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 ?