UMTS-Power-Control.ppt

POWER CONTROL in UMTS

AgendaWhat is Power Control?Importance of Power Control in UMTS RAN.Power Control on Common ChannelsPower Control on Dedicated ChannelsDownlink Power ControlUplink Power ControlPower Control during Soft HandoverPower Control during Compressed ModePower Control during Inter Frequency HandoverPC ScenariosPower Control on HSDPA and EUL

Few Basics.COVERAGE

CAPACITY

QUALITY

POWER

Understanding Power Control LOWER Power Per User HIGHER Number of Users

HIGHER Power Per User LOWER Number of Users

Interference No or Improper Power Control leads to High interference that impacts Coverage, Capacity and Quality

Power Ctrl ON OFF

What is Power Control?Any Spread Spectrum Technology like WCDMA is limited by Interference. Internal Interference is the biggest challenge that need to be controlled.Interference impacts the three important aspects of WCDMA Networks, the Coverage, Capacity and QualityPower Control is a process that minimizes the internal interference as low as possible, there by providing best possible capacity and quality.It controls the downlink and uplink transmit power and makes sure that only the required amount of power being transmitted.Power Control works on a per connection basis.

Importance of Power ControlMinimizes the Interference and there by enhances capacity and quality.It helps allowing as many users as possible while keeping the interference as minimum as possibleIt maintains the quality of all radio connections by controlling the transmit power in both the links.Power Control aims at using the minimum required SIR for the quality of connection to remain sufficient. No excessive quality.Power Control on common channels ensures that their coverage is sufficient for call setupIt provides protection against slow fading and fast fading.Efficient power control avoids the near-far problem.Power control works efficiently during transmission gap in compressed mode by bring the SIR back close to the target SIR.It helps reducing the battery consumption

Power Control on Common ChannelsCall Setup and Cell Reconfiguration Downlink

PCPICHAlways transmitted in a cellIts power is set to an absolute value thru parameter primaryCpichPower . Every other channels power is expressed as an offset relative to this parameter

PCCPCHIt carries the BCH and multiplexed with SCHbchpower is the parameter that determines it power and is expressed as an offset relative to PCPICH Power.

SCHSCH consists of Primary SCH and Secondary SCH that are used in the cell search process.Paramters primarySchPower and secondarySchPower determine the power. These are set as an offset relative to PCPICH Power

All the channels above are having fixed power set thru configurable parameters

Power Control on Common ChannelsCommon Transport Channel Setup Downlink

AICHThis Channel carries the AI Response to the PRACH Preambles.Its power, aichPower is expressed as an offset relative to PCPICH.

SCCPCH Carrying FACHmaxFach1Power determines power if the FACH Carries a logical control channel like BCCH, CCCH or DCCHmaxFach2Power determines the power the FACH Carries a logical traffic channel (DTCH)SCCPCH Carrying PCHpchPower is the parameter that sets power for this channel.

PICHThis channel carries the paging indicators.pichPower is the parameter that sets power for this channel.

All the channels above are having fixed power that are configurable through parameters.

Power Control on Common Channels

Power Control on Common ChannelsCommon Transport Channel Setup Uplink - PRACHPRACHs power is dynamic depends on the RF Conditions of the UEInitial power on PRACH is determined as per the below equationP_PRACH = L_PCPICH + RTWP + constantValueCprachWhere, L_PCPICH Path loss estimated by UE RTWP Received Total Wideband Power (uplink interference) as measured by RBSconstantValueCprach a configurable parameter used by UE to calculate the initial power on PRACH.RTWP and constantValueCprach are provided through System Information broadcasted in the cell to UE

Power Control on Common ChannelsPower Ramping on PRACH

powerOffsetP0powerOffsetP0powerOffsetPpm

PRACH

AICH

Preamble Sequence

1st transmitted preamble

Increase power until heard

Once preamble is heard, increase power for message

Message (Control Part)

Preamble heard and Acquisition Indicator sent

UL

DL

Power Control on Common ChannelsRecommended Values for the parameters related to PRACH

Power Control on Dedicated ChannelsDownlink Power ControlUplink Power Control

UMTS Power Control

Open Loop Power Control

Inner Loop

Closed Loop Power Control

Outer Loop

Power control during initial network access

Fast power control for compensation of signal fluctuations

Slow power control for ensuring the desired signal power level

Downlink Power ControlDL Outer Loop PC / Target SIR EstimationDL Inner Loop Power Control+/-RXPwr = Pwr +/- 0.5 dBBLERUERBSRNCPower Controlled DPCCH/DPDCHTPC CommandsDL SIRTARGET SIRInitial DPCCH/DPDCH Power EstimationDL OPEN LOOP PC

Downlink Power Control Open LoopOpen loop power control enables a reliable radio connection setup and minimal impact on existing connections.

Downlink Power Control Open LoopRNC Calculates the initial DPDCH power that is transmitted using the below equation and transfers the result to RBS.

PL_DL_DPDCH = primaryCpichPower + (dlInitSirTarget Ec/No_PCPICH) + cBackOff + 10log(2/SF_DL_DPDCH)Where,primaryCpichPower downlink output power used for PCPICH in the cell where connection is setup Ec/No_PCPICH measured Ec/No on PCPICH at UE dlInitSirTarget required initial SIR target for downlink DPDCH SF_DL_DPDCH Spreading factor c BackOff a correction factor for the initial power. Higher this value the better call setup reliability.

Downlink Power Control Open LoopWhen a measured Ec/No is not available, the system defines a default value, configurable through ecNoPcpichDefaultInitial power of downlink DPCCH is related to the initial power of DPDCH by certain offsets as belowP_DL_DPCCH_TFCI = (P_DLDPDCH + pO1)P_DL_DPCCH_TPC = (P_DLDPDCH + pO2)P_DL_DPCCH_PILOT = (P_DLDPDCH + pO3)pO1, pO2 and pO3 are power offsets to the respective fields. pO2 is usually higher than other two. Reason being the TPC Commands are to be decoded before combining when UE receives multiple TPC from different RBS.

Downlink Power Control Open LoopRecommended values for parameters related to Downlink Open loop power control

Downlink Power Control Inner LoopInner Loop is the fastest power control in UMTS that works at a speed of 1500 Hz (One TPC per slot in DL DPCH. There are 15 slots in a 10 ms frame that accounts to 1500 TPC commands per secUE measures SIR of DPCCH/DPDCH and compares with its target SIR. Based on the difference UE sends TPC commands, in uplink DPCCH, either to increase or decrease the downlink tx power on DPCCH/DPDCHRBS monitors the TPC commands received on uplink DPCCH and regulates the DPCCH/DPDCH tx power accordingly. The power step Delta TPC is +/- 0.5 dB (1.0dB if the link is established over lur, thro DRNC) The power regulation is constrained within the downlink Power Limits set by Capacity Management (calculated by RNC)

Downlink Power Control Inner LoopMeasured SIR >= Target SIR UE Sends TPC to reduce powerMeasured SIR < Target SIR UE Sends TPC to increase power

Downlink Power Control Outer LoopDownlink Outer loop power control is taken care by the UE using its own proprietary algorithmRNC sets a quality target in terms of BLER (Block Error Rate), which is determined through the configurable parameter blerQualityTargetDl This BLER quality target is signaled by RNC to UEUEs proprietary algorithm calculates the target SIR using the BLER sent by RNC.This target SIR is used in the UEs Downlink Inner Loop Power Control

Downlink Power Control Power LimitsMaximum downlink tx power depends upon the radio connection type, specially the data rate. RNC calculates the max power limit using the following configurable parameters and data rate table shown.minPwrMax & minimumRateinterPwrMax & interRatemaxPwrMax & maxRate

Radio Connection TypeMaximum Radio Link RatePS384/HS3700SRB14800AMR 12.215900CS6467700PS64/6470900MultiRAB (CS64 + PS8/8)76100PS64/384406900

Downlink Power Control Power LimitsMinimum Downlink Tx power is set using the configurable parameter minPwrRl, which is relative to the CPICH Power

maxPwrMax

interPwrMax

minPwrMax

MinRate

InterRate

Max Rate

Max DL TX Power (dB)

Maximum RL Rate (bps)

CPICH + 3.8dB

CPICH + 4.8dB

CPICH + 0dB

15900(AMR12.2)

77600

406900(PS384)

The max power for services with RL rate 15.9kbps or less is 0dB stronger than CPICH Tx Pwr

The max power for services with RL rate 406.9kbps or greater is 4.8dB stronger than CPICH Tx Pwr

Uplink Power Control

Uplink Power Control Open LoopPower_ULDPCCH_INIT = DPCCH_POWER_OFFSET RSCP_PCPICH

Uplink Power Control Open LoopRNC uses the equation below to calculate the DPCCH_POWER_OFFSETDPCCH_POWER_OFFSET = primaryCpichPower + RTWP + ulInitSirTarget 10log(SF_DPCCH) + cPOWhere:primaryCpichPower CPICH transmit power RTWP Received Total Wideband Power (uplink interference) measured by RBS ulInitSirTarget Initial value for Uplink SIR Target defined as per the SF of DPDCH SF_DPCCH Spreading Factor for DPCCH cPO a correction parameter used to set the uplink DPCCH Power offset to a conservative level to avoid excessive UL InterferenceInitial Power in DPDCH is determined using a predefined relative power offset (gain factor) between DPCCH and DPDCH. This factor is described in 3GPP standards. UTRAN informs this gain factor to UE.

Uplink Power Control Inner LoopUplink Inner loop power control works at the same speed as that of the down link inner loop power control, ie) 1500 Hz.RBS measures the SIR from the uplink DPDCH / DPCCH and compares the measured SIR value with the target SIR.Based on the difference in SIR, RBS sends TPC commands to UE, in downlink DPCCH, either to increase or decrease the UEs DPDCH / DPCCH tx powerUE monitors the TPC commands and increases or decreases the tx power in steps +/- 1dB

Uplink Power Control Inner LoopMeasured SIR >= Target SIR UE Sends TPC to reduce powerMeasured SIR < Target SIR UE Sends TPC to increase power

Uplink Power Control Outer LoopOuter Loop power control in downlink is to adjust the SIR target that the UE will attempt to maintain.By adjusting target SIR, consequently the UL tx power levels, the outer loop power control aims at providing the just required quality.RBS sends the CRC of DPCCH to RNC.RNC uses the CRC to estimate the quality of DPCCH and decides if the target SIR is to be increased or decreased.And then the new target SIR is passed on to the RBS, to be used in the inner loop power control.The fastest possible outer loop pc is 50 times per sec. This speed is possible in AMR Voice as it sends CRCs every 20ms.

Uplink Power Control Outer LoopThere are two methods available in determining the target SIRConstant Step RegulatorJump RegulatorConfigurable parameter ulOuterLoopRegulator determines which method to use

Uplink Power Control Outer LoopConstant Step Regulator

NBR_OF_CRC_OK depends on the BLER target defined through blerQualityTargetUlulSirStep is a configurable parameter that determines how much increase to be done in target SIR when a erroneous CRC is received.

CRC

SIR Target

NBR_OF_CRC_OK

ulSirStep

Uplink Power Control Outer LoopJump Regulator

UP_DOWN_STEP_RATIO depends on the BLER Target. While reducing SIR target, following formula is usedSIRtargetnew = SIRtarget + ulSirStep [-X/(Z*UP_DOWN_STEP_RATIO)+Y/Z]Z the total number of received transport blocksX number of transport blocks with CRC = OKY number of transport blocks with CRC = NG

CRC

SIR Target

ulSirStep

UP_DOWN_STEP_RATIO

ulSirStep

Power Control during SHO - DownlinkInitial Power Setting at SHO

Aim of power control at SHO is to equalize the power from different RBS, while compensating for differences in PCPICH power so that every signal received at the cell border is at the same strength.Biggest challenge in equalizing power is setting the initial power for a new link being added into active set. Following formula is used to set the initial powerP_DL_DPDCH = primaryCpichPower + (dlInitSirTarget Ec/No_PCPICH) + cSho + 10log(2/SF_DL_DPDCH)primaryCpichPower Downlink PCPICH tx power of the cell to be added Ec/No_PCPICH measured Ec/No on PCPICH of the cell to be added. If measured value is not available, then ecNoPcpichDefault is used dlInitSirTarget required Initial SIR Trget for downlink DPDCH cSho Correction factor that takes into account the handover margin (mSho) and configurable parameter initShoPowerParam

Power Control during SHO - DownlinkTypically, when a new link is added into active set the PCPICH Ec/No of the cell to be added will be lower than the Best cells PCPICH Ec/No already in the active set, by a Handover margin.Hence the new cell will have to transit a bit high initial power in order to make the UE receives the same strength from all its cells in the active set.Once the new cell is added into active set, then the new cell starts monitoring the TPC commands of the UE and start adjusting the tx power using inner and outer loop power controls.In case of a softer handover, the downlink Inner Power Control of the existing branch is directly applied to the new branch.During SHO, all cells in the active set receive the same TPC and acts accordingly. But any one cell receives TPC erroneously that could impact the transmit power of that cell which will lead to power drift that eventually lead to uncoordinated links.There is a mechanism called Power Balancing which prevents this Power Drift issue.

Power Balancing (Power Drift Prevention)

Power Balancing with DL Inner Loop PCPower Balancing can function in parallel with the Downlink inner loop power control. The power correction suggested by Power balancing will be added before adjusting the down link tx power by Downlink power control.There are 4 methods in Power Balancing configured thro parameter dlPcMethod.dlPcMethod = FIXED. Here Both Power Balancing and Downlink Ineer Power Control are disabled. Downlink power is kept a constant configurable level fixedPowerDldlPcMethod = NO BALANCING. Downlink Inner loop is active but Power balancing is always off.dlPcMethod = BALANCING. Downlink Inner loop PC is always active. Power Balancing runs in parallel when there is more than one radio link in the active set.dlPcMethod = FIXED BALANCING. DL Inner loop PC is active as long as there is only one radio link involved. Moment an additional radio link is added into active set, DL Inner Loop PC is disabled and Power Balancing is activated. In this case, downlink power converges to a configurable reference power level, fixedRefPower

Power control during SHO - Uplink

Power control during SHO - UplinkDuring a softer handover, UE should receive identical TPC commands from two or more cells of the same Node B. Because the fading conditions are expected to be the same over multiple links from the same Node B.Knowing this fact, UE soft combines the TPC commands before making any decision on the value of the TPC bit.After combining the TPC bit is read and decision made either to increase or decrease the power.

DL PC during Compressed ModeHigher Layer Scheduling (HLS ) For Non real time services. Amount of data scheduled to be transmitted is reduced during compressed frames.Lower Spreading Factor (SF/2) SF is reduced by 2 during compressed frame.

DL PC during Compressed ModeAim of PC during Compressed Mode is to recover the SIR close to the target SIR as quickly as possible after each transmission gap.To achieve this, PC increases the downlink power and the RRC signaling increases the downlink target SIR used by PC algorithm in UE.In a compressed mode frame, except during transmission gaps, RNC estimates the k-th TPC command and adjusts the current downlink power P(k-1) as per the below formulaP(k) = P(k - 1) + PTPC(k) + PSIR(k) + P bal(k)PTPC (k) is the k-th power adjustment due to the Inner Loop Power ControlPSIR (k) is the k-th power adjustment due to the downlink SIR target variationPbal (k) [dB] is the correction due to the Power Balancing PTPC is calculated using a step size Delta STEP instead of Delta TPC. And Delta STEP is set to 2*Delta TPC during the Recovery Period Length (RPL) after each transmission gapRPL is expressed as number of slots and set to minimum of {transmission gap legth, 7 slots}

UL PC during Compressed ModeUL inner loop PC increases the target SIR to help recovering after each transmission gap during compressed mode.All the serving cells in the active set estimate the SIR of received uplink DPCC, generate TPC and transmit them one per slot except downlink transmission gap, according to the following ruleIf estimated SIR >= SIRcm_target RBS sends TPC with power DOWNIf estimated SIR < SIRcm_target RBS sends TPC with power UPSIRcm_target is the target SIR during Compressed Mode, which is increased to compensate the interruption in PC during transmission gaps and for differences in number of pilot bits in uplink DPCCH

UL PC during Compressed ModeCompressed and Non-compressed frames in uplink DPCCH can have different pilot bits per slot, because of transmission gap during compressed mode.But nevertheless the total pilot energy per slot to be maintained. To accomplish this an additional Delta Pilot is added to the uplink transmit powerDelta Pilot = 10Log10 (N pilot,prev /N pilot,curr )N pilot,prev PIlot bits in the most recent slotN pilot,curr PIlot bits in the current slotThus an increase in uplink pilot power will increase the received SIR per slot. But this is not the actual improved channel conditions. Because the SIR target is also increased by Delta SIRPILOT as given belowDelta SIRPILOT = 10Log10 (Npilot,N /N pilot,currframe )Npilot,N No. of PIlot bilts per slot in a non-compressed uplink frameN pilot,currframe - No. of PIlot bilts per slot in the current uplink frame

PC during Inter Frequency HandoverSetting a proper initial power on DL DPCCH/DPDCH at IFHO enables reliable radio link setup at the target frequency and minimal impact on existing connectionsRNC calculates the initial DL power and transfer the result to the RBS with target frequency. RNC uses following equation

P_DL_DPDCH = prmaryCpichPower + (dlInitSirTarget Ec/No_PCPICH) + cNbifho + 10log(2/SF_DL_DPDCH)Where, cNbiho a configurable correction factor that takes non-blind IFHO margin into account. This parameter can be used to set the initial power at a conservative value.Initial Downlink DPCCH power is calculated thro DPDCH power using few offsets pO1, pO2, pO3 as seen in the DL open loop power control

Power Control ScenariosRADIO LINK SETUP When a dedicate radio link is established from a common channel or during a IRAT HO, the following PC steps are takenSet Initial Downlink Power DL Open Loop PCSet Power limits DL Max Power and Min PowerSet Initial Uplink Power UP Open Loop PCSet Initial Uplink SIR Target thro configurable parameters ulInitSirTargetSrb, ulInitSirTargetLow, ulInitSirTargetHighStart Uplink Outer Loop Power ControlStart Downlink Inner Loop Power ControlStart Uplink Inner Loop Power Control

Power Control ScenariosRAB ESTABLISHMENT When a RAB is added to the existing connection, following steps related PC are takenSet Power Limits DL max and min powerDownlink Inner Loop PC is already running and no changes made as a result of addition of a service. Any requirements for increased power is handled thro regular downlink power updates thro TPC coming on uplink DPCCH.Set Initial Uplink SIR Target thro configurable parameters ulInitSirTargetSrb, ulInitSirTargetLow, ulInitSirTargetHighStart Uplink Outer Loop power controlUplink Inner Loop PC is already running and no changes made as a result of addition of a service. Any requirements for increased power is handled thro regular uplink power updates thro TPC coming on downlink DPCCH.

Power Control ScenariosSOFT HANDOVER When a radio link is added to the active set, following actions related to power control are taken.Set Initial Downlink Power DL open loop PCSet Power Limits downlink max and min transmit powerStart Power Balancing Upon a radio link addition no uplink PC actions are initially required. This is because the UEs power is already controlled by existing links. After the new link obtains uplink synchronization, the RBS issues TPC based on the SIR thro uplink inner loop PC. When a radio link is removed from the active set, the power balancing is stopped. No other actions taken.

Power Control ScenariosINTER FREQUENCY HANDOVER When a IFHO occurs, following actions related to power control are taken.Set Initial Downlink Power DL open loop PCSet Power Limits downlink max and min transmit powerSet Initial Uplink Power UL open loop PCSet Initial Uplink SIR Target thro configurable parameters ulInitSirTargetSrb, ulInitSirTargetLow, ulInitSirTargetHighStart DL Inner Loop PCStart UL Outer / Inner Loop PC

A Sample of Power Control Procedure

HSDPA Power ControlHS-DPCCHHS-DPCCH power is given as an offset relative to DPCCHPower offset factors depend on the number of RLS as shown

HSDPA Power ControlHS-DPCCHRecommended values for power offsets are shown belowHigher offsets given when UE is in soft handoff. The reason during SHO, HS-DPCCH will be received by only one cell which transmits HS-DSCH. But HS-DPCCH will be power controlled by TPCs from all cells in the active set. If the HS-DSCH transmitting cell is not the best cell in the active set, then HS-DPCCHs performance may degrade. So, a higher power off set is given to HS-DPCCH to combat this issue.

HSDPA Power ControlHS-SCCHPower of HS-SCCH is fixedConfigurable parameter hsScchMaxCodePower determines the power level and it is expressed as an offset relative PCPICH

HS-DSCHInitial Power is set in the same way as the Open loop power control of DCHThere is no further power controlThe channel quality is controlled by adaptive modulation and coding formats. CQI on HS-DPCCH is used to estimate the current channel conditions.

EUL Power ControlDownlink Channels Fixed power depends on configurable parameters which are relative to PCPICHE-AGCH (Absolute Grant Ch.) - maxUserEhichErgchPowerDl E-RGCH (Relative Grant Ch.) - maxUserEhichErgchPowerDl E-HICH (HARQ Information Ch.) - maxEagchPowerDl

Uplink ChannelsE-DCH (E-DPCCH, E-DPDCH) Initial Power is set as an relative offset to UL DPCCH using gain factors.Inner and Outer power control works in the same way as that of UL DCH.

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