syncob

SyncobCollaborative Time Synchronization

in Wireless Sensor Networks

Albert Krohn1, Michael Beigl2, Christian Decker3, Till Riedel3

1Particle GmbH, Germany2 DUS/Universität Braunschweig, Germany

3 TecO/Universität Karlsruhe, Germany

Syncob: Collaborative Time Synchronization in Wireless Sensor Networks08.06.07 - INSS '07 2

Motivation: Collaborative Sensing

Wireless Sensor Networks Collaborative monitoring Duty cycle

100ms every 2sec Short time to communicate

Important: synchronization


Motivation: Ultrasound location

∆t

Distance: time of flight Nodes only measure time Approx. 10µs per 3mm Accurate synchronization Global timestamps for tracking


Sync times at different layers


Physical Layer Synchronization

On PHY: Only radio propagation delays Very deterministic Accurate synchronization

Simple for single source of synchronization

More complicated for distributed operation All nodes re-synchronize their neighbors Need for coordination


Conflicting sync

Random access Especially

problematic in dense scenarios

Can be resolved via CSMA Hidden or Exposed Terminal Problems Can make synchronization unstable


Conflict-free sync

Can use CDMA,FDMA for beacons Difficult to choose non-overlapping codes/freq Inefficient for mobile scenarios

Multiple access Good for

static topologies


SynCob: Collaborative Sync

Collaborative time synchronization Send simultaneously

on same frequency band Use principles of cooperate transmission Receiver can still decode the synchronization Support for ad-hoc, mobile scenarios Implementation for low-cost hardware

Collaborativesynchronization


Related Work

Link Layer: e.g. LTS, mini-sync Physical layer: RBS, BITMac

BitMAC Collision-free

synchronization Proposes “or” on PHY

“Identical transmissions by two senders with small synchronization errors. The receiver will see slightly stretched “1”bits and slightly compressed “0”bits ”Source: Ringwald,M. ,Römer K.: BitMac A Deterministic, Collision-Free and Robust MAC Protocol for Sensor Nodes. EWSN 2005


Sync symbol sequence

10101010101011001S1S0S1S0S1S0S1S0S1S0S1S0S1S1S0S0S1

No channel or source coding! Superimpose two sync symbols Special case of cooperative transmission:

Narrow band radio Can be used with FSK,ASK or OOK Here: Narrow band binary OOK/ASK

S1= active S0= inactive


Excursion:superimposed radio

10101010101011001 OR 10101010101011001


Superimposing sync sequences

10101010101011001 OR 10101010101011001

10101010100011001


Superimposing sync sequences


Destructive interference

We do not have an “OR” behavior on the channel !!


Using signatures to handle interference

Alternative: ML energy-detector See also:Albert Krohn, et. al.:The implementation of non-

coherent cooperativetransmission for WSNs. INSS 06.

Spread spectrum Add noise to

carrier

Simulation of 2 signals with power 1


Distributed Synchronization

No predefined roles

Each node is responsible for: Establishing sync Keeping up the sync Rate control

No additional communication channel necessary No cooperate in ad-hoc manner


Resynchronization

∆ T = T0(1

1−k−

11+k

)

ttol = t∆ init +tresync (2k

1−k2 ) ≈ t∆ init + tp · 2k

ttol = t∆ init +tp · 2k +t

Maximum initial offset t∆init after sync Maximum tolerable offset t

tol Quartz accuracy k Oscillating Period T

0 Oscillating difference

ttol=t∆init+tresync

für k2<<1: ttol=tresync

2k+t∆init

2k

1−k 2


Implementation: Particle AwareCon Protocol


Implementation: Particle AwareCon Protocol

Particle Computer Sensor Nodes TR1001 Transceiver OOK/ASK 8bit 5MHz PIC18F6720 MCU (t∆init=0.2µs) 10 ppm Quarz (k=10 *10-6)

S1 and S0 24µs, data rate 125kbit/s (ttol

=4µs)

tresync

=(ttol-t∆init)/2k=190ms

Framesize 13ms =>4% every 14 frames(Current Syncob/Awarecon synchronizes every slot and changes status to unsynchronized after 7 for stability)


Issues/ Future work

Over-sized loops Synchronization returns over multiple hops Limit maximum time-shift Assumptions about physical

and topological layout necessary

Concurrent island Two synchronized networks join Collision Detection/Resolution Single channel approach Preference based election


Conclusion

High Accuracy < 10µs Syncob suited for WSN fusion and coordination Can be used for sound based location No additional coordination necessary Ideal for mobile ad-hoc scenarios

Averages sync collaboratively Locally adapts to network density


Question?

syncob

Technology

synchronization unstable

synchronization support

time of flightnodes

sensor nodes

physical layer synchronization

small synchronization

time shifts t

sec short time