timing for iot applications - cambridge wireless€¦ · 01/05/2019 · time sources available for...

Timing for IoTapplications

Chris Marshall

1 May 2019

Introduction

IoT devices

absolute and relative timing

Sources of timing

wireless provision of time

location and time

Some hybrid timing examples

u-blox AG2

Timing for Internet of Things applications

u-blox AG3

Internet of ThingsApplications

Hours Hours-Minutes Sec-mSec µSec and better

1st Gen 2nd Gen 3rd Gen 4th Gen

Mechanization,

water power,

steam power

Mass production,

assembly line,

electricity

Computer and

automation

Cyber physical

systems

• Lots of devices, sensors

• Connected (e.g. wirelessly)

• Often static

• Often indoors

• Application from shared information

• Measurements and data have value through knowledge of its time

• Instructions and control require coordination of actuators

The relevance of timing in industrial revolutions

u-blox AG4

What’s a nanosecond between friends?What are IoT applications looking for

Time Performance

Class

Accuracy Purpose

T1 ±1 ms Time tagging of events

T2 ± 100 µs Time tagging of zero crossings and

of data for the distributed

synchrocheck. Time tags to

support point on wave switching.

T3 ±25 µsInstrument transformer

synchronization (sampled values)T4 ±4 µs

T5 ±1 µs

• “Accuracy of ±1ms”

• for comparison of events

• for slow measurements

• “Accuracy of ±1µs”

• for comparison of waveforms

Time performance requirements laid out in IEC 61850

u-blox AG5

Accuracy and precisionA reminder about terminology

• Precision is about repeatability

• affected by

• jitter

• signal level

• bandwidth of the signal being measured

• Accuracy is about bias

• affected by propagation delays

• and with what we are comparing our measurement

• … relative timing

A high quality watch set to the wrong time may be precise, but inaccurate

PrecisionHigh Low

Acc

ura

cyH

igh

Low

u-blox AG6

A framework for thinking about applications Relative and absolute timing

• Time

• how precisely?

• relative to what?

• and how accurately?

A framework for expressing IoT device timing requirements

Group of devices

Single device

Absolute time

Relative time

u-blox AG7

Sources of timing

u-blox AG8

Sources of timing informationFor a wireless IoT device

• Local clock

• Wireless signal

• Messages from a server

• GNSS

u-blox AG9

Local clockProviding on-board timekeeping

Local timer LTE signal Network timing GNSS

Elapsed time locally Relative time everywhere Absolute time everywhere Absolute and relative time

outdoors

Pros

• Always available

provided sufficient

power

• Widely available, even

underground

• Excellent short term

stability

• Widely available for

connected devices

• Excellent accuracy

• Excellent long term

stability

Cons

• Applicability limited by

drift, temperature and

aging

• Absolute timing may be

undefined

• Observed time depends

on signal propagation

distance

• Limited by variable

latency, depending on

the network, which

affects timing accuracy

• Limited to the cases

where signal reception

is not obstructed (not

indoors and not

underground)

• Commonly at the heart of a device

• Compromises between performance and power consumption

• Subject to temperature drift and aging

Time sources available for cellular modem IoT applications

u-blox AG10

Wireless signalsTerrestrial transmissions that are available



outdoors

Pros


provided sufficient

power


underground


stability


connected devices



stability

Cons



aging


undefined



distance



the network, which





indoors and not

underground)

• Broadcast transmitters for frequency and time

• Cellular transmitters (LTE) plentiful

• of increasingly good quality

• but uncontrolled absolute timing

• signal anyway received in IoT modem, so measurement “for free”

• Good for relative timing

• Accuracy is affected by signal propagation


u-blox AG11

Providing time in different locationsWireless distribution affected by propagation delay

• Relative delay between devices∆𝑡= Τ𝑑𝐴 − 𝑑𝐵 𝑐

• The location of the devices might not be known

• Location of the transmitter might not be known

• might be nearer 𝐴, or nearer 𝐵

• Give an uncertainty (bias) in the relative accuracy

• though for many applications this is fine100m → ±0.3s

Cellular base station

Unknown clock

modem on sensor𝐴

𝐵

𝑑𝐵

𝑑𝐴

u-blox AG12

Network timing servicesMessage from the cloud



outdoors

Pros


provided sufficient

power


underground


stability


connected devices



stability

Cons



aging


undefined



distance



the network, which





indoors and not

underground)


• Time messages sent over the network

• Available to anything that’s connected

• Affected by the network delays

• Excellent for general orientation

u-blox AG13

GNSSTime and position



outdoors

Pros


provided sufficient

power


underground


stability


connected devices



stability

Cons



aging


undefined



distance



the network, which





indoors and not

underground)

• Available world-wide

• Perfect for interfacing to third parties

• Performance much improved with Assistance provided by the connectivity

• But the signals are very weak –indoor operation is tricky

• Outstanding performance, with excellent accuracy


u-blox AG14

Solving for position as well as timeTo obtain the propagation delay

• GNSS is a positioning system

• the transmitter locations are known

• the receiver (sensor) locations are solved

• Needs to receive signals from 4 satellites

• to find (latitude, longitude, altitude, time)

• Difficult indoors

Obtaining position and time from four satellites

Unknown clock

Receiver

x

y

z

t

u-blox AG15

Solving for position as well as timeTo obtain the propagation delay

• GNSS is a positioning system

• the transmitter locations are known

• the receiver (sensor) locations are solved

• Needs to receive signals from 4 satellites

• to find (latitude, longitude, altitude, time)

• Difficult indoors

• If have an estimate position of receiver, this becomes a known

• then1 satellite is sufficient to find time

• giving improved coverage and reliability

Obtaining time from a single satellite, with the device in an estimated location.

Unknown clock

Receiver

t

u-blox AG16

Hybrid timingIn practice combine sources, depending on circumstances

• To meet application requirements for accuracy, precision, jitter, robustness…

cellular modem

absolute timeeverywhere

relative timing

everywhere

accurate time

outdoors

elapsed time locally

u-blox AG17

Examples

u-blox AG18

MetersSubstantially autonomous devices

• Each device acts on its own

• Task is to measure intervals of time

• Absolute timing accuracy is not particularly important

• Important are cost, coverage, reliability

• Time set and updated over the network

• Operational timing maintained by a local clock providing autonomy

• For high precision, could use LTE signals for measuring timing intervals

Group of devices

Single device

Absolute time

Relative time

Local clockWireless signalNetwork messageGNSS

u-blox AG19

Factory of the future, and signal handlingMultiple devices in a locality, acting in concert

• Multiple devices (often indoors, and usually fairly close together)

• Synchronised processing of signals or information in a distributed application

• Examples

• Fault finding

• Average speed measurement

• Audio streaming

• Cellular signal can provide a common shared time base for measurements

• Network timing to time-stamp UTC of events

Group of devices

Single device

Absolute time

Relative time


u-blox AG20

Wide area sensor networkLarge scale, wide area deployment

• Outdoors

• Examples

• Seismology

• Forest fire detection

• Utilities

• Financial transactions

• GNSS

• Local clock for low power

Group of devices

Single device

Absolute time

Relative time


u-blox AG21

Distributing timing indoorsWith reference to a wireless signal

• Wireless signal measured by a GNSS receiver in a good location

• Wireless signal timing then used nearby

𝑑𝐵

𝑑𝐴

• Accuracy determined by difference in propagation distances

Group of devices

Single device

Absolute time

Relative time


u-blox AG22

Maintaining timing for robustnessWhen an interruption in the primary source

• Wireless signal measured by a GNSS receiver when conditions are good

• Wireless signal timing then used when GNSS signal lost

• Accuracy determined by drift in wireless signal transmitter

GPS LTE

Timing drift over 1 hour after disconnection of GPS antenna

1s

-1s

Group of devices

Single device

Absolute time

Relative time


u-blox AG, For internal use23

Relative timing for indoor sensorsExperiment comparing the time of events at separate modems

• Measure the speed of a «hotwheels» car:

• two separate modem application boards with optical sensor detect the car passing

• each has a module with CellTimeTM, they keep time using LTE and time-stamp events

• the modems send time stamps to a server,application calculates and outputs speed of car

• Illustrates

• hybrid timing, with initial calibration by GNSS

• timing indoors using the LTE signal

• relative timing between two cellular modems

Relative timing between two modems

8.37km/h

u-blox AG24

ConclusionsTiming for IoT applications

• Think about the application

• precision

• relative timing accuracy

• absolute timing accuracy

• usage model and environment

• Robustness, power consumption, cost…

• Address with hybrid timing, using

• the local clock

• wireless signals

• network messages

• GNSS

For further discussion, see https://www.u-blox.com/en/white-papers

u-blox AG25

Thank youfor your attention

timing for iot applications - cambridge wireless€¦ · 01/05/2019 · time sources available for...

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