-BSSPAR- Power Control

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BSSPAR

BSSPAR- Power Control

Training Document


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The information in this document is subject to change without notice and describes only theproduct defined in the introduction of this documentation. This document is intended for theuse of Nokia Networks' customers only for the purposes of the agreement under which thedocument is submitted, and no part of it may be reproduced or transmitted in any form ormeans without the prior written permission of Nokia Networks. The document has been

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Nokia Networks' liability for any errors in the document is limited to the documentarycorrection of errors. Nokia Networks WILL NOT BE RESPONSIBLE IN ANY EVENT FORERRORS IN THIS DOCUMENT OR FOR ANY DAMAGES, INCIDENTAL ORCONSEQUENTIAL (INCLUDING MONETARY LOSSES), that might arise from the use of thisdocument or the information in it.

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Other product names mentioned in this document may be trademarks of their respectivecompanies, and they are mentioned for identification purposes only.

Copyright Nokia Oyj 2003. All rights reserved.

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Contents

1 Module Objectives................................................................................4

2 Introduction To Power Control ...........................................................52.1 Strategy 52.2 PC Threshold Comparison And PC Command....................................7

3 Power Control Algorithms.................................................................113.1 MS/BTS Power Increase Due To Signal Level..................................123.2 MS/BTS Power Increase Due To Signal Quality................................153.3 BTS Power Decrease Due To Signal Level ......................................18

3.4 BTS Power Decrease Due To Signal Quality ....................................203.5 MS Power Decrease Due To Signal Level ........................................243.6 MS Power Decrease Due To Signal Quality .....................................263.7 MS Power Optimisation.....................................................................303.8 Conclusions 31

4 Power Control Exercise.....................................................................33

5 Key Learning Points...........................................................................34

6 Review Questions ..............................................................................37

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1 Module Objectives

At the end of the module, the participant will be able to:

State the purpose and the important considerations for power control

in GSM networks

List the steps involved in the power control process

Explain the difference between fixed and variable power-change step-

size

Discuss the Power Control Algorithms that are used to increase or

decrease the MS or BTS transmit power based on received signal levels and

quality

Name the parameters that are used for Power Control

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2 Introduction To Power Control

In a communication link, when one side is received well by the other side, it is

beneficial to reduce the transmit power by a suitable amount. This is to maintainthe quality of the communication link while at the same time reducing the

interference caused on other calls in surrounding areas, thus increasing the

spectral efficiency and capacity of the network. Uplink power control also

extends the mobiles' battery endurance. Power control refers to the mechanism

that is used to modify, within some limits, the transmit power of the radio at the

Mobile Station, and Base Station.

In GSM, uplink and downlink power control is carried out independently.

Furthermore, power control is applied independently on each mobile call or

transaction. Depending on the MS power class, the range of power levels

specified for uplink power control is between 20 and 30 dB of attenuation in

steps of 2 dB. The range used for downlink control is manufacturer dependentand in steps of 2 dB.

An operator may choose to apply power control in one direction, in both

directions, or none at all. However, all MS must support power control to

comply with the GSM specs.

Thus, power control is used for two purposes:

a) Decrease the power consumption of each Mobile Stations in the uplink

direction and hence achieve a longer serving time for the Mobile Station

rechargeable battery

b) Decrease interference in both uplink and downlink directions by using the

lowest possible transmitting power in the Mobile Station and BTS, soincreasing network capacity and spectral efficiency.

2.1 Strategy

Power control can be used in the downlink direction in every TRX, except in a

TRX with the BCCH RTSL. This is because each MS is continually measuring

the RX level of the adjacent cell BCCHs in any one of the RTSL of the BCCH

TRX, and because the BTS has to send system information messages

continuously at full planned power on the BCCH to all MS in the cell andneighbouring cells. The Mobile Station can use power control on each

frequency continuously, if needed.

In order to use BTS power control, the parameter PowerCtrlEnabled (PENA)

(POC) should be enabled on a cell by cell basis by the operator.

When using power control, enough margin has to be reserved for Rayleigh

fading and it has to be taken into account that handover always has higher

priority than power control.

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REASONS

Optimize Uplink and Downlink QOS decrease interference

Decrease power consumption of the Mobile

STRATEGY

Handled by the BSC

Enough margin against Rayleigh fading

HO has always higher priority than POC

Controlled by interval

Increase and decrease act independently (can be fixed or variable step

size)

BTS and MS apply Power Control independently

BCCH TRX doesn't use Power Control

DL/UL Power Control can be disabled

Initial POC level used by MS in new cell after HO, is determined by

the BSC - default is max permitted level:

MsTXPwrMaxGSM (PMAX1)(BTS)(5..43)(33dB) for GSM850-900

MsTXPwrMaxGSM (PMAX2)(BTS)(0..36)(30dB) for GSM1800-1900

and when accessing the CCH of a cell:

MsTXPwrMaxCCH (TXP)(BTS)(5..43/0..30/0..32)(33/30/30dB) for

GSM850-900/1800/1900

Optionally POC/HOC processes can optimise the initial RF power in

case of intra BSC HO

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2.2 PC Threshold Comparison And PC Command

After every SACCH multiframe period, the BSC compares each of the

processed measurement results (averages) with the relevant power control

thresholds.

MeasurementsMeasurements

BtsMeasAverage

AveragingWindow SizeAdjCellAllAdjacentCellsAveragedNumberOfZeroResults

AveragingAveraging

AveragingAveraging

BookkeepingBookkeeping

ho/pc_Averaging_Lev/Qual_UL/DLWindowSize

Weight

msDistanceAveragingParameterWIndowSize

DTXMode

MeasurementsMeasurements

Power Control ?

EnaFastAveCallSetupEnaFastAvePCEnaFastAveHOMS +BTS

MS

Figure 1. Overview

If the power control (PC) threshold comparison indicates that the MS or the

BTS needs an increase or decrease in RF power, then the BSC sends a PC

command to the MS/BTS including the new transmission power level of the

MS/BTS.

When the BSC defines the new transmission power level of the MS, it takes

into account both the RF power capability and the revision level of the MS.

The BSC may send the PC command simultaneously both to the MS and the

BTS or to one of them. Power control for the MS and BTS runs

independently.

The minimum and maximum MS transmission powers are determined on cell-

by-cell basis. The maximum transmission power that an MS may use in the

serving cell is controlled by the parametermsTxPwrMax The minimum MS

transmission power is controlled by the parameter minMSTxPwr (PMIN)

(BTS).

The maximum transmission power of the BTS is controlled by the parameter

BsTxPwrMax (PMAX)(POC). The parameter BsTxPwrMin (PMIN)(POC)

indicates the minimum transmission power of the BTS.

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powerC ontrolInterval 0 30 sec

powerIncrStepSize 2, 4, 6 dBpowerRedStepSize 2, 4 dBpowerControlEnabled Y / N

Parameter Value

Uplink Level

U plink Q uality A V_ R XQ U AL _ UL _ PC

AV_RXLEV_UL_PC

Downlink Level

Downlink Quality AV_RXQUAL_DL_PC

AV_RXLEV_DL_PC

POWER CONTROL

UPLINK

POWER CONTROL

UPLINK

TH RESHO LDCOMPARISON

S eparate Averaging P arameters

For Handover and for Power Control

POWER CONTROL

DOWNLINK

POWER CONTROL

DOWNLINK

P OCINTERVAL

Figure 2. Overview

The range of the BTS transmission power is 30 dB to 0 dB of attenuation

applied to the maximum peak power of the base station. The parameter

PowerCtrlEnabled (PENA)(POC) indicates whether the BTS power control is

enabled. When the power control is enabled, it concerns every transceiver of the

BTS with the exception of the BCCH (broadcast control channel) transceiverwhich always transmits with the maximum power level (parameter

BsTxPwrMax (PMAX)(POC)).

In order to prevent repetitive power changes for the same MS/BTS, there is a

timer for the minimum time interval between the changes in the RF output

power level. The timer is controlled by the parameterPowerControlInterval

(INT)(POC), which is the minimum interval between the changes in the RF

power level. The averaging and PC threshold comparison do not stop during

this time but the PC commands are not possible.

The BSC observes the power changes from the measurement results it receives

from the BTS. The measurement result includes the RF power level, which the

MS and the BTS have used during the previous SACCH multiframe period. Ifthe MS does not change its output power in time, the BSC sends the PC

command once more. The power control of the MS does not stop even if the

MS did not change its RF output power. If the BTS does not change its output

power with the first PC command, the BSC does not send any further PC

commands to the BTS.

The measurement results (uplink or downlink) preceding the MS/BTS power

change are not valid after the power change. If the scaling of measurement

results is disabled (selected by means of the parameterEnaFastAvePC (EFP)

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(HOC), the averaging and threshold comparison based on those measurement

results (uplink/downlink) must start from the beginning after the power change

(this concerns both Handover and Power control).

When the scaling of measurement results is enabled, the BSC scales the relevantmeasurement results preceding the power change so that they correspond to the

new transmission power level of the MS/BTS and thus the averaging and

threshold comparison can continue without interruption, with the exception of

the PC threshold comparison which always starts from the beginning after the

power change.

Parameter Value

pcUpper/LowerThresholdsLevULrxLevel

pxnx

pcUpper/LowerThresholdsLevDLrxLevel

pxnx

pcUpper/LowerThresholdsQualULrxQual

pxnx

pcUpper/LowerThresholdsQualDLrxQual

pxnx

-110 ...-47 dBm1 ... 321 ... 32

-110 ... -47 dBm1 ... 321 ... 32

0 ... 71 ... 321 ... 32

0 ... 71 ... 321 ... 32

Figure 3. POC Parameters

LowerLEV UpperLEV

UpperQUAL

LowerQUAL

Applicable in bothDownlinkand Uplink Directions

Figure 4. Safety Region

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bsTxPwrMax 0 30 dBbsTxPwrMin 0 30 dB

minMsTxPower 0 36 dBmmsTxPwrMax 0 36 dBm

Parameter Value

30 dBRange

SystemDependentRange

Attenuations

P ower Values

Figure 5. Power Control Ranges

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3 Power Control Algorithms

Once the comparison of the averaged values with the corresponding thresholds

has indicated the need for the MS or the BTS to increase or decrease theirtransmission power, the BSC has to determine the size of the increase/decrease.

This size is calculated by the Power Control algorithm, starting from a fixed

power change step size, on the basis of the averaged values, of the relevant

thresholds and in some cases of the current (non-averaged) measured values.

Fixed power change step size is selected by means of the parameter

powRedStepSize (RED)(POC)(2 or 4 dB), which impacts on the size of the

step for the decrease of the MS/BTS transmission power, and the parameter

powIncrStepSize (INC)(POC)(2, 4 or 6 dB), which impacts on the size of the

step for the increase of the MS/BTS transmission power.

In some cases the size of the increase or decrease corresponds to the fixedpower change step size, while in other cases a variable power change step size

is calculated from the fixed power change step size.

The BSC uses a variable power change step size for increasing and decreasing

the MS transmission power and for increasing the BTS transmission power in

such situations where the required power change is so large that the use of the

fixed step size would require several power control commands and a lot of time.

By using the variable power change step size instead of the fixed step, it is

possible to reach the required power level in one step. A detailed explanation of

the variable power change step size can be found in the flowcharts below.

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3.1 MS/BTS Power Increase Due To Signal Level

IF

AV_RXLEV_UL/DL_PC


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RXLEV_UL/DL + 2* PowIncrStepSize


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In other words if two "regular" steps are not enough to go above the lower

thresholds then a variable step is used to increase the transmission power and

bring the received level at the threshold.

It should be noted that RXLEV_UL/DL is the current uplink signal levelmeasured by the BTS/MS and not the averaged value. This is due to the fact

that the averaged values tend to follow the raw measurements with a certain

delay that is longer when the averaging windows gets large.

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3.2 MS/BTS Power Increase Due To Signal Quality

The parameter powerCtrlEnabled (PENA)(POC) enables the BTS power

increase.

The following parameters pcLowerThresholdsQualUL and

pcLowerThresholdsQualDL are used in comparison with the averaged values

of uplink/downlink signal quality to trigger the power control.

RxQual (0 7) is the threshold level for the MS/BTS power increase.

The range is from 0 to 7

Nx (1 32) is the total number of averages to be taken into account

before decision is possible.

Px(1 32) is the number of averages out of total averages that have to be

lower than or equal to the threshold, before power increase is possible.

The BSC compares the averaged measurement result

AV_RXQUAL_UL/DL_PC with PcLowerThresholdsQualUL/DL.

AV_RXQUAL_UL/DL_PC => PcLowerThresholdsQualUL/DL

If this condition is met Px times out of Nx then the power control is triggered.

NOTE: For 14.4 kbit/s data (see High Speed Circuit Switched Data and 14.4

kbit/s Data Services in BSC /13/), the BSC compares the averaged

measurement result AV_RXQUAL_UL_PC with the power control threshold

pcLowerThresholdsQual144 instead of the "standard"pcLowerThresholdsQualUL.

The BSC always uses a variable power change step size for increasing the

MS/BTS transmission power due to signal quality.

The BSC is able to calculate the variable power change step size by means of

two alternative algorithms, taking into account the cause of the bad signal

quality. The cause in fact may be interference or low signal level; in order to

cope with the most significant cause of bad quality, the BSC selects the largest

step size.

The first way is based on the distance between the current quality and therelevant threshold:

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Only variable step size

Two different Algorithms

Largest increase is considered

PWR_INCR_STEP =

(1+MAX(0,Qa))*PowIncrStepSize

where

Qa = RXQ UAL_ UL -PcL owerThresholdsQualU L

PWR_INCR_STEP =

(1+MAX(0,Qa))*PowIncrStepSize

where

Qa = RXQ UAL_ UL -PcL owerThresholdsQualU L

PWR_INCR_STEP =PcLowerThresholdsLevUL- RXLEV_ ULPWR_INCR_STEP =PcLowerT hresholdsLevUL- RXLEV_UL

Based on Current Level

Based on C urrent Quality

IF : RXLE V_ UL + 2*PowIncrStepSize< =PcLowerT hresholdsLevULIF : RXLE V_U L + 2*PowIncrStepSize< =PcLowerThresholdsLevUL

LARGEST INCREALARGEST INCREAS

Figure 9. MS Power increase due to signal quality

Only variable step size

Two different Algorithms

Largest increase is considered

PWR_ INCR_STEP = (1+MAX(0,Qa))*PowIncrStepSize

where

Qa = RXQUAL_DL -PcLowerThresholdsQualDL

PWR_ INCR_STEP = (1+MAX(0,Qa))*PowIncrStepSize

where

Qa = RXQUAL_ DL -PcLowerThresholdsQualDL

PWR_INCR_STEP =PcLowerThresholdsLevDL- RXLEV_DLPWR_INCR_STEP =PcLowerThresholdsLevDL- RXLEV_DL

Based on Current Level

Based on Current Quality

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The size of the variable power change step PWR_INCR_STEP is calculated in

the following way:

PWR_INCR_STEP = PcLowerThresholdsLevUL/DL - RXLEV_UL/DL

RXLEV_UL/DL is the current uplink signal level measured by the BTS/MS

It should be noted that a low received level doesn't necessarily correlate to bad

quality. In such a case, the application of the fixed step in increasing the

transmission power can be a reasonable possibility.

When the power control is triggered by quality, the situation is more critical as

the radio connection is probably suffering from the transmission power being

too low. Therefore, a more aggressive action is taken by the BSC that always

applies a variable step. This variable step is affected by the distance of the

current quality from the threshold and by the distance of the current level from

the relevant threshold.

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3.3 BTS Power Decrease Due To Signal Level

The parameterpowerCtrlEnabled (PENA)(POC) should be enabled for BTS

power control.

The following parameters pcUpperThresholdsLevDL and

pcUpperThresholdsQualDL are used in comparison with the averaged values

of downlink signal level and quality measurements to trigger the power control.

They are composed of three elements as follows:

The decrease of the transmission power of the BTS due to level is triggered by

RxLev (-110 -47 dBm) is the threshold level for the BTS power

decrease.





The BSC compares the averaged measurement result AV_RXLEV_DL_PC

with PcUpperThresholdsLevDL

AV_RXLEV _DL_PC >= PcUpperThresholdsLevDL.

If the condition is met for Px averaged values out of Nx then the power control

due to level is triggered.

ifVariableDL StepUse= N PWR_DECR_STEP =PowRedStepSize

(no variable step size)

PcUpperThresholdsLe

P ower Control Triggered

VariableDL StepUse Y/N

Parameter Value

Figure 11. BTS Power decrease due to signal level

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New BSC specific parameter, VariableDLStepUse (VDLS)(BSC), indicates if

the variable step size is used, when downlink power is decreased. The

parameter has two values, 'yes' and 'no'. The default value is 'no', in other words

the variable step size is not in use.

New TRX specific parameter, OptimumRxLevDL (LEVD)(TRX), indicates

the optimum downlink RF signal level which both ensures adequate speech/data

quality and does not cause downlink interference. The parameter is used by the

power control of the BTS.

The range is from -109 dBm to -47 dBm, the use of the parameter is disabled

when the value is 'not used'. The default value is 'not used'.

IfVariableDL StepUse= Y

PcUpperThresholdsLe

Power Control Triggered

If R XLE V_ DL - 2*PowRedStepSize>=

PcUpperThresholdsLevDL

PWR_DECR_STEP =

MIN((RXLEV_DL -

PcUpperThresholdsLevDL),10)

(Variable step size)

Else

PWR_DECR_STEP =

PowRedStepSize

RXLEV_DL is the current signal levelmeasured by the MS RXLE V_DL AV_R XLEV_ DL_P C ( usedfor threshold comparison )

Figure 12. BTS Power decrease due to signal level

The transmission power of the BTS is decreased of a quantity given by the fixed

or variable power change step size, based on the distance between the threshold

and the current received value uplink. In other words if

RXLEV_DL - 2* PowRedStepSize >= PcUpperThresholdsLevDL

the transmission power of the BTS is decreased by using the variable power

change step size; otherwise, the fixed power change step size is used.

The size of the variable power change step PWR_DECR_STEP is calculated inthe following way:

PWR_DECR_STEP = MIN((RXLEV_DL - PcUpperThresholdsLevDL), 101)

RXLEV_DL is the current downlink signal level measured by the MS.

1It must be noted that the power decrease step is limited to 10 dB at a time due to

limitations in some mobile phones.

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3.4 BTS Power Decrease Due To Signal Quality

The parameterpowerCtrlEnabled (PENA)(POC) should be enabled for BTS

power control.

The parameter pcUpperThresholdsQualDL is used in comparison with the

averaged values of downlink quality measurements to trigger the power control.

They are composed of three elements as follows:

The decrease of the transmission power of the BTS due to quality is triggered

by

RxQual (0 7) is the threshold level for the BTS power decrease.

The range is from 0 to 7





In the same way AV_QUAL_DL_PC and PcUpperThresholdsQualDL are

compared by the BSC.

AV_RXQUAL_DL_PC =< PcUpperThresholdsQualDL.

If the condition is met for Px averaged values out of Nx then the power control

due to quality is triggered.

VariableDLStepUse=Y /N (S9 new feature)

OptimumRxLevDL=-109-47 dBm/N

IfVariableDLStepUse=N

PWR_DECR_STEP =PowRedStepSize (no variable step size)

The decrease in power does not take place if there is the possibility that it wthe thresholdPcLowerThresholdsLevDL(the safety margin is 6dB)

Figure 13. BTS Power decrease due to signal quality

The BSC will determine the power change step size by using two alternative

algorithms. The algorithm is selected by means of the new parameter

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OptimumRxLevDL (LEVD)(TRX) which is controlled on a transceiver-by-

transceiver basis.

There are two different algorithms based on whether OptimumRxLevDL(LEVD)(TRX) is used or not. If the resulting receive level downlink

(RXLEV_DL) gets close to PcLowerThresholdLevDL (as a result ofthe BTS power decrease) there could be a consecutive increase due to level

which will lead to triggering the decrease again. To avoid this "ping pong"

effect BSC makes sure before decreasing the power due to signal quality, that

RXLEV_DL is at least 6 dB higher than the

PcLowerThresholdLevDL , i.e. 6 dB Margin is in-built in BSC

Calculation based on non-defined optimum downlink RF signal level

The transmission power of the BTS is decreased of a quantity given by the fixed


and the current received value uplink.

If RXLEV_DL 2 *PowRedStepSize >= PcUpperThresholdsLevDL

the transmission power of the BTS is decreased by using the variable power


The size of the variable power change step PWR_DECR_STEP is calculated in

the following way:

PWR_DECR_STEP = MIN ((RXLEV_DL - PcUpperThresholdsLevDL),

101)

RXLEV_DL is the current downlink signal level measured by the MS. The

parameterPcUpperThresholdsLevDL is the threshold (signal strength) levelfor the BTS power decrease.

If VariableDLStepUse = Y and

If OptimumRxLevDL = < not defined >

If RXLEV_DL - 2*PowRedStepSize >= PcUpperThresholdsLevDL

PWR_DECR_STEP =MIN((RXLEV_DL -PcUpperThresholdsLevDL) ,10)


Else

PWR_DECR_STEP = PowRedStepSize

RXLEV_UL is the current signal level measured by the BTS

RXLEV_UL AV_RXLEV_UL_PC (used for threshold

comparison)

1It must be noted that the power decrease step is limited to 10 dB at a time due to

limitations in some mobile phones

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Calculation based on defined optimum downlink RF signal level

If the optimum downlink RF signal level has been defined for the transceiver by

means of the parameter OptimumRxLevDL (LEVD)(TRX), the variablepower change step size will be based on:

averaged downlink signal quality

quality threshold for BTS power decrease

the optimum downlink RF signal level

current downlink signal level

In this case, the BSC uses merely a variable power change step size for

decreasing the BTS transmission power. The size of the variable power change

step PWR_DECR_STEP is calculated in the following way:

PWR_DECR_STEP = MIN ((

MIN (PwrDecrLimit, MAX (MAX (0, RXLEV_DL - OptimumRxLevDL),

(PwrDecrQualFactor + MAX (0,Qa)) * PowRedStepSize))), 10)

where Qa = PcUpperThresholdsQualDL - AV_RXQUAL_DL_PC

PWR_DECR_STEP =

MIN ((MIN{PwrDecrLimit, MAX[ MAX (0, R XLEV _DL -OptimumRxLevDL),(PwrDecrFactor+ MAX(0, Qa)) *PowRedStepSize]}),10)

where

Qa =PcUpperThresholdsQualDL- AV_RXQUAL_DL_P C

PWR_DECR_STEP =

MIN ((MIN{PwrDecrLimit, MAX[ MAX (0, RXLE V_DL -OptimumRxLevDL),(PwrDecrFactor+ MAX(0, Qa)) *PowRedStepSize]}),10)

where

Qa =PcUpperThresholdsQualDL- AV_RXQUAL_DL_P C

IF :optimumRxL evDL NIF :optimumRxLevDL N

IfVariableDL StepUse=Y andIfOptimumRxLevDL= defined

Figure 14. Power Decrease Step Calculation

The parameter PwrDecrLimit/Band0-2 is the maximum size of the variable

power decrease step:

PwrDecrLimitBand0 (PD0)(POC) indicates the maximum size of the power

decrease step when the BTS power is decreased due to signal quality and the

averaged signal quality (bit error rate) is better than 0.2% (quality band 0). The

values range from 0 to 38 dB with a step size of 2 dB.

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PwrDecrLimitBand1 (PD1)(POC) indicates the maximum size of the power

decrease step when the BTS power is decreased due to signal quality and the

averaged signal quality (bit error rate) is between 0.2% and 0.4% (quality band

1). The values range from 0 to 38 dB with a step size of 2 dB.

PwrDecrLimitBand2 indicates the maximum size of the power decrease step

when the BTS power is decreased due to signal quality and the averaged signal

quality (bit error rate) is worse than 0.4 (quality bands from 2 to 7). The values

range from 0 to 38 dB with a step size of 2 dB.

AV_RXQUAL_DL_PC is the averaged signal quality band for power control

and the parameter PcUpperThresholdsQualDL indicates the quality band

which corresponds to the quality threshold for the BTS power decrease

The parameterPwrDecrQualFactor (PDF)(POC) indicates whether the power

decrease takes place when the current downlink signal level (RXLEV_DL) is

lower than the optimum downlink RF signal level (OptimumRxLevDL) and the

averaged signal quality (AV_RXQUAL_DL_PC) equals to the quality thresholdPcUpperThresholdsQualDL.

Additionally it should be noted that the power decrease due to quality does not

take place if there is a possibility that it would trigger the threshold

pcLowerThresholdsLevDL. A safety margin is used which is equal to 6 dB.

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3.5 MS Power Decrease Due To Signal Level

The parameter pcUpperThresholdsLevUL is used in comparison with the

averaged values of uplink signal level measurements to trigger the power

control. As usual, the threshold is composed of three parts:

rxLev (-110 -47 dBm) is the threshold level for the MS power

decrease.



Px (1 32) is the number of averages out of total averages that have to be

greater than or equal to the threshold before power decrease is possible

If R XLEV _U L - 2*PowRedStepSize>=PcUpperT hresholdsLevUL

PWR _DECR _STE P = RXLEV_UL -

PcUpperThresholdsLevUL


Else

PWR_DECR_STEP =

PowRedStepSize

R XLE V_ UL is the current signal level

measured by the BT S

RX LEV _U L AV_ RX LEV_ UL_ PC ( used forthreshold comparison )

PcUpperT hresholdsLe

P ower Control Triggered

Figure 15. MS Power decrease due to signal level

The BSC compares the averaged measurement result AV_RXLEV_UL_PC

with pcUpperThresholdsLevUL.

AV_RXLEV _UL_PC >= PcUpperThresholdsLevUL.If at least Px averages out of Nx averages are greater than or equal to the

threshold RxLev, the power control due to level is triggered.

The transmission power of the MS is decreased of a quantity given by the fixed


and the current received value uplink. In other words if

RXLEV_UL - 2* PowRedStepSize >= PcUpperThresholdsLevUL

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the transmission power of the MS is decreased by using the variable power


The size of the variable power change step PWR_DECR_STEP is calculated in

the following way:PWR_DECR_STEP = RXLEV_UL - PcUpperThresholdsLevUL

where (again) the current uplink signal level RXLEV_UL is considered. The

reason for using the current value of the uplink level measured by the BTS

instead of the averaged value AV_RXLEV _UL_PC, is that the average is

always delayed with respect to the raw values and consequently the power

decrease might result too small when e.g. the MS is approaching the BTS.

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3.6 MS Power Decrease Due To Signal Quality

Two different algorithms are used based on OptimumRxLevUL (LEV)(TRX) being used or not. If the resulting RXLEV_UL would get too close toPcLowerThresholdLevUL (as a result of the decrease) there could be aconsecutive increase due to level, which will lead to triggering the decrease

again. To avoid this "ping pong" effect BSC makes sure before decreasing the

power due to signal quality that RXLEV_UL is at least 6 dB higher than the

PcLowerThresholdLevUL , i.e. 6 dB Margin is in-built in BSC

LowerLEV UpperLEV

UpperQUAL

LowerQUAL

Power decrement due to qualityPower increment due to level

Figure 16. Ping Pong Effect

The parameter pcUpperThresholdsQualUL is used for comparing the

averaged values of uplink signal quality measurements for triggering the power

control. As all the other thresholds related with power control, this parameter is

composed of three parts:RxQual (0 7) is the threshold level for the MS power increase. The

range is from 0 to 7





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The condition

AV_RXQUAL_UL_PC = PcUpperThresholdsLevUL

PWR _DECR_ STEP = RXLEV_UL -PcUpperT hresholdsLevUL


else

PWR_DECR_STEP =PowRedStepSize

Same as in the MS P ower decrease dSignal Level,

but Triggered by different condition (qu

Figure 17. MS Power decrease due to signal quality

OptimumRxLevUL = 'not used'

Fixed step is the default, but if the signal level is very high, besides theexcessive signal quality, the variable power change step size is used. In other

words if:

RXLEV_UL - 2* PowRedStepSize >= PcUpperThresholdsLevUL

then a variable step is used. The size of the variable power change step is

calculated in the following way:

PWR_DECR_STEP = RXLEV_UL - PcUpperThresholdsLevUL

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where RXLEV_UL is the current uplink signal level measured by the BTS and

not the averaged value.

PcUpperT hresholdQualUL= 1

0

2

4

6

8

10

12

14

16

-109

-107

-105

-103

-101

-99

-97

-95

-93

-91

-89

-87

-85

-83

-81

-79

-77

-75

-73

-71

-69

-67

-65

-63

RxLev_U

Pwr_

Decr_Step

B = Max ( 0 , RXLEV_UL - Opt imumRxLevUL ) C = (PwrDecrFac tor + Max(0,Qa)) *PwrRedStepSize Min(Max(B;C) , PwrDecrLim

PWR_DECR_ STEP =

MIN[PwrDecrL imit, MAX(MAX (0, RXLEV_ UL -OptimumRxL evUL),

(PwrDecrFactor+ MAX(0, Qa)) *PowRedStepSize) ]

where Qa =PcUpperThresholdsQualUL- AV_RXQUAL_UL_PC

PWR_DECR_STEP =

MIN[PwrDecrL imit,MAX(MAX (0, RXLEV_UL -OptimumRxLevUL),

(PwrDecrFactor+ MAX(0, Qa)) *PowRedStepSize)]

where Qa =PcUpperT hresholdsQualUL- AV_RXQUAL_UL_PC

IF : optimumRxLevUL NIF :optimumRxLevUL N

PwrDecrL imitBand0: i fAV_RXQUAL_UL_P C = 0

PwrDecrLimit= 10dB

PwrDecrL imitBand1: i f

AV_RXQUAL_UL_P C = 1

PwrDecrL imitBand2: i fAV_RXQUAL_UL_P C = 2

Figure 18. MS Power decrease due to signal quality

The philosophy behind this solution is that the condition of good quality doesn't

necessarily correspond to a high-received level and in such a case, the

application of the fixed step is a reasonable choice.

If the received level is above the threshold pcUpperThresholdsLevUL , then

the step in the power decrease can be easily and safely determined. In this case,

the step size calculation is based on a different threshold than the one that

triggered the power control.

OptimumRxLevUL 'not used'

If the optimum uplink RF signal level has been defined for the transceiver by

means of the parameteroptimumRxLevUL (LEV)(TRX), the variable power

change step size will be based on: the averaged uplink signal quality

the quality threshold for the MS power decrease

the optimum uplink RF signal level

the current uplink signal level.

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In this case, the BSC always uses the variable power change step size for

decreasing the MS transmission power. The variable step is calculated as

follows:

PWR_DECR_STEP = MIN (PwrDecrLimit, MAX (MAX (0, RXLEV_UL -OptimumRxLevUL), (PwrDecrQualFactor + MAX (0,Qa)) *

PowRedStepSize))

The formula is quite complicated and can be simplified as follows:

PWR_DECR_STEP = MIN (PwrDecrLimit, MAX (A, B))

The parameterpwrDecrLimit indicates the maximum possible reduction to the

power of the MS and in reality takes three different values depending on the

value of the averaged quality that triggered the power control.

pwrDecrLimitBand0 (PD0)(POC)(0, 2, 38) maximum size of the power

decrease, when the averaged signal quality (bit error rate) is better than 0.2%

(quality band 0).pwrDecrLimitBand1 (PD1)(POC)(0, 2, 38) maximum size of the power

decrease, when the averaged signal quality (bit error rate) is between 0.2%

and 0.4% (quality band 1).

pwrDecrLimitBand2 (PD2)(POC)(0, 2, 38) maximum size of the power

decrease, when the averaged signal quality (bit error rate) is worse than 0.4%

(quality bands 2-7)

The term A = MAX (0, RXLEV_UL - optimumRxLevUL), generates a

decrease in the MS transmission power that would bring the received level

Uplink (not the averaged level, but the current received level) to the optimum

level as defined by the parameteroptimumRxLevUL (LEV)(TRX)(-109 dBm

to -47 dBm). It can be noted that A may be equal to 0.

The term B = (PwrDecrQualFactor + MAX (0,Qa)) * PowRedStepSize

where Qa = pcUpperThresholdsQualUL - AV_RXQUAL_UL_PC

takes into account the distance in quality between the averaged quality and the

threshold that triggered the power control. This distance is multiplied by the

powRedStepSize (RED) POC)(2 , 4).

The parameterpwrDecrQualFactor (PDF)(POC)(2 , 4) is used to have always

B > 0. In fact if Qa=0 and A=0, then the calculation of the variable step gives

PWR_DECR_STEP = PwrDecrQualFactor * PowRedStepSize

The parameter allows the operator to avoid that there is no reduction of

transmission power in the MS when good quality uplink is encountered.

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3.7 MS Power Optimisation

Normally MS accesses the TCH with the maximum Tx Power allowed in the cell:

msTxPwrMax

When power optimization is employed

MS_TXPWR_ OPT =MsTxPwrMax- MAX ( 0, (RXLEV_UL -OptimumRxLevUL) )

ParameterOptimumRxLevULmust be defined for each TRX in the cell. If there aredifferent values defined for different TRXs then maximum value is considered in thecalculation.

RXLEV_UL is measured during signalling phase

OptimumRxLevUL -109 -47 / N dBm

Parameter Value

Figure 19. Call Set-up

Normally MS uses the maximum Tx Power allowed in the target cell

msTxPwrMax

When power optimization is employed

MS_TXPWR_ OPT =MsTxPwrMax- MAX( 0, (AV_RXLEV_UL_HO + (MsTxPwrMax-

MS_TXPWR) -OptimumRxLevUL)

ParameterOptimumRxLevULmust be defined for each TRX in the cell. If differentvalues then maximum is considered

Example: AV_RXLEV_UL_HO= -75 dBm

OptimumRxLevUL= -80 dBm MS_TXPWR_OPT = 33 dBm -MAX( 0, -75 dBm+80 dBm)

MS_TXPWR_MAX= 33 dBm = 33 dBm -5 dB = 28 dBm

MS_TXPWR = 33 dBm

Figure 20. Intracell Handover

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Normally MS uses the maximum Tx Power allowed in the target cell

msTxPwrMax

When power optimization is employed;

MS_TXPWR_ OPT(n) =MsTxPwrMax(n)- MAX ( 0, (AV_RXLEV_NCELL(n) -MsPwrOptLevel) )

ParametermsPwrOptLevel is defined on a per adjacent cell basis

msPwrOptLevel -110 -47/N dBm

Parameter Value

6 dB

Handover

Serving Cell DL

Adjacent Cell DL

Adjacent

Cell ULmsPwrOptLevel

Affects Uplink

Either Uplink signal equalsdownlink signal

Or Differences in UL / DLconsidered when definingmsOptPwrLevel

Figure 21. Internal Intercell Handover

3.8 Conclusions

When trying to understand the power control algorithm, it should be kept in

mind that the whole process is composed of four steps:

Measurements done by the MS and by the BTS

Measurement processing in the BSC

Threshold comparison

Calculation of the power change

The last step may appear complicated due to the large differentiation in the

formulas used by the algorithm. The situation is different when the power

control is triggered by quality (good or bad) or by level (high or low) and

depending on the transmission power being required for the MS or the BTS.

It is possible to get a more clear idea by noting that in all cases a range of good

values is defined by an upper threshold and lower threshold. The power control

works tries to keep the received level and the received quality into that range by

changing the transmitter power "on the other side" of the radio connection.

In case of power control due to level, the averaged value is out of the range (or

on its border) and the action is taken to bring it within the defined band. The

variable step is calculated very easily in this case.

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When the quality is out of range, then issue is more complicated because the

reason for good (or bad) quality could be a very high (or too low) received

power. Therefore, the calculation of the power change step size involves both

level and quality.

For the MS, in particular a dedicated parameter optimumRxLevUL (LEV)

(TRX) is used to define what can be considered an optimum level to be

received by the BTS. When used, this parameter affects the calculation of the

step used in the power decrease.

Finally

The power decrease due to quality does not take place if there is a possibility

that it would trigger the threshold pcLowerThresholdsLevUL/DL. A safety

margin is used which is equal to 6 dB.

The power control of the BTS can be disabled by means of the parameter

powerCtrlEnabled (PENA)(POC).

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4 Power Control Exercise

1) Look-up the default and recommended values for the thresholds named on

this diagram, and compare those values to the values that are shown on thediagram.

2) State the optimum range of values for RX level & RX quality, for both UL

& DL.

0

1

2

3

4

5

6

7

-110

-108

-106

-104

-102

-100

-98

-96

-94

-92

-90

-88

-86

-84

-82

-80

-78

-76

-74

-72

-70

-68

-66

-64

-62

-60

-58

-56

-54

-52

-50

dBmdBm

QualityQuality

RxlevAccMinRxlevAccMin

RxlevAccMinRxlevAccMin(n)(n)

PcLowerThresholdLevULPcLowerThresholdLevUL /DL/DL

PcUpperThresholdLevULPcUpperThresholdLevUL /DL/DL

PcLowerThresholdQualULPcLowerThresholdQualUL /DL/DL

PcUpperThresholdQualULPcUpperThresholdQualUL /DL/DL

No Action NeededNo Action Needed

Increase PowerIncrease Power

Decrease PowerDecrease Power

PcUpperThresholdQualUL/DL

Power decrease

No action needed

PcUpperThresholdLevUL/DL

PcLowerThresholdLevUL/DL

PcLowerThresholdQualUL/DL

RxLevMinCell(n)

RxLevAccMin

Power increase

Thresholds

Actions

Figure 22. Power Control Thresholds and Actions Uplink & Downlink

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5 Key Learning Points

Power control (PC): is a method of battery charge endurance improvement and

interference reduction. The aim is to maintain a good link quality at lowestpossible transmit powers. It can be applied in uplink and downlink direction. It

is controlled by the BSC and performed in both base station and mobile station.

When power control is enabled, it concerns every transceiver of the BTS with

the exception of the BCCH (broadcast control channel) transceiver which

always transmits with the maximum power level. This is because

a) Mobiles detecting a BCCH carrier in the power-up procedure need a

constant carrier signal

b) Every MS is also continuously measuring the RX level of the adjacent cell

BCCHs so this signal has to be always at its highest value in every RTSL

of the BCCH TRX.

When in dedicated or connected mode, the mobile reports on a regular basis

received signal power of the serving cell to the base station. BS commands the

mobile to reduce/ increase its transmit power in incremental steps of 2 dB.

Power control can be level-based, quality based or both.

Level-based power control means the BTS aims for a target RX level. Transmit

power of mobiles and BS is regulated such, that the received signal is always

near the target level.

In order to use BTS power control, an operator should enable it on a cell by cell

basis using the parameter Power Control Enabled:

PowerCtrlEnabled (PENA)(POC)

Power control algorithm is composed of the following steps:

a) Measurements done by the MS and by the BTS and sent to BSC

b) Measurement processing in the BSC

c) Threshold comparison: a threshold is compared with BTS measurements. A

range of good values is defined by an upper threshold and lower threshold

for signal level and quality for uplink and downlink. The power control

works tries to keep the received level and quality into this range by

changing the transmitter power

d) Calculation of the power change: comparison indicates that the MS or theBTS needs an increase or decrease in RF power

The BSC sends a PC command to the MS/BTS including the new transmission

power level of the MS/BTS

The minimum and maximum MS transmission powers are determined on cell-

by-cell basis by the parameters

MsTxPwrMaxGSM

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minsTxPwrMin (PMIN)(BTS)

The maximum and minimum transmission power of the BTS is controlled by

the following parameters respectively

BsTxPwrMax (PMAX)(POC)

BsTxPwrMin (PMIN)(POC)

The range of the BTS transmission power is from 30 dB to 0 dB of attenuation

from the maximum peak power of the base station transmitter.

To prevent repetitive changes of RF output power for the MS/BTS, a minimum

time interval between the changes can be set using the parameter:

PowerControlInterval (INT)(POC)

Fast Averaging for power control allows only measurements after a Power

Change to be used for averaging and it is enabled using the parameter:

EnaFastAvePC (EFP)(HOC))

The BSC determine the size of the increase or decrease of transmit power using

fixed step size or variable step size (used in situations where the required power

change is so large that the use of the fixed step size would require several power

control commands and a lot of time)

Fixed transmit power change step size for MS/BTS is selected using parameter

powRedStepSize (RED)(POC)(2 or 4 dB)

powIncrStepSize (INC)(POC)(2, 4 or 6 dB)

Variable transmit power step size is enabled using parameter:

VariableDLStepUse (VDLS)(BSC)(Yes/No): default value = 'no'

There are a number of possibilities during Power Control

1. The MS/BTS power increase due to signal level is carried out when the

following condition is satisfied Px averaged values out of Nx values

AV_RXLEV_UL/DL_PC


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2. The MS/BTS power increase due to signal quality is carried out when the

following condition is satisfied Px averaged values out of Nx

AV_RXQUAL_UL/DL_PC


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6 Review Questions

Q1. Which of the following is true about Power Control?

A. It reduces battery lifetime.

B. It results in increased interference.

C. It aims to maintain a good link quality at lowest possible transmit powers.

D. It is downlink direction only.

E. It is performed by BTS in both BSC and mobile station.

Q2. Why is power control not used on the TRX that transmits the BCCH?

A. Mobiles detecting a BCCH carrier in the power-up procedure need a

constant carrier signal.

B. Every MS is also continuously measuring the RX level of the adjacent cell

BCCHs so this signal has to be always at its highest value.

C. It can reduce battery lifetime for MS.

D. All of the above

E. Choices A and B.

Q3. Which of the following steps are used in Power control algorithm?

A. Measurements done by the MS and by the BTS and sent to BSC.

B. Measurement processing in the BSC

C. Threshold comparison.

D. Calculation of the power change.

E. All of the above.

Q4. Which parameter does an operator use to enable BTS power control on a

cell by cell basis?

A. PowerCtrlEnabled (PENA)(POC).

B. MsTxPwrMax

C. MsTxPwrMin

D. BsTxPwrMax (PMAX)(POC).

E. BsTxPwrMin (PMIN)(POC)

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Q5. Which parameter sets the maximum BTS transmission powers?

A. PowerCtrlEnabled (PENA)(POC).

B. MsTxPwrMax

C. MsTxPwrMin

D. BsTxPwrMax (PMAX)(POC).

E. BsTxPwrMin (PMIN)(POC)

Q6. Why is there a need for a variable step size in Power Control?

A. The BSC determines the size of the increase or decrease of transmit power.

B. Using fixed step size would violate threshold comparison.

C. When the required power change is so large that the it would require several

power control commands and hence a lot of time.

D. To prevent repetitive changes of RF output power for the MS/BTS.

E. It is a requirement of fast averaging.

Q7. To prevent repetitive changes of RF output power for the MS/BTS, a

minimum time interval between the changes can be set using the parameter

A. VariableDLStepUse (VDLS)(BSC)(Yes/No)

B. powRedStepSize (RED)(POC)(2 or 4 dB)

C. powIncrStepSize (INC)(POC)(2, 4 or 6 dB)D. EnaFastAvePC (EFP)(HOC))

E. PowerControlInterval (INT)(POC)

Q8, Fast Averaging for power control is enabled using the parameter:



C. powIncrStepSize (INC)(POC)(2, 4 or 6 dB)

D. EnaFastAvePC (EFP)(HOC))E. PowerControlInterval (INT)(POC)

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Q9. Fixed step size for MS/BTS transmit power decrease is selected using

parameter



C. powIncrStepSize (INC)(POC)(2, 4 or 6 dB)

D. EnaFastAvePC (EFP)(HOC))


Q10. Fixed transmit power increase step size for MS/BTS is selected using

parameter


B. powRedStepSize (RED)(POC)(2 or 4 dB)C. powIncrStepSize (INC)(POC)(2, 4 or 6 dB)

D. EnaFastAvePC (EFP)(HOC))


Q11. Variable transmit power step size is enabled using parameter:



C. powIncrStepSize (INC)(POC)(2, 4 or 6 dB)D. EnaFastAvePC (EFP)(HOC))


Q12. Which of the following is true about Px and Nx in PC Algorithms?

A. Nx represents lower threshold parameter for uplink.

B. Px representlower threshold parameter for downlink.

C. They are threshold level used for comparison.

D. Nx is number of averages taken into account before decision ispossible.

E. Px is the number of averages that have to be lower than or equal to

the threshold, before power change is possible.

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Q13. Which TRX parameter indicates the optimum downlink RF signal level

ensures both adequate speech/data quality and does not cause downlink

interference

A. PcLowerThresholdsQualUL/DLB. PcLowerThresholdsQualUL/DL

C. PcUpperThresholdsLevUL

D. PcUpperThresholdsQualDL

E. OptimumRxLevDL (LEVD)(TRX)

Q14. Which threshold parameter is used to decrease MS power due to signal

level?

A. PcLowerThresholdsQualUL/DL

B. PcLowerThresholdsQualUL/DL

C. PcUpperThresholdsLevUL

D. PcUpperThresholdsQualDL

E. PcUpperThresholdsQualDL:

Q15. Which condition has to be satisfied for Px averaged values out of Nx when

the MS power is decreased due to signal quality?

A. AV_RXLEV_UL_PC

Date post:	04-Apr-2018
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