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• Capacity & Coverage Users Capacity Capacity Cell Coverage

• Capacity & Quality Capacity Quality (interference ) Quality (Ec/Io , Rx Level )

Capacity

• Coverage & Quality Quality (Ec/Io ) Cell Coverage

Capacity

Quality Coverage

COST

WCDMA Coverage Capacity Quality Relation

For a soft capacity limited system, the Erlang capacity cannot be calculated from the Erlang B formula, since it would give too pessimistic results

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• 1 DL Power • 2 Signal quality (Ec/Io, RX level)• 3 Iub Transmission • 4 (OVSF)Code Resource • 5 CE Resource

WCDMA Capacity Calculation

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5 W for

power control

4 W for

common channel

11 W for Traffic channel

1 DL power analysis

DL total power is 20W

If DL power utilization is over 75%,DL power congestion.

DL Loading=DL used power/Total power

Reserved power Usable power,75% of total power

If the user use the max service ,one cell can support 2 384K subscriber,one site can only support 6

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• 3G:The DL Cell power is dynamic. If User is far away the cell or the signal quality is bad, the cell need to use higher power to maintain it service .So the max and min user will be change. The DL power utilization will affect the cell user number and traffic

• 2G:The DL power is fixed. It will not affect the cell user number and traffic

• If one subscriber want to access one cell ,it will increase the DL power utilization.

• So the DL power is the main resource in WCDMA system.

• Question :If the DL power is the main resource, can we use high power RRU to

improve it ?how 60W or 80W RRU? Answer:No, we can’t do like this. Because the WCDMA is self interference system.

If one cell improve it’s TX power, it will increase the Io, so the Ec/Io will become very bad.

In WCDMA, We need to control the coverage to improve the Ec/Io quality

1 DL power analysis

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2 Signal quality analysis

Cell capacity Cell Interference Cell Ec/Io Cell Ec/Io

Cell A

Cell BCell capacity

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MAX interference level (Io)

Eb/No

Ec/Io range

Ebit

Processing Gain

Echip

Service Ec / Io= Eb / No –PG (Processing Gain)

2 Signal quality analysis

Spreading and Despreading

If user TCH Ec/Io is lower than 6.2 dB, the cell can’t provide 384k service

Other user interference

singal

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• 3G: WCDMA is a self interference system The capacity is affected by RX level and Ec/Io. Ec/Io is the main factor to affect cell capacity.

• 2G: The capacity is affected by RX level.

2 Signal quality analysis

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3 Iub Transmission

384k user in one site

site throughput

12 5.8Mbps

• If the DL power isn’t in congestion, and the DL Ec/Io is higher than -9.2 dB, then one site throughput can reach 5.8Mbps.

• As we know, The mobility of subscribers, fluctuation of the radio signals, the variety of service and the complexity of the network

system has made Ec/Io and DL power in unstable situation, it is very hard to let the site throughput reach 5.8Mbps.

one site can’t support 12 users if DL Ec/Io is very bad or DL power is not enough

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4 OVSF Resource

SF = 1 SF = 2 SF = 4

Cch,1,0 = (1)

Cch,2,0 = (1,1)

Cch,2,1 = (1,-1)

Cch,4,0 =(1,1,1,1)

Cch,4,1 = (1,1,-1,-1)

Cch,4,2 = (1,-1,1,-1)

Cch,4,3 = (1,-1,-1,1)

OVSF:Orthogonal Variable Spreading Factor

• In WCDMA system, Channelisation code uses OVSF code, for keeping the orthogonality of different subscriber physical channels.

• One cell only has one OVSF tree

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4 OVSF Resource

• One site’s OVSF can support 21 384K subs if DL power is enough or the Ec/Io is good.

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5 CE Resource

Case study

• In this case ,users number is limited by the DL CE, because it is 64.• But one site capacity can support 256 DL CE, it can support more subs

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DL power is the main resource in WCDMA system.

So we need to monitor it’s utilization, if the DL power utilization is reach 70%,we can say the site utilization is 100%

DL Power Utilization =DL used power/Total power

Summary

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Typical CapacityTypical Capacity

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CE Consuming of differenct serviceCE Consuming of differenct service

Thank You