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Synchronization of RFID Readers for Dense RFID Reader Environments

Alfio R. GrassoAuto-ID Lab @ Adelaide

School of Electrical & Electronic Engineering

University of AdelaideAustralia

alf@eleceng.adelaide.edu.au

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AUTO-ID LABS

Main Author Kin Seong Leong (PhD Candidate)

Co-Authors Prof Peter Cole (PhD Supervisor) Ms Mun Leng Ng (PhD Candidate) Mr Alfio Grasso

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AUTO-ID LABS User Requirements †

100’s to 1000’s of readers possible Industrial Parks 2 km radius 100’s readers per DC

DC → 50 -100 dock doors 2 readers per door

Cluster of DC in Industrial Parks

† User Requirements for European RFID Regulations at UHF 30 Nov 2005

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AUTO-ID LABS

Challenges in Dense RFID Reader Environments

Performance Maximal coverage Minimal interference / data loss

Regulations Frequency Hopping Spread Spectrum

(USA) Listen Before Talk (Europe)

Standards EPCglobal C1G2

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AUTO-ID LABS FCC Environment

FCC Part 15.247 UHF RFID Band (902 to 928 MHz)

4W EIRP 50 channels, 500 kHz wide

902.5 to 927.5 MHz Maximum dwell time 0.4 s (400 ms) Each channel used in 2.0 s interval

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AUTO-ID LABS Listen Before Talk (LBT)

ETSI 302 208 865 to 868 MHz 200 kHz channels 2W ERP – only 10 channels LBT

Listen for 5 ms Various thresholds 2W ERP, threshold is -96 dBm

Use sub-band for 4s

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AUTO-ID LABS LBT Threshold

-126 dBW = -96 dBm

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AUTO-ID LABS EPCglobal C1G2

Dense Reader Mode (Annex G) Reader

Transmissions in even channels

Tag replies in odd channels

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AUTO-ID LABS Deployment Problems

Distance between antennas, to be lower than -96 dBm threshold

Front

Back

Side

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Proposed Solution Reader Synchronisation

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AUTO-ID LABS

Reader SynchronizationThe Concept

Readers start to “Listen” at the same time and start to “Talk” at the same time.

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AUTO-ID LABS Synchronisation Schemes

Not a subject of the paper, but could be Hardwired Network Controlled

NTP Protocol designed to synchronise the clocks of

computers over a network

Radio controlled Separate channel (use one of the 5 lower

powered channels) Master reader, issues synchronisation pattern

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AUTO-ID LABS Case Study

Warehouse Depot in Europe ETSI EN 302 208

865 to 868 MHz 15 sub-bands, 200 kHz wide Only 10 sub-bands at 2W ERP

EPCglobal Class 1 Generation 2 Interrogator transmissions are located in even-

numbered channels Tag backscatter located in odd-numbered

channels

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AUTO-ID LABS Combining EN302-208 and C1G2

Interrogators can use 5 of the channels for TransmissionIf all readers in a facility are networked and configured to listen at the same time, then at the end of the listen period, they can all synchronously startReaders will then naturally stay synchronised, within the sub-band

Even if different readers are on for different time, they will all start again in synchronism, once the last reader has finished.

Thus we can have several readers occupying the same channel, even if they are close together.

Readers assigned geographically to 1 of 5 sub-bands

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AUTO-ID LABS Dock Door Example

The next slide depicts 6 dock doors, with each door requiring 2 readersEach reader is networked, and connected to some synchronisation deviceEach reader is assigned to operate in one of the 5 even numbered sub-bandsEach reader listens for the required time, and if the sub-band is free, starts TransmissionsEach transmission must terminate within 4 seconds, after which the reader releases the sub-band for at least 100 ms

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Sub-band 2

Sub-band 4

Sub-band 6

Sub-band 8

Sub-band 10

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AUTO-ID LABS Other Mechanisms

Reducing the overall time for talking. While it is possible to talk for 4 seconds, readers and

applications should be configured to talk for only the time necessary to capture tag data.

Reducing the power output. While the maximum allowed 2W ERP can be useful in single

or small population reader environments, in dense reader populations this higher power may not be necessary.

Use of external sensors. The sensor determines that an object may be in the near

vicinity, to turn on (enable) an RFID reader.

The use of RF opaque or RF absorbing materialsDynamic Channel Assignments

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AUTO-ID LABS

Example with sensorstruck 3 absent, so no RF

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AUTO-ID LABS

RF Opaque / RF Absorbing Materials

RF A b so rb in g M a te ria ls

2 8 4 1 0 6 2 8 4 1 0 6 2 8

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AUTO-ID LABS

Switching Channel Assignments

C h a n n e l

8 2 6 1 0 4 8 2 6 1 0 4 8 2 C h a n n e l

2 8 4 1 0 6 2 8 4 1 0 6 2 8

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AUTO-ID LABS Conclusions

Reader densities in Distribution Centres > 100’s

ETSI EN302-208 LBT @ -96 dBm Threshold

C1G2 Even channels for Tx => 5 sub bands

Synchronisation