Energy-Efficient Low Power Listening in Noisy Environments

Chenyang  Lu  Cyber-­‐Physical  Systems  Laboratory  Department  of  Computer  Science  and  Engineering  

Wireless  Sensors  in  Noisy  Environments  

Source:  AT&T  Labs  

2  

Noise  in  Residen4al  Environments Ø  Spectrum usage of 2.4 GHz band in six apartments and Bryan Hall.

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CCA  

Low  Power  Listening  Ø  Effective energy efficient MAC approach in clean environmentsØ  BoX-MAC-2: Default MAC protocol of TinyOS

4  

Sender  

Recipient  

Packet  

RX  

ACK  

False  Wakeup  Caused  by  Noise  

5  

Recipient  RX  

CCA  

Time  out  

Sender  

Noise  

False  Wakeup  Rates  in  Homes  

Ø Measurement in an apartmentq  Ideal: duty cycle < 1%, lifetime > 1 month.

q  Real: duty cycle > 4%, lifetime < 1 week.

Ø  This is a wide spread problem!

 

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Wakeup  Check  

K  readings  ≥  wakeup  threshold  à  wakeup  

Poll  CCA  pin  N  Bmes  

Increase  wakeup  threshold

0      1      1      1      1      1      1      0      0      0                              

RSSI

wakeup  threshold

0      0      0      1      0      0      0      0      0      0    

wakeup  threshold

RSSI

No  wakeup  

Idea:  Adapt  Wakeup  Threshold  

wakeup  threshold

RSSI min  packet

max  noise

min  packet  RSS  >  wakeup  threshold  >  max  noise  

RSSI min  packet max  noise

No  wakeup  threshold  will  work,  but  it  will  be  a  bad  link  anyway  

Impact  on  False  Wakeup  Rate

 

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False wakeup rate under interference of an 802.11n router

Objec4ves  

1.  Maintain link reliability (ETX threshold) 2.  Reduce false wakeup rate (wakeup rate threshold)

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AEDP:  Adap4ve  Energy  Detec4on  Protocol

Ø  Controlled variablesq  ETX: recent expected transmission count in a sliding window.

q  WR: recent wakeup rate in the sliding window

q  WRL: long-term average wakeup rate à battery life  

Ø  Adapt  wakeup  threshold  T  online  q  if ETX > threshold à T=Tmin

q  if ETX ≤ threshold•  WR > threshold à T=T+△T

•  WR ≤ threshold AND WRL > threshold à no change.•  WR ≤ threshold AND WRL ≤ threshold à T=T-△T

q  Reset T periodically to react to network changes

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Cri4cal  Flaw  of  BoX-­‐MAC-­‐2  in  TinyOS

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•  Sender: gap between transmissions is 8ms•  >> time needed to receive a (soft) ACK (≤2.4ms)

•  Receiver: wakeup check lasts 8.8ms (poll CCA pin 400 times)

•  >> time needed to receive an entire packet (<4.3ms)!

•  Contradict the intended short wakeup check à energy waste!

Sender  

Recipient  

ACK  

RX  

8ms

CCA  

8.8ms

Fix  for  BoX-­‐MAC-­‐2  in  TinyOS •  Sender: reduce gap between transmissions from 8msà2.8ms

•  > time needed to receive a (soft) ACK (≤2.4ms)•  Receiver: shorten wakeup check 8.8msà2.9ms (poll CCA pin 115 times)

•  < time it takes to receive an entire packet (<4.3ms)

•  Shorter wakeup check à lower duty cycle

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Sender  

Recipient  (fixed)  

RX  

CCA   ACK  

Duty  Cycle  in  an  Apartment

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AEDP reduces mean duty cycle by•  45.5% over fixed BoX-MAC-2•  65.1% over the default BoX-MAC-2

Effects  of  Links’  Signal  Strength

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•  Stronger links benefit more•  Does no harm to weak links

Comparison  with  A-­‐MAC

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•  AEDP wins at low data rates•  AEDP wakeup check: 2.9ms •  A-MAC polling cost: 6.2ms

•  A-MAC wins at high data rates•  Avoids sending duplicated packets to wake up receiver

Related  Works

Ø  Optimize CCA to avoid collision [Brodsky SigComm’09, Boano EWSN’10]

Ø  AEDP focuses on CCA threshold’s role in wakeup

Ø  Enhance reliability under interference [Srinivasan TOSN’10, Liang SenSys’10]

Ø  AEDP focuses on energy efficiency in noisy environments

Ø  Receiver-initiated MAC [Sun SenSys’08, Dutta SenSys’10]

Ø  AEDP addresses sender-initiated MACØ  Complementary to each other for different data rates

Ø  ContikiMAC performs two CCA checks spaced slightly apart [Dunkels ’11]

Ø  AEDP performs only one CCA check à more efficient

Ø  SoNIC classifies specific types of interference sources [Frederik IPSN’13]

Ø  AEDP is a generic approach for all noise

Conclusion Ø  False wakeups compromise energy efficiency of Low Power

Listening in noisy environments.

Ø AEDP: Adaptive Energy Detection Protocol for LPL q  Self-tunes wakeup threshold online.

q  Implemented in TinyOS 2.1.1 on TelosB.

q  Fixes critical flaw of BoX-MAC-2 implementation.

Ø AEDP mitigates the impact of noise on radio duty cycles. q  Significantly reduces false wakeup rate under LPL.

q  Adapts to links with different signal strength.q  Outperforms A-MAC for low data rate applications.

M. Sha, G. Hackmann and C. Lu, Energy-Efficient Low Power Listening for Wireless Sensor Networks in Noisy Environments, ACM/IEEE International Conference on Information Processing in Sensor Networks (IPSN'13), April 2013.