Understanding the Limitations of Transmit Power Control for Indoor WLANs

1. 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 IMC 2007

2. Low Power High Power Transmission Power Control Energy Efficiency Spectral Efficiency IMC 2007

3. 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) IMC 2007

4. 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 IMC 2007

5. 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 IMC 2007

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

7. 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 IMC 2007

8. 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 IMC 2007

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

10. Dimensions of Power Control IMC 2007

11. 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] IMC 2007

12. 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? IMC 2007

13. Limitations Use of fine grained power levels works well in the absence of RSS variations IMC 2007

14. Limitations Use of fine grained power levels works well in the absence of RSS variations However, RSS variations are significant in typical wireless scenarios IMC 2007

15. RSS Variations Multipath, fading, shadowing External Interference Indoor Environments Outdoor Environments IMC 2007

16. Indoor Environments (Multi-path Dominates) IMC 2007

17. RSS Variations (Outdoor) 20% packets are received at RSS of 22dBm overlap IMC 2007

18. RSS Variations (Indoors) 40,50,60 mw have significant overlap IMC 2007

19. RSS Variations Outdoors Indoors IMC 2007

20. 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 IMC 2007

21. 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 IMC 2007

22. Experimental Testbed LOS NLOS IMC 2007

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

24. Practical Transmit Power Control Sample sufficient number of packets at each power level IMC 2007

25. Practical Transmit Power Control Sample sufficient number of packets at each power level Characterize RSS distribution IMC 2007

26. 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 IMC 2007

27. Characterizing RSS distribution What is the minimum sample size to accurately capture RSS distribution? IMC 2007

28. 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 IMC 2007

29. 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 ? IMC 2007

30. 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 IMC 2007

31. Online Mechanism IMC 2007

32. Evaluation accuracy of RSS distributions obtained with Online Mechanism NLOS Light LOS Light LOS Heavy NLOS Heavy IMC 2007

33. 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 IMC 2007

34. Experimental Testbed 12 8 10 11 IMC 2007

35. The final outcome Number of power levels 3 1 2 3 Feasible Power Levels at four receivers in the testbed IMC 2007

36. 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 IMC 2007

37. Sample Applications • Joint power and data rate adaptation converges much faster with Model-TPC IMC 2007

38. End user performance (1) Goodput for end user in the power-data rate adaptation process IMC 2007

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

40. Future Work • Use our model as a module in previously proposed Transmit Power Control mechanisms IMC 2007

41. 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 IMC 2007

42. Future Work • Use our model as a module Transmit Power Control mechanisms • Build a practical transmit power control mechanism using the guidelines discussed here IMC 2007

43. Questions ? IMC 2007