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Power drifting needs to be taken into account in the downlink power control mechanism during a soft handov

Figure 25: Power drifting

The basic operation of the DL fast closed loop power control is as follows:

The UE measures the received SIR of the downlink dedicated physical channel every transmitter

power control cycle. Each transmitter power control cycle takes 0.667 ms (1500 Hz) which is sametime as one slot period. The measured SIR value is compared to a SIR target value in each slot time

When the measured SIR value is higher than the SIR target value, the transmitter power control

command (TPC) is set to "0" and when the measured SIR value is lower than the SIR target value, t

TPC command is set to "1".

The UE inserts the value of the TPC command to the next slot of the uplink DPCCH.

The WCDMA BTS either decreases or increases the transmission power of the dedicated physical channel

based on the received TPC value. The adjustment is done for every slot, that is each 0.667 ms, 1500 Hz.

In the event of a soft handover, the UE sends the same power control command value to all BTSs involved i

the handover and each BTS detects the value on its own. Because of detection errors, the power control

commands might be decoded incorrectly some of the base stations and the power level is increased instead

decreased or vice versa. As a result, the DL transmission power of radio links at different base stations start

to drift apart and the power values received at the UE are unbalanced.

The power balancing algorithm controlled by the RNC works together with the DL fast closed loop power

control in the BTS as long as the soft handover situation lasts. Power balancing is located in the handover

control functional unit and the measurement messages for power balancing are terminated in the handover

control. The figure Functional split of the power balancing functionalityshows the functional split of the powe

balancing function.

Figure 26: Functional split of the power balancing functionality

The figure Ideal power control without power balancingshows the ideal power control situation where power

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balancing is not needed as no DL fast closed loop power control commands are misinterpreted. The cell pow

is in balance. This ideal situation is not possible in real radio networks.

Figure 27: Ideal power control without power balancing

The figure Real situation with misinterpreted PC commandsshows how the cell power becomes unbalanced

because of misinterpreted DL fast closed loop power control commands in real radio networks. If too many

misinterpreted DL fast closed loop power control commands occur in a soft handover situation, the DL

transmission power of one radio link may rise up while the transmission power of another radio link goes dow

Figure 28: Real situation with misinterpreted PC commands

In figure Power balancing in work, power balancing is in use and misinterpreted DL fast closed loop power

control commands do not occur in the shown adjustment period. The cell power is close to the reference

power. The figure shows a situation where the UE is not moving and the DL transmitted powers are set almo

to the same level. It is assumed that the CPICH powers are equal.

Figure 29: Power balancing in work

The power balancing algorithm works independently of the inner and outer loop power control mechanisms.

Each adjustment period, power balancing tries to correct the DL transmission powers to the level of the

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reference power that was defined in the SRNC before the current adjustment period.

The reference power is common to all base stations. Each time new measurement data is received from the

base stations, it is calculated in the SRNC as follows:

Select the highest of the averaged DL transmission powers.

Subtract the highest of the averaged DL transmission powers by the internal PrefSubtractvalue.

This system is not aware of any individual misinterpreted DL fast closed loop power control command but it

assumes that these commands are misinterpreted. The balancing algorithm causes under and over

estimations of the current transmitted power but the DL fast closed loop power control adjusts those

mistakes immediately. The result is a destabilized power control and the attempt to drive DL transmission

powers of radio links on different base stations to the same level.

If a radio link is in compressed mode, UL fast closed loop power control commands can get lost. When

compressed mode ends, such situations are corrected immediately by fast closed loop power control and

there is no need for power balancing to correct this error.

Activation of power balancingDeactivation of power balancingThe DL power control requestUsage of the power balancing adjustment Type in the BTS and the DRNCUpdating the reference transmission power during the soft handoverSending the new reference transmission power to the BTSPower balancing algorithm in the BTSReliability check for DL TPC commands during soft handover

DN03471612 Id: 0900d80580749ba1 2010 Nokia Siemens Networks

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