power control

Power Control

Basics

BSNL 3G Validation

Commercial in confidence 2008-11-272

Introduction

� Power Control is essential for the smooth operation of a WCDMA system, because all users share the same radio frequency band through the use of different codes.

� Uplink Power Control is used to alleviate the channel fading effect, to adjust the transmit power of the UE so that the signals received at the Radio Base Station (RBS) from all UEs have equal Signal to- Interference Ratio (SIR) at the same bit rate (this is to counteract the near-far problem), and to reduce the co-channel interference caused by simultaneous users.

� Downlink Power Control is used to minimize the transmitted powerof the RBS and to compensate for channel fading in the downlink.

� Its objective is to maximize capacity by minimizing power and interference.

� Three types of power control algorithms are employed in WCDMA:– Open Loop Power Control

– Inner Loop Power Control

– Outer Loop Power Control


WCDMA Power Control loops


Power Control – Basic Types

Power control on

COMMON CHANNELS

ensures there is sufficient

coverage to establish

connections and transfer

date on common

transport channels

Power control on

DEDICATED CHANNELS

(DCH) ensures sufficient

connection quality while

minimizing impact on other

connections.

Cell set-up and cell re-configuration

Common transport channel setup and re-configuration

Radio Link Setup

Soft Handover (SOHO)

Power Balancing

Compressed Mode

Inter-Frequency Handover


Common Channel – Power setting example


Power Control on common channels-Uplink

This open loop power control function ensures that the random access does not cause too much interference

1. Initial preamble from UE uses the UL power calculated as:

P_PRACH = L_PCPICH + RTWP + constantValueCprachwhere:

L_PCPICH is PCPICH DL path loss

RTWP is UL interference measured by the RBS

2. If the RBS does not detect the preamble, the UE sends a new preamble with an UL power that is

powerOffsetP0 higher than the previous preamble.

3. When the RBS detects a preamble, it sends acknowledgement indication (AI) to the UE, which then sends

the RACH control part of the message with an UL power that determined by the last preamble and an offset powerOffsetPpm.

1 2 2 3

Parameter preambleRetransMax decides how many times a preamble shall be re-transmitted with increased UL

power within one cycle. When the maximum is reached, a new preamble cycle is started. Parameter maxPreambleCycle

determines how many preamble cycles shall be allowed before the access is aborted.


Open Loop Parameters-example

� After the first preamble, if no acknowledgement is received for the successive preambles, a preamble power ramping is adopted with step equal to the parameter ‘powerOffsetPO’ which may be set between 1 and 8 dB in steps of 1 dB.

� Since the default value is 3 dB each preamble will be ramped up by 3 dB from the previous until an Acquisition Indication Channel (AICH) is received or the maximum number of attempts has been made.

� The maximum number of attempts is set with the parameter ‘preambleRetransMax’, which can be set from 1 to 64 with a default of 8.

� If the maximum number of attempts has been exceeded another preamble cycle may be started and a new value for P_PRACH calculated.

� The maximum number of preamble cycles is set with the parameter ‘maxPreambleCycle’ which can be set from 1 to 32 with a default of 4.

� If a positive AICH is received the UE will transmit the RACH message control part on the Q branch of the modulator at a power above the last preamble by a value set by the parameter ‘powerOffsetPpm’.

� This parameter can be set –5 and 10 dB in steps of 1 dB with a default value of –4dB.


Inner Loop Power Control

� The aim of uplink Inner Loop Power Control is to

maintain all connections at a sufficiently good SIR value.

� The use of uplink Inner Loop Power Control by all UEs

reduces the total amount of radiated power in the

network.

� This means that uplink interference in the network is reduced and UE battery life increased.

� In the downlink Inner Loop Power Control is used to

minimize the transmission power of the RBS and hence

increase capacity.


Power Control on DCH – Inner Loop

BLER = Block Error Rate

SIR = Signal to Interference Ratio

TPC = Transmit Power Control

P(Startvalue)

Open loop

P(SIR-Target,UL)

P(SIR-Target, DL)

Inner loop

DL-TPCUL-TPC

SIR-Target,DL

BLER-Measured,DL

DL-Outer loop

RNC

SIR-Target,UL

SIR-Error,UL

UL-Outer loop

Initial Power Setting in UL and DL uses Open

Loop. It ensures reliable connection setup, minimal impact on existing connections (UL)

and avoids excessive power (DL).

The RNC and UE uses Outer

Loop power control to calculate UL and DL quality targets to

which the UE and RNC shall

adjust its transmitted power.

The RBS and UE uses Inner

Loop to send UL and DL TPC’s(transmit power commands).

The TPC’s are determined by

the outer loop power control.


Uplink Inner Loop Power Control on DCH- Summary

• If estimated UL SIR >= target UL SIR, then RBS send power DOWN command

• If estimated UL SIR < target UL SIR, then RBS send power UP command

• UE always power UP/DOWN in steps of 1 dB

• In SOHO:

• if all radio links in active set send power UP command, the UE powers UP by 1 dB

• If at least one radio link in the active set sends power DOWN command, the UE powers DOWN by 1 dB.

• The TPC command is sent to the uplink Power Control algorithm in the UE, and the uplink DPCCH/DPDCH power is increased o decreased based on the TPC command which will be ‘+1’ for ‘up and ‘-1’ for ‘down


Outer Loop Power Control

•The Serving Radio Network Controller (SRNC) will constantly monitor the CRCs of the uplink Transport Blocks (TB) and estimate the Block Error Rate (BLER).

•If it discovers that this BLER is either higher or lower than what is required for the RAB, it will change the SIR target for Inner Loop Power Control. •In doing so the uplink BLER for the service is maintained, regardless of UE environment and mobility, as shown in Figure a above.•The UE must perform the same function on the downlink TBs and change the target for

the Downlink Inner Loop Power Control. This will maintain the downlink BLER for the service regardless of the UE environment and mobility, as shown in Figure b above.

Fig a Fig b


Downlink Initial Power Setting on DCH

This open loop power control function

calculates the initial DL power of the physical channel that carries user data (DPDCH) and the physical channel that carries information needed to keep the connection running. Initial DL power at DCH

establishment

P_DL_DPDCH = primaryCpichPower + (dlInitSirTarget – Ec/No_PCPICH) + cBackOff + 10 log (2/SF_DL_DPDCH)

where:

• primaryCpichPower is DL power for PCPICH in the cell where the connection is established

• Ec/No is the measured DL Ec/No reported by the UEWhen the EcNo is not available (e.g. at IRAT HO), the default Ec/No is set to ecNoPcpichDefault

• dlInitSirTarget is the signal-to-interference target for the DL DPDCH

• SF_DL_DPDCH is the spreading factor for the DL connection

• cBackOff is used to provide a more conservative initial output power or to increase the call setup reliability

• The initial power of downlink DPCCH is related to the initial power of DPDCH, by means of a series of offsets according to the following equations:

• P_DL_DPCCH_TFCI = (P_DL_DPDCH + pO1)

• P_DL_DPCCH_TPC = (P_DL_DPDCH + pO2)

• P_DL_DPCCH_PILOT = (P_DL_DPDCH + pO3)


INITIAL DOWNLINK DPDCH/DPCCH POWER� The initial downlink power calculation is done in the RNC.

� The result is transferred to the RBS.

� The initial downlink DPDCH power setting algorithm uses the Spreading Factor, measured Ec/No on PCPICH, transmitted power on PCPICH, and the service requirements as input to the calculation for downlink power for DPDCH, as in the equation below.

� P_DL_DPDCH = primaryCpichPower – Ec/No_PCPICH + dlInitSirTarget + 10 log (2/SFDL_DPDCH) + CBackOff dBm

� Where:

� primaryCpichPower is the downlink output power used for the PCPICH.

� Ec/No_PCPICH is a UE measurement that is received in the RRC CONNECTION REQUEST message.

� This is optional so if the measurement is not available, the ‘ecNoPcpichDefault’ parametervalue is used.

� dlInitSirTarget is the required initial SIRtarget for the downlin kDPDCH.

� It is a configurable parameter. SFDL_DPDCH is the Spreading Factor for the downlink DPDCH.

� cBackOff is a parameter configurable by the operator.

� The purpose is to back off the Open Loop Power Control estimate to a conservative starting point.

� To calculate this power RNC must get the ‘Ec/No_PCPICH’ from the ‘RRC CONNECTION REQUEST’message to complete the formula or use the default value set by the parameter ‘ecNoPcpichDefault’ as shown in the figure below

Example Initial DPDCH parameters


SETTING THE INITIAL DOWNLINK DPCCH POWER

•For the downlink DPCCH, the initial power is related to that of the downlink DPDCH, by

means of a series of offsets according to the equations in Figure above.•Using the default values the power is doubled (+3dB) when the Transmit Power Control (TPC) bits are being transmitted.


UPLINK SIGNAL TO INTERFERENCE RATIO (SIR)

3GPP SIR Definition

Note

ISCP is normally approximated to the Received Total

Wideband Power (RTWP).


Power Balancing

� When using Downlink Outer or Inner Loop Power Control during Soft Handover, the downlink power of the radio links involved iscoordinated by starting the radio link at the correct power level and then by receiving the unique TPC command sent by the UE.

� However, the TPC commands received at different RBSs are affected by different errors, so the downlink output power of the different radio links starts to drift in an uncoordinated mannerbetween the RBSs.

� The extent of downlink power drift can be reduced through the Downlink Power Balancing procedure.

� The operator can activate this algorithm by setting the parameter ‘dlPcMethod’ to ‘3’ (Balancing).

� This algorithm is used only when a connection is in Soft Handover.

� If ‘dlPcMethod’ was set to ‘2’ (No Balancing) only downlink inner loop power control is activated.

� If ‘dlPcMethod’ is set to “1” (Fixed) the Dl Power balancing is turned off and also the inner loop power control and if ‘dlPcMethod’ is set to 4 (fixed balancing) the DL Power balancing is switched on but a fixed value is used as a Dl reference Power.


DL Power Balancing

Prevents different radio links involved in

a connection from transmitting different DL power.

dlPcMethod decides Power Balancing method:

•FIXED•NO BALANCING•BALANCING•FIXED BALANCING


DL Power Control in Compressed Mode

SF/2

+Increased DL power

Increased DL SIR target

� Compressed Mode is defined as the mechanism whereby certain idle periods are

created in radio frames so that the UE can perform measurement during these periods

� UTRAN supports two different type of transmission time reduction methods to obtain the UL and DL compressed mode gap:

– Higher layer scheduling (HLS)

– Lower spreading factor (SF/2)

� HLS used for Packet RABs and SF/2 method used for Circuit RABs


Initial Power at Inter-Frequency Handover

Establishes a reliable radio link on target

frequency while minimizing the impact on existing connections on that frequency

P_DL_DPDCH

f1 f2

P_DL_DPDCH = primaryCpichPower + (dlInitSirTarget – Ec/No_PCPICH) + cNbifho + 10 log (2/SF_DL_DPDCH)

where:

• Ec/No_PCPICH is measured Ec/No on target frequency received in the UE. If no measurement value is available, parameter ecNoPcpichDefault is used

• cNbifho is used to set the initial power at a more conservative level to take into account the inter-frequency HO margin.


Downlink Initial Power Setting at SOHO

P_DL_DPDCH = primaryCpichPower + (dlInitSirTarget – Ec/No_PCPICH) + cSho + 10 log (2/SF_DL_DPDCH)

where:

• cSho is a function of the Soft HO margin + initShoPowerParam

+Compensates for difference in Ec/No due to handover margin at SOHO, ideally

allowing all carriers in the active set to transmit on the same DL power.


Handover – Soft handover

� Soft/Softer Handover provides the UE with the ability to add, remove, and replace radio links with the same frequency.

� In Soft Handover the UE is connected to more than one Radio Base Station (RBS) simultaneously.

� At least one radio link is always active and there is no interruption in the dataflow during the actual handover.

� The signals are received in the UE and combined in the RAKE receiver to give protection against fading.

� In Softer Handover the UE communicates with one RBS through

� several radio links, the Softer Handover is a handover between two or more cells of the same RBS.

� WCDMA systems must use soft or softer handover to reduce interference caused by near-far problems resulting from UEs at cell borders


� Fig 2a shows the effect of not using soft handover in a WCDMA system.

� As the UE moves away from RBS 1 towards RBS 2 the signal received at RBS 2 may exceed its received power target and cause excessive UL interference in that cell.

� Since the UE is not connected to RBS 2, the base station has no way of reducing the transmit power of the UE.

� This excessive UL interference at RBS 2 could ultimately lead to dropped connections in RBS 2.

� Once the connection undergoes a hard handover to RBS 2, power control messages from RBS 2 can be used to reduce the UE transmit power and therefore reduce the interference.

� Soft and softer handovers allow the UE to be power controlled by

� both base stations, which eliminates this excessive interference (see Fig 2b)


Power Control – Soft handover

Fig 2a Received power without soft handover

Fig 2b-received power with soft handover.


� While connected to RBS1 only the UE acts on power control commands from that base station alone, which maintains the receive power target for that cell.

� As the UE moves closer to RBS2 there will come a point when the threshold ‘t_add’is exceeded and RBS2 is added to the active list.

� From this point on, the call is said to be in soft handover. � The UE is now responding to power control messages from both base stations. � However, it initially ignores the power increase commands from RBS 1, but

responds to the power decrease commands from RBS 2.� In fact, the UE will only increase its power when requested to do so by BOTH base

stations and will reduce its power when requested by EITHER base station.

� In the example, the “control” of the output power of the UE is effectively changing back-and-forth between the two base stations.

� UEs in soft handover will cause less interference in the system and the more cells involved in the handover the lower the interference.

� This is why soft handover is said to improve capacity since lower UL interference results in an increased UL air interface capacity.

� The effect on the downlink capacity is not as clear-cut because although there is some macro diversity gain (meaning that the UE on average will ask for less power then compared to a case where it is only connected to one base station), there are still two downlinks that have to be transmitted on.


Channel Switching - Example

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