On the (In)feasibility of Fine Grained Power Control

1. 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 MobiCom SRC ‘06

2. Low Power High Power Transmission Power Control Energy Efficiency Spectral Efficiency MobiCom SRC ‘06

3. 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 MobiCom SRC ‘06

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 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 MobiCom SRC ‘06

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

6. 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 MobiCom SRC ‘06

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 A2. Few carefully chosen, environment dependent, discrete power levels are practically usable MobiCom SRC ‘06

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

9. 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] MobiCom SRC ‘06

10. 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? MobiCom SRC ‘06

11. Limitations Use of fine grained power levels works well in the absence of RSS variations MobiCom SRC ‘06

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

13. RSS Variations Multipath, fading, shadowing External Interference MobiCom SRC ‘06

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

15. RSS Variations Multipath, fading, shadowing External Interference 40,50,60 mw have significant overlap MobiCom SRC ‘06

16. RSS Variations Multipath, fading, shadowing External Interference with without MobiCom SRC ‘06

17. 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 MobiCom SRC ‘06

18. 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 MobiCom SRC ‘06

19. RSS variations are environment dependent Line of Sight Non Line of Sight Non Line of Sight with Hotspot Interference Non Line of Sight with controlled interference MobiCom SRC ‘06

20. Practical Transmit Power Control Sample sufficient number of packets at each power level MobiCom SRC ‘06

21. Practical Transmit Power Control Sample sufficient number of packets at each power level Characterize RSS distribution MobiCom SRC ‘06

22. 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 MobiCom SRC ‘06

23. Characterizing RSS distribution What is the minimum sample size to accurately capture RSS distribution? MobiCom SRC ‘06

24. 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 MobiCom SRC ‘06

25. 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 ? MobiCom SRC ‘06

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

27. Online Mechanism MobiCom SRC ‘06

28. Evaluation accuracy of RSS distributions obtained with Online Mechanism MobiCom SRC ‘06

29. 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 MobiCom SRC ‘06

30. Experimental Testbed 12 8 10 11 MobiCom SRC ‘06

31. The final outcome Number of power levels 3 1 2 3 Feasible Power Levels at four receivers in the testbed MobiCom SRC ‘06

32. 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 MobiCom SRC ‘06

33. Future Work • Use our model as a module in previously proposed Transmit Power Control mechanisms MobiCom SRC ‘06

34. 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 MobiCom SRC ‘06

35. 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 MobiCom SRC ‘06

36. Questions ? MobiCom SRC ‘06