Sep 27, 2006

MobiCom SRC ‘06

On the (In)feasibility of Fine Grained Power Control

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

Sep 27, 2006

MobiCom SRC ‘06

Energy Efficiency

Spectral Efficiency

Transmission Power Control

High Power Low Power

Sep 27, 2006

MobiCom SRC ‘06

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

Sep 27, 2006

MobiCom SRC ‘06

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 cardsOur 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

Sep 27, 2006

MobiCom SRC ‘06

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

Sep 27, 2006

MobiCom SRC ‘06

The EssenceQ. 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

Sep 27, 2006

MobiCom SRC ‘06

The EssenceQ. 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 infeasibleA2. Few carefully chosen, environment dependent, discrete power levels are practically usable

Sep 27, 2006

MobiCom SRC ‘06

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

Sep 27, 2006

MobiCom SRC ‘06

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

Sep 27, 2006

MobiCom SRC ‘06

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?

Sep 27, 2006

MobiCom SRC ‘06

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

Sep 27, 2006

MobiCom SRC ‘06

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

However, RSS variations are significant in typical wireless

scenarios

Sep 27, 2006

MobiCom SRC ‘06

Multipath, fading, shadowing

External Interference

RSS Variations

Sep 27, 2006

MobiCom SRC ‘06

Multipath, fading, shadowing

External Interference

RSS Variations

overlap20% packets are received at RSS of 22dBm

Sep 27, 2006

MobiCom SRC ‘06

Multipath, fading, shadowing

External Interference

RSS Variations

40,50,60 mw have significant overlap

Sep 27, 2006

MobiCom SRC ‘06

RSS Variations

Multipath, fading, shadowing

External Interference

without with

Sep 27, 2006

MobiCom SRC ‘06

• 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

Sep 27, 2006

MobiCom SRC ‘06

The Essence - Part IQ. 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

Sep 27, 2006

MobiCom SRC ‘06

Line of Sight

Non Line of Sight

Non Line of Sight with controlled interference

Non Line of Sight with Hotspot Interference

RSS variations are environment dependent

Sep 27, 2006

MobiCom SRC ‘06

Practical Transmit Power Control

Sample sufficient number of packets at each power level

Sep 27, 2006

MobiCom SRC ‘06

Practical Transmit Power Control

Sample sufficient number of packets at each power level

Characterize RSS distribution

Sep 27, 2006

MobiCom SRC ‘06

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

Sep 27, 2006

MobiCom SRC ‘06

Characterizing RSS distributionWhat is the minimum sample size to accurately capture RSS distribution?

Sep 27, 2006

MobiCom SRC ‘06

Characterizing RSS distributionWhat 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

Sep 27, 2006

MobiCom SRC ‘06

Characterizing RSS distributionWhat 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 ?

Sep 27, 2006

MobiCom SRC ‘06

Online MechanismNormalized Kullback-Leibler Divergence (NKLD)Quantifies the distance or relative entropy between two distributions

Operating point

Sep 27, 2006

MobiCom SRC ‘06

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

Sep 27, 2006

MobiCom SRC ‘06

Evaluation accuracy of RSS distributions obtained with Online Mechanism

Sep 27, 2006

MobiCom SRC ‘06

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

Sep 27, 2006

MobiCom SRC ‘06

Experimental Testbed

810

11

12

Sep 27, 2006

MobiCom SRC ‘06

The final outcome

Feasible Power Levels at four receivers in the testbed

3

2

1

3

Number of power levels

Sep 27, 2006

MobiCom SRC ‘06

The Essence – Part IIQ. 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 infeasibleA2. Few carefully chosen, environment dependent, discrete power levels are practically usable

Sep 27, 2006

MobiCom SRC ‘06

Future Work Use our model as a module in

previously proposed Transmit Power Control mechanisms

Sep 27, 2006

MobiCom SRC ‘06

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

Sep 27, 2006

MobiCom SRC ‘06

Future Work Use our model as a module Transmit

Power Control mechanisms Study interdependence between

power and data rates, in view of few discrete power levels

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

Sep 27, 2006

MobiCom SRC ‘06

Questions ?