Sep 27, 2006MobiCom 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 Sciences University of Wisconsin-Madison University of Wisconsin-Madison

Sep 27, 2006MobiCom SRC ‘06 Energy Efficiency Spectral Efficiency Transmission Power Control High Power Low Power

Sep 27, 2006MobiCom 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, 2006MobiCom 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 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

Sep 27, 2006MobiCom SRC ‘06 The Essence Q. What granularity of power control is practically usable and how do we determine these discrete power levels ?

Sep 27, 2006MobiCom SRC ‘06 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

Sep 27, 2006MobiCom SRC ‘06 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

Sep 27, 2006MobiCom SRC ‘06 In this talk, we substantiate these claims and build an empirical power control model on the basis of these guidelines

Sep 27, 2006MobiCom 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, 2006MobiCom 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, 2006MobiCom SRC ‘06 Limitations Use of fine grained power levels works well in the absence of RSS variations

Sep 27, 2006MobiCom SRC ‘06 Limitations Use of fine grained power levels works well in the absence of RSS variations However, RSS variations are significant in typical wireless scenarios

Sep 27, 2006MobiCom SRC ‘06 Multipath, fading, shadowing External Interference RSS Variations

Sep 27, 2006MobiCom SRC ‘06 Multipath, fading, shadowing External Interference RSS Variations overlap 20% packets are received at RSS of 22dBm

Sep 27, 2006MobiCom SRC ‘06 Multipath, fading, shadowing External Interference RSS Variations 40,50,60 mw have significant overlap

Sep 27, 2006MobiCom SRC ‘06 RSS Variations Multipath, fading, shadowing External Interference without with

Sep 27, 2006MobiCom 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, 2006MobiCom SRC ‘06 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

Sep 27, 2006MobiCom 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, 2006MobiCom SRC ‘06 Practical Transmit Power Control Sample sufficient number of packets at each power level

Sep 27, 2006MobiCom SRC ‘06 Practical Transmit Power Control Sample sufficient number of packets at each power level Characterize RSS distribution

Sep 27, 2006MobiCom 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, 2006MobiCom SRC ‘06 Characterizing RSS distribution What is the minimum sample size to accurately capture RSS distribution?

Sep 27, 2006MobiCom SRC ‘06 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

Sep 27, 2006MobiCom SRC ‘06 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 ?

Sep 27, 2006MobiCom SRC ‘06 Online Mechanism Normalized Kullback-Leibler Divergence (NKLD) Quantifies the distance or relative entropy between two distributions Operating point

Sep 27, 2006MobiCom SRC ‘06 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

Sep 27, 2006MobiCom SRC ‘06 Evaluation accuracy of RSS distributions obtained with Online Mechanism

Sep 27, 2006MobiCom 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, 2006MobiCom SRC ‘06 Experimental Testbed

Sep 27, 2006MobiCom SRC ‘06 The final outcome Feasible Power Levels at four receivers in the testbed Number of power levels

Sep 27, 2006MobiCom SRC ‘06 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

Sep 27, 2006MobiCom SRC ‘06 Future Work  Use our model as a module in previously proposed Transmit Power Control mechanisms

Sep 27, 2006MobiCom 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, 2006MobiCom 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, 2006MobiCom SRC ‘06 Questions ?