Synchronization of RFID Readers for Dense RFID Reader Environments

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Synchronization of RFID Readers for Dense RFID
Reader Environments

• Alfio R. Grasso
• Auto-ID Lab _at_ Adelaide
• School of Electrical Electronic Engineering
• University of Adelaide
• Australia
• alf_at_eleceng.adelaide.edu.au

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• 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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User Requirements

• 100s to 1000s of readers possible
• Industrial Parks 2 km radius
• 100s 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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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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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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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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LBT Threshold
-126 dBW -96 dBm
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EPCglobal C1G2

• Dense Reader Mode (Annex G)
• Reader Transmissions in even channels
• Tag replies in odd channels

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Deployment Problems

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

Side
Back
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Proposed Solution Reader Synchronisation
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Reader SynchronizationThe Concept

• Readers start to Listen at the same time and
  start to Talk at the same time.

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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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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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Combining EN302-208 and C1G2

• Interrogators can use 5 of the channels for
  Transmission
• If 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 start
• Readers 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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Dock Door Example

• The next slide depicts 6 dock doors, with each
  door requiring 2 readers
• Each reader is networked, and connected to some
  synchronisation device
• Each reader is assigned to operate in one of the
  5 even numbered sub-bands
• Each reader listens for the required time, and if
  the sub-band is free, starts Transmissions
• Each 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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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 materials
• Dynamic Channel Assignments

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Example with sensorstruck 3 absent, so no RF
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RF Opaque / RF Absorbing Materials
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Switching Channel Assignments
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Conclusions

• Reader densities in Distribution Centres
• gt 100s
• ETSI EN302-208
• LBT _at_ -96 dBm Threshold
• C1G2
• Even channels for Tx gt 5 sub bands
• Synchronisation