yin power control in cellular systems

8/13/2019 Yin Power Control in Cellular Systems

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1 S-72.333 Postgraduated Course in Radio Communication

P ow er C on trol in C ellular Sys tem

Hongying Yin

Feb. 12nd2002

Helsinki University of Technology


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Po w er C ontro l in W C D M A (G enera l)

Maintain the link quality in uplink and downlink byadjusting the powers

Mitigate the near far effect by providing minimumrequired power level for each connection

Power control provides protection against shadowing

and fast fading Minimisies the interference in the network, thus

improves capacity and quality

The battery life of the MS can be extended


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N ear-Far P roblem


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Pow er C ontro l Loops in W C DM A

O pen loop p ow er contro l for initial power setting

C losed loo p pow er con trol Inn er (Fast) pow er con trol (1500 H z)

makes Eb/N0 requirements lower equalizes received powers at BS in reverse link (avoid near-far)

introduces peaks in the transmit power

O uter pow er control loo p at a much slower rate, across the Iub interface in reverse link Adjusts the SIR target to achieve a target FER/BER


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O pen Lo op P ow er C ontrol (1 )


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Open loop power control relies on the assumptionthat the power-loss in the uplink and downlink

channel is identical.

The MS Tx power is adjusted as follows:

Measure the received power from the BS (afterRAKE finger comb.)

The power-loss in the downlink channel isdetermined based on knowledge of the BS Txpower.

Adjust the MS Tx power according to the estimatedpower-loss.


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Purpose: to set the initial transmitted power of PRACH in theUL and FACH in the DL

UE determines the uplink preamble power of PRACHUE PRACH first preamble power =

Transmission power of CPICH (broadcast on BCH) -Downlink Ec/Io measurement of active cell on CPICH (measured by

UE) +

Total received wideband interference power at WCDMA BTS(broadcast on BCH) +Required received SIR at the WCDMA BTS (broadcast on BCH)

BTS of downlink FACH is fixed and set on a per-cell basis

(RAN1) Open loop PC is a part of random access procedure for

PRACH channel



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O pen Lo op P ow er control (5 )

Rando m Access Procedure UE transmits the first preamble with the power determined by UL open loop

PC

If the UE does not detect any acquisition indicator in AICH, it increases the

preamble Tx power by a specified offset Po If the UE detects the positive indicator in AICH, it transmits the random

access message, 3 or 4 access slots after the UL access slot of the lasttransmitted preamble

The Tx power of the control part of random access message should be pp-mhigher than the last transmitted preamble power

The required power offset values for random access procedure Power offset between the preamble and control part message in PRACH(pp-m) Power offset when no acquisition indicator is not received in AICH (power ramp

step)(Po)Preamble

Expectedanswer Answer

Random Access Message

Po


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Inner Lo op (closed ) P ow er Con trol (1)


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U pLink Inn er Loo p (C losed) Pow er

C ontro l (3)

PC frequency 1500 Hz

PC step 1dB (0.5 2 dB)

PC delay approx. one slot

UE sets the power on UL DPCCHand UL DPDCH on following way:

TPC = 1 --> increase power by 1 dBTPC = 0 --> decrease power by 1 dB

UL DPCCH

UE

Measure received Eb / No on UL DPCCH

Compare measured Eb / No withEb / No setpoint value received

from UL outer loop PC

Measured Eb / No < Eb / No setpoint --> TPC bit = 1Measured Eb / No => Eb / No setpoint --> TPC bit = 0

BS

Send TPC bit on DL DPCCH

Changed power on UL DPCCH


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D ow nL ink Inn er Lo op Po w er C on trol (4 )

UE measures the SIR on DL DPCCH during the pilot period (inSHO after MRC)

Before generating TPC commands UE checks DPC_MODE(downlink power control mode)

DPC_MODE parameter is a UE specific parameter controlled by thenetwork

If DPC_MODE is 0, UE sends unique TPC command in each slot

in the first available TPC field in the UL DPCCH If DPC_MODE is 1, repeats the same TPC commands over 3 slot

but a new TPC command is sent at the beginning of the frame DPC_MODE is set to 0in RAN1, RNC sends it to UE in RRC

Upon receiving the TPC commands BS adjusts its downlinkDPCCH/DPDCH power accordingly


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G ain o f Inn er P ow er Co ntrol (5)

Speech performance FER=1% (8kbps 10ms interleaving) with 2 branchreceiver antenna diversity in uplink

Slow power control = no power control in simulations = correct averagepower

Slow power control Fast 1.5kHz powercontrol

Gain from fastpower control

ITU Pedestrian A 3 km/h 11.3dB 5.5dB 5.8dB

ITU Vehicular A 3 km/h 8.5dB 6.7dB 1.8dB

ITU Vehicular A 50 km/h 6.8dB 7.3dB -0.5dB

Slow power control Fast 1.5kHz powercontrol

Gain from fastpower control

ITU Pedestrian A 3 km/h 11.3dB 7.7dB 3.6dB

ITU Vehicular A 3 km/h 8.5dB 7.5dB 1.0dB

ITU Vehicular A 50 km/h 6.8dB 7.6dB -0.8dB

The gain from the fast power control is larger for low mobilespeeds than for high mobile speeds in received powers than intransmitted powers if only little multipath diversity is available

Receivedpower

Transmittedpower


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O uter Lo op P ow er C on trol (1)


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O uter Lo op P ow er C on trol (2)


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O uter Loop Pow er C ontro l

f f

[ ]dBnntEbNoSetpoinntEbNoSetpoi +=+ )()1(

sFsf th=

where

where is 1 if there is a frame error according to CRC and is0 if the frame was correct. Fth is the wanted FER and s is the

step size which is a parameter.

Basic function


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U pLink O uter Lo op Po w er C ontrol (3 )

BTS 1

BTS 2

R N C

Macro diversitycombining

Outer looppower control

Iub

Fast powercontrol 1.5 kHz

SIR target for fastpower control

10-100 Hz

Received data

Outer loop PC adjusts

the SIR targets requireddepending on UE speedand multipathpropagation environmentto achieve requiredquality with min. power


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U pLink O uter Lo op Po w er C ontrol (4 )

The outer loop is responsible of controlling the Eb/N0set point so that the required quality is achieved withminimum power. The quality requirement of the linkis usually given in long term average FER or BER.

Uplink located in RNCThe initial Eb/No set-point and changes to that are sent

from RNC to BS.

outer loop must be prevented to run to abnormally high orlow Eb/No setpoints


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Com parison o f PC in W CD M A vs . CD M A

The closed loop power control (i.e. fast power control) is that itworks better in the WCDMA as the signal is spread to widerbandwidth.

CDMA traditionally has fast power control only in the uplinkand not in the downlink, whereas WCDMA has fast powercontrol for both the uplink and downlink.

The outer loop (quality) loop power control is much morecritical in the WCDMA as it contains radio bearers of verydiverse types and very different requirements. TraditionalCDMA algorithms are developed for the speech bearers

mainly, although recent development allows other bearers aswell.


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Abbreviat ion

AICH: acquisition indicator channel;

BER: bit error ratio;

BLER: block error ratio; block error rate;

PRACH:packet random access channel; physical random access channel;

FACH; forward access channel;

SIR: signal-to-interference ratio;

FER: frame error rate; CCTrCH: coded composite transport channel;

DPCCH:Dedicated Physical Control Channel

TFCI: transport format combination indicator; TDD: time division duplex ;

SHO: soft handover ; soft handoff /US/ ;


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References

Holma & Toskala: "WCDMA for UMTS, Radio Access forThird Generation Mobile Communications", J ohn Wiley &Sons.

T. Ojanpera &etc. wideband CDMA for Third GenerationMobile Communications, Artech House Publishers.

A. Viterbi: CDMA Principle pf Principle Spectrum

CommunicationAddison Wesley. Nokia Internal training materials for CDMA/WCDMA.

http://www.utdallas.edu/~torlak/wireless/projects/tran.pdf

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