Oct 26, 2007

IMC 2007

Understanding the Limitations of Transmit

Power Control for Indoor WLANs

Vivek Vishal Shrivastava Dheeraj Agrawal

Arunesh Mishra Suman Banerjee

Tamer Nadeem (Siemens Research)

Department Of Computer SciencesDepartment Of Computer Sciences

University of Wisconsin-MadisonUniversity of Wisconsin-Madison

Oct 26, 2007

IMC 2007

Energy Efficiency

Spectral Efficiency

Transmission Power Control

High Power Low Power

Oct 26, 2007

IMC 2007

Received Signal Strength

The signal strength measured at the receiver

Transmitted at Power Pt

Received at Power Pr

RSS is a good indicator of “bit error rates” and “delivery probabilities” (Reis et al

Sigcomm 2006)

Oct 26, 2007

IMC 2007

Transmission Power Control A wide variety of power control algorithms have

been proposed in literature

Few have made it to practice

This gap has been attributed to lack of sophisticated hardware

Absence of fine grained power levels in current state of the art wireless cards

Oct 26, 2007

IMC 2007

Transmission Power Control A wide variety of power control algorithms have

been proposed in literature

Few have made it to practice

This gap has been attributed to lack of sophisticated hardware

Absence of fine grained power levels in current state of the art wireless cards

Our claim: Even if fine-grained power control wasavailable in wireless cards, no algorithm will be ableto take advantage of it in any practical setting due to significant RSS variations

Oct 26, 2007

IMC 2007

The Essence

Q. What granularity of power control is practically usable and how do we determine these discrete power levels ?

Oct 26, 2007

IMC 2007

The Essence

Q. What granularity of power control is practically usable and how do we determine these discrete power levels ?

A1. In practical settings, significant overlap between RSS for different power levels makes fine grained power control infeasible

Oct 26, 2007

IMC 2007

The Essence

Q. What granularity of power control is practically usable and how do we determine these discrete power levels ?

A1. In practical settings, significant overlap between RSS for different power levels makes fine grained power control infeasible

A2. Few carefully chosen, environment dependent, discrete power levels are practically usable

Oct 26, 2007

IMC 2007

In this talk, we substantiate these claims and build an empirical power control model on the basis of these guidelines

Oct 26, 2007

IMC 2007

Dimensions of Power Control

Oct 26, 2007

IMC 2007

PCMA [Infocom ‘01]

Other approaches: SHUSH[WICON ‘05], IPMA[SCC 2003]

• An interesting work that proposed use of power control for throughput enhancement

• Designed power controlled medium access

• Receiver finds optimum power and sends a feedback to the transmitter

• Use of out-of-band busy tones to silence neighbors

Some Existing Power Control Approaches

Oct 26, 2007

IMC 2007

PCMA [Infocom ‘01]

Other approaches: SHUSH[WICON ‘05], IPMA[SCC 2003]

• One of the first works to use power control for throughput enhancement

• Designed power controlled medium access

• Receiver finds optimum power and sends a feedback to the transmitter

• Use of out-of-band busy tones to silence neighbors

Some Existing Power Control Approaches

Works well with fine grained power control

What happens if RSS variations are present?

Oct 26, 2007

IMC 2007

Limitations

Use of fine grained power levels works well in the absence of RSS variations

Oct 26, 2007

IMC 2007

Limitations

Use of fine grained power levels works well in the absence of RSS variations

However, RSS variations are significant in typical wireless

scenarios

Oct 26, 2007

IMC 2007

Multipath, fading, shadowing

External Interference

RSS Variations

Outdoor Environments

Indoor Environments

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IMC 2007

Indoor Environments (Multi-path Dominates)

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IMC 2007

RSS Variations (Outdoor)

overlap

20% packets are received at RSS of 22dBm

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IMC 2007

RSS Variations (Indoors)

40,50,60 mw have significant overlap

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IMC 2007

RSS Variations

Outdoors Indoors

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IMC 2007

• Receiver cannot distinguish two transmit power levels with significant overlap

• Only transmit power levels with minimum overlap be used together

• Needs some number of packets (>1) to characterize RSS distribution

Implications of RSS variations

Oct 26, 2007

IMC 2007

The Essence - Part I

Q. What granularity of power control is practically usable and how do we determine these discrete power levels ?

A1. In practical settings, significant overlap between RSS for different power levels makes fine grained power control infeasible

Oct 26, 2007

IMC 2007

Experimental Testbed

NLOS

LOS

Oct 26, 2007

IMC 2007

Line of Sight (no interference)

Non Line of Sight (no interference)

RSS variations are environment dependent

LOS Heavy

NLOS Heavy

LOS light

NLOS Light

Non Line of Sight (with interference)

Line of Sight (with interference)

Oct 26, 2007

IMC 2007

Practical Transmit Power Control

Sample sufficient number of packets at each power level

Oct 26, 2007

IMC 2007

Practical Transmit Power Control

Sample sufficient number of packets at each power level

Characterize RSS distribution

Oct 26, 2007

IMC 2007

Practical Transmit Power Control

Sample sufficient number of packets at each power level

Characterize RSS distribution

Operate on power levels with non-overlapping RSS distributions

Oct 26, 2007

IMC 2007

Characterizing RSS distribution

What is the minimum sample size to accurately capture RSS distribution?

Oct 26, 2007

IMC 2007

Characterizing RSS distribution

What is the minimum sample size to accurately capture RSS distribution?

– RSS variations are typical of a particular indoor environment

– Different number of packets may be required to accurately capture RSS distribution

– Brute Force : Capture very large number of packets for determining RSS distribution

Oct 26, 2007

IMC 2007

Characterizing RSS distribution

What is the minimum sample size to accurately capture RSS distribution?

– RSS variations are typical of a particular indoor environment

– Different number of packets may be required to accurately capture RSS distribution

– Brute Force : Capture very large number of packets for determining RSS distributionCan we do better ?

Oct 26, 2007

IMC 2007

Online MechanismNormalized Kullback-Leibler Divergence (NKLD)

Quantifies the distance or relative entropy between two distributions

Operating point

LOS light

NLOS light

NLOS heavy

LOS heavy

Oct 26, 2007

IMC 2007

Online Mechanism

Calculate distribution of RSS for n, n + δ

Compute divergence using statistical tools like NKLD

Sample n + δ packets

If divergence < thresholdreturn the distribution

Oct 26, 2007

IMC 2007

Evaluation accuracy of RSS distributions obtained with Online Mechanism

LOS Light

LOS Heavy

NLOS Light

NLOS Heavy

Oct 26, 2007

IMC 2007

Online Mechanism Sample sufficient number of packets, to

capture RSS distribution with some accuracy

Profile different available power levels

Find the power levels with non overlapping RSS distribution

Repeat this procedure periodically to cope up with large scale variations in channel conditions

Oct 26, 2007

IMC 2007

Experimental Testbed

810

11

12

Oct 26, 2007

IMC 2007

The final outcome

Feasible Power Levels at four receivers in the testbed

3

2

1

3

Number of power levels

Oct 26, 2007

IMC 2007

The Essence – Part II

Q. What granularity of power control is practically usable and how do we determine these discrete power levels ?

A1. In practical settings, significant overlap between RSS for different power levels makes fine grained power control infeasible

A2. Few carefully chosen, environment dependent, discrete power levels are practically usable

Oct 26, 2007

IMC 2007

Sample Applications Joint power and data rate adaptation

converges much faster with Model-TPC

Oct 26, 2007

IMC 2007

End user performance (1)Goodput for end user in the power-data rate adaptation process

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IMC 2007

End user performance (2)

Cumulative distribution of goodput achieved by end user for adaptation at Location T1 in the testbed

Oct 26, 2007

IMC 2007

Future Work Use our model as a module in

previously proposed Transmit Power Control mechanisms

Oct 26, 2007

IMC 2007

Future Work

Use our model as a module in previously proposed Transmit Power Control mechanisms

Study the interdependence between power and data rates, in view of few discrete power levels

Oct 26, 2007

IMC 2007

Future Work

Use our model as a module Transmit Power Control mechanisms

Build a practical transmit power control mechanism using the guidelines discussed here

Oct 26, 2007

IMC 2007

Questions ?