Power control in Ad hoc networks (ongoing work)

1. Power control in Ad hoc networks(ongoing work) Zhang Jun Computer Science Department The Hong Kong University of Science and technology AoE Meeting Friday 10 December 2004, HKUST

2. What is Ad hoc network • A collection of wireless or mobile nodes communicating in multi-hop • Self configuring • No infrastructure • Usually limited energy supply

3. RTS DATA Sender SIFS SIFS CTS SIFS ACK Receiver NAV(RTS) Other NAV(CTS) Dominant MAC protocol for wireless ad hoc networksIEEE802.11 CSMA/CA protocol

4. What is Power control • In IEEE802.11 MAC protocol: • Each node transmits packets at the same power level: maximal possible power level. • Transmission power may be higher than enough • May generate too much interference and unnecessary energy consumption • Power control: • Each node determines an appropriate transmission power level to ensure that the transmitted packet is received correctly

5. Targets of power control • Improve network throughput* • Reduce overall energy consumption* • Improve fairness • Reduce packet latency • Partial Combination of above targets • Etc.

6. Why power control helps to improve throughput? • Reduce data retransmission probability • With a good assignment of transmission power, each transmitter guarantees its transmission in a low number of attempts and reduces its interference on other nodes. • Increase spatial reuse ratio • Transmission range is proportional to transmission power • Number of simultaneous transmission is inversely proportional to average transmission range

7. How to reduce energy consumption? Energy consumption in Ad Hoc network • Sensing • Receiving • Transmitting • Idling Similar energy consumption Depends on transmission power Little energy consumption • Reducing transmission power • Reducing retransmission count • Reducing number of nodes in sensing mode

8. Necessary and sufficient condition to receive packetsuccessfully • Pr≥ Rxthreshold • Pr: received power level • Rxthreshold : minimal necessary power level • Pr ≥ SIRthreshold * Pnoise • Pnoise: noise power level at receiver side • SIRthreshold : signal to interference ratio (SIR) threshold

9. Classification of power control Algorithms • Deterministic power control algorithms • Transmission power is determined by a some equation base on several parameters (such as busy tone signal strength, received packet power level, node degree, …) • Adaptive power control algorithms • Each node adaptively changes its transmission power based on the network performance (packets loss rate, average access time,…)

10. Some deterministic power control algorithms • BASIC power control algorithm [1] • Power control algorithm with multiple channels [2] • Power control algorithm with busy tone channel [3] [4]

11. BASIC Power Control algorithm [1] • RTS/CTS are sent at max power • DATA/ ACK are sent at minimal required power Pt=Fpath* Rxthreshold* c • Pt : minimal required transmission power • Pmax: maximal power level • Fpath:path loss factor (Fpath=Pt/Pr) • Rxthreshold : minimal necessary power level to decode packets • c: constant

12. Power Control Dual channels (PCDC) [2] • Multiple channels: • RTS/CTS channel • DATA channel • ACK channel • RTS, CTS, ACK are transmitted at Pmax • DATA from node s to node t are transmitted at C(t) *SIRthreshold*Pnoise(t)*Fpath(s,t) • Fpath(s,t): path loss factor between s and t • C(t): a safety factor determined by node t • Pnoise(t) : noise power level at node t

13. Power control with busy tone channel [3] [4] • Busy tone channel • Narrow band • Only signal strength rather than content is known • Does not collide with data channel • Each node broadcasts its data-channel noise level information by busy tone • Busy tone signal strength inversely proportional to data-channel noise power, or • Busy tone transmitted at maximum power

14. Power control with busy tone channel [3] [4] • Transmission power requirement for • RTS: set to avoid collision at other receivers. Inferred from received busy tones. • CTS: • In [3] maximal power • In [4] computed by max{Fpath*RXthreshold,SIRthreshold*Pnoise*Fpath}

15. Power control with busy tone channel [3] [4] • DATA: both [3] and [4] use max{Fpath*RXthreshold, SIRthreshold*Pnoise*Fpath} • ACK: maximal power Pnoise is known from the busy tone signal Fpath is calculated by Fpath= Psend/received

16. Possible drawback for deterministic power control algorithms • May need extra hardware support (busy tone, multiple channels) • The noise power level estimation may not be accurate enough • noise power level when receiver receives RTS and when receives DATA may be different (RTS and CTS affects the noise on the receiver side) • noise power level changes with time • The safety factor c(t) is heuristic and may not work for certain scenarios

17. Adaptive power control algorithm • Adaptively changes transmission power on a packet by packet basis • Increase/decrease transmission power when • Too many packets lost /Very few packets lost [5] • Average access time is very large/ small [6]

18. Possible drawback for adaptive power control algorithms • Increase/decrease transmission power too frequently /too rarely • How to determine the initial transmission power? • Falsely increases transmission power when it is not necessary • When a RTS times out, • the receiver channel is busy (receiving data, or NAV set) there is no need to increase power • the transmission power of RTS is not large enough • Ignore the relationship between the transmission power of sequential packets. (RTS<->CTS<->DATA<->ACK)

19. Proposed Class of Correlative power control (C2PC) algorithms Intuition • Noise level depends on how many transmitters generate interference • The number of transmitters around the receiver depends on the transmission range of the last control packet sent by the receiver • There exists a relationship between the necessary transmission power for RTS, CTS, DATA, ACK.

20. A B d RCTS,B RRTS,A Correlative power control algorithm Given the Tx power of RTS from A to B, what is the appropriate Tx power for a CTS from B to A to be received correctly • Basic definition • PRTS,B = PRTS,A/d4 • PCTS,A = PCTS,B/d4 • R4RTS,A = PRTS,A/Rxthreshold • R4CTS,B = PCTS,B/Rxthreshold • R4avg = Pavg / Rxthreshold • gain(A,B) = PRTS,B/PRTS,A • gain(B,A) = PCTS,A/PCTS,B Px,t : power of packet x at location t Rx,t : transmission range of packet x from transmitter t A B RTS CTS DATA ACK

21. A B d RCTS,B RRTS,A Correlative power control algorithm • Requirement (1) • RRTS,A ≥ d • RCTS,B ≥ d • Requirement (2) • PCTS,A≥Pnoise,A*SIRthreshold • And… A B RTS CTS DATA ACK

22. Worst case average noise level estimation assumes • All nodes within transmission range of the RTS are silenced by the RTS => only nodes outside RRTS,A interfere with the CTS • Any active node transmits at an average power Pavg • Any active node silences all nodes within its range • All nodes in the network that are not silenced are active => Active node density ∆ ≤ 1/ R2avg Pnoise,A =

23. Requirement to receive CTS successfully PRTS,A ≥ Rxthreshold / gain(A,B) PCTS,B ≥ Rxthreshold / gain(B,A) PCTS,B *(PRTS,A/Pavg)1/2 ≥ Rxthreshold*SIRthresohld*π/gain(A,B) A B d RCTS,B RRTS,A Correlative power control algorithm

24. Requirement for successful RTS-CTS-DATA-ACK handshaking • We can derive similar correlative requirement between • PCTS,B and PDATA,A • PDATA,A and PACK,B

25. Requirement for successful RTS-CTS-DATA-ACK handshaking • We finally obtain • Four Path loss constraints (1)-(4) • PRTS,A ≥ Rxthreshold / gain(A,B) • PCTS,B ≥ Rxthreshold / gain(B,A) • PDATA,A ≥ Rxthreshold / gain(A,B) • PACK,B ≥ Rxthreshold / gain(B,A) and …

26. Requirement for successful RTS-CTS-DATA-ACK handshaking • Three correlative constraints (5)-(7) • PCTS,B *(PRTS,A/Pavg)1/2 ≥ Rxthreshold*SIRthresohld*π/gain(A,B) • PDATA,A *(PCTS,B/Pavg)1/2 ≥ Rxthreshold*SIRthresohld*π/gain(B,A) • PACK,B *(PDATA,A/Pavg)1/2 ≥ Rxthreshold*SIRthresohld*π/gain(A,B) • If Pavg and PRTS are known PCTS, PDATA and PACK can be calculated

27. Deterministic correlative power control (CPC) algorithm • Let • U = Px,s* Py,r(1/2) • Pavg equal to Pmax • (x,y) in {(RTS,CTS),(CTS,DATA),(DATA,ACK)} • s is the sender of packet x • r is the sender of packet y • Assign the transmission power of RTS to be Pmax • Calculate U from the correlative constraints (5)-(7); assign appropriate transmission power for RTS, CTS, DATA, ACK • Ensure that power assignment fulfills path loss constraints (1)-(4)

28. Asymmetric Adaptive CPC algorithm(Implementation ongoing) • Let • U = Px,s* Py,r(1/2) • V=Pavg (may be initialized to Pmax) • (x,y) in {(RTS,CTS),(CTS,DATA),(DATA,ACK)} • s is the sender of packet x • r is the sender of packet y • Assign the transmission power of RTS to be Pmax • Calculate U from the correlative constraints (5)-(7); assign appropriate transmission power for RTS, CTS, DATA, ACK • Ensure that power assignment fulfills path loss constraints (1)-(4) • Change V adaptively • Increase/decrease V when packet loss too frequent/too rare

29. Symmetric CPC algorithm (Implementation ongoing) • Assume symmetric channel between sender/receiver => PRTS = PCTS = PDATA = PACK As a consequence we can calculate the power requirement Px,y = (const2 * Pavg)1/3 (8) const = Rxthreshold*SIRthresohld*π/gain(A,B) • V=Pavg (may be initialized to Pmax) • (x,y) in {RTS,CTS,DATA,ACK} X {sender, receiver} • Calculate Transmit power from (8) • Ensure that power assignment fulfills path loss constraints (1)-(4) • Estimation of V: • Bound V by Pmax • Increase/decrease V when packet loss too frequent/too rare (Adaptive CPC)

30. Algorithm evaluation • Experiment scenario • 50 nodes, 10 end to end random flows • Flows type: TCP or CBR • Network topology • Grid network (7 x 7 network with an extra node) • Sensor network( Grid network with a single sink) • Random topology network • Rand sensor network (Random topology network with a single sink)

31. Throughput comparison for TCP traffic Similar throughput

32. Throughput comparison for CBR traffic Deterministic CPC outperform others in most case Deterministic CPC is more stable than other power control algorithm.

33. Energy consumption comparison for TCP traffic Power control saves energy BASIC saves energy most.

34. Energy consumption comparison for CBR traffic Power control saves energy BASIC saves energy most. ( in the cost of huge throughput reduction)

35. Throughput/Energy consumption ratio comparison for TCP traffic The higher ratio is, the less energy consumption is needed for transmitting one bit.

36. Throughput/Energy consumption ratio comparison for CBR traffic Deterministic CPC outperforms others. Deterministic CPC is more stable.

37. Conclusion • Deterministic correlative power control algorithm outperforms IEEE802.11 MAC protocol or achieves similar performance in throughput, energy consumption. • Deterministic correlative power control algorithm is more stable than BASIC and adaptive power control algorithm. • Will the adaptive feature improve the correlative power control algorithm performance?

38. Reference • [1] A Power Control MAC Protocol for Ad Hoc Networks (MobiCom2002) Eun-Sun Jung, Nitin H. Vaidya • [2] Power Controlled Dual Channel (PCDC) Medium Access Protocol for Wireless Ad Hoc Networks (INFOCOM 2003) AlaaMuquattash and Marwan Krunz • [3]A Power Controlled Multiple Access Protocol for Wireless Packet Networks (INFOCOM 2001) Jeffrey P. Monks, Vaduvur Bharghavan and Wen-mei W. Hwu • [4] Intelligent Medium Access for Mobile Ad Hoc Networks with Busy Tones and Power Control(IEEE Journal on Selected Area in Communications 2000) Shu-Lin Wu, Yu-Chee Tseng, and Jang-Ping Sheu • [5] Distributed Power Control in Ad-hoc Wireless Networks (PIMRC 2001)Sharad Agarwal Srikanth et. al. • [6] Load Sensitive Transmission Power Control in Wireless Ad-hoc Networks (GLOBECOM 2002) Seung-Jong Park and Raghupathy Sivakumar