05 power control mo v3.1

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V3 © Nokia Networks Oy 1 (22)

Power Control

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Power Control

Contents

5 Power Control.............................................................. 45.1 Module Objectives......................................................... 45.2 Introduction.................................................................... 45.2.1 Purpose.......................................................................... 45.2.2 MS Power Classes......................................................... 55.2.3 General Parameters....................................................... 65.2.4 General Strategy............................................................ 95.3 Power Change due to Signal Level ..............................125.3.1 Power Increase ............................................................125.3.2 Power Decrease ..........................................................135.4 Power Change due to Signal Quality ...........................155.4.1 Power Increase ............................................................155.4.2 Power Decrease ..........................................................165.5 Power Optimisation ......................................................185.5.1 For Power Control ........................................................185.5.2 For TCH Allocation .......................................................215.5.3 For Handover ...............................................................22

© Nokia Networks Oy 3 (22)

Power Control

5 Power Control

1.2 Module Objectives

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

Explain the motivation for power control

Indicate the BSS parameters required for power control in general (output power levels, fixed step sizes)

Describe the principle steps to be executed for power control (averaging, triggering, power change step size estimation)

Discuss the algorithms used to estimate the power change step size, when the power has to be increased / decreased due to signal level / quality

Explain the motivation for power optimisation

Indicate the BSS parameters required for power optimisation additionally

Discuss, how power control and traffic channel allocation are modified by power optimisation

1.3 Introduction

1.3.1 Purpose

In the dedicated mode, both the BTS and the MS transmit with a power as low as possible for the following reasons (see also Fig. 5-1):

On the uplink, the power consumption of each MS is decreased and thus a longer service time for the rechargeable battery is achieved.

Both on the uplink and downlink interference is reduced, so that the network capacity and the spectral efficiency are improved.

To obtain an optimised low power level, special mechanisms are applied, which are discussed in the following sections. This power control is carried out independently for uplink and downlink, and also independently for each call.


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The operator can choose, whether power control shall be applied on the uplink, the downlink, on both or not at all. Nevertheless power control must be supported by any MS according the GSM specification.

1.3.2 MS Power Classes

The maximum output power with which a MS may transmit in a cell, depends on its power class, and also on the frequency band. Altogether five power classes have been defined. Class 1 offers the highest maximum output power, class 5 the lowest one. In GSM 900, a class 1 MS may transmit with a power of up to 39 dBm, in GSM 1800 with up to 36 dBm (33 dBm, GSM 1900).

Power GSM 900 & GSM 850

DCS 1 800 PCS 1 900 Tolerance (dB)

class Nominal Maximum output

Nominal Maximum output

Nominal Maximum output

for conditions

power power power normal extreme

1 - - - - - - 1 W (30 dBm) 1 W (30 dBm) ±2 ±2,52 8 W (39 dBm) 0,25 W (24 dBm) 0,25 W (24 dBm) ±2 ±2,5

3 5 W (37 dBm) 4 W (36 dBm) 2 W (33 dBm) ±2 ±2,5

4 2 W (33 dBm) ±2 ±2,5

5 0,8 W (29 dBm) ±2 ±2,5

There is also a minimum output power, with which a MS must transmit in any case. In GSM 900, it is 13 dBm for a phase 1 MS, and 5 dBm for a phase 2 MS. In GSM 1800 and 1900, the minimum output power is 0 dBm.

The available power range is divided into several levels separated by 2 dB steps (exception is the …,30, 32, 33dBm for GSM1900).

The output power of the BTS has to follow certain limits as well. The available power range again is divided into 15 steps à 2 dB, giving a range of 30 dB.


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Fig. 5-1: Power control motivation

1.3.3 General Parameters

1.3.3.1 Power Control Activation

DL Power control is activated by using the parameter powerCtrlEnabled (PENA)(POC)(Y,N)(Y). UL PC may be enabled/disabled by setting msTxPwrMax and msTxPwrMin value.

1.3.3.2 Output Power Limits

To control the output power limits, the following parameters are used. A short summary of them is given by Fig. 5-2.

Parameter Remark

msTxPwrMaxGSM (PMAX1)(BTS)(5..39)(33)

Maximum MS output power in dBm for GSM 900/850on TCH

msTxPwrMaxGSM1x00 (PMAX2)(BTS)(0..36/0..32, 33)(30/30)

Maximum MS output power in dBm for GSM 1800/1900 on TCH


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msTxPwrMaxCCH (TXP1)(BTS)(5..39)(33)

Maximum MS output power in dBm for GSM 900/850 on CCH

msTxPwrMaxCCH1x00 (TXP2)(BTS)(0..36/0..32,33)(30/30)

Maximum MS output power in dBm for GSM 1800/1900 on CCH

minMSTxPower (PMIN)(BTS)(5..39/0..36/0..32)(5/0/0)

Minimum MS output power in dBm for GSM 900,850/1800/1900

bsTxPwrMax (PMAX1)(POC)(0..30)(0)

Maximum BTS output power as (minimum) attenuation of the peak power in dB for GSM 900/850

bsTxPwrMax1x00 (PMAX2)(POC)(0..30)(0)

Maximum BTS output power as (minimum) attenuation of the peak power in dB for GSM 1800/1900

bsTxPwrMin (PMIN)(POC)(0..30)(0)

Minimum BTS output power as (maximum) attenuation of the peak power in dB for any GSM band

bsTxPwrOffset (POFF)(POC)(0..30)(0)

Additional attenuation in d for super reuse TRx for any GSM band


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Fig. 5-2: Power control parameters (output power limits)

1.3.3.3 Power Change Step Sizes

When the MS or BTS shall change its power, this can be done by steps of fixed or variable size. The fixed step size is controlled by the following parameters. A short summary of them is given by Fig. 5-3.

Parameter Remark

powerIncrStepSize (INC)(POC)(2,4,6)(4)

Step size in dB to increase the output power of the MS

powerRedStepSize (RED)(POC)(2,4)(2)

Step size in dB to reduce the output power of the MS

Fig. 5-3: Power control parameters (power change step sizes)

1.3.3.4 Power Control Interval

Both MS and BTS must wait a minimum time between consecutive power changes. This interval is controlled by the parameter powerCtrlInterval (INT)(POC)(0..31)(2). The unit is s.


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1.3.4 General Strategy

1.3.4.1 Measurement Processing and Averaging

As just discussed in the previous chapter, both the MS and the BTS measure continuously the signal level and quality on the link. The MS measures the signal level RXLEV_DL and signal quality RXQUAL_DL of the downlink, while the BTS the signal level RXLEV_UL and signal quality RXQUAL_UL of the uplink. To get reliable results, which are not affected by short term fluctuations, values averaged over a certain number of SACCH periods are estimated. The principle idea is shown by Fig. 5-4.

Fig. 5-4: Power control strategy (measurement averaging)

1.3.4.2 Triggering

The averaged values are compared with the following threshold values:

Averaged Value Threshold Value

RXLEV_DL pcUpperThresholdLevelDL (UDR)(POC)(-110..-47)(-70)

pcLowerThresholdLevelDL (LDR)(POC)(-110..-47)(-85)

RXQUAL_DL pcUpperThresholdQualDL


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(UDR)(POC)(0..7)(0)

pcLowerThresholdQualDL (LDR)(POC)(0..7)(3)

RXLEV_UL pcUpperThresholdLevelUL (UUR)(POC)(-110..-47)(-70)

pcLowerThresholdLevelUL (LUR)(POC)(-110..-47)(-85)

RXQUAL_UL pcUpperThresholdQualUL (UUR)(POC)(0..7)(0)

pcLowerThresholdQualUL (LUR)(POC)(0..7)(3)

The unit of the thresholds related to signal level is dBm. Each measurement is compared with an upper and lower threshold. For this comparison not only the last averaged value, but the last Nx ones are taken into account. If at least Px of the Nx ones exceed a threshold, a change of power is required. The principle idea is shown by Fig. 5-5.

The ranges and default settings for Nx and Px are the following:

Threshold Nx Px

pcUpperThresholdLevelDL Nx(UDN)(POC) (1..32)(1)

Nx(UDP)(POC) (1..32)(1)

pcLowerThresholdLevelDL Nx(LDN)(POC) (1..32)(1)

Nx(LDP)(POC) (1..32)(1)

pcUpperThresholdQualDL Nx(UDN)(POC) (1..32)(32)

Nx(UDP)(POC) (1..32)(32)

pcLowerThresholdQualDL Nx(LDN)(POC) (1..32)(4)

Nx(LDP)(POC) (1..32)(3)

pcUpperThresholdLevelUL Nx(UUN)(POC) (1..32)(1)

Nx(UUP)(POC) (1..32)(1)

pcLowerThresholdLevelUL Nx(LUN)(POC) (1..32)(1)

Nx(LUP)(POC) (1..32)(1)

pcUpperThresholdQualUL Nx(UUN)(POC) (1..32)(32)

Nx(UUP)(POC) (1..32)(32)

pcLowerThresholdQualUL Nx(LUN)(POC) (1..32)(4)

Nx(LUP)(POC) (1..32)(3)


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Fig. 5-5: Power control strategy (triggering)

1.3.4.3 Calculation of Power Change Step Size

If a power change is triggered, the required step size has to be estimated. If the desired power can be achieved by 1 or 2 commands, the fixed step size may be used. Otherwise a variable step size is calculated. The algorithm to do this depends somewhat on the threshold triggering the power control command.

1.3.4.4 Possible Scenarios

The thresholds introduced just above correspond to the following scenarios shown by Fig. 5-6.


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Fig. 5-6:Power control strategy (scenarios)

1.4 Power Change due to Signal Level

1.4.1 Power Increase

If one of the thresholds pcLowerThresholdLevelDL and pcLowerThresholdLevelUL is exceeded, a power increase is required for the BTS and MS, respectively.

If the actual signal level RXLEV_DL/UL follows the condition

RXLEV_DL/UL > pcLowerThresholdLevelDL/UL - 2 powerIncrStepSize

the fixed step size can be used. Otherwise a variable step size PWR_INCR_STEP must be applied, which is calculated as follows:

PWR_INCR_STEP = pcLowerThresholdLevelDL/UL - RXLEV_DL/UL

Now the power change is done by one command. The whole procedure is summarized by Fig. 5-7.


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Fig. 5-7: Power increase due to signal level

1.4.2 Power Decrease

If one of the thresholds pcUpperThresholdLevelDL and pcUpperThresholdLevelUL is exceeded, a power decrease is required for the BTS and MS, respectively.

If the actual signal level RXLEV_DL/UL follows the condition

RXLEV_DL/UL < pcUpperThresholdLevelDL/UL + 2 powerDecrStepSize

the fixed step size can be used. Otherwise a variable step size PWR_DECR_STEP must be applied, which is calculated as follows. There are different algorithms for the BTS and MS.

1.4.2.1 Variable Step Size for BTS

For the BTS the following relationship must be used:

PWR_DECR_STEP = Min (RXLEV_DL - pcUpperThresholdLevelDL,10)

The reason for this is, that some kinds of MS do not support a BTS power decrease of more than 10 dB in one step.

The use of the variable step size for BTS power decrease must be activated by the control parameter variableDLStepUse (VDLS)(BSC)


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(Y,N)(N). Note, that the default setting is N, i.e. the variable step size is not activated. The whole power decrease procedure for the BTS is summarized by Fig. 5-8.

Fig. 5-8: Power decrease due to signal level (BTS)

1.4.2.2 Variable Step Size for MS

For the MS the relationship is completely analogous to the power increase:

PWR_DECR_STEP = RXLEV_UL - pcUpperThresholdLevelUL

The whole power decrease procedure for the MS is summarized by Fig.5-9.


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Fig. 5-9: Power decrease due to signal level (MS)

1.5 Power Change due to Signal Quality

1.5.1 Power Increase

If one of the thresholds pcLowerThresholdQualDL and pcLowerThresholdQualUL is exceeded, a power increase is required for the BTS and MS, respectively.

Here always a variable step size PWR_INCR_STEP is used. It is calculated by two ways, considering the main reasons for the low signal quality.

One reason may be interference. For this purpose PWR_INCR_STEP is calculated from the actual signal quality RXQUAL_DL/UL, the threshold and the fixed step size according the following relationship:

PWR_INCR_STEP = (1 + Max (0,QUAL)) * powerIncrStepSize

QUAL = RXQUAL_DL/UL - pcLowerThresholdQualDL/UL


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Another reason may be simply a too low power. Thus the variable step size given here is compared with that one arising from a power below the threshold pcLowerThresholdLevelDL/UL (see power increase due to signal level). For the power increase the larger of the two variable step sizes will be used, i.e. the dominating reason for the low signal quality will be taken into account. The whole procedure is summarized by Fig. 5-10.

Fig. 5-10: Power increase due to signal quality

1.5.2 Power Decrease

If one of the thresholds pcUpperThresholdQualDL and pcUpperThresholdQualUL is exceeded, a power increase is required for the BTS and MS, respectively.

If power optimisation is not used, the power decrease is carried out exactly by the same way as that one due to signal level - i.e. not only the signal quality, but also the signal level must be too high. Otherwise a more complicated algorithm is applied, which is explained in the following section.

Power decrease due to signal quality can lead to a ping-pong effect. If the signal level is already low, a further decrease can trigger a power increase due to signal level again. Therefore a power decrease due to


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signal quality will be done only, if the actual RXLEV_DL/UL is at least 6 dB above pcLowerThresholdLevelDL/UL. The whole procedure is summarized by Fig. 5-11, the ping pong effect is shown by Fig. 5-12. A summary about all power change algorithms is given by Fig. 5-13.

Fig. 5-11: Power decrease due to signal quality (no power optimisation)


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Fig. 5-12: Power decrease due to signal quality (ping pong effect)

Fig. 5-13: Power control summary

1.6 Power Optimisation

The parameters optimumRxLevDL (LEVD)(TRX)(-109..-47)() and optimumRxLevUL (LEV)(TRX)(-109..-47)() allow to define an optimum receive level (in dBm) for the downlink and uplink, respectively. This way a sufficient quality can be achieved for voice and data simultaneously. Furthermore, interference on the downlink and uplink can be avoided. If no values are set (the default setting), the parameters are deactivated

1.6.1 For Power Control

The use of power optimisation introduces further parameters. A short summary of them is given by Fig. 5-14.

Parameter Remark

pwrDecrLimitBand0 Maximum power decrease step size in dB, if


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(PD0)(POC)(0..38)(38) the signal quality is 0 (BER < 0.2%)

pwrDecrLimitBand1 (PD1)(POC)(0..38)(38)

Maximum power decrease step size in dB, if the signal quality is 1 (0.2 % BER < 0.4%)

pwrDecrLimitBand2 (PD2)(POC)(0..38)(38)

Maximum power decrease step size in dB, if the signal quality is 2 or worse (BER 0.4%)

powerDecrQualFactor (PDF)(POC)(0,1)(1)

If this parameter is activated (by setting it to 1, which is the default), the MS decreases power due to signal quality, even if the uplink receive level is below the optimum level

Fig. 5-14: Power control parameters (power optimisation)

1.6.1.1 Power Decrease of the MS due to Signal Quality

Power optimisation is applied to power control only, if a power decrease due to signal quality is required. In comparison to the standard algorithm, the calculation of the variable step size becomes quite complicated. The formula now has the following form:

PWR_DECR_STEP = Min (pwrDecrLimit, Max (A,B))

pwrDecrLimit is the maximum power decrease step size just defined above. The terms A and B consider the possible reasons of the too high signal quality, which are too high power or very low interference.

A = Max (0, RXLEV_UL - optimumRxLevUL)


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would bring the actual uplink receive level exactly to the optimum level.

B = (powerDecrQualFactor + Max (0,QUAL)) * powerDecrStepSize

with QUAL = pcUpperThreshold - <RXQUAL_UL>

considers the difference between the average uplink receive quality and the threshold quality, multiplied with the fixed power decrease step size.

The larger of the two terms is adopted as power decrease step. If it is larger than pwrDecrLimit, however, the predefined maximum step size is used.

1.6.1.2 Power Decrease of the BTS due to Signal Quality

As already mentioned, some kind of MS do not support a BTS power decrease of more than 10 dB. Therefore the formula just given above has to be modified according to

PWR_DECR_STEP = Min [Min (pwrDecrLimit, Max (A,B)),10]

A and B have the same meaning as just explained above, but all variables now are related to the downlink and not to the uplink. The calculation of the variable step size both for the MS and BTS is summarized by Fig. 5-15.

Fig. 5-15: Power decrease due to signal quality (with power optimisation)


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1.6.2 For TCH Allocation

Power optimisation modifies the estimation of the maximum acceptable interference level (see chapter Radio Resource Management, section TCH Allocation). For call set up and intra cell handover now the following relationship is used:

MAX_INTF_LEV = max (min (RXLEV_UL + msTxPrwMax - MS_TXPWR, optimumRxLevUL), RXLEV_UL + msTxPrwMin - MS_TXPWR) - cnThreshold

The variables have the same meaning as without power optimisation. For inter cell handover the modified expression is:

MAX_INTF_LEV = max (min (AV_RXLEV_NCELL (n) - rxLevBalance, optimumRxLevUL (n)), AV_RXLEV_NCELL (n) - rxLevBalance + msTxPrwMin (n) - msTxPrwMax (n)) - cnThreshold (n)

AV_RXLEV_NCELL (n) is the averaged uplink receive level in adjacent cell n. The meaning of the other variables is the same as without power optimisation (see Fig. 5-16).

Fig. 5-16: TCH allocation (with power optimisation)


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1.6.3 For Handover

Power optimisation does not have only an effect on power control and traffic channel allocation, but also on intra cell and internal inter cell handover. This will be shown in the chapter ‘Handover Control and Adjacencies’.


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