Syncob

Syncob
Collaborative Time Synchronization
in Wireless Sensor Networks

Albert Krohn1, Michael Beigl2, Christian Decker3, Till Riedel3

Particle GmbH, Germany
1
2
DUS/Universität Braunschweig, Germany
3
TecO/Universität Karlsruhe, Germany

08.06.07 - INSS '07 Syncob: Collaborative Time Synchronization in Wireless Sensor Networks 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


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

Multiple access

Good for
static topologies

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


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

Collaborative
synchronization


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

10101010101011001
S1S0S1S0S1S0S1S0S1S0S1S0S1S1S0S0S1


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
 S = active S = inactive
1 0


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

Spread spectrum

Add noise to
carrier

Simulation of
2 signals with
power 1


Alternative: ML energy-detector

See also:Albert Krohn, et. al.:The implementation of non-
coherent cooperativetransmission for WSNs. INSS 06.


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


Time shifts

∆t


not trivial for detector to make binary decision

Signal boarders get fuzzy

See again:Albert Krohn, et. al.:The implementation of non-
coherent cooperativetransmission for WSNs. INSS 06.


Resynchronization
 Maximum initial offset t∆init after sync
 Maximum tolerable offset ttol

Quartz accuracy k
 Oscillating Period T0
1 1

Oscillating difference ∆ T = T 0( − )
2k
2
1−k 1+ k
1−k 
 ttol=t∆init+tresync

2k
 für k2<<1: ttol=tresync2k+t∆init = t∆ init + tresync
ttol (
1 − k2


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)


Implementation:
Sync propagation time
Sync to network Sync to single partner

1
cumulated probability functions

0 20 Delay [ms] 100


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

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