energy-efficient time synchronization achieving nanosecond accuracy in wireless networks

Energy-Efficient Time SynchronizationAchieving Nanosecond Accuracy

in Wireless Networks

Kyeong Soo (Joseph) Kim(With S. Lee and E. G. Lim@XJTLU)

Department of Electrical and Electronic EngineeringXi’an Jiaotong-Liverpool University

2016 ICIOT-5GMTGuangzhou University27-28 November, 2016

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Outline

Introduction

Time and Space in Synchronization

Energy-Efficient Time Synchronization for AsymmetricWireless Networks

Simulation Results

Next Steps: Extension to Multi-Hop Time Synchronization

Conclusions

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Next . . .IntroductionTime and Space in SynchronizationEnergy-Efficient Time Synchronization for AsymmetricWireless Networks

Hardware and Logical Clock ModelsEffect of Clock Skew on Measurement Time EstimationAsynchronous Source Clock Frequency Recovery atSensor Nodes: One-Way Clock Skew Estimation

Simulation ResultsPerformance of One-Way Clock Skew EstimationPerformance of Measurement Time Estimation andEnergy EfficiencyEffect of Bundling of Measurement Data

Next Steps: Extension to Multi-Hop Time SynchronizationConclusions

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An Asymmetric Wireless Network

HeadeNode

Internet

RemoteeUser

SensoreNodes

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Head Node

I A base station that serves as agateway between wired andwireless networks.

I A center for fusion of datafrom distributed sensors.

I Equipped with a powerfulprocessor and supplied powerfrom outlet.

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Sensor Node

I Measuring data and/or detectevents with sensors andconnected to a WSN onlythrough wireless channels.

I Limited in processing andbattery-powered.

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Design Goals

I Achieving sub-microsecond time synchronizationaccuracy

I Through propagation delay compensation.I With higher energy efficiency at battery-powered

sensor nodesI Minimize the number of packet transmissions and the

amount of computation at sensor nodes.

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Effects of Time and Space

The effects of time and space are so closely related thatthey cannot be easily separated from each other as in thefollowing examples:I Synchronization and localization accuracies.

I In time-based localization.I e.g. Time of arrival (TOA).

I Clock offset and propagation delay.I In one-way synchronization.

I e.g. Flooding time synchronization protocol (FTSP).

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Synchronization and Localization Accuracies

I AccuraciesI 1 ms↔ 300 kmI 1µs↔ 300 mI 1 ns↔ 30 cmI 1 ps↔ 0.3 mm

I Time-based localization schemesI Time of arrival (TOA)I Time difference of arrival (TDOA)

I A special variation of TDOA with virtual anchors doesnot require synchronization among devices.⇒ See the next slide.

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TDOA with Virtual Anchors 1

Anchor

Agent

Virtual

Anchors

1E. Leitinger et al., IEEE J. Sel. Areas Commun., vol. 33, no. 11, pp.2313–2328, Nov. 2015.

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Clock Offset and Propagation DelayCan the receiver distinguish between the following twocases if θ = d?

Packet with

Timestamp T

�

� � �

vs. Packet with

Timestamp T

�

� � ��

TX

RX

TX

RX

• �: Clock offset

• �: Propagation delay

I Answer is “No”.I Two-way message exchanges needed for delay

compensation.13 / 49





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Conventional Two-Way Message Exchanges I

Master

sHead Node)

Slave

sSensor Node)Measurement

Interval of Time Sync. si.e., 2-Way Message Exchange)

……

Report

Request

Response

Report

Measurement

T1

T2

T4

T3

I Sensor nodes transmit “Request” messages forsynchronization.

I In addition to measurement data packets.15 / 49

Conventional Two-Way Message ExchangesII

I The sensor node can estimate its clock offset w.r.t. thehead node and synchronize its clock to that of thehead node:

I Clock offset: θ̂ =(T2 − T1) − (T4 − T3)

2.

I Propagation delay: d̂ =(T2 − T1) + (T4 − T3)

2.

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Reverse Two-Way Message Exchanges I

Master

sHead Node)

Slave

sSensor Node)

Beacon/Request

sMeasurement)

Report/Response

T1 T4

T3T2

d

tm

��

I Sensor nodes do not transmit any other messagesexcept “Request/Response” messages.

I If there are no measurement data, sensor nodes justreceive messages.

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Reverse Two-Way Message Exchanges II

I The head node can estimate the clock offset of thesensor node, but the sensor node cannot.

I As a result, the information of all sensor node clocksis centrally managed at the head node.

I “Response” (synchronization) and “Report”(measurement data) messages can be combined tosave the number of message transmissions from thesensor node.

I Optionally measurement data and correspondingtimestamps can be bundled together in a“Report/Response” message when there are no stricttiming requirements.

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Next . . .

Introduction




Simulation Results


Conclusions19 / 49

Hardware Clock Model

Time Ti of the hardware clock of the ith sensor node at thereference time t is modeled as a first-order affine function:

Ti(t) = (1 + εi)t + θi,

whereI (1 + εi) ∈ R+: Clock frequency ratio.2

I θi ∈ R: Clock offset.

2εi is called a clock skew in the literature.20 / 49

Logical Clock Model

Time Ti of the logical clock of the ith sensor node athardware clock time Ti(t) is modeled as a piecewise linearfunction: For tk<t≤tk+1 (k=0, 1, . . .),

Ti

(Ti(t)

)= Ti

(Ti(tk)

)+

Ti(t) − Ti(tk)1 + ε̂i,k

− θ̂i,k,

whereI tk: Reference time when a kth synchronization occurs.I ε̂i,k: Estimated clock skew from the kth

synchronization.I θ̂i,k: Estimated clock offset from the kth

synchronization.

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Next . . .

Introduction




Simulation Results


Conclusions22 / 49

Measurement Time Estimation Error:Conventional Two-Way Message Exchanges

Master

sHead Node)

Slave

sSensor Node)Measurement

Request

Response

Report

s1

s2≈s3

s4

d

��

tm

I When Tm�d,∆t̂Conv.

m ∼ Tm × ∆ε̂i,

where ∆ε̂i is the clock skew estimation error.23 / 49

Measurement Time Estimation Error:Reverse Two-Way Message Exchanges

Master

sHead Node)

Slave

sSensor Node)

Beacon/Request

sMeasurement)

Report/Response

T1 T4

T3T2

d

tm

��

I When Tm�d,

∆t̂Rev.m ∼

Tm

2× ∆ε̂i.

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Next . . .

Introduction




Simulation Results


Conclusions25 / 49

Message Departure and Arrival Times

I Let td(k) (k=0, 1, . . .) be the reference time for the kthmessage’s departure from the head node.

I td(k) also denotes the value of the timestamp carriedby the kth message.

I Then the arrival time of the kth message with respectto the ith sensor node’s hardware clock is given by

ta,i(k) = Ti (td(k)) + d(k) = (1 + εi)td(k) + θi + d(k),

whereI d(k): One-way propagation delay in terms of the ith

sensor node’s hardware clock.

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Joint Maximum Likelihood EstimatorsFor a white Gaussian delay d(k) with known mean d andvariance σ2,

θ̂MLi (k) =

t2d · ta,i − td · tdta,i

t2d −

(td

)2− d,

R̂MLi (k) =

tdta,i − td · ta,i

t2d −

(td

)2,

where

I x ,∑k

j=0x(j)k

,

I xy ,∑k

j=0x(j)y(j)

k.

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Regression through The Origin (RTO) Model

The problem of asynchronous source clock frequencyrecovery (SCFR) can be formulated as a linear RTO modelas follows: For k = 1, 2, . . .,

t̃a,i(k) = (1 + εi)t̃d(k) + d̃(k),

whereI t̃a,i(k),ta,i(k)−ta,i(0),I t̃d(k),td(k)−td(0),I d̃(k),d(k)−d(0).

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Cumulative Ratio (CR) Estimator

R̂CRi (k) =

t̃a,i(k)t̃d(k)

= Ri +d̃(k)t̃s(k)

,

whereI Ri: Ratio of the ith sensor node hardware clock

frequency to that of the reference clock (i.e., 1+εi).

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Next . . .

Introduction





Conclusions31 / 49

Estimated Clock Skews with Gaussian Delays: σ=1 ns

5 10 15 20 25 30 35 40 45 50Number of Messages

10−21

10−20

10−19

10−18

10−17

10−16

10−15

MSE

RLSCRJoint MLEGMLLE (Two-Way)LB for CRCRLB for Joint MLELB for GMLLE

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Estimated Clock Skews with Gaussian Delays: σ=1µs


10−15

10−14

10−13

10−12

10−11

10−10

10−9

MSE

RLSCRJoint MLEGMLLE (Two-Way)LB for CRCRLB for Joint MLELB for GMLLE

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Estimated Clock Skews with AR(1) Delays3: σ=1µs


10−14

10−13

10−12

10−11

10−10M

SE

RLSCRJoint MLEGMLLE (Two-Way)

3ρ = 0.6.34 / 49

Estimated Clock Skews with AR(1) Delays: σ=1 ms


10−8

10−7

10−6

10−5

10−4M

SE

RLSCRJoint MLEGMLLE (Two-Way)

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Next . . .

Introduction





Conclusions36 / 49

Estimated Frequency Ratio (Sensor Node) andMeasurement Time (Head Node): SI=100 s

-4E-11

-2E-11

0E+00

2E-11

4E-11

Freq

uenc

yD

iffer

ence

[ppm

]

Proposed (w/ CR)Two-Way (w/ GMLLE)

0 500 1000 1500 2000 2500 3000 3500

Time [s]

-1E-02

-8E-03

-6E-03

-4E-03

-2E-03

0E+00

2E-03

4E-03

Mea

sure

men

tTim

eE

rror

[s]

Proposed (w/ CR)Two-Way (w/ GMLLE)Two-Way

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Estimated Frequency Ratio (Sensor Node) andMeasurement Time (Head Node): SI=1 s

-4E-11

-2E-11

0E+00

2E-11

4E-11

Freq

uenc

yD

iffer

ence

[ppm

]


0 500 1000 1500 2000 2500 3000 3500

Time [s]

-1E-04

-8E-05

-6E-05

-4E-05

-2E-05

0E+00

2E-05

4E-05

Mea

sure

men

tTim

eE

rror

[s]


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Estimated Frequency Ratio (Sensor Node) andMeasurement Time (Head Node): SI=1 ms

-4E-11

-2E-11

0E+00

2E-11

4E-11

Freq

uenc

yD

iffer

ence

[ppm

]


0 500 1000 1500 2000 2500 3000 3500

Time [s]

-1E-06

-8E-07

-6E-07

-4E-07

-2E-07

0E+00

2E-07

4E-07

Mea

sure

men

tTim

eE

rror

[s]


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Effect of SI on Time Synchronization andEnergy Consumption4

Synchronization Skew Estimation Measurement Time NTX NRXScheme MSE Estimation MSE

ProposedSI=100 s 8.8811E-25 5.8990E-19 100 36SI=1 s 9.1748E-25 5.4210E-19 100 3600SI=10 ms 1.0887E-24 4.7684E-19 100 360100

Two-Way with GMLLESI=100 s 1.9021E-24 4.7784E-19 136 36SI=1 s 1.7034E-24 6.1452E-19 3700 3600SI=10 ms 9.0992E-25 4.0485E-19 360100 360000

Two-WaySI=100 s

N/A3.4900E-05 136 36

SI=1 s 3.4564E-09 3700 3600SI=10 ms 3.3638E-13 360100 360000

4Estimations are for the samples taken after 360 s (i.e., one tenth ofthe observation period) to avoid the effect of a transient period.

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Next . . .

Introduction





Conclusions41 / 49

Effect of Bundling on Measurement Time Estimation5

0 500 1000 1500 2000 2500 3000 3500

Time [s]

-2.0E-09

-1.0E-09

0.0E+00

1.0E-09

2.0E-09

Mea

sure

men

tTim

eE

rror

[s]

NBM=1NBM=2NBM=5NBM=10

5SI = 1 s.42 / 49

Effect of Bundling on Time Synchronization andEnergy Consumption

Synchronization Scheme Measurement Time NTX NRXEstimation MSE

Proposed

NBM = 1 5.4210E-19 100 3600NBM = 2 5.1116E-19 50 3600NBM = 5 3.7504E-19 20 3600NBM = 10 2.6468E-19 10 3600

I In interpreting the results, the following should betaken into account:

I The bundling increases the length of messagepayload.

I The increased message payload also can affect theframe errors and the number of retransmissions.

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Multi-Hop Extension through Gateways

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Challenges and Opportunities

I Tradeoff between time-translating andpacket-relaying gateways..

I The multi-hop extension should be implementedtogether with a routing protocol.

I As in LEACH protocol6 and its many variations, theenergy efficiency is also critical in the formation of ahierarchy and the selection of cluster heads (i.e., thegateway nodes in the multi-hop extension of theproposed scheme).

6W. R. Heinzelman et al., Proc. HICSS’00, Jan. 2000, pp. 1–10.46 / 49

Preliminary Results

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Conclusions

I Propose an energy-efficient time synchronizationscheme for asymmetric wireless networks achievingsub-microsecond time synchronization accuracy.

I Also, discuss the optional bundling of measurementdata in a “Report/Response” message.

I Topics for further study includeI Extension to multi-hop synchronization through

packet-relaying or time-translating gateway nodes;I Energy-delay tradeoff and the effect of frame errors

and retransmissions in bundling of measurementdata.

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