dynamic-bts-power-control

User Description, Dynamic BTS PowerControl

USER DESCRIPTION

E

Copyright

© Ericsson AB 2002. All rights reserved.

Disclaimer

The contents of this document are subject to revision without notice due tocontinued progress in methodology, design and manufacturing.

Ericsson shall have no liability for any error or damages of any kind resultingfrom the use of this document.

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User Description, Dynamic BTS Power Control

Contents

1 Introduction 1

2 Glossary 1

2.1 Concepts 1

2.2 Abbreviations and Acronyms 1

3 Capabilities 2

3.1 Interference 2

3.2 Battery backup power consumption 2

3.3 Receiver saturation 2

3.4 Quality and signal strength impact 2

4 Technical description 3

4.1 General 3

4.2 Algorithm 4

4.3 Handover power boost 11

4.4 Power regulation example 12

4.5 GPRS/EGPRS 13

4.6 AMR FR Power Control 13

4.7 Main changes in Ericsson GSM system R10/BSS R10 14

5 Engineering guidelines 14

5.1 Interactions with other features 14

5.2 Frequency planning aspects 15

5.3 Recommendations 16

6 Parameters 26

6.1 Main controlling parameters 26

6.2 Parameters for special adjustments 26

6.3 Value ranges and default values 27

7 References 28

8 Appendix A 28

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1 Introduction

With the Dynamic BTS Power Control feature the output power of a BaseTransceiver Station (BTS) can be controlled during a connection. The controlstrategy is to maintain a desired received signal strength and quality in themobile station (MS).

This User Description describes the BTS Power Control and AMR PowerControl algorithm for circuit switched connections only.

2 Glossary

2.1 Concepts

MeasurementReport

Message consisting of measurements done by the MS,which is sent from the MS to the BTS.

MeasurementResult

Message consisting of the Measurement Report andmeasurements done by the BTS, which is sent fromthe BTS to the BSC.

2.2 Abbreviations and Acronyms

AMR Adaptive Multi Rate

BCCH Broadcast Control Channel

C/I Carrier to Interference Ratio

CNA Cellular Network Administration

DTX Discontinuous Transmission

GPRS General Packet Radio Service

LRP Locating Reference Point

SDCCH Stand Alone Dedicated Control Channel

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3 Capabilities

3.1 Interference

The aim with BTS Power Control is to increase the number of MSs withsufficiently good C/I. BTS Power Control will improve C/I if traffic is maintained,or maintain C/I when traffic is increased or tighter frequency re-use is realised.The gain is obtained by a reduction of the over all interference level (I) in thenetwork.

When BTS Power Control is used in all BTSs in the network, the total amount ofradiated power is reduced compared to when it is not used. This implies thatthe downlink co- and adjacent channel interference in the network is reduced.Since MSs with low signal strength or bad quality use full BTS output power,reduced interference level imply increased C/I for these connections. On theother hand, the C/I is decreased for connections with high signal strengthand good quality since they are subjected to a reduced BTS output power.Reduction of C/I will not affect the speech quality of these connections sincethey have a margin to the lowest tolerable C/I.

Frequency Hopping, together with BTS Power Control and DTX improve thepossibilities to achieve very tight frequency reuse, see further User Description,Discontinuous Transmission and User Description, Frequency Hopping.

3.2 Battery backup power consumption

If the power supply for the base station is cut off, a battery backup is used.When BTS Power Control is used the battery consumption is reduced and themaximum possible speech time will increase.

3.3 Receiver saturation

The high signal energy from BTSs transmitted to MSs that are close mightsaturate the MS receiver. The sensitivity of the receiver will then decrease andthe speech quality become poor. If the output power of the concerned BTSsis lowered, the risk for this kind of radio frequency blocking is reduced. Thereceiver might still be blocked if an MS is very close to the base station, but theprobability for this is significantly reduced.

3.4 Quality and signal strength impact

Both quality and signal strength is considered by the algorithm. Quality is theestimated bit error rate which is represented by rxqual. Signal strength is

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represented by rxlev. Bad quality as well as low signal strength will increasethe output power of the BTS.

4 Technical description

4.1 General

Important notice: The algorithms in MS Power Control and BTS Power Controlare the same.

In Figure 1 on page 3, the BTS output power and the signal strength in the MSversus path loss between a BTS and an MS is shown. A BTS can only transmitat distinct power levels, this is illustrated in the figure.

Path lossRegulation area

BTS output power

Maximumpower level

Minimumpower level

MS received power

BTS output power MS received power

Figure 1 Base station output power and MS signal strength versus path loss.Quality is not taken into account.

When a connection has low path loss (left part of Figure 1 on page 3 ), theBTS transmits at its lowest possible power level. Although the MS receives asignal that exceeds the desired value, the BTS can not reduce the transmittedpower any further. Conversely, when a connection experiences high path loss(right part of Figure 1 on page 3), the BTS transmits at the maximum allowedpower level for the cell. The power cannot be increased even if the receivedsignal strength in the MS is low. Note that this is dependent on the path losscompensation used (see Section 4.2.4 on page 8 ).

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When quality is taken into account the output power is regulated up or downdepending on the received quality (see Figure 2 on page 4 ). The base stationpower then varies with the quality measured by the MS. When an MS has lowrxqual (high quality) the base station sends on low power and when an MShas high rxqual, on high power. The higher the rxqual, the higher the powerand vice versa.

rxqual

BTS output power

0 1 2 3 4 5 6 7

Maximumpower level

Minimumpower level

Figure 2 Example of BTS output power versus rxqual. Signal strength isnot taken into account.

4.2 Algorithm

4.2.1 General

Dynamic BTS Power Control is performed for Traffic channels (TCHs) as wellas for SDCCHs. Power control of the SDCCHs is enabled with the switchSDCCHREG. All time slots on the BCCH frequency are transmitted on fullpower, i.e. there is no Power Control of these time slots.

During a call, the MS measures the downlink signal strength and quality. Thesemeasurements are sent to the BTS in the Measurement Report and furtheron to the BSC in the Measurement Result message where they are used forcalculation of a new BTS output power.

The measurements from the Measurement Result that are used in the DynamicBTS Power Control algorithm are shown in Table 1 on page 4.

Table 1 Measurements used by BTS Power Control

Data description Source

signal strength downlink full set (1) MS

signal strength downlink subset (1) MS

quality downlink full set (1) MS

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Table 1 Measurements used by BTS Power Control

Data description Source

quality downlink subset (1) MS

power level used by BTS BTS

DTX used by BTS or not BTS

(1) The MS performs signal strength and signal quality measurements on thedownlink. Measurements are made on the full set of frames (full set), as well ason the subset of frames where there is always traffic (subset). Which of the setswill be used depends on whether DTX downlink has been used or not, during themeasurement period (see also User Description, Discontinuous Transmission ).

The minimum time period between two consecutive power orders is controlledby the parameter REGINTDL. REGINTDL is set in units of SACCH periods(480 ms) between 1 and 10.

The BTS is able to change its output power on a time slot basis. The resolutionin output power is in steps of 2 dB and the maximum configurative changeis 30 dB.

For a single connection, the maximum change per SACCH period is also 30 dB.

Down regulation can be limited to 2 dB per SACCH period by means of theparameter STEPLIMDL. The default value of this parameter is OFF.

The Dynamic BTS Power Control algorithm consists of three stages:

1 Preparation of input data

The output power level used in the latest measurement period is convertedfrom a relative scale. A decision is taken about which set of measurements(full set or subset (1)) to use. Signal strength and quality are compensatedfor frequency hopping and power control.

2 Filtering of measurements

Measurements are filtered in exponential non-linear filters in order toeliminate variations of temporary nature.

3 Calculation of power order

Two power orders are calculated according to the algorithm using twodifferent parameter settings. The one with the maximum power order(minimum attenuation) is chosen. A number of constraints (according tohardware limitations and parameter settings) are applied to the chosenpower order.

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4.2.2 Preparation of input data

The output power level used by the BTS (TRU) at SACCH period k, is given byPLused as a number of 2 dB steps downwards from the configured output power.

BTS (TRU) output power (k) (dBm) = BSPWRT - 2 * PLused (1)

In the Measurement Result message, the BTS sends information about whetherDTX (see User Description, Discontinuous Transmission ) has been usedduring the measurement period or not. This information is used by the BSCto decide which set of downlink measurements, full set or subset, to use onTCHs. The subset of measurements should be used if DTX was used duringthe measurement period by the BTS. On SDCCHs the full set of measurementsare always used.

To be able to use the desired quality (QDESDL) and the measured rxqual inthe calculations, both must be converted to C/I expressed in dB according toTable 2 on page 6. The mapping between rxqual and C/I is non-linear due tothat faster regulation is needed for low and high rxqual values.

Table 2 Table with relations due to non-linear rxqual to C/I mapping

QDESDL [dtqu] 0 10 20 30 40 50 60 70

rxqual 0 1 2 3 4 5 6 7

C/I [dB] 23 19 17 15 13 11 8 4

QDESDL defines a desired value for rxqual that the regulation will aim for inthe regulation process and is given in dtqu (deci-transformed quality units).Difference between dtqu and rxqual is a factor of ten. If QDESDL is not equal tothe values given in Table 2 on page 6, linear interpolation is used to realize C/I.

Example of QDESDL interpolation:

If QDESDL = 35 then C/I = 15+(13-15)*0,5 = 14 dB

QDESDL expressed in C/I is called QDESDL_dB which is the value used inthe calculations.

The BCCH frequency is not subjected to power control. When frequencyhopping (User Description, Frequency Hopping ) is applied and the BCCHfrequency is included in the hopping set, the BTS output power will varyfrom burst to burst depending on which frequency the burst is sent on. Acompensation is necessary to obtain a correct estimation of the measuredsignal strength, see eq. 2.

SSTCH = SSM - (BSPWR-BSTXPWR +2*PLused ) / Nf (2)

where SSTCH is the signal strength on the down regulated TCH carriers, SSMthe measured signal strength reported by the MS, BSPWR is the BTS output

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power on the BCCH frequency in the LRP (see User Description, Locating), BSTXPWR is the BTS output power on the TCH frequencies in the LRP(see User Description, Locating ) and Nf is the number of frequencies in thehopping frequency set. The compensation is performed if the BCCH frequencyis included in the hopping set and if the MS measures on the BCCH frequency.All signal strength measurements are compensated before the filtering (seeSection 4.2.3 on page 7).

SSTCH is also compensated for power control according to eq. 3.

SS_COMP = SS TCH + 2*PLused (3)

where SS_COMP is the signal strength compensated for both down regulationand frequency hopping.

If the BSC does not receive the Measurement Result from a BTS, the powerregulation is inhibited for that connection. At the same time the REGINTDLcounting is suspended. When a Measurement Result is received again, powerregulation and REGINTDL counting are resumed.

The signal strength filter will not be updated when signal strength results(measured in the Measurement Report) are missing. This means that theoutput from the signal strength filter is held until the next value is received.

Missing quality values in the Measurement Report are set to the worst possiblevalue. This means that missing quality values are interpreted as rxqual = 7.

If information about the BTS power level used is missing in the MeasurementReport, the missing values are set to the latest calculated power order.

4.2.3 Filtering of measurements

The filtering for both signal strength and quality is done with exponentialnon-linear filters. SSFILTERED in eq. 4 is the filtered signal strength compensatedfor down regulation, i.e. the signal strength that would have been received bythe MS if no power control was used. SSFILTERED is defined as:

SSFILTERED (k) = b * SS_COMP(k) + a * SSFILTERED (k-1) (4)

where b and a (b = (1-a)) represent the filter coefficients, SS_COMP is thesignal strength compensated for both down regulation and frequency hoppingand k is a sequence number. Coefficient a is given by the length of theexponential filter (see Appendix A). Each filter length (L) corresponds to acertain value of a, and L is determined in the following way:

if SS_COMP(k) < SSFILTERED (k-1)

then L = SSLENDL

else L = SSLENDL * UPDWNRATIO / 100 (5)

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where L is rounded upwards to SACCH periods. When the length exceeds 30SACCH periods, the length is set to 30.

To enable calculating and sending the power order immediately after assignmentor handover, the filter is initiated with SSFILTERED (k-1) = SSDESDL. This leadsto that the regulation starts immediately after the first valid Measurement report.

Quality filtering is performed in the same way as for signal strength i.e. withexponential non-linear filters. The filtering is done according to eq. 6.

QFILTERED (k) = b * Q_COMP(k) + a * QFILTERED (k-1) (6)

where QFILTERED is the filtered quality compensated for down regulation, i.e. theestimated C/I (in dB) that would have been received by the MS if no powercontrol was used. Q_COMP is the compensated quality part according to eq. 7.

Q_COMP = RXQUAL_dB + 2*PLused (7)

where RXQUAL_dB is the measured rxqual transformed to C/I (in dB) accordingto Section 4.2.2 on page 6.

The coefficient a in eq. 6 above is given by the length of the exponential filter(see Appendix A) in the same way as for the signal strength case, only that thistime L is determined in the following way:

if Q_COMP(k) < QFILTERED (k-1)

then L = QLENDL

else L = QLENDL * UPDWNRATIO / 100 (8)

where L is rounded upwards to SACCH periods.

To enable calculating and sending the power order immediately after assignmentor handover, the quality filter is initiated with QFILTERED (k-1) = QDESDL_dB.

4.2.4 Calculation of power order

The calculation of the power order is made in three steps:

1 The two basic power orders are calculated.

2 Certain constraints are applied.

3 The output data is finally converted to power order units before it istransmitted to the BTS as a power order.

The actual information sent to the BTS is the power level, PLused, accordingto Section 4.2.6 on page 10.

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The basic power orders for regulation (pu1 and pu2) are given by the followingexpression:

pui = α i * (SSDESDL - SSFILTERED ) + βi * (QDESDL_dB - QFILTERED ) (9)

i = 1, 2

where the parameters α i and βi are defined as follows:

α1 = LCOMPDL / 100 (pathlosscompensation)

(10)

β1 = QCOMPDL / 100 (quality compensation) (11)

α2 = 0.3 (pathlosscompensation)

(12)

β2 = 0.4 (quality compensation) (13)

The parameters α i and βi control the compensation of path loss and quality.The parameters α1 and β1 can be set by means of LCOMPDL and QCOMPDLwhile parameters α2 and β2 are fixed. These values have been optimised toget the regulation towards the noise floor fast without jeopardising the quality.The setting of α2 and β2 is however not critical since these parameters merelyserve as a limitation for regulation close to the noise floor (see Section 4.4on page 12).

The two power orders are calculated simultaneously (eq. 9) and the one withthe highest value (minimum down regulation) is used. This resulting powerorder is called the unconstrained power order, pu.

pu = max(pu1 ,pu2 ) (14)

4.2.5 Power order constraints

Dynamic power range limitation is applied if the unconstrained power order isoutside the dynamic range:

• The highest allowed power order is zero (0). This corresponds to full poweraccording to BSPWRT .

• The lowest allowed power order is given by the minimum of

a 30

b BSPWRT - (Miminum BTS output power (HW limit))

c BSTXPWR - BSPWRMIN

Note that even if the actual output power BSPWRT in the BTS is set to theminimum value, lower power levels can actually be achieved when BTS Power

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Control is active. For an RBS2000 GSM900 MHz with minimum output powerpossible to configure equal to 35 dBm (BSPWRT: 35 to 47 dBm, odd valuesonly), the lowest achievable output power is 47 - 30 = 17 dBm when BTS powercontrol is active.

4.2.6 Conversion of output data

The new power order has to be converted from the internal dBm scale to PLusedrepresentation before it can be transmitted to the BTS. In reality this means thatthe constrained power order is quantisized in steps of 2 dB according to:

PLused = Int(-pu/2 ) [0..15]

where PLused is the power level. PLused = 0 represents full power and PLused =15 represents 30 dB down regulation.

The power is always truncated to a higher value (lower down regulation).

4.2.7 Regulation procedure

When a TCH connection is set up, maximum configurative output power isalways used, for example in the following situations:

• assignment of a TCH.

• assignment failure or handover failure.

• intra-cell handover and subcell change.

• inter-cell handover.

Down regulation always starts after the first valid Measurement report (seeSection 4.2.3 on page 7). The response time for up regulation is controlled bythe parameters QLENDL and SSLENDL. QLENDL determines the responsetime on high interference and SSLENDL on signal strength drops. The valuesof QLENDL and SSLENDL corresponds to a 90 % rise time of the exponentialfilters.

The response time for down regulation is determined by the expressionsQLENDL *UPDWNRATIO /100 and SSLENDL *UPDWNRATIO /100 whereUPDWNRATIO is the ratio between up- and down regulation speed. Thisresults in a quick up regulation and a smooth down regulation.

UPDWNRATIO is a BSC exchange property.

When a power order is sent it takes REGINTDL SACCH periods before the nextpower order can be sent. If this power order differs from the previous one, itis sent. If it does not differ from the previous one, a new order is calculatedevery SACCH period until a different power order is obtained. Then that orderis sent, and REGINTDL SACCH periods must elapse before a new order canbe sent again.

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4.2.8 Multislot configuration

If the TCH channel is a part of a channel combination, it can be either a main,bi-directional or a uni-directional channel.

If the channel is a main channel in a multislot configuration, the differencebetween the computed power order and the previous power order must exceeda hysteresis of two dB before a new power order is sent.

BTS power regulation on bi-directional channels is done independently of theother channels.

For uni-directional channels BTS Power Control is activated without startingnormal power regulation. No Measurement reports will be received foruni-directional channels. Instead the BTS power value of the main channel isdistributed to the uni-directionals in the multislot configuration.

In a multislot configuration only the main channel is affected by the handoverpower boost, see Section 4.3 on page 11.

See further User Description, Channel Administration and User Description,High Speed Circuit Switched Data (HSCSD) .

4.3 Handover power boost

With Handover power boost, the handover command is sent by the BSC/BTSto the MS on maximum configurative power. Handover command includesinformation about which uplink power the MS shall use in serving cell. TheMS then acknowledges the handover command using maximum configurativepower. In case of a HO failure, the HO failure message is also sent onmaximum configurative power. When handover power boost is triggered,normal regulation is inhibited until the MS has received the handover command.The BTS ignores all BTS or MS power orders sent by the BSC in the servingcell until the MS has acknowledged the handover command.

The speech/channel coding and interleaving in GSM is very robust. A smallnumber of bursts/frames can be lost without speech degradation (the numberdepends on the error distribution). Power Control should therefore also be usedfor connections close to the cell border. Since the signaling for the handoverprocedure (e.g. Handover Command) is more critical and error-sensitive,it should be sent on maximum power in order to maximise the handoverperformance.

HOPB is useful when the SS quickly drops, for example when the MS movesaround a street corner. In this case, due to the system delay and the limitedup-regulation speed, the signaling would be sent on a too low power withoutHOPB. Thus in order to maximise the probability of a successful handover,Handover Power Boost should be used.

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Since the maximum configurative power is only used for a short time beforethe handover, activating HOPB has a minor impact on the overall interferencelevel in the network.

Note that HOPB only improves the HO performance if power control is activated.

Handover power boost is activated by setting the state variable HPBSTATE.

4.4 Power regulation example

The most important thing for good comprehension of the BTS Power Controlalgorithm is to understand how the two algorithms work in parallel and howdifferent settings of the available parameters will influence the regulation. Theequations given in Section 4.2.4 on page 8 can be used to find out how muchthe output power will be down regulated for a certain signal strength and quality.But to get an overview picture of the algorithm as a whole, the dependencebetween signal strength, quality and down regulation must be understood.

A suitable way of studying these three quantities is in a three dimensional plotdescribing the static behaviour of the algorithm. Let quality (rxqual) and signalstrength (rxlev) constitute a two dimensional plane that, if BTS Power Control isnot active, holds all traffic in the network. Let us now introduce down regulationin the third dimension. If BTS Power Control is activated, the surface thenbecomes raised for those values of rxqual and rxlev where the algorithm allowsdown regulation. As an example of this, see Figure 3 on page 12 which showsa principal figure for downregulation.

01

23

45

67 0

10

20

30

40

50

600

5

10

15

20

RxLev

RxQual

Dow

n re

gula

tion

(dB

)

1

2

3

Figure 3 Principal figure for downregulation

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As it can be seen in Figure 3 on page 12, the surface is raised for rxlev > 14and rxqual < 5. The down link for MSs in this area is down regulated. The levelof the down regulation is shown on the z-axis.

Note that rxqual and rxlev in Figure 3 on page 12 corresponds to the measuredvalues collected from the Measurement Report before any compensationhas been done.

The static behaviour is calculated by assuming an initial down regulation of zeroand that the path loss to the MS is constant. Then, for a certain value of initialRxLev and RxQual (a point in the x-y plane), the down regulation (z-value)settles after some iterations. Repeating the calculations over the entire x-yplane produces the graphs above.

For the recommended setting, SSDESDL and QDESDL are set to -90 and30 respectively. These two values define the point (marked 1 in Figure 3 onpage 12) on the two dimensional plane (quality vs signal strength plane) wherethe two separate planes (marked 2 and 3 in Figure 3 on page 12) of thealgorithm meet. Plane 2 regulates the MSs towards the noise floor (low signalstrength) and plane 3 towards quality. The position of these planes in the threedimensional plot is determined by SSDESDL and QDESDL or SSDESDLAFRand QDESDLAFR for AMR Power Control, see Section 4.6 on page 13. Notethat figure 3 shows the down regulation without the truncation of the powerorder, to illustrate the two different algorithms (planes).

The parameters QCOMPDL and LCOMPDL decide about the angles of plane3 towards the two dimensional plane (quality vs signal strength). QCOMPDLsets the angle along the QDESDL -value and LCOMPDL along the SSDESDL-value. The angles of plane 2 are fixed (see Section 4.2.4 on page 8).

4.5 GPRS/EGPRS

GPRS/EGPRS BTS Power Control is not supported in BSS R10. Full outputpower is used on all GPRS/EGPRS channels.

4.6 AMR FR Power Control

4.6.1 General

Adaptive Multi Rate (AMR) is a speech and channel codec feature for full ratechannels that makes it possible to acheive improved speech quality for mobileconnection as well as better capacity, see User Description, Adaptive Multi Rate.

The AMR Power Control is used to minimize the interference in the radionetwork for AMR FR connections, by reducing the output power of the AMRFR connections.

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4.6.2 AMR Power Control Algorithm

The AMR Power Control is based on the the Dynamic BTS Power Control andDynamic MS Power Control respectively, see User Description, Dynamic MSPower Control.

The AMR FR speech coding is more robust and can perform well on low C/Ilevels. This results in a possibility to down regulate the output power of AMRFR connections more than for non-AMR connections. This means that AMRFR Power control parameter set can be set more aggressive than for non-AMRparameter setting. To be able to set the parameter more aggressive for AMRFR connections, two new parameters are implemented SSDESDLAFR andQDESDLAFR in the Dynamic BTS Power Control. This means that the twopower orders for AMR FR connections are calculated according to:

pui = α i * (SSDESDLAFR – SSFILTERED) + βi *(QDESDLAFR_dB – QFILTERED)i = 1,2

(15)

The QDESDLAFR_dB is QDESDLAFR expressed in C/I (in dB) accordingto Section 4.2.2 on page 6.

Then the remaining calculations in Section 4.2.4 on page 8 are the same.

4.7 Main changes in Ericsson GSM system R10/BSS R10

AMR Power Control is introduced.

5 Engineering guidelines

5.1 Interactions with other features

The gain of BTS Power Control increases in high capacity systems utilizinga tight frequency reuse. The primary application is a system that uses acombination of Dynamic BTS Power Control, Dynamic MS Power Control,Frequency Hopping and DTX. The mutual interaction between these featuresprovides a very powerful method to increase system performance, and therebysystem capacity (see further in User Description, Discontinuous Transmission, User Description, Frequency Hopping and User Description, Dynamic MSPower Control ).

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Preferably, power regulation should be configured to be performed before anintra-cell handover occurs. Also power regulation should be configured toalways occur before a bad quality urgency handover is attempted.

The desired regulation performance can be achieved through a well balancedcombination of the following:

• the BTS Power Control parameters SSDESDL and QDESDL that set thelimits for how close to the noise floor (how low rxlev ) and how high ininterference (how high rxqual ) BTS down regulation can be performed.

• the AMR FR Power Control parameters SSDESDLAFR and QDESDLAFRthat set the limits for how close to the noise floor (how low rxlev) and howhigh in interference (how high rxlev) AMR FR down regulation can beperformed.

• the quality compensation factor QCOMPDL and the path loss compensationfactor LCOMPDL that determine the angles of inclination of plane 3 inFigure 3 on page 12.

• the intra-cell handover area defined by QOFFSETDL andQOFFSETDLAFR, (User Description, Intra Cell Handover ).

• the threshold triggering bad quality urgency handovers, QLIMDL andQLIMDLAFR(User Description, Locating ).

• the lengths of the locating quality filter QLENSD (User Description, Locating), and the power control quality filter, QLENDL.

Example:

QDESDL = 30, QOFFSETDL = 5 and QLIMDL = 55.

With this setting, full power will always be used before an intra-cell or urgencyhandover occurs.

5.2 Frequency planning aspects

In order to utilize BTS Power Control in an optimum way, it is preferable to use adedicated BCCH band. This means that a BCCH carrier is never used as a TCHcarrier and vice versa. The level of interference will in this way be decreasedfor all TCH carriers. The BCCH carriers are unaffected, but will, depending onthe frequency plan, experience less adjacent channel interference from thedown regulated TCH carriers.

The BCCH carriers can either be allocated in a contiguous BCCH band or in astaggered BCCH band. In a contiguous band, carrier no. 1-15 can for examplebe used as BCCH carriers whereas in a staggered band, for example everysecond frequency can be used as BCCH carriers (1,3,5..31). There are prosand cons with both these strategies. For BTS Power Control it is probablybeneficial to use the contigous BCCH band since when using staggered BCCH,

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the down regulated TCH carriers in between BCCH carriers will suffer fromadjacent channel interference from the, on full power always transmitting,BCCH carriers.

In a network with tight reuse and if the BCCH carriers are allocated in acontigous band, it is beneficial to use a more aggressive setting than therecommended, e.g. by increasing QCOMPDL to 65.

5.3 Recommendations

5.3.1 General

When attempting to decrease the downlink co-channel and adjacent channelinterference in the system, the BTS Power Control feature should beconsidered. However, since downlink power regulation is never performed onBCCH carriers, the impact of downlink regulation will be greater in systemshaving three or more Transceivers (TRXs) per cell.

When introducing BTS Power Control into a system it is recommended to beginwith moderate settings for the controlling parameters. The majority of thegain obtained from using power control originates from the first decibels ofregulation. Therefore, a good strategy is to down regulate many connectionswith a few dB. To get the best effect it is important to reduce the BTS outputpower for as many connections as possible, also those connections to MSs inthe cell border regions being closest to neighbouring users. For such MSshowever the interference levels are often considerable, and great care has tobe taken not to degrade such calls.

5.3.2 Tuning of the algorithm

The shown down regulation in Figure 3 on page 12 and in the graphs in thissection is a target regulation that the algorithm aims for. It is important tounderstand that the down regulation is determined by the combination of theparameters SSDESDL and QDESDL or SSDESULAFR and QDESULAFR forAMR FR connections, not one of the parameters alone. Since the environmentchanges quickly, and the filtering of signal strength and quality introducesdelays, the target down regulation is never reached directly.

The recommended strategy (see Figure 3 on page 12) is a good parametersetting that is not particularly aggressive according to any regulation strategy.By changing the parameters, the regulation can be made more aggressivetowards quality or signal strength or combinations depending on the needsof the customer.

Note, it is not recommended to limit the down regulation with the parameterBSPWRMIN. If used, the parameter will seriously limit the regulation towardsinterference and also introduce a delay in the regulation algorithm. Instead it

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is recommended to use a more restrictive parameter setting e.g. according toFigure 6 on page 18.

To get a regulation that is more aggressive towards quality (i.e. allows higherinterference before it regulates up to full power), QDESDL can be set to ahigher value e.g. QDESDL = 40. This will lead to, if no other parameters arechanged, an increase of the raised surface in Figure 3 on page 12 that growsmainly to the right (towards worse quality) but also a little bit to the left (towardslower signal strength). And if the inclination of plane 3 is left unchanged, theresult is also an upwards shift of this plane. As an example, Figure 4 on page17 shows more aggressiveness towards quality, signal strength and downregulation compared to Figure 3 on page 12. Still, the only parameter that hasbeen changed is QDESDL.

01

23

45

67 0

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5

10

15

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RxLev

SSDESDL:90 QDESDL:40 LCOMPDL: 5 QCOMPDL:55

RxQual

Dow

n re

gula

tion

(dB

)

Figure 4 Aggressive parameter setting towards quality. This setting is ratheraggressive, also towards signal strength and down regulation. Only parameterQDESDL has been changed compared to recommended setting (see Figure 3).

For the parameter setting in Figure 4 on page 17, the quality part of the powercontrol will always fully compensate for bad quality. Full power should bereached quickly in case of high rxqual (rxqual = 5, 6 or 7). This is in order tominimise the risk of having poor speech quality due to too much down regulationand also prevent unnecessary intra-cell handovers and urgency handovers.Hence, a shorter quality filter might be needed (see Section 5.3.4 on page 24).

As an example of more aggressive regulation towards signal strength,study Figure 5 on page 18. The only parameter changed compared to therecommended setting is SSDESDL which is set to -97. For this setting thedownlink for MSs with rxlev = 10 and rxqual = 0 is down regulated 4 dB. Notethat this might sound a bit more aggressive than it is, since at this low signalstrength, noise will impose occasional bit errors to the connection. This willmake the regulation to “bounce” on the noise floor. Very few connections willthen manage to be as much as 4 dB down regulated. Instead most connectionswill alter between 0 and 2 dB down regulation.

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01

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67 0

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RxLev


RxQual

Dow

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(dB

)

Figure 5 Aggressive regulation towards low rxlev. MSs with low signalstrength also get down regulated in case of good quality.

As an example of a more careful regulation strategy see Figure 6 on page 18.This shows how QDESDL can be decreased compared to the recommendedsetting to get a very moderate setting. Maximum 10 dB down regulation isthen allowed.

01

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67 0

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RxLev


RxQual

Dow

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Figure 6 Moderate parameter setting. Only parameter QDESDL has beenchanged compared to recommended setting (see figure 3)

To compensate for this low setting of QDESDL, one alternative could be toallow more down regulation for those MSs that have good quality. Figure 7 onpage 19 show how this can be done. The parameter QCOMPDL is increasedand as a result the inclination of plane 3 is changed. The algorithm then allows

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more down regulation for MSs with good quality but is still careful when it comesto regulation towards bad quality.

01

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67 0

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RxLev


RxQual

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Figure 7 Moderate parameter setting, more aggressive towards downregulation.

Another way of changing the inclination of plane 3 would be to change the pathloss compensation parameter LCOMPDL . In Figure 8 on page 19 LCOMPDLhas been set to 10 while all other parameters are the same as in Figure 6on page 18. This results in that the MSs with high signal strength regardlessof quality gets more down regulated.

01

23

45

67 0

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5

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RxLev

SSDESDL:90 QDESDL:20 LCOMPDL:10 QCOMPDL:55

RxQual

Dow

n re

gula

tion

(dB

)

Figure 8 Moderate parameter setting with path loss compensation factorLCOMPDL set to 10. This results in a very aggressive behaviour towardsdown regulation.

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With the setting in Figure 8 on page 19, plane 3 has become very large anddominating. This setting has regulation towards signal strength and is moreaggressive towards down regulation. The maximum down regulation is here 18dB compared to 14 dB for the old recommended setting.

Important notice: The default values given in Table 3 on page 27 are also NOTrecommended to use!

5.3.3 Examples of parameter settings

Below are some examples of static behaviour with different parameter settingsshown. The first figure illustrates the recommended setting, and the rest of theexamples are sorted in order of increasing “aggressiveness”. These examplescan all be considered as recommendations for different “aggressiveness” levels.

01

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67 0

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RxLev


RxQual

Dow

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Figure 9 The recommended setting.

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01

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Figure 10

01

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Figure 11

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01

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RxLev


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Figure 12

01

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45

67 0

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RxLev

SSDESDL:92 QDESDL:20 LCOMPDL:10 QCOMPDL:55

RxQual

Dow

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(dB

)

Figure 13

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01

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Figure 14

01

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Figure 15

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01

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Figure 16

5.3.4 Filter tuning

Generally for up regulation, the BTS Power Control quality filter QLENDL canbe set to a value between 2 or 5. This is fairly uncritical since instability in thecontrol loop has not shown to be a problem with this control strategy. Thereforeit is better to have a short power control quality filter since the response to badquality then becomes quick. It is not useful to set QLENDL = 1. This would onlylead to extremely nervous behaviour resulting in less average down regulation.Tests have shown that the difference in fast up regulation between QLENDL =2 and QLENDL = 3 is insignificant.

In order to avoid unstable behaviour, the down regulation must be slow. Testshave shown that a filter with lengths between 6 and 9 is good. Of course longerfilters can also be used. This would result in an even more cautious behaviour.The filter length on the down regulation is determined by parameters QLENDLand UPDWNRATIO. UPDWNRATIO sets how much longer the down regulationfilter is compared to the up regulation filter in percent. It is recommended to usehigh UPDWNRATIO instead of using STEPLIMDL. As an example of how thesystem reacts to bad quality, see Figure 17 on page 25.

Example:

QLENDL is 2 and UPDWNRATIO is 600.

This gives 2 SACCH periods filter length for up regulation and 2*600% = 2*6 =12 SACCH periods filter length for down regulation.

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0 5 10 15 200

2

4

6

0 5 10 15 20

15

10

5

0

rxqu

alD

own

regu

latio

n [d

B]

Time [seconds]

Figure 17 Step response to bad quality. Parameter setting QLENDL = 3and UPDWNRATIO = 300 was used. Note the logarithmic behaviour of thedown regulation.

The BTS Power Control signal strength filter is less critical. The regulation isdone in the same way as for quality filtering. The length of the up regulation filteris set by the parameter SSLENDL and for the down regulation by SSLENDLand UPDWNRATIO. For up regulation SSLENDL = 3 is recommended. Theparameter UPDWNRATIO should be tuned for the quality filter. If it is tunedfor quality filtering, it is also valid for signal strength filtering. Thus, for downregulation, a filter length of 6 to 9 is recommended but longer filter lengths canbe used if necessary. See also Figure 18 on page 25.

0 2 4 6 8 10 12 14 16 18 20110

100

90

80

70

60

50

40

0 2 4 6 8 10 12 14 16 18 20

15

10

5

0

Dow

n re

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tion

[dB

]

Time [seconds]

Sig

nal s

tren

gth

[dB

m]

Figure 18 Step response to low signal strength. Parameter setting SSLENDL= 3 and UPDWNRATIO = 300 was used. Aggressive parameter setting gave16 dB down regulation before the low signal strength occurred. Note thelogarithmic behaviour of the down regulation.

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REGINTDL should be set to REGINTDL = 1 in order to make the up regulationquick in bad quality situations.

6 Parameters

6.1 Main controlling parameters

SSDESDL defines the target value for the desired signal strength measured bythe receiver in the MS at the outer rim of the regulation area. The parameteris set per subcell.

QDESDL defines the target value for the desired quality level measured by thereceiver in the MS. It is measured in rxqual units and transformed into dB unitsbefore is used in the algorithm. The parameter is set per subcell.

SSDESDLAFR defines the target value for the desired signal strength for AMRFR connection measured by the receiver in the MS at the outer rim of theregulation area. The parameter is set per subcell.

QDESDLAFR defines the target value for the desired quality level for AMR FRconnection measured by the receiver in the MS. It is measured in rxqual unitsand transformed into dB units before is used in the algorithm. The parameteris set per subcell.

LCOMPDL is the parameter that determines how much of the path loss thatshall be compensated for in the algorithm that regulates towards quality. Theparameter is set per subcell.

QCOMPDL is the parameter that determines the weight of the qualitycompensation. This parameter ranges between 0 and 100 and is set persubcell.

6.2 Parameters for special adjustments

REGINTDL defines the regulation interval. The parameter is set per subcell.

SSLENDL defines the length of the signal strength filter. The parameter is setper subcell.

QLENDL defines the length of the quality filter. The parameter is set per subcell.

SDCCHREG is a switch for the regulation of SDCCH channels. The switchis set per subcell.

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BSPWRMIN defines the minimum allowed output power for the BTS on thenon-BCCH frequencies. The parameter is set per subcell.

BSTXPWR defines the maximum allowed power level for BTSs in the currentsubcell. The parameter is also used in Locating, see User Description, Locating.

BSC exchange properties

UPDWNRATIO is the ratio between the up- and down regulation speed.

STEPLIMDL is a switch that makes it possible to limit the down regulationto 2 dB per SACCH period.

6.3 Value ranges and default values

Table 3

Parameter name Defaultvalue

Recommendedvalue Value range Unit

SSDESDL (1) -70 -90 -110 to -47 dBm

QDESDL 20 30 0 to 70 dtqu

SSDESDLAFR (1) -70 -90(3) -110 to -47 dBm

QDESDLAFR 20 40(3) 0 to 70 dtqu

LCOMPDL 70 5 0 to 100 %

QCOMPDL 30 55 0 to 100 %

REGINTDL 5 1 1 to 10 SACCHperiods

SSLENDL 5 3 3 to 15 SACCHperiods

QLENDL 8 3 1 to 20 SACCHperiods

SDCCHREG OFF ON ON, OFF

BSPWRMIN -20 -20 -20 to +50 dBm

BSTXPWR (2) 0 to 80 dBm

UPDWNRATIO 200 300 100 to 700 %

STEPLIMDL OFF OFF ON, OFF

(1) SSDESDL and SSDESDLAFR takes the corresponding positive value inMML commands and CNA.

(2) The value of this parameter is highly dependent on the cell planning. Nodefault value is provided.

2778/1553-HSC 103 12/4 Uen D 2005-02-14


(3) These recommended values are based on assumptions/simulations andhave not been live tested.

7 References

1 User Description, Discontinuous Transmission

2 User Description, Frequency Hopping

3 User Description, Locating

4 User Description, Intra Cell Handover

5 User Description, Dynamic MS Power Control

6 User Description, Channel Administration

7 User Description, High Speed Circuit Switched Data (HSCSD)

8 User Description, Adaptive Multi Rate

8 Appendix A

Filter coefficients

Table 4 Coefficients for the exponential filters used.

Filter length L Filter coefficient a

1 0.1000

2 0.3162

3 0.4642

4 0.5623

5 0.6310

6 0.6813

7 0.7197

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Table 4 Coefficients for the exponential filters used.

Filter length L Filter coefficient a

8 0.7499

9 0.7743

10 0.7943

11 0.8111

12 0.8254

13 0.8377

14 0.8483

15 0.8577

16 0.8660

17 0.8733

18 0.8799

19 0.8859

20 0.8913

21 0.8962

22 0.9006

23 0.9047

24 0.9085

25 0.9120

26 0.9152

27 0.9183

28 0.9211

29 0.9237

30 0.9261

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