Transcript slides

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
Oct 26, 2007
IMC 2007
Transmission Power Control
Spectral
Efficiency
Energy
Efficiency
High Power
Low Power
Oct 26, 2007
IMC 2007
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)
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 was
available in wireless cards, no algorithm will be able
to 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
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IMC 2007
Dimensions of Power Control
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IMC 2007
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]
Oct 26, 2007
IMC 2007
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?
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
RSS Variations
Multipath, fading,
shadowing
External
Interference
Indoor
Environments
Outdoor
Environments
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IMC 2007
Indoor Environments (Multi-path Dominates)
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IMC 2007
RSS Variations (Outdoor)
overlap
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20% packets are
received at RSS of
22dBm
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RSS Variations (Indoors)
40,50,60 mw
have
significant
overlap
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RSS Variations
Outdoors
Oct 26, 2007
Indoors
IMC 2007
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
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
LOS
NLOS
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IMC 2007
RSS variations are environment
dependent
LOS light
Line of Sight
(no interference)
NLOS Light
Non Line of Sight
(no interference)
NLOS Heavy
Non Line of Sight
(with interference)
Line of Sight
(with interference)
LOS Heavy
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
Oct 26, 2007
Characterize
RSS
distribution
IMC 2007
Practical Transmit Power Control
Sample sufficient
number of packets
at each power level
Oct 26, 2007
Characterize
RSS
distribution
IMC 2007
Operate on power
levels with nonoverlapping RSS
distributions
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 distribution
Can we do better ?
Oct 26, 2007
IMC 2007
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
Oct 26, 2007
IMC 2007
Online Mechanism
Sample n + δ packets
If divergence < threshold
return the distribution
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Calculate distribution
of RSS for n, n + δ
Compute divergence using
statistical tools like NKLD
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Evaluation accuracy of RSS distributions obtained
with Online Mechanism
NLOS Light
LOS Light
LOS Heavy
Oct 26, 2007
NLOS Heavy
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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
12
10
8
11
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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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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
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End user performance (1)
Goodput for end user in the power-data rate
adaptation process
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End user performance (2)
Cumulative distribution of goodput achieved by
end user for adaptation at Location T1 in the
testbed
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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
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IMC 2007
Questions ?
Oct 26, 2007
IMC 2007