Cambridge Consultants LtdScience Park, Milton Road,Cambridge, England CB4 0DW

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12 October 2006 S4989-P-139 v1.0

Tim WhittakerCambridge Consultants

Zonal Location and Asset Tracking with ZigBee Technology (using RSSI)Finding it cheaply !

© 2006 Cambridge Consultants Ltd.

Commercially Confidential This version of this Presentation contains ideas and information which are proprietary to Cambridge Consultants Limited: it is given to you in confidence. You are authorised to open and view any electronic copy we send you of this document within your organisation and to print a single copy. Otherwise the material may not in whole or in part be copied, stored electronically or communicated to third parties without the prior written agreement of Cambridge Consultants Limited.

212 October 2006 S4989-P-139 v1.0

Location and Asset Tracking with ZigBee

ZigBee Introduction

Demonstration telemetry & location system

Indoor propagation effects

Practical location issues

Summary and conclusion

312 October 2006 S4989-P-139 v1.0

802.16

Burst data rate bits/s

Pow

er c

onsu

mpt

ion

in d

uple

x

1k 10k 100k 1M 10M 100M100

1W

0.1W

10mW

802.11a /HiperLAN2

10W

WANProprietary and new standards

802.11bDECT

LAN

ZigBee

UWB

PAN

RFID/NFC

3GHSDPA/EVDO3GGPRSGSM

EDRBluetooth

Range

The wireless standards landscape continues to expand downwards – to lower power & data rate

412 October 2006 S4989-P-139 v1.0

The ZigBee protocol was developed for low-power telemetry & control by industry specialists

• ZigBee is a powerful standard for wireless ad-hoc mesh networking, optimised for low power, low data rate applications

• ZigBee network control is

• resilient to link interruptions, • supports self-configuration,• self-repair of the mesh configuration

• PHY/MAC layers are defined in IEEE standard 802.15.4 (wireless personal area network)

512 October 2006 S4989-P-139 v1.0

ZigBee applications are becoming widespread

• The ZigBee market is expected achieve commercial quantities in 2006 and beyond

• Home control and automation• Building security and monitoring• Telemetry

– Industrial– Medical

• Industrial control

• New applications involving mobile devices will gain value in being able to locate one or more nodes

612 October 2006 S4989-P-139 v1.0

ZigBee physical layer is IEEE standard 802.15.4

• IEEE 802.15.4 defines properties of the WPAN physical layer operating on three ISM frequency bands

• However, the most popular band is the internationally-allocated 2.4GHz band

• Performance details are:

– Frequency band 2400-2483.5MHz– TX power 0dBm to +20dBm (+4dBm is common)– RF channels 16, spaced by 5MHz– Raw data rate 250kbps– Modulation, DSSS 2Mchip/s (x8 spread, O-QPSK)– RX sensitivity –85dBm or better (–92dBm is common)– Typical indoor range 20 – 30m

712 October 2006 S4989-P-139 v1.0

2.4 GHz Frequency 2.483 GHz

802.11b channel Channels for ZigBee / 802.15.4 Networks

Medium Access Control (MAC) and Network layers are also fully defined

• MAC layer (802.15.4) defines timing, framing and RF channel management, designed to

• co-exist in the frequency domain with other services

• co-exist in time, using WiFi-like procedure (CSMA/CA)

• Network layer (ZigBee Alliance) provides routing functionality:

• Network formation, and repair when needed• Discovery of routes between multiple nodes• Store and forward of messages from end to end

812 October 2006 S4989-P-139 v1.0

Location and Asset Tracking with ZigBee

ZigBee Introduction

Demonstration telemetry & location system

Indoor propagation effects

Practical location issues

Summary and conclusion

912 October 2006 S4989-P-139 v1.0

The problem with indoor radio is fading

• Shadowing effects by obstacles give blank spots in communication

• Multipath fading (diagram)

• gives additional deep dips in the received signal,

• changes with time

• This makes reliable communication, and reliable location, more of a challenge

-90.0

-80.0

-70.0

-60.0

-50.0

-40.0

-30.0

0 15 30 45 60 75

Time / seconds

Rec

eive

d Si

gnal

Lev

el /

dBm

Multipath fading of received signal, with time or distance

1012 October 2006 S4989-P-139 v1.0

Fading Distribution Statistics

0

10

20

30

40

50

60

70

80

90

100

-110

-100

-90

-80

-70

-60

-50

-40

Received Level (dBm)

Perc

ent o

f sam

ples

> s

peci

fied

leve

l

.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

100.00%

Nor

mal

ized

Fre

quen

cy o

f Occ

uran

ce

Measured CDF Nakagami

Median Level = -57.1 dBm

However, the statistics of multipath fading can be turned to our advantage

• The histogram of a number of signal level readings has an asymmetric form (diagram)

• We can use this to estimate the ‘true’ signal level:

• Take a number of readings over time

• Accumulate a peak value• Modal value =

peak value – constant

1112 October 2006 S4989-P-139 v1.0

An understanding of propagation is needed to design a network of mobile and fixed nodes

• Propagation models used for cellular network coverage prediction can be adapted for indoor coverage

• We need to understand the transition from inverse square law to higher-order losses

• For a radio signal over a ground plane, loss ∝ distance4

• On average, for indoor propagation, loss ∝ distance3 (approx.)

• We need to allow for shadow fading in link budget

• We must also allow for multipath fading

• e.g. assume that loss of 20% of data packets is acceptable• because these fades appear and disappear quite quickly, a re-try should

succeed

1212 October 2006 S4989-P-139 v1.0

Static node positioning should avoid blank spots

Blank spots

Static node

Predicted Indoor RF Coverage (24x40m)

1312 October 2006 S4989-P-139 v1.0

A small change improves coverage considerably

Blank spots now overlaid in wall

Static node moved

Predicted Indoor RF Coverage (24x40m)

1412 October 2006 S4989-P-139 v1.0

Other measures could be taken to improve coverage and location accuracy

• Increase concentration of Static nodes

• improves probability of hearing a given mobile• but increases data volume in the system

• Slower tracking can use ‘up-fades’

• Can wait for an up-fade to lift device out from bad coverage for a particular path

• Would give more data for interpolation

• We might also re-use Mobile nodes as location sensors, where better accuracy is traded off against

• higher duty cycle (⇒ higher average power taken)• more data sent through the network

1512 October 2006 S4989-P-139 v1.0

Location and Asset Tracking with ZigBee

ZigBee Introduction

Demonstration telemetry & location system

Indoor propagation effects

Practical location issues

Summary and conclusion

1612 October 2006 S4989-P-139 v1.0

The practical location system is based on a ZigBee mesh structure

• Gateway node – for access to main controller

• Static nodes (ZigBee routers) are at fixed points around building

• on a 20-30m grid, • (free space range of 70m is reduced

inside buildings)• used for base coverage and

triangulation

• Mobile nodes (ZigBee RFD) are tracked by this system

1712 October 2006 S4989-P-139 v1.0

What accuracy can we achieve?

• Signal strength indication gives only zonal location

• Basic accuracy 5m rms, due to noise & measurement resolution• Achieved by comparing signal strengths from at least three Static stations• We can improve zonal accuracy by

– accumulating more signal strength values for our algorithm,– smaller cell radii, – using sectored antennas,– Using more prior knowledge (Bayesian inference)

• Key requirement – being able to report RSSI for each received packet –is not supported by all ZigBee platforms

• For higher location accuracy, other schemes can be used (e.g. time-of-flight; angle-of-arrival) with resolutions ~50cm

1812 October 2006 S4989-P-139 v1.0

Capacity of the system depends on update rate

• Each location attempt requires

• a number n of RSSI readings (r bytes) by each of s Static nodes• each Static node to send this data back to the Gateway node in a packet

with v overhead bytes• this data to be acknowledged and re-sent at each of h hops• which generates s . h .(n.r + 2v) bytes in total

• A ZigBee channel delivers 250kbit/s x 33%÷8 = 10.3 kbytes/s

• assuming no other data payload !

44,0001 hour (3600 s)

73060 seconds

242 seconds

Max # mobile stationsInterval between location updates

1912 October 2006 S4989-P-139 v1.0

System design of the ZigBee network needs to consider additional effects

• Network adaptation is required

• Network must re-learn optimum routes as the mobile tags move• In practice, ZigBee adaptation rate is much faster than typical mobile

node movement

• Application and duty cycle should match the type of device

• patient health data and location would need regular updates• a medical instrument locked in a cupboard needs infrequent access• a bed-side call/panic button needs fast signalling when activated

• Battery life is, as always, inversely proportional to the duty cycle

• Duty cycle is the sum of ‘payload’ and ‘location’ transmissions

2012 October 2006 S4989-P-139 v1.0

Location and Asset Tracking with ZigBee

ZigBee Introduction

Demonstration telemetry & location system

Indoor propagation effects

Practical location issues

Summary and conclusion

2112 October 2006 S4989-P-139 v1.0

Our demonstration system is based on a semi-static ZigBee network

• We take advantage of ZigBee’sregular network maintenance

• A mix of “Static” and “Mobile” nodes provide background coverage and tagging devices for mobile tracking and control

• Overlay service can add location on to an existing ZigBee telemetry network

2212 October 2006 S4989-P-139 v1.0

Screenshot of ZigBee Mobile Telemetry System

2312 October 2006 S4989-P-139 v1.0

… with ‘payload’ breathing monitor application shown as overlay

2412 October 2006 S4989-P-139 v1.0

Location and Asset Tracking with ZigBee

ZigBee Introduction

Demonstration telemetry & location system

Indoor propagation effects

Practical location issues

Summary and conclusion

2512 October 2006 S4989-P-139 v1.0

Summary and conclusion

• An mobile telemetry system is demonstrated, based on a semi-static ZigBee telemetry network

• Zonal location based on RSSI is good enough for general asset tracking and simple control for paging and alerts

• Simultaneous operation is possible of static services (building control) and mobile services (tracking and paging)

• Single-chip ZigBee transceivers offer practical and cost-effective solutions for indoor telemetry.