On the Infeasibility of Fine Grained Power Control

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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 Sciences
• University of Wisconsin-Madison

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Transmission Power Control
Energy Efficiency
Spectral Efficiency
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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

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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
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The Essence

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

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

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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 SHUSHWICON 05, IPMASCC
2003
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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 SHUSHWICON 05, IPMASCC
2003
Works well with fine grained power control What
happens if RSS variations are present?
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Limitations
Use of fine grained power levels works well in
the absence of RSS variations
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Limitations
Use of fine grained power levels works well in
the absence of RSS variations
However, RSS variations are significant in
typical wireless scenarios
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RSS Variations
Multipath, fading, shadowing
External Interference
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RSS Variations
Multipath, fading, shadowing
External Interference
20 packets are received at RSS of 22dBm
overlap
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RSS Variations
Multipath, fading, shadowing
External Interference
40,50,60 mw have significant overlap
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RSS Variations
Multipath, fading, shadowing
External Interference
with
without
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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 (gt1) to
  characterize RSS distribution

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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
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RSS variations are environment dependent
Line of Sight
Non Line of Sight
Non Line of Sight with Hotspot Interference
Non Line of Sight with controlled interference
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Practical Transmit Power Control

Sample sufficient number of packets at each power
level
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Practical Transmit Power Control

Sample sufficient number of packets at each power
level
Characterize RSS distribution
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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
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Characterizing RSS distribution

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

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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

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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 ?
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Online Mechanism
Normalized Kullback-Leibler Divergence
(NKLD) Quantifies the distance or relative
entropy between two distributions
Operating point
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Online Mechanism
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• Evaluation accuracy of RSS distributions
  obtained with Online Mechanism

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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

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Experimental Testbed
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8
10
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The final outcome
Number of power levels
3
1
2
3
Feasible Power Levels at four receivers in the
testbed
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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
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Future Work

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

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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

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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

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Questions ?