10 Power Control

7/27/2019 10 Power Control

1/49

Slide 1

WCDMA Radio Network

Optimisation

Power control

WCDMA Radio Network Optimisation

Chapter 10


2/49

Slide 2

Power control

Introduction

Impact on power control

Power control mechanisms Open loop power control

Power control on downlink common

channels Closed loop power control

Outer loop power control

WCDMA Radio Network Optimisation


3/49

Slide 3

Power control

Introduction

IMS Architecture and Services R1A


4/49

Slide 4

Introduction

Every user in a CDMA network increases

interference

The impact of interference can be reducedwith the use of power control

The objective of power control is to use just

enough power so that Eb/No requirements

are met



5/49

Slide 5

Power control

Impact of power control



6/49

Slide 6

Impact of power control:

Interference control


User 1User 2


7/49Slide 7




(EbN

0

)User2

=W

Rb

*(S

I)

User2=

W

Rb

*

PT 2

L2

PN+

PT 1

L1

(EbN

0

)User1

=W

Rb

*(S

I)

User1=

W

Rb

*

PT 1

L1

PN+

PT 2

L2

(EbN

0

)User1

=W

Rb

*

1L

1

PN

PT

+1

L2

>L2 and PT1=PT2=PT then,


8/49Slide 8



Near-far effect is not present in downlink

direction

But power control should try to minimise thetransmitted power as this is a shared

resources among all the users



9/49Slide 9


Coverage and capacity

Directly impacted by interference generated.

We use the term soft capacity to indicate that

capacity can be compromised for coverage This impact is called cell breathing



10/49Slide 10


Quality of connections

Power control increase power in situations

when the quality is poor

It is important that only just enough power istransmitted



11/49Slide 11

Power control

Power control mechanisms



12/49Slide 12


In ideal situation, Eb/No should be the input

to power control algorithm

Eb/No is very difficult to obtain. SIR is used instead of Eb/No

The target is set in accordance with the

following relation:


(SI)Min

=R

b

W*(Eb

N0

)Min


13/49Slide 13



Node B

Open Loop Power Control

Inner Loop Power Control Closed Loop Power Control

Outer Loop Power Control

UE

RNC


14/49


15/49Slide 15

Open loop power control

Power control in UL and DL are practically

correlated

Open loop power control does not require tosend any TPC command to be sent.

Also no feed back mechanism is available

Works in a cycle:

Measure-Read-Decide

Defines only the initial value of transmitted

power



16/49Slide 16

Open loop power control:

Uplink open loop power control Requires UE measurements and some control

parameters from the network.

The involved steps are:

1. The mobile measures the received power from the base

station.

2. The mobile reads the base station transmit power of the

common pilot from the broadcast channel.

3. The mobile estimates (calculates) the minimum transmit

power necessary to access the cell and makes an attemptat a slightly lower power.

4. If this attempt is unsuccessful, that is, there is no

response from the base station, it will increase the power

in steps and retry.IMS Architecture and Services R1A


17/49Slide 17


Uplink open loop power control


3)Transmit

satC

alcula

tedPo

wer

UE 1

RBS

1) UE measures Pilot

2) Reads interference

level from Broadcast

channel

4) The Power is rampedup until a response is

heard or maximum

number of re-attempts

is reached

onnect on esta s e w t

minimum interference to other user

Dedicated Channel at just enought power


18/49Slide 18


Uplink open loop power control

Power on DPCCH:

DCCH_Power_offset is calculated by RNC andprovided to UE during RRC connection setup.

Where,

SIRDPCCH- initial target SIR from admission control

algorithm

SFDPCCH- spreading factor of corresponding DPCCH


DPCCH_Initial_power = DPCCH_Power_offset - CPICH_RSCP

DPCCH_Power_offset= CPICH_Tx_power + UL_interference+ SIRDPCCH

-10log (SFDPCCH

)


19/49Slide 19


Downlink open loop power control Used to set the initial power of downlink channels based on

measurement from the UE

The exact algorithm is not standardized.


2)Tran

smitsatCa

lculate

dPow

er

UE 1

UE 2

RBS3) The Power is ramped

up until a response is

heard or until a certain

maximum power is

reached

1) Uses parameters to

calculate required power

Dedicated Channel at just enought power

Minimum downlink

power used to setup a

connection thus

maximizing downlinkcapacity


20/49Slide 20


Downlink open loop power control

Power of DPDCH and DPCCH fields are not

same. They are relative to DPDCH power


DL TxPower

Time

Data 2Pilot

TPC P01

P0

3

Timeslot (0,667 ms)

DL DPCH

Data 1

TFCI

P02


21/49Slide 21

Power control

Power control for downlink common

channels



22/49Slide 22

Power control for downlink common

channels Power of PCPICH is set in dBm and during cell planning

Power of every other downlink common channels are

expressed relative to PCPICH

The parameters are set during radio planning

FACH can have different offsets depending on what logicalchannel (control or traffic) is transmitted


ower

Time

DataPilot

TFCIP01(db)

P03(db)

2560 CHIPS


23/49

Slide 23

Power control

Closed loop power control



24/49

Slide 24


Transmitted power is adjusted based on measured

S/I

A radio connection must be up and running to

provide feedbacks.

The cycle can be explained as:

Measure-Command-React

This power control has a rate of 1.5 kHz in WCDMA There are two types of closed loop power control:

Inner loop

Outer loop



25/49

Slide 25



DL SIR Target

adjustment

x ower

Adjustment

UL SIR Target

adjustment

RNC

DL Outer Loop

Power Control

Inner Loop

Power Control

UL Outer Loo

Power Control

PT,UL

PT,DLBLERUL

BLERDL


26/49

Slide 26

Power control

Inner-loop power control



27/49

Slide 27

Inner loop power control

Feed back from the opposite direction is

necessary.

Applicable to DCH channels in UL and DL The power control evaluation are

independent in UL and DL direction



28/49

Slide 28

Inner-loop power control:

Uplink inner loop power control

Used to set the power of uplink DPCH

Tries to maintain SIRtarget

SIRtarget

is compared to SIRest

If SIRest SIRtarget then the TPC command to transmit is

"0" which is translated to TPC_cmd=-1. This means

power down command is sent.

If SIRest SIRtarget then the TPC command to transmit is

"1" which is translated to TPC_cmd=1. This means

power up command is sent.IMS Architecture and Services R1A


29/49

Slide 29



The change in DPCCH power with respect to its

previous value is derived by the UE and is denoted

by DDPCCH (in dB)

In any case, the maximum power cannot be morethan UE power class or maximum power allowed in

the cell

The major objective of this power control is to

combat Rayleigh fading or so called fast fading



30/49

Slide 30



Two algorithms, selected by parameter

PowerControlAlgorithm

Algorithm 1:

When UE speed is low and step size is 1 or 2 dB

Algorithm 2:

When UE speed is very high. Effectively, power control is

turned off.

Because at high speeds, inner loop power control cannot really

follow fast fading, rather introduces noise in UL transmission. The step size, is a layer 1 parameter which is derived from

the UE-specific higher-layer parameter "TPC-StepSize".

When "TPC-StepSize" =dB1, TPC=1 dB

and "TPC-StepSize" =dB2, TPC=2dBIMS Architecture and Services R1A

DTPC


31/49

Slide 31


Uplink inner loop power controlDuring soft handover:


SRNC

TPC1

TPC2

UE

Algorithm 1 or 2

to Combine

TPC1 and TPC2

Macro Diversity

Combining and

Splitting

NODE B1

NODE B2

UL TPC2 Commands

UL TPC1 Commands

SIRmeasured1 vs SIRtarget

SIRmeasured2

measured1

SIRmeasured1 vs SIRtarget


32/49

Slide 32



During compressed mode:

During compressed mode the same algorithms are

applied with some adjustments

However, there is a mechanism to recover SIR target

after the transmission gap is finished.

A new target is used: SIRcm_target , but the algorithm is

similar.

SIRcm_target is comparatively bigger value than the

normal situation to compensate for the interruption



33/49

Slide 33


Downlink inner loop power control

Power control dynamic range in DL


3 dB

28 dB

DL PC DynamicRange

DL Total Power

Dynamic Range

max mum

output power

Maximum code

channel power

No traffic

channels activate

Minimum code

channel ower


34/49

Slide 34


Downlink inner loop power control IfSIRest SIRtarget then the TPC command to transmit is "0".

IfSIRest < SIRtarget then the TPC command to transmit is "1".

The UE shall check the downlink power control mode

(DPC_MODE) before generating the TPC command:

IfDPC_MODE= 0 : the UE sends a unique TPC command in

each slot and the TPC command generated is transmitted in the

first available TPC field in the uplink DPCCH.

IfDPC_MODE= 1 : the UE repeats the same TPC command

over 3 slots and the new TPC command is transmitted such that

there is a new command at the beginning of the frame, unless

uplink discontinuous transmission is activated, in which case the

UE shall behave as forDPC_MODE= 0 .

The DPC_MODE parameter is a UE specific parameter controlled

by the UTRAN.



35/49

Slide 35



The TPC commands are sent on the uplink DPCCH. The

power control of a DPCCH and its corresponding DPDCHs in

the downlink is performed simultaneously and by the same

amount.

The relative power difference between the DPCCH and the

TFCI, TPC and pilot fields of the downlink DPCCH are

determined by PO1, PO2 and PO3, respectively.

In soft handover, the UE transmit power is reduced if the PC

signalling quality is improved by setting a higher power for the

DPCCH than for the DPDCH in the downlink.

The downlink power control step size DTPC can take four

values: 0.5, 1, 1.5 or 2 dB. It is mandatory for UTRAN to

support DTPC of 1 dB, while support of other step sizes is

optional. The parameter is set during radio network planning.



36/49

Slide 36



During soft handover:


SRNC

UE

Macro Diversity

Combining and

Splitting

NODE B1

NODE B2

DPC_MODE=0: TPC Decision On each SlotDPC_MODE=1: TPC Decision upon 3 Slots

DPC_MODE=0: TPC Decision On each Slot

DPC_MODE=1: TPC Decision upon 3 Slots

DL SIR Measured vs SIR Targe

TPC=0 or 1

DPC_MODE=1: Same TPC

Command is Repeated over

3 Consecutive Slots

DL TPC


37/49

Slide 37



Power drifting: Due to signalling errors in the air interface, the Node Bs may

detect this power control command in a different way.

It is possible that one of the Node Bs lowers its transmissionpower to that UE while the other node Bs increases its

transmission power.

This behaviour leads to a situation where the downlink powers

start drifting apart; this is referred to here as power drifting.

Power drifting is not desirable, since it mostly degrades the

downlink soft handover performance.



38/49

Slide 38



Power drifting: RNC can take initiative to provide a single reference power to

every cell in active set so that they do not drift further away.

This reference power is adjusted periodically



39/49

Slide 39



Compressed mode: Aim of DL power control during compressed mode is to recover

as fast as possible a SIR close to the target SIR after each

transmission gap. A special algorithm is used to calculate the new value of the

power control command.

However, the step size is double compared to without

compressed mode situation.



40/49

Slide 40

Power control




41/49

Slide 41


Maintains the quality of the connection

based on bearer service in question

Modifies the SIRtarget for inner loop powercontrol

SIR target needs to be changed due to UE

speed or a change in multipath propagation

Frequency of outer loop power control varieswithin 10 to 100 Hz



42/49

Slide 42

Outer loop power control:

General outer loop power control algorithm


Decrease

SIRtarget

Increase

SIRtarget

Received quality

Better thanRequired quality

NoYes


43/49

Slide 43


Uplink outer loop power control


UE

NODE B1

NODE B2

Outer Loop

Power Control

RNC

Radio Link 1

Radio Link 2

Each Radio Link

Has Only One

Common Inner Loop

Power Control For All

Services

Estimated Quality

of each Service

Set SIRtargetAccording to the

Service Requiring

Higher Target

Speech

Video

Web

Inner Loop

Power Control

Data

Data

Data

Macro Diversity

Combining

SIRtarget

SIRtarget


44/49

Slide 44


Downlink outer loop power control

The algorithm runs in the UE

The specific algorithm is implementation specific.

Because the handset manufacturer has control over

the process

The value of the downlink outer loop power control

target in the UE is controlled by admission control in

RNC.

This target can be modified during connection

Node B does not need to increase power at the

request from UE



45/49

Slide 45


Downlink outer loop power control

Compressed mode:

SIR target is adjusted in the node B and in the UE

during compressed frames compared to normal

situation

Below is the representation of the SIR target in the

compressed mode.

SIRcm_target=SIRtarget+SIRpilot+SIRcoding

Where,

SIRpilotandSIRcodingtake respectively into account

the reduction of pilot symbols in compressed mode

and the mechanism for generating the gaps.



46/49

Slide 46

Power control

Outer loop anti-windup feature:



47/49

Slide 47



Under some situation, it is possible that the

transmitter uses maximum power, still CRC fails and

SIRtarget continues to increase

If, after such a situation, conditions become normal(receiver moves closer to the transmitter), SIR target

becomes very high and inner loop power control tries

to adjust in such a way, that the transmitted power is

higher than necessary to achieve desired BLER



48/49

Slide 48




To avoid such situations a so called "anti-windup"

mechanism may be employed to limit how high the

SIR target may rise above the measured achievedSIR.

If the difference between the SIR estimated by uplink

Inner Loop Power Control and the SIR target, known

as SIRerror, over a 60 msec period, is equal to orgreater than 2 dB, the node B will send a message to

the Serving RNC to suspend the uplink outer loop

Power Control algorithm.



49/49



A typical scenario where the UE enters a tunnel

and looses radio contact is illustrated

NODE B

SIR Error

Tim

1

2

60

ms

UE

Documents

10 Power Control