Demonstration of an Indoor Real-Time Location System with Optical Fibre Backbone

Chin-Pang Liu, Yanchuan Huang, Tabassam Ismail, Paul Brennan and Alwyn Seeds

UCL Department of Electronic and Electrical EngineeringUniversity College London

Torrington Place, London WC1E 7JE, United Kingdom

chliu@ee.ucl.ac.uk

Outline

• Introduction

• Principle of location finding and algorithm

• The active transmit-only tag

• Experimental arrangement, signal processing & results

• Conclusion and acknowledgements

Introduction

• Tracking of individuals and goods is often required for safety, security and asset management purposes.

• GPS works fine outdoors but not indoors, e.g. airport passenger terminal.

• WLAN, cellular and Bluetooth do not have sufficient accuracy (>50m)

• In The Intelligent Networked Airport (TINA) project, it is envisaged that

– An optical fibre backbone will carry growing amount of data traffic at airports;

– Air passengers will be given RFID embedded boarding passes so they can be tracked.

• First demonstration of an indoor location system with optical fibre backbone

Location Finding by Multilateration

• A process of finding the tag location from the measured time difference of arrival (TDOA)

of the tag signal at known coordinates.

• Each TDOA between two AUs represents all possible locations along a parabola in 2-D.

AU 3

AU 2

AU 1

RFIDtag

Parabola betweenAU 1 and AU 2Parabola between

AU 2 and AU 3

Parabola betweenAU 1 and AU 3

• An optimisation method is employed to find the tag position.

Multilateration Algorithm

AU1 (0,0) & AU2 (0,15.6) AU1 (0,0) & AU3 (6.6,7.8) AU2 (0,15.6) & AU3 (6.6,7.8)

• First pre-calculate the three sets of TDOAs (td12, td13, td23) on a grid representing the venue.

• Then form an error function E(x,y) with the measured TDOAs (TD12, TD13, TD23)

• Finally find x and y so that E(x,y) is minimised. Corresponding x and y are then the tag’s coordinates.

The Transmit-Only Tag

Battery powered Analog Devices AD9910 direct digital synthesizer (DDS) evaluation board.Programmed FM chirp from 216.5 MHz to 300 MHz with 900 MHz sampling clock.

• Measured output frequency variation with time.

• Only the 80 s down-chirp is used for the TDOA measurement.

Frequency (Hz)

900 MHzClock

Fundamental chirp Image chirps

Bandpass filtered and amplified

DDS output spectrum

Experimental Arrangement

2390 MHz LO

ISM band bandpass

filters

Low passfilters

GPIBinterface

Microwavemixers

Real-timeoscilloscope

AU 3

AU 2

AU 1RFIDtag

1 2 3

Hub

Optical fibre

-3 dBm

2390 MHz LO

ISM band bandpass

filters

Low passfilters

GPIBinterface

Microwavemixers

Real-timeoscilloscope

AU 3

AU 2

AU 1RFIDtag

1 2 3

Hub

Optical fibre

-3 dBm

Test venue: A 6.6m-by-15.6m area within a large café.

Measurement of TDOA

• Time difference of arrival between two similar chirp signals can be found by multiplying them together.

However, if the received signals contain multipath interference, this method will fail!

1-sMHz 04375.1

s 80

MHz 5.83

t

f

AU 1

RFIDtag

AU 2 T2 T1

T2 T1 = t

Time

Freq.

f

t

tfAU1

tfAU2

FFT

Freq.

Amplitude

ftftf AU1AU2

Key Signal Processing Steps

• Each AU received signal is first multiplied with a pre-recorded “perfect” reference chirp in a matched filter operation.

• After FFT, line-of-sight signal has the lowest frequency.

AU

1, A

mp

litu

de

(V

2 )A

U 2

, Am

plit

ud

e (

V2 )

AU

3, A

mp

litu

de

(V

2 )

Frequency (Hz)

Frequency (Hz)

Frequency (Hz)

Well separatedindirect signals

Nearbyindirect signal

Line-of-sight

Line-of-sight

Line-of-sightPeak amplitude

Peak amplitude

Peak amplitude

Screen Capture of the Labview Interface

Measured frequency differences between antennas provide TDOA information used to calculate the tag location.

Map of the measurement area and visualisation of the tag location.

Actualtag coordinates (m)

Measuredtag coordinates (m)

Location System Result Summary

AU 3

AU 2

AU 1

Y (

m)

X (m)

Upper no.: Mean error distance (m)Lower no.: Standard deviation (m)

• 30 measurements taken @ each of 30 chosen locations

• Mean error distance: 1.1 m or better @ 29 out of 30 locations

• Overall positional error: 0.72 m RMS

TINA Showcase Demonstration

• Extension of existing single tag detection to two tags using both positive and negative chirp rates simultaneously.

• This demo lays the foundation for future location systems capable of identifying and locating large numbers of active tags carried by air-passengers, staff, vehicles and/or equipment at airports.

Conclusions

• First indoor real-time location system with an optical fibre backbone.

• Overall positional error: 0.72 m RMS.

• Dedicated ICs can reduce tag size, costs and power consumption.

• An additional AU can improve accuracy and reliability by providing

information redundancy.

• Use of smart antennas could provide angle of arrival (AOA)

information and together with the TDOA data make the system more

robust.

Acknowledgements

• UK EPSRC Grant (EP/D076722/1) as part of The INtelligent Airport (TINA) project.

• The authors would like to thank ZinWave for providing the hub and antenna units.