Oct 26, 2007IMC 2007 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 University of Wisconsin-Madison

Oct 26, 2007IMC 2007 Energy Efficiency Spectral Efficiency Transmission Power Control High Power Low Power

Oct 26, 2007IMC 2007 Received Signal Strength  The signal strength measured at the receiver Transmitted at Power Pt Received at Power Pr RSS is a good indicator of “bit error rates” and “delivery probabilities” (Reis et al Sigcomm 2006)

Oct 26, 2007IMC 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, 2007IMC 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, 2007IMC 2007 The Essence Q. What granularity of power control is practically usable and how do we determine these discrete power levels ?

Oct 26, 2007IMC 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, 2007IMC 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, 2007IMC 2007 In this talk, we substantiate these claims and build an empirical power control model on the basis of these guidelines

Oct 26, 2007IMC 2007 Dimensions of Power Control

Oct 26, 2007IMC 2007 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

Oct 26, 2007IMC 2007 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?

Oct 26, 2007IMC 2007 Limitations Use of fine grained power levels works well in the absence of RSS variations

Oct 26, 2007IMC 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, 2007IMC 2007 Multipath, fading, shadowing External Interference RSS Variations Outdoor Environments Indoor Environments

Oct 26, 2007IMC 2007 Indoor Environments (Multi-path Dominates)

Oct 26, 2007IMC 2007 RSS Variations (Outdoor) overlap 20% packets are received at RSS of 22dBm

Oct 26, 2007IMC 2007 RSS Variations (Indoors) 40,50,60 mw have significant overlap

Oct 26, 2007IMC 2007 RSS Variations Outdoors Indoors

Oct 26, 2007IMC 2007 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

Oct 26, 2007IMC 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, 2007IMC 2007 Experimental Testbed NLOS LOS

Oct 26, 2007IMC 2007 Line of Sight (no interference) Non Line of Sight (no interference) RSS variations are environment dependent LOS Heavy NLOS Heavy LOS light NLOS Light Non Line of Sight (with interference) Line of Sight (with interference)

Oct 26, 2007IMC 2007 Practical Transmit Power Control Sample sufficient number of packets at each power level

Oct 26, 2007IMC 2007 Practical Transmit Power Control Sample sufficient number of packets at each power level Characterize RSS distribution

Oct 26, 2007IMC 2007 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

Oct 26, 2007IMC 2007 Characterizing RSS distribution What is the minimum sample size to accurately capture RSS distribution?

Oct 26, 2007IMC 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, 2007IMC 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, 2007IMC 2007 Online Mechanism Normalized Kullback-Leibler Divergence (NKLD) Quantifies the distance or relative entropy between two distributions Operating point LOS light NLOS light NLOS heavy LOS heavy

Oct 26, 2007IMC 2007 Online Mechanism Calculate distribution of RSS for n, n + δ Compute divergence using statistical tools like NKLD If divergence < threshold return the distribution Sample n + δ packets

Oct 26, 2007IMC 2007 Evaluation accuracy of RSS distributions obtained with Online Mechanism LOS Light LOS Heavy NLOS Light NLOS Heavy

Oct 26, 2007IMC 2007 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

Oct 26, 2007IMC 2007 Experimental Testbed

Oct 26, 2007IMC 2007 The final outcome Feasible Power Levels at four receivers in the testbed Number of power levels

Oct 26, 2007IMC 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, 2007IMC 2007 Sample Applications  Joint power and data rate adaptation converges much faster with Model-TPC

Oct 26, 2007IMC 2007 End user performance (1) Goodput for end user in the power-data rate adaptation process

Oct 26, 2007IMC 2007 End user performance (2) Cumulative distribution of goodput achieved by end user for adaptation at Location T1 in the testbed

Oct 26, 2007IMC 2007 Future Work  Use our model as a module in previously proposed Transmit Power Control mechanisms

Oct 26, 2007IMC 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, 2007IMC 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

Oct 26, 2007IMC 2007 Questions ?