WCDMA Power Control (1)

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WCDMA Power Control Principle

ISSUE 1.0

2 Internal Use

Chapter 1 Power OverviewChapter 1 Power Overview

Chapter 2 Power Control AlgorithmChapter 2 Power Control Algorithm

3 Internal Use

Purpose of power controlPurpose of power control

Purpose of power controlPurpose of power control

Uplink channelUplink channel: To overcome the near-far effect.

Downlink channelDownlink channel : Overcome fast fading and the interferences of adjacent cells.

Power control must be used in CDMA system to ensure every user transmits

at minimum power, thus maximizing network capacity

4 Internal Use

Power control classificationPower control classification

Power control classification：

Open loop Power controlOpen loop Power control

Closed loop Power controlClosed loop Power control

Uplink inner power control

Downlink inner-power control

Uplink outer power control

Downlink outer power control

The purpose of inner loop power control in WCDMA system is to maintain a certain

signal-to-interference ratio of transmission signal power when the signals reach the

receiving end.

However, for different multi-path environments, even if the mean signal-to-

interference ratio is kept above a certain threshold, the communication quality

requirement (BER) can not always be satisfied .

5 Internal Use

The Relationship between Transmitted Power and The Relationship between Transmitted Power and Received Power after Power Control Methods IntroducedReceived Power after Power Control Methods Introduced

0 200 400 600 800-20

-15

-10

-5

0

5

10

15

20

Time (ms)

Rel

ati

ve

po

wer

(d

B)

Channel

Transmitted power

Received power

6 Internal Use

Benefit of Power ControlBenefit of Power Control

Benefit of power control

Overcome uplink near-far problem

Ensure good communication quality in uplink and downlink by adjust

transmission power

Overcome the influences of unfavorable factors such as fast fading, slow fading

on radio channels

Decrease network interference, increase the capacity and quality of network

In other word, the purpose of power control is to ensure the QoS with minimum

power in the CDMA system.

7 Internal Use

Power control methods adopted for various physical channelsPower control methods adopted for various physical channels

Power control methods adopted for various physical channels "X" – can be applied, "–" – not applied

Physical channel

Open loop power control

Inner loop power control

Outer loop power Control

No power control process, power is specified by upper

layers.

DPDCH － X X －

DPCCH X X X －

PCCPCH －－－ X

SCCPCH －－－ X

PRACH X －－－

AICH －－－ X

PICH －－－ X

8 Internal Use

Chapter 1 Power OverviewChapter 1 Power Overview


9 Internal Use

1.Open loop power control1.Open loop power control

2.Inner-loop power control2.Inner-loop power control

3.Outer loop power control3.Outer loop power control


10 Internal Use

Open Loop Power Control OverviewOpen Loop Power Control Overview

Purpose

UE estimates the power loss of signals on the propagation path by

measuring the downlink channel signals, then calculate the transmission

power of the uplink channel.

Mainly used for initial connection setup.

Under the FDD mode, fast fading of the uplink channel is unrelated to fast

fading of the downlink channel.

11 Internal Use

Open Loop Power Control OverviewOpen Loop Power Control Overview

Disadvantage of open loop power control

This power control method is rather vague

Application scenarios of open loop power control

In the range of a cell, signal fading caused by fast fading is usually more

serious than that caused by propagation loss. Therefore, open loop

power control is applied only at the beginning of connection setup,

generally in setting the initial power value.

12 Internal Use

Open Loop Power Control of PRACHOpen Loop Power Control of PRACH

Random access procedure of PRACHRandom access procedure of PRACH : - UE transmit a preamble using the selected uplink access slot, signature, and preamble

transmission power. - Then ,UTARN response by sending AI if the preamble is received. - Next, UE transmit the message part if the AI is received. - But, if UE does not receive the AI from UTRAN in τp-p period, a next preamble will be

transmitted.

AICH accessslots RX at UE

PRACH accessslots TX at UE

One access slot

p-a

p-mp-p

Pre-amble

Pre-amble

Message part

Acq.Ind.

13 Internal Use

Open Loop Power Control of PRACHOpen Loop Power Control of PRACH

The initial value of PRACH power is set through open loop power control

Preamble_Initial_Power = PCPICH DL TX power Preamble_Initial_Power = PCPICH DL TX power －－ CPICH_RSCP + UL CPICH_RSCP + UL

Interference + Constant ValueInterference + Constant Value

ParametersParameters

PCPICH DL TX power, UL interference and Constant are given in system information.

The value of CPICH_RSCP is measured by UE

PCPICH DL TX power is very closed to the downlink coverage ability,

which is already given in cell setup.

UL interference can be measured by NodeB, then it will be reported to RNC.

Constant Value is the threshold of preamble message. This value has to be

analysed very carefully.

14 Internal Use

Open loop power control of PRACHOpen loop power control of PRACH

NO. Parameter Parameter meaning

1 Power Offset Pp-m The power offset of the last access preamble and message control part. This value plus the access preamble power is the power of the control part

2 Constant Value This parameter is the correction constant used for the UE to estimate the initial transmission power of PRACH according to the open loop power

3 PRACH Power Ramp Step This parameter is the ramp step of the preamble power when the UE has not received the capture indication from NodeB

4 Preamble Retrans Max This parameter is the permitted maximum preamble repeat times of the UE within a preamble ramp cycle

Power Ramp Step

Pp-m

10ms/20ms

Preable_Initial_power

15 Internal Use

Open loop power control of PRACHOpen loop power control of PRACH

Different Constant Values for different stage of WCDMA network

lifecycle. Take the beginning stage for example:

Constant Value can be greater (-16dB or -15dB) so that the preamble

message can be received easier by UTRAN

The power ramp step could be greater so that the possibility which the

preamble message can be received correctly will be higher

With the development of network, the number of users increased

very fast. On this stage, the Constant value could be less 1dB.

16 Internal Use

Open loop power control of PRACHOpen loop power control of PRACH Open loop power control of PRACHOpen loop power control of PRACH

Application scenariosApplication scenarios

1. CCCH : RRC Connection Request

Open loop power control of PRACH

5. Downlink Synchronisation

UE Node BServing RNS

Serving RNC

DCH - FP

Allocate RNTISelect L1 and L2parameters

RRC RRC

NBAP NBAP

3. Radio Link Setup Response

NBAP NBAP

2. Radio Link Setup Request

RRC RRC

7. CCCH : RRC Connection Set up

Start RX description

Start TX description

4. ALCAP Iub Data Transport Bearer Setup

RRC RRC

9. DCCH : RRC Connection Setup Complete

6. Uplink Synchronisation

NBAP NBAP

8. Radio Link Restore Indication

DCH - FP

DCH - FP

DCH - FP

17 Internal Use

Open loop power control of DL DPCCHOpen loop power control of DL DPCCH

The DL DPCCH open loop power control can be calculated by the

following formula:

P =P = （（ Ec/Io)Ec/Io)ReqReq - CPICH_Ec/Io + PCPICH - CPICH_Ec/Io + PCPICH

ParametersParameters

(Ec/Io)req is the required Ec/Io, which should be satisfied for UE to be able to

received the message from dedicated channel correctly

CPICH_Ec/Io is measured by UE, then it is given to UTRAN by RACH

PCPICH is the transmission power of CPICH

Comments

Similar to UL, the (Ec/Io)Req value should be considered very carefully

Because there is no power ramp in the initial DL DPCCH, the initial power should

be satisfied with the requirements. Therefore, this value can be greater than the

one from simulation to ensure the success ratio

18 Internal Use

Open loop power control of DL Open loop power control of DL DPCCHDPCCH Open loop power control of DL Open loop power control of DL DPCCHDPCCH



Open loop power control of DPCCH



Serving RNC

DCH - FP

Allocate RNTISelect L1 and L2 parameters

RRC RRC

NBAP NBAP


NBAP NBAP


RRC RRC





RRC RRC



NBAP NBAP


DCH - FP

DCH - FP

DCH - FP

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Open loop power control of UL DPCCHOpen loop power control of UL DPCCH

The UL DPCCH open loop power control can be calculated by the following formula:

DPCCH_Initial_powerDPCCH_Initial_power ＝＝ PCPICH DL TX power - PCPICH DL TX power - CPICH_RSCPCPICH_RSCP

+ UL interference + + UL interference + DPCCH_SIRtargetDPCCH_SIRtarget

ParameterParameter PCPICH DL TX power is the transmission power of CPICH CPICH_RSCP can be measured by UE UL interference can be measured by NodeB

Comments The DPCCH_SIRtarget value should be considered very carefully.

It reflects the lowest requirement for decoding the DPCCH in a certain multiple path environment

20 Internal Use

Open loop power control of UL Open loop power control of UL DPCCHDPCCH Open loop power control of UL Open loop power control of UL DPCCHDPCCH



Open loop power control of DPCCH



Serving RNC

DCH - FP

Allocate RNTISelect L1 and L2parameters

RRC RRC

NBAP NBAP


NBAP NBAP


RRC RRC





RRC RRC



NBAP NBAP


DCH - FP

DCH - FP

DCH - FP

21 Internal Use





22 Internal Use

CClose loop power controllose loop power control

Weakness of open loop power control

Open loop power control can decided the initial power, but it is not accurate

For WCDMA-FDD system, the uplink fading is not related to the downlink

one because of the big frequency interval of them

Therefore, the path loss and interference estimated by downlink can not reflect

the one in uplink completely. But, the close loop power control can solve this

problem

The advantages of close loop power control

Can control the transmission power of uplink and downlink very fast, and

decrease interference in system.

Maintains a higher quality of service

Why the close loop power control is neededWhy the close loop power control is needed

23 Internal Use

IInner-loop power controlnner-loop power control

The receivers calculate the SIR by estimating the power strength

and the current interference. Then, compare this one to SIRtarget

If SIRSIRmeasured <measured < SIRtarget SIRtarget, send TPC = TPC = 11 to inform receivers increase

its transmission power

If SIRSIRmeasured >measured > SIRtarget SIRtarget, send TPC = 0TPC = 0 to inform receivers decrease

transmission power

The receiver that receives the TPC will adjust the transmission power by

algorithms. The inner loop power control can convergence the

estimated SIR to SIR target

The principle for Inner-loop power controlThe principle for Inner-loop power control

24 Internal Use

IInner-loop power controlnner-loop power control

In 3GPP protocol, two algorithms are adopted in the inner-loop

power control of uplink DPCCH

PCA1 : PCA1 : uplink power control step is tpc=△tpc=△ 11dB or dB or 22dBdB

PCA2 : PCA2 : uplink power control step is tpc=△tpc=△ 11dBdB

The power control adjustment range in DPCCH is

△△ DPCCH= tpc×TPC_cmd△DPCCH= tpc×TPC_cmd△

TPC_cmd is achieved by different algorithm

The power offset shows the difference of transmission power of UL

DPCCH and UL DPDCH

The adjustment range of DPDCH is the same as the DPCCH.

The power offset is decided by the signaling from higher layer

IInner-loop power control Algorithmnner-loop power control Algorithm

25 Internal Use

Uplink-inner loop power controlUplink-inner loop power control

NodeB compares SIRSIRmeasuredmeasured with the preset target signal-to-interference ratio

(SIRtargetSIRtarget).

NodeB

UETransmit TPC

Inner-loop

set SIRtar

1500Hz 1500Hz

Each UE has own loop Each UE has own loop

26 Internal Use


2

2

d

c

DPDCH/DPCCH structureDPDCH/DPCCH structure

The power ratio of DPCCH to DPDCH is

Pilot N pilot bits

TPC NTPC bits

DataNdata bits

Slot #0 Slot #1 Slot #i Slot #14

Tslot = 2560 chips, 10 bits

1 radio frame: T f = 10 ms

DPDCH

DPCCHFBI

N FBI bitsTFCI

N TFCI bits

Tslot = 2560 chips, N data = 10*2 k bits (k=0..6)

27 Internal Use


The uplink DPCCH SIR should be estimated by different serving cells.

In each time slot, the TPC can be generated by the following rules:

No soft handover

If SIR estimation > SIR target,

then TPC = 0 to decrease the transmission power

If SIR estimation < SIR target,

then TPC = 1 to increase the transmission power

Soft handover

In one time slot, UE received several TPC, then combine then.

Comments

In the last situation, the PCA decides how TPC_cmd are combined.

The PCA has two methods. UTRAN decides which one is used.

TPCTPC

28 Internal Use


UE can adjust the UL DPCCH transmission power with △△ tpctpc step

according to the received TPC_cmdTPC_cmd

The step △△ tpctpc can be 1dB or 2dB, which is decided by UTRAN

If the TPC_cmd = 1 ， the UL DPCCH and UL DPDCH transmission

power should be increased △ tpc

If the TPC_cmd = -1 ， the UL DPCCH and UL DPDCH transmission

power should be decreased △ tpc

If the TPC_cmd = 0 ， the UL DPCCH and UL DPDCH transmission

power should be decreased △ tpc

29 Internal Use


UE in Non - Soft handover caseUE in Non - Soft handover case

UE receives only one TPC

If TPC = 0, TPC_cmd = -1

If TPC = 1, TPC_cmd = 1

PCA1PCA1

30 Internal Use


UE in Soft handoverUE in Soft handover

UE receives several TPCs in one time slot and combine them to get one TPC_cmd

by the following two steps:

First, combine the TPCs from one RLS

Then, combine the TPCs from different RLS

Comments

The TPC from RLSi is Wi

PCA1PCA1

31 Internal Use


Wi can be achieved by the following rules

If the TPC = 0, Wi=0

If the TPC = 1, Wi ＝ 1

Assume UE has N RLSs ， N TPC can be obtained after

combination, W1 、 W2…WN. The combination method for these

N TPCs from N RLSes can be described as following formula

TPC_cmd = γ (W1, W2, … WN)

γ function should satisfied:

If one Wi = 0, TPC_cmd = -1

If all Wi are 1 ， TPC_cmd = 1

PCA1PCA1

32 Internal Use


UE in non-soft handover modeUE in non-soft handover mode

Only one TPC is received in one time slot. The power control can be done once by

each 5 time slots. Each frame is divided 3 groups of 5 time slots. In the first 4 time

slots, the TPC_cmds are 0, which means the power does not change. In the 5th

time slot, the TPC_cmd can be achieved by the following rules:

If all TPC are 0, the TPC_cmd is -1 and the transmission will decrease 1dB;

If all TPC are 1, the TPC_cmd is 1 and the transmission will increase 1dB;

Otherwise, TPC_cmd=0.

TPC （ RX） TPC_cmd

0000 0 0000 -1

1111 1 0000 1

else 0000 0

PCA2PCA2

33 Internal Use


UE in soft handoverUE in soft handover

TPC_cmd can be achieved by the following two steps

First, combine the TPC from the same RLS

N TPCi (i = 1,2......N) obtained from N RLSs in each time slot

The N TPC_cmds from different RLS can be achieved by the above mentioned rules.

Assume each final TPC_cmd from N RLS are TPC_tempi （ i = 1,2......N ）

TPC_cmd in the fifth time slot can be obtain by the following rules ：

TPC_cmd (5th slot) = γ(TPC_temp1, TPC_temp2, …, TPC_tempN)

where γ is defined as follows:

or

otherwise, TPC_cmd = 0.

PCA2PCA2

N

ii cmdTPCtempTPC

N 1

1_,5.0_1

N

ii cmdTPCtempTPC

N 1

1_,5.0_1

34 Internal Use


The control frequency

PCA1, the power control frequency is 1500Hz

PCA2, the power control frequency is 300Hz

Application scenarios

When UE is moving with high speed (80Km/h), the fast inner-loop

power control can not catch up with the fast fading, but will produce

negative gain. In this situation, PCA2 is preferred.

Comparison between PCA1 and PCA2Comparison between PCA1 and PCA2

35 Internal Use

Downlink Inner-loop power controlDownlink Inner-loop power control

NodeB

Transmit TPC

Measure SIR and compare

Inner-loop

1500Hz

36 Internal Use

Downlink inner-loop power controlDownlink inner-loop power control

Inner-loop power control of downlink DPCCH include two types: - Inner-loop power control in compressed mode, - Inner-loop power control in non-compressed mode.

Timeslot structure of Downlink DPCH :

－ PO3 defines the power offset of the TFCI bit in the downlink DPCCH to DPDCH.

－ PO2 defines the power offset of the TPC bit in the downlink DPCCH to DPDCH.

－ PO1 defines the power offset of the Pilot bit in the downlink DPCCH to DPDCH.

－ The values of PO1 、 PO2 and PO3 are defined by RNC.

37 Internal Use


First, UE should estimate the downlink DPDCH/DPCCH power and the

current SIR

Then, UE can generate TPC by comparing SIRestimated to SIRtarget

If SIRestimated > SIRtarget ; TPC = 0 (decrease power)

If SIRestimated < SIRtarget ; TPC = 1 (increase power)

The step of DL inner-loop power control could be 0.5, 1, 1.5 or 2dB

38 Internal Use


When UE is not in soft handover TPC generated by UE is transmitted in TPC domain of UL channel

When UE in soft handover, two power control modes can be used, which is

decided by DPC_mode: DPC_MODE ＝ 0 ， UE will transmit TPC in every slot

DPC_MODE ＝ 1 ， UE will transmit the same TPC in every three time slot

When the downlink channel is out of synchronization, UE will transmit

TPC=1 because UE can not measure the downlink SIR

Upon reception of TPC, UTRAN will adjust the downlink power of

DPCCH/DPDCH. But the transmission power can not higher than

Maximum_DL_Power and not less than Minimum_DL_Power neither.

Power control in different statePower control in different state

39 Internal Use





40 Internal Use

OOuter-loop power controluter-loop power control

The limitation of inner loop power control

The purpose of inner loop power control of the WCDMA system is to

maintain a certain signal-to-interference ratio of transmission signal

power when the signals reach the receiving end.

The character of outer-loop power control

The Qos which NAS provide to CN is BLER, not SIR

The relationship between inner-loop power control and outer-loop

power control

SIR target should be satisfied to be able to decoding receive signal correctly.

But different radio path radio environment has different requirement on SIR

Therefore, the outer-loop power control can adjust the SIR to get a stable

BLER in the changeable radio environment

41 Internal Use

Uplink outer loop power controlUplink outer loop power control

NodeB UE

Transmit TPC

Measure and compare SIR

Inner-loop

Set SIRtarget

get the good quality service data get the good quality service data

Out loop

RNC

Measure received data and

compare BLER

Set BLERtarget

10-100Hz

42 Internal Use

NodeB

set SIRtar

Transmit TPC

Measure and compare SIR

Measure and compare BLER

Outer loop

Inner loop L1

L3

10-100Hz1500Hz

Downlink outer loop power controlDownlink outer loop power control

43 Internal Use

Outer loop power controlOuter loop power control

SIR target SIR target adjustment step adjustment step

etBLERt

etBLERtBLERmeastepSIRAdjustSoefficientSIRAdjustcSIRtar

arg

arg**

44 Internal Use

WCDMA Power Control (1)

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