Power Saving and Power Management in WiFi and Bluetooth Networks

1. Power Saving and Power Managementin WiFi and Bluetooth Networks Prof. Yu-Chee Tseng Dept. of Comp. Sci. & Infor. Eng. National Chiao-Tung University (交通大學 資訊工程系 曾煜棋)

2. Outline • Power control: • S.-L. Wu, Y.-C. Tseng, and J.-P. Sheu, "Intelligent Medium Access for Mobile Ad Hoc Networks with Busy Tones and Power Control", IEEE Journal on Selected Areas in Communications, 18(9):1647-1657, Sep. 2000. • Power management: • Y.-C. Tseng, C.-S. Hsu, and T.-Y. Hsieh, "Power-Saving Protocols for IEEE 802.11-Based Multi-Hop Ad Hoc Networks", Computer Networks, Elsevier Science Pub., Vol. 43, No. 3, Oct. 2003, pp. 317-337. • WiFi vs Bluetooth: • T.-Y. Lin and Y.-C. Tseng, "An Adaptive Sniff Scheduling Scheme for Power Saving in Bluetooth", IEEE Wireless Communications, Vol. 9, No. 6, Dec. 2002, pp. 92-103.

3. Introduction: Basic Concept

4. Introduction • Battery is a limited resource in any portable device. • becoming a very hot topic is wireless communication • Power-related issues: • PHY: transmission power control • MAC: power mode management • Network Layer: power-aware routing

5. Transmission Power Control • tuning transmission energy for higher channel reuse • example: • A is sending to B (based on IEEE 802.11) • Can (C, D) and (E, F) join? Yes! No!

6. Power Mode Management • doze mode vs. active mode • example: • A is sending to B (based on 802.11) • Does C need to stay awake?

7. N2 N1 SRC DEST + – + – + – + – + – + – N3 N4 Power-Aware Routing • routing in an ad hoc network with energy-saving in mind • Example: in an ad hoc network

8. Intelligent Medium Access for Mobile Ad Hoc Networks with Busy Tones and Power Control S.-L. Wu, Y.-C. Tseng, and J.-P. Sheu, IEEE J. of Selected Areas on Communications (JSAC)

9. Abstract • A New MAC Protocol • based on RTS/CTS • with Busy Tones • with Power Control

10. Power Control • Use an appropriate power level to transmit packets. • to increase the possibility of channel reuse • to increase channel utilization • Example: • (a) without power control: • the transmissions from C to D and from E to F are prohibited. • (b) with power control: • all these can coexist.

11. How to Tune Power Levels • Assumptions: • A mobile host can choose on what power level to transmit a packet. • On receiving a packet, the physical layer can offer the MAC layer the power level on which the packet was received. • Suppose Pt and Pr are the power levels a packet is sent and received, respectively. • l = carrier wavelength • n = path loss coefficient (typically 2 ~ 6) • d = distance between sender and receiver • gt and gr: antenna gains at the sender and receiver sides, respectively

12. Note: during a short period, the values of n and d can be treated as a constant. This makes power control possible. • Let Pmin be the minimum power level to decode a packet. • Suppose X sends an RTS to Y with power Pt. • If Y wants to reply a CTS to X with a power level PCTS, such that X receives the packet at the smallest power level Pmin, then we have: • Dividing the above formulas, we have:

13. BTr BTt frequency data channel control channel General Rules in This Paper • Busy Tone (BT) • Senders should send BTt, but gauge any BTr. • Receivers should send BTr, but gauge any BTt. • General Rules: • Data packet and BTt: transmitted with power control. • CTS and BTr: transmitted at the normal (largest) power. • RTS: at a power level based on how strong the BTr are around the requesting host. • Channel Model:

14. A is sending to B. A’s data packet and BTt at the minimal level (yellow circle). B’s BTr at the largest level (white circle). C intends to send to D. C hears no BTr. D hears not BTt. So the transmission can be granted (pink circle). Illustrative Example (I) D C A B

15. Now we moe C into A’s circle. A is sending to B. A’s data packet and BTt at the minimal level (yellow circle). B’s BTr at the largest level (white circle). C intends to send to D. C hears no BTr. D hears no BTt. So the transmission can be granted (pink circle). Illustrative Example (II) D C A B

16. Next we move D into A’s circle. A is sending to B. A’s data packet and BTt at the minimal level (yellow circle). B’s BTr at the largest level (white circle). C intends to send to D. C hears no BTr. D hears A’s BTt. So the transmission can NOT be granted (pink circle). Illustrative Example (III) D C A B

17. A is sending to B. A’s data packet and BTt at the minimal level (yellow circle). B’s BTr at the largest level (white circle). C intends to send to D. C hears A’s BTt and B’s BTr. D hears no BTt. The transmission can be granted if C controls its transmission power (pink circle). Illustrative Example (IV) D C A B

18. A is sending to B. A’s data packet and BTt at the minimal level (yellow circle). B’s BTr at the largest level (white circle). C intends to send to D. C hears A’s BTt and B’s BTr. D hears no BTt. The transmission can be granted if C controls its transmission power (pink circle). Illustrative Example (V) D C A B

19. Many Transmission Pairs with Power Control and Busy Tones D C E B A F BTt and DATA: yellow circles BTr: white circles

20. The Protocol • Pmax: the maximum transmission power • Pmin: the minimum power to distinguish a signal from a noise • Pnoise: the maximum power at which an antenna will regard a signal as a noise • Pmin - Pnoise should be a very small value • Basic “Power” Rules: • Data packet and BTt: transmitted with power control. • CTS and BTr: transmitted at the largest power Pmax. • RTS: at a power level based on how strong the BTr are around the requesting host.

21. Detailed Protocol • On a host X intending to send a RTS to Y, • X senses any receive busy tone BTr around it • X sends a RTS on the control channel at power level Px: • If there is no BTr, let Px = Pmax. • O/w, let Pr be the power level of BTr that has the highest power among all heard BTr’s. • The RTS should not go beyond the nearest host that is currently receiving a data packet. • Pmax is used because BTr is always transmitted at the maximal power.

22. On Y receiving X’s RTS, • Y senses any transmit busy tone BTt around it. • If there is any BTt, then Y ignores this RTS. • O/w, Y does the following: • reply with a CTS at the maximum power Pmax • turn on its receive busy tone BTr at the maximum power Pmax • On X receiving Y’s CTS, • X transmits its data packet at power Px. • X turns on its transmit busy tone BTt at power Px. • Pr is the power level at which X receives Y’s CTS. Px is the minimal possible power level to ensure that Y can correctly receive the data packet.

23. Many Transmission Pairs with Power Control and Busy Tones BTr BTt RTS D C CTS E B A F H G

24. Analysis • Scenario: • A is currently sending to B. • Another pair, C and D, is intending to communicate. • Goal: We want to find out the probability that C can send to D. • Through complicated calculus, we find that …

25. When BC < rmax • INTC(Ra, Rb, AB) = the intersection of the circles centered at a and b • Ra = radius of the circle centered at a • Rb = radius of the circle centered at b • AB = distance of a and b • The probability that C can send to D when A is sending to B: • i.e., the coverage of Rc excluding the coverage of Ra • Fig. 6

27. cont... • Integrating over  = 0 .. 2, and then over CB = 0 .. rmax • Integrating over AB = 0 .. rmax, we have the final result • On the contrary, the DBTMA has probability of 0.

28. When rmax < BC < 3rmax • Main difference: C’s RTS will be sent with max. power. • The probability that C can send to D when A is sending to B: • See Fig. 7: • At point C1, node C can always send. • At point C2, node C can’t send if D is in A’s range.

30. cont... • Integrating over  = 0 .. 2, and then over CB = rmax..3rmax • Integrating over AB = 0 .. rmax, we have the final result

31. cont. • On the contrary, the DBTMA has a success probability of X change to rmax

32. Discrete Power Control • The levels of power provided by hardware may not be infinitely tunable. • We may have a discrete number of power levels. • Theorem: • Given a fixed integer k, evenly spreading the k power levels will be the best choice. • I.e., (1/k)*Pmax, (2/k)*Pmax, (3/k)*Pmax, …, (k/k)*Pmax.

33. Simulation Parameters • Simulation parameters • physical area = 8km  8km • max transmission distance (rmax) = 0.5 or 1.0 km • number of mobile hosts = 600 • Speed of mobile hosts 0 or 125 km/hr. • length of control packet = 100 bits • link speed = 1 Mbps • transmission bit error rate = 10-5/bit

34. (a) rmax = 0.5 km (b) rmax = 1.0 km Simulation Results:Channel utilization vs. traffic load

35. Channel utilization vs. data packet length at various traffic loads

36. Channel Utilization vs. Number of Power Levels • rmax = 1 km; arrival rate = 200 or 400 packets/ms; packet length = 1 or 2 Kbits • So 4 to 6 levels will be sufficient.

37. Channel Utilization vs. Traffic Load • mobility = 0 km/hr and 125 km/hr • The transmission distance rmax = 1.0 km

38. Short Conclusion • a new MAC protocol • power control on top of RTS/CTS and busy tones • Channel utilization can be significantly increased because the severity of signal overlapping is reduced.

39. Power Mode Management in IEEE 802.11 Y.-C. Tseng, C.-S. Hsu, and T.-Y. Hsieh, "Power-Saving Protocols for IEEE 802.11-Based Multi-Hop Ad Hoc Networks", Computer Networks, Elsevier Science Pub., Vol. 43, No. 3, Oct. 2003, pp. 317-337 (also in INFOCOM).

40. Power Consumption • IEEE 802.11 power model • transmit: 1400 mW • receive: 1000 mW • idle: 830 mW • sleep: 130 mW

41. Power Mode Management • Power modes in IEEE 802.11 • PS and ACTIVE • Problem Spectrum: • infrastructure • ad hoc network (MANET) • single-hop • multi-hop ad hoc networks

42. Infrastructure Mode • two power modes: active and power-saving (PS)

43. Beacon Interval Beacon Interval ATIM Window ATIM Window power saving state active state ATIM Host A Beacon BTA=2, BTB=5 data frame power saving state Host B ACK ACK Beacon Ad Hoc Mode (Single-Hop) • PS hosts also wake up periodically. • interval = ATIM (Ad hoc) window

44. Problem Statement(Multi-Hop MANET) • Clock Synchronization: • a difficult job due to communication delays and mobility • Neighbor Discovery: • by inhibiting other's beacons, hosts may not be aware of others’ existence • Network Partitioning: • with unsynchronized ATIM windows, hosts with different wakeup times may become partitioned networks

45. A D D C ╳ F F Network Partition ╳ B E E ╳ ╳ Network-Partitioning Example Host A ATIM window Host B Host C Host D Host E Host F

46. What Do We Need? • PS protocols for multi-hop ad hoc networks • Fully distributed • No need of clock synchronization (i.e., asynchronous PS) • Always able to go to sleep mode, if desired

47. Features of Our Design • Guaranteed Overlapping Awake Intervals: • two PS hosts’ wake-up patterns always overlap • no matter how much time their clocks drift • Wake-up Prediction: • with beacons, derive other PS host's wake-up pattern based on their time difference

48. Structure of a Beacon Interval • BI: beacon interval (to send beacons) • AW: active window • BW: beacon window • MW: MTIM window (for receiving MTIM) • listening period: to monitor the environment Beacon Int. (BI) Act. Win. (AW) BW MW listening BW MW listening

49. Three Protocols • Based on the above structure, we propose three protocols • Dominating-Awake-Interval • Periodical-Fully-Awake-Interval • Quorum-Based

50. Beacon Interval Beacon Interval Host A ╳ ╳ Host B Beacon Interval Beacon Interval P1: Dominating-Awake-Interval • intuition: impose a PS host to stay awake sufficiently long • “dominating-awake” property