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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 Sciences University of Wisconsin-Madison. Low Power.

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understanding the limitations of transmit power control for indoor wlans

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 Sciences

University of Wisconsin-Madison

IMC 2007

transmission power control

Low Power

High Power

Transmission Power Control

Energy Efficiency

Spectral Efficiency

IMC 2007

received signal strength
Received Signal Strength
  • The signal strength measured at the receiver

Received at Power Pr

Transmitted at Power Pt

RSS is a good indicator of “bit error rates” and “delivery probabilities” (Reis et al Sigcomm 2006)

IMC 2007

transmission power control1
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

IMC 2007

transmission power control2
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 was

available in wireless cards, no algorithm will be able

to take advantage of it in any practical setting due to significant RSS variations

IMC 2007

the essence
The Essence

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

IMC 2007

the essence1
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

IMC 2007

the essence2
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

IMC 2007

slide9

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

IMC 2007

some existing power control approaches
Some Existing Power Control Approaches

PCMA [Infocom ‘01]

  • 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

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

IMC 2007

some existing power control approaches1
Some Existing Power Control Approaches

PCMA [Infocom ‘01]

  • 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

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

Works well with fine grained power control

What happens if RSS variations are present?

IMC 2007

limitations
Limitations

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

IMC 2007

limitations1
Limitations

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

However, RSS variations are significant in typical wireless scenarios

IMC 2007

rss variations
RSS Variations

Multipath, fading, shadowing

External Interference

Indoor Environments

Outdoor Environments

IMC 2007

rss variations outdoor
RSS Variations (Outdoor)

20% packets are received at RSS of 22dBm

overlap

IMC 2007

rss variations indoors
RSS Variations (Indoors)

40,50,60 mw have significant overlap

IMC 2007

rss variations1
RSS Variations

Outdoors

Indoors

IMC 2007

slide20

Implications of RSS variations

  • 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

IMC 2007

the essence part i
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

IMC 2007

experimental testbed
Experimental Testbed

LOS

NLOS

IMC 2007

rss variations are environment dependent
RSS variations are environment dependent

LOS light

Line of Sight (no interference)

NLOS Light

Non Line of Sight (no interference)

Non Line of Sight (with interference)

NLOS Heavy

Line of Sight (with interference)

LOS Heavy

IMC 2007

practical transmit power control
Practical Transmit Power Control

Sample sufficient number of packets at each power level

IMC 2007

practical transmit power control1
Practical Transmit Power Control

Sample sufficient number of packets at each power level

Characterize RSS distribution

IMC 2007

practical transmit power control2
Practical Transmit Power Control
  • Operate on power levels with non-overlapping RSS distributions

Sample sufficient number of packets at each power level

Characterize RSS distribution

IMC 2007

characterizing rss distribution
Characterizing RSS distribution

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

IMC 2007

characterizing rss distribution1
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

IMC 2007

characterizing rss distribution2
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

Can we do better ?

IMC 2007

online mechanism
Online Mechanism

Normalized Kullback-Leibler Divergence (NKLD)

Quantifies the distance or relative entropy between two distributions

LOS light

NLOS light

NLOS heavy

Operating point

LOS heavy

IMC 2007

slide32
Evaluation accuracy of RSS distributions obtained with Online Mechanism

NLOS Light

LOS Light

LOS Heavy

NLOS Heavy

IMC 2007

online mechanism2
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

IMC 2007

experimental testbed1
Experimental Testbed

12

8

10

11

IMC 2007

the final outcome
The final outcome

Number of power levels

3

1

2

3

Feasible Power Levels at four receivers in the testbed

IMC 2007

the essence part ii
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

IMC 2007

sample applications
Sample Applications
  • Joint power and data rate adaptation converges much faster with Model-TPC

IMC 2007

end user performance 1
End user performance (1)

Goodput for end user in the power-data rate adaptation process

IMC 2007

end user performance 2
End user performance (2)

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

IMC 2007

future work
Future Work
  • Use our model as a module in previously proposed Transmit Power Control mechanisms

IMC 2007

future work1
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

IMC 2007

future work2
Future Work
  • Use our model as a module Transmit Power Control mechanisms
  • Build a practical transmit power control mechanism using the guidelines discussed here

IMC 2007

slide43

Questions ?

IMC 2007

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