Dec 8 th, RTSS 2004, Presented by Ajay TirumalaPower Point created by Qixin Wang and Ajay Tirumala Lightning: A fast and lightweight acoustic localization

Dec 8th, RTSS 2004, Presented by Ajay Tirumala Power Point created by Qixin Wang and Ajay Tirumala

Lightning:A fast and lightweight acoustic localization protocol using low-end wireless

micro-sensors.

Qixin Wang, Rong Zheng, Ajay Tirumala, Xue Liu and Lui Sha.


Outline of the presentation

• Demand

• Observations and Solution Heuristics

• Protocol Details

• Theorems and Experiment Results

• Demo Video

• Conclusion


Demand

• Want a fast, deterministic (i.e.O(1) response time) acoustic event localization scheme.

• Fits low-end wireless micro-sensor networking.

• Proximity localization, i.e. electing the closest sensor, is good enough.


Observations

• Sound sources are often directional and of unknown intensities. This implies:– Intensity-based localization is not desirable.– Time-Of-Arrival (TOA) is a more reliable metric. When

sensors are densely deployed, Line-Of-Sight to the closest sensor can usually be guaranteed, therefore earliest TOA closest sensor.

• Radio (RF) wave travels much faster than acoustic wave.– When a lightning strikes, people see the lightning

before hearing the rumbling of the thunder.


Solution Heuristic I

• When sound reaches the closest sensor, the closest sensor should immediately announce the event (via RF broadcast) to all other sensors and suppress them even before they hear the sound.


Observation

• Immediate Data Packet (DP) RF broadcast is not practical, because of collisions.– To think several sensors, all almost the same

distance to the sound source, try to broadcast data packets at almost the same time.


Solution Heuristic II

• Do NOT use data packet broadcast, broadcast RF burst instead.

RF burst is not susceptible to overlapping.


Observation

• If there are multiple closest sensors, there can be multiple election winners.

• How to guarantee every time there is only one winner?


Solution Heuristics III

• Color the sensors, to differentiate RF burst duration, to break ties.– It is proven, with regular sensor layout and

proper coloring, it is guaranteed to always elect one winner sensor.

– (To be included in our upcoming publications) Empirically, even with random sensor layout and without coloring, the number of winners is still well limited.


Remark

• Heuristics I, II and III lead to the design of Basic Lightning Protocol.


Observation

• Energy cost is a concern.– Currently, a sensor has to have RF on all the

time to listen to possible RF bursts. Remember a sensor to be suppressed receives RF burst before hears the sound.

• How to have RF module sleep during most of the time and only be turned on when there is an acoustic event?


Solution Heuristics IV

• RF Sleep during usual time.• When hears a sound, turn on RF and RF listen

for Δdefer sec, to make sure all other sensors that can hear the sound have turned on their radios. Then carry out the same procedure as Basic Lightning Protocol.

• Equivalent to the sound takes place Δdefer sec later in real-world, and Basic Lightning Protocol is deployed.


Remark

• Heuristics IV leads to the design of Energy-Efficient Lightning Protocol.


Basic Lightning Protocol• Regular sensor layout with

certain coloring• All sensors are initially in RF-

listen mode• Beep recognized:

– broadcast RF burst without backoff for iTburst.

– Listen for RF bursts for Tb.• No other burst recognized

“elected”• Other burst recognized –

”supressed”• During RF listen

– RF burst recognized• Enter supressed mode

• Reenter RF listen mode after basic timer expires.

RF Listen

Beep recognizedRF burst

for i.Tburst

Post burst listen

Burst recognizedSupressedSet reset timer

No burst recognizedElected

set reset timer

Timer expiresBurst recognized

Timer expires


Energy-efficient lightning protocol

• All sensors initially in RF sleeping

• When beep is recognized– Listen for RF bursts but defer

bursting for Δdefer

– If no burst is recognized, transmit RF burst without backoff ， for iTburst sec.

– Post burst RF listen • No other burst recognized

“elected”• Other burst recognized –

”suppressed” – If burst is recognized in RF

listen – enter suppressed mode• When reset timer expires,

return to RF Sleeping mode

RF Sleeping

Beep recognizedRF listen for

defer

Post burst listen

Burst recognizedSupressedSet reset timer

No burst recognizedElected

set reset timer

Timer expires

Timer expires

Burst recognized

RF burst

for i.Tburst


Key Properties

• Theorem 1,2: Both Basic and Energy-Efficient Lightning Protocol elect a unique winner with deterministic localization distance error.

• Corollary 1,2: Both have a short and O(1) time bound for the election.


Experiment results

• Implemented on U.C. Berkeley MICA Motes• Directional Sound Source


Experiment results

• Comparable, or even better accuracy than ideal (no pkt loss) data packet based localization.


Experiment Results

• Fast: in the sense that the Upper Bound of Lightning Protocol Election Delay is shorter than the Lower Bound of Data Packet Election Protocol

0

10

20

30

40

50

60

Max Min Mean

LightningColor 4 (ms)

DP - ClosestFirst (ms)

DP - Any first(ms)


Experiment Results

• Deterministic Election Delay, while data packet localization’s election delay are random due to MAC contention.


Experiment Results

• O(1) broadcasts– Theoretical: Guaranteed to be <= 4.– Experiment: Never more than 2.

Number of bursts in lightning protocol

1 2 > 2

% of localization trials 81.4 18.6 0


Demo video (Qixin Wang)

URL: http://www-rtsl.cs.uiuc.edu/papers/LightningDemo.html

See reference [13]

http://www-rtsl.cs.uiuc.edu/papers/LightningDemo.html


Conclusion• Fast and Deterministic:

– O(1) election delay. – O(1) number of broadcasts.

• Lightweight: – only involves simple comparison (<>) operation; – no clock synchronization needed; – simple network stack.

• Comparable, or even better localization accuracy than ideal data packet scheme.

• Robust: – Immune to RF broadcast overlapping; – Handles directional sound source

• Energy Efficient: – only turns on RF module when there is an acoustic event.


Thank you!

Documents

Dec 8 th, RTSS 2004, Presented by Ajay TirumalaPower Point created by Qixin Wang and Ajay Tirumala Lightning: A fast and lightweight acoustic localization