Pulsar: A Wireless Propagation-Aware Clock Synchronization Platform - - PowerPoint PPT Presentation

Pulsar: A Wireless Propagation-Aware Clock Synchronization Platform

Adwait Dongare, Patrick Lazik, Niranjini Rajagopal, Anthony Rowe RTAS ’17, Pittsburgh PA April 19, 2017

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Accurate Timekeeping

2

Antiquity:
 Navigation Sensing:
 LIGO Communication:
 GSM Databases

Spanner

Kdb+

Motivating Applications

3

sec msec nsec μsec

Physics Robotics Control TDMA Sound OS Scheduling Structural
 Vibration Speed-of-Light
 Localization Distributed
 Antenna Arrays Virtual Reality

time

Human Response

c = 299792458 m / s
 1 nsec: 30 cm

Overview

• Motivation
• The Pulsar Platform
• Analysis and Evaluation

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Focus: Next-Generation Wireless (1/2)

5 Spectrum (Hz) Space (bits/sec/Hz/M2) Spectral Efficiency (bits/sec/Hz)

2X 2X >10X

Bell Labs, “The Future X Networks”, 2016

Focus: Next-Generation Wireless (2/2)

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apple.com, airport extreme

Spatial Multiplexing

• Currently
• Multiplexing on single access

point (AP)

• Carefully matched signal path
• Software-defined radios (SDR)

are popular research platforms

• Accurate synchronization →


Multiplexing on multiple APs

How Is It Done Today?

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10 MHz 1 PPS

Clock Synchronization

• Time → events
• Clock → counting “regular” events
• Clock synchronization → agreement on start & counts per epoch
• Time synchronization → agreement with standard reference (like UTC)

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David Allan ToF = L / cwire 1 10 tdavid tallan L 51 62

propagation
 delay propagation-aware 10.000 62.003

Propagation Delay

• Two-way messaging
• Network Delay (e.g. NTP)
• Propagation Time Delay
• Can compensate for

propagation delay if distances are known

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M D2 D1

D12 D11

Overview

• Motivation
• The Pulsar Platform
• Analysis and Evaluation

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Ideal Time-Transfer Platform

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Reference

{0, t0}

Ideal
 Platform

event i

Ideal Clock

{xi, ti}

Ideal Clock

act j, {xj, tj} tj

Objective: Time-transfer - sharing a time reference across locations Capabilities

• Perfect event timestamps {x, t}
• Perfectly-timed actions with

timestamp Requirements

• Timing: Ideal clocks
• Ranging: Infinite bandwidth

Our Pulsar Platform

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• Stable clock:


Chip-scale atomic clock

• Ultra-wideband ranging radio:


15.6 ps hardware timestamps

• Glue logic:
• Low-jitter phase-locked loop
• ARM processor
• Phase measurement unit
• We implement time-of-flight

(propagation-aware) clock synchronization

Chip%Scale%% Atomic%Clock% Fout% DW1000% LMX2571% Fin% PPSout% PPSin% Antenna% Tune% ARM%K22F% RPI%2%Header%

Hardware repository:
 https://upverter.com/WiselabCMU/eab20f02c4d4f096/Pulsar-V2/

Overview

• Motivation
• The Pulsar Platform
• Analysis and Evaluation

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Problem: Message Passing for
 Clock Synchronization

• Frequency estimation:

• ne-way messaging

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ref
 node child
 node t’i-1:TX t”i-1:RX t”i:RX t’i:TX t”R(i-1):TX t”R(i):TX t’R(i-1):RX t’R(i):RX

ref
 clock child
 clock

∆ti

TOF =

• t0

R(i):RX − t0 i:TX

• −
• t00

R(i):TX − t00 i:RX

• 2

= < =

yi = f child

i

f reference

i

= t00

i:RX − t00 i-1:RX

t0

i:TX − t0 i-1:TX

• Time-of-flight estimation:


two-way messaging

• Clocks must remain stable

between message exchanges

• Accurate timestamps

• Clock accuracy → δf/f (ppm)
• Clock stability → accuracy
• ver time: Allan variance
• Clock synchronization will

degrade over time

2 −4 −3.5 −3 −2.5 −2 −1.5 −1 −0.5 0.5 1 4 6 8 10 12 14 16 Time (day) Frequency (ppm)

Problem: Clock Stability

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• D. Mills, Computer Network Time Synchronization

2

y(⌧) = 1

2 ⌦ (¯ yi − ¯ yi1)2↵

i

= < =

100 101 102 103 104 105 10−3 10−2 10−1 100 101 102 Micro PPS Lan Allan deviation (ppm) Time interval (s)

Time between frequency samples(yi, yi-1)

Allan Deviation

16 10-2 10-1 100 101 102

= (sec)

10-12 10-11 10-10 10-9 10-8 10-7

Allan Deviation <y(=)

Quartz TCXO CSAC CSAC Datasheet

f r e q u e n c y 
 l i n e Allan intercept phase
 line

Time between frequency samples
 OR Averaging time

time ⟷ phase

Problem: Phase Synchronization

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38.4 MHz

UWB Radio (DW1000) radio clock

reset

Phase Lock Loop Clock (CSAC)

10 MHz 1 PPS

MCU

message & 
 timestamp tuning

• Precise timing for e.g. SDR
• Frequency input: 10 MHz
• Phase/time input: 1 PPS


✓

??

Problem: Clock Phase Offset

• Radio clock does not reset

exactly at desired time

• Electronic rise time
• Digital I/O time

discretization
 (38.4 MHz ~ 26 nsec)

• Offsets are constant after PLL

lock

• PMU for δtCPO measurement

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time Voltage

1 PPS→reset 38.4 MHz

δtCPO

transition
 level

time

Pulsar Architecture

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38.4 MHz

UWB Radio (DW1000) radio clock

reset

Phase Lock Loop Clock (CSAC)

10 MHz 1 PPS

MCU

message & 
 timestamp tuning

PMU

δtCPO ΔtTOF ΔtPPS Δf

Synchronization Protocol

• Proof-of-concept protocol
• Algorithm
• Frequency synchronization
• Phase synchronization
• Phase bootstrap
• Time distribution tree
• Timestamp variance as a

simple link metric

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• 5

5 10 15 20 25

• 2

2 4 6 8 10 12

• 8
• 7
• 6
• 5
• 4
• 3
• 2

Log (variance) Meters Meters 2 (Master) 3 4 5 6 7 1

M D2 D1

D12 D11

Evaluation

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• 1

1 2 3 4 5

Error in " tPPS : corrected (nsec)

20 40 60 80

counts

7 =2.12 nsec < =0.84 nsec

ϵt

Reference Node Child
 Node

ground
 truth estimate

ΔtPPS ΔtPPS:estimate

Logic Analyzer

ΔtPPS = ΔtPPS:estimate + ϵt

1PPS 1PPS

Software repository:
 http://git.wise.ece.cmu.edu/adwait/pulsar_freertos

Future Work

• Testing lower cost OCXOs (~$100/

clock)
 e.g. CW-OH200 Series

• Phase-measurement unit integration
• Testing other flexible and robust

synchronization protocols

• SDR synchronization with Pulsar

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Conclusions

• Pulsar: Platform & protocol

for better than 5 nsec clock synchronization
 (below 26 nsec digital time discretization of radio components)

• End-to-end evaluation and

analysis of timing errors

• Provide precise timing for

an application (SDR) to enable spatial multiplexing

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Chip%Scale%% Atomic%Clock% Fout% DW1000% LMX2571% Fin% PPSout% PPSin% Antenna% Tune% ARM%K22F% RPI%2%Header%

!

Thank you

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