Dynamic Data Rate and Transmit Power Adjustment in IEEE 802.11 Wireless LANs

Pierre Chevillat, Jens Jelitto, and Hong Linh TruongIBM Zurich Research Laboratory

International Journal of Wireless Information NetworksVol. 12, No. 3, July 2005

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Outline

Introduction Joint power and data rate adaptation

Rate selection Dynamic threshold

High-Performance (HP) mode Low-Power (LP) mode Performance evaluation Conclusion

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Introduction

To maximize the throughput or minimize the transmission delay High data rate is a right choice in many cases

But high data rate may lead to an excessive number of retransmissions Low SNR

SS

RRHighHigh

LowLow

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Introduction (cont.)

Dynamically data rate selection could achieve a better throughput

Better medium utilization Adapt to the channel condition

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Introduction (cont.)

For battery-powered devices Transmission power awareness is crucial to

save energy and prolong battery life

When the distance to receiver is small Transmission power adjustment also helps

reduce interference with neighbors

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Goal

Adjusting jointly the transmit power level and the data rate to the radio channel conditions

This method is fully compatible with the 802.11 wireless LAN standard Do not rely on the RTS/CTS protocol

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Goal (cont.)

Optimize the throughput and transmit power for a given channel condition To transmit as many bits with as little energy a

s possible

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Rate selection

There are four parameters in this algorithm

s, f, Smax and Fmax

Concatenate successful transmissions countConcatenate successful transmissions count

Concatenate failure transmissions countConcatenate failure transmissions count

Successful thresholdSuccessful threshold

Failure thresholdFailure threshold

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Rate selection (cont.)

While the transmitter receiving an ACK frame s is increased by one

f resets to zero

Similarly, while the transmission is failed s resets to zero

f is increased by one

If s / f reaches Smax / Fmax the data rate is increased/decreased the power level is decreased/increased

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Dynamic threshold adaptation

The values of Smax and Fmax are critical for the

performance of the link adaptation scheme

Fmax is set to 1

Immediate adaptation for the worse channel Decrease the data rate or increase the transmis

sion power

The optimum choice for Smax depends on the c

hannel dynamics

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Dynamic threshold adaptation (cont.)

11 11

2a2a 2b2b

Rapidly Slowly

Rate increased

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High-Performance mode

The primary goal for this mode is to support the highest possible data rate

Smax : rate increased

Reduce the transmit power level

Fmax : power level increased

Reduce the transmission rate

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High-Performance mode (cont.)

Failure

Success

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High-Performance mode (cont.)

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Low-Power mode

Use the lowest possible transmit power level Only adjust the transmit rate if we can’t decrease the

power level

Parameters dualities rate → pow ++ → - - min → max ratecrit → powcrit

pcnt → rcnt PMax → RMax

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Rate Only mode

Transmit power level is fixed

Data rate is adjusted using the dynamic threshold adaptation scheme

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Performance Evaluations

Simulator C++

Packet length 1000 Bytes

Power level Max: +10dBm, Min: -10dBm

Power Step Up: +5dB, Down: -2dB

Thresholds S1=3, S2=10, FMax=1

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Throughput –distance between two nodes

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Average transmit power – distance between two nodes

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Power efficiency – distance between two nodes

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Mean Transfer Time (20M)

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Avg. Power and Power Efficiency

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Conclusion

This paper have introduced new and simple but powerful dynamic link adaptation schemes

These schemes utilize ACK frame for adjusting the link parameters No feedback is required from the receiver side

All the schemes are fully compatible with the 802.11 wireless LAN standard

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Conclusion (cont.)

These schemes should be applied to different services or requirements Ex: Power, performance

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Thank You !Thank You !