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Interference in RFID Systems

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co-channel operation, but other problems remain. ... Unmodulated (CW) interference limits the modulation depth and corrupts tag's ... – PowerPoint PPT presentation

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Title: Interference in RFID Systems


1
Interference inRFID Systems
  • Rene MartinezIntermec TechnologiesUnited States
    Delegation

2
RFID Interference Paths
3
Interference Terminology and Approach
  • Terminology
  • Source-on-victim nomenclature A sources
    transmission can interfere with a victims
    reception and corrupt the victims capability to
    decode information
  • Rejection is a quantitative measure of rejecting
    an interfering signal while receiving the desired
    signal
  • Approach
  • RFID tags have stringent demands on cost, size,
    and power consumption, so tags should ultimately
    limit interference performance of RFID system
  • Analyze reader-on-tag interference, and use this
    to set expectations on reader-on-reader
    interference.

4
Tag and Reader Interference Rejection
  • Tags principally reject interference using
    amplitude selectivity
  • Tags receive and select the stronger signal as
    the desired reader signal, and tags successfully
    decode if the desired signal is large enough
    relative to the interfering signal.
  • Tags reject a weaker signals from an interfering
    reader, and the rejection secondarily depends on
    frequency difference between the desired and
    interfering readers.
  • Readers principally reject interference using
    frequency to select and receive tag signals from
    a desired frequency.
  • If the interference is outside of the desired
    frequency channel, the reader can decode the
    desired signal even if the interfering signal is
    larger.
  • If the interference is inside of the desired
    frequency and larger than the desired signal, the
    reader can not receive.

Ears
Eyes
5
Interference Rejection Ratio
  • The interference rejection ratio (IRR) measures a
    receivers capability to reject interference, and
    it often depends on the offset frequency between
    the interfering and desired signals
  • The Interference Rejection Ratio is the maximum
    acceptable ratio of interference-to-desired power
    for a tag or reader to operate
  • Better rejection, e.g. reader
  • Appears as higher values on the IRR dB scale
  • IRR gt 0dB means successful reception even if the
    interferer is stronger than the desired signal
  • Worse rejection, e.g. tag
  • Appears as lower values on the IRR dB scale
  • IRR lt 0dB means successful reception only if the
    desired signal is stronger than the interfering
    signal

6
IRR Measurement Data for Passive Tags
Based on measurements on passive tags, active
tagscould have IRR from -10dB to -6dB
7
Interference distances
8
Range for Interfering Reader
  • Graph assumes free space and passive tag is 3
    meters from desired reader. Also assumes antenna
    boresights for interfering and desired readers
    are aligned

9
Interfering Distances
10
Reader Rejection Dependence on Tag
  • where the terms and estimates for the terms are
  • Pxmt is the readers conducted transmit powers
    (30dBm)
  • Mloss is the backscatter modulation loss of the
    tag (6dB)
  • Pdt is the desired reader signal into the tag
    -10dBm for passive tags -30dBm for battery
    tags
  • Grdr is the readers antenna gains (6dBi)
  • Gtag is the tags antenna gain (2dBi)
  • IRRtag is the tag interference rejection ratio
    0.02 to 0.25 for passive tags (-17dB to -6dB)
    0.10 to 0.25 for battery tags (-10dB to -6dB)

11
Reader Interference Rejection
  • Reader IRR based on reader-to-tag interference
  • Using measured and estimated IRR for
    tags Reader IRR is 32 to 40dB for passive tag
    readers Reader IRR is 58 to 60dB for battery tag
    readers
  • Reader IRR based only on rdr-to-rdr interference
  • Reader IRR equals the ratio of the maximum
    interference (0dBm) to the minimum signal from a
    tag Reader IRR is 56dB for passive tag
    readers Reader IRR is 96dB for battery tag
    readers

12
Conclusion
  • Analysis exclusively based on reader-on-reader
    interference for battery tags RFID systems leads
    to a stringent reader rejection ratio of 90dB or
    higher however, an analysis that includes
    reader-on-tag interference leads to a much more
    lenient ratio of 60dB or better
  • Mitigation of reader-on-tag interference, such as
    temporal synchronization, may be needed, and
    mitigation will similarly alleviate
    reader-on-reader interference.

13
BACKUP SLIDES FORREADER-ON-TAG INTERFERENCE
  • Adrian WitherspoonRene MartinezIntermec
    Technologies

14
Received Signal into Tag No Interference
15
Signal into Tag 4th Channel Interference
16
Signal into Tag 2nd Channel Interference
17
Signal into Tag Co-channel Interference
18
Simplified View of Tag Demodulation
  • Regardless of complexity, all UHF RFID tags have
    amplitude demodulation and detect an average of
    the RF envelope
  • The demodulator rejects interference if the beat
    frequency is significantly faster than the
    demodulator (fbeat gtgt 1/RC)
  • As beat frequency decreases (fbeat 1/RC or
    fbeat lt 1/RC), the demodulators filter does not
    assist in rejecting interference

19
Reader-to-tag Interference Test Setup
  • Data sets collected with using same Gen2 tag chip
    mfg, different chips, 50-100 max range, and
    modulating/CW interference

20
Co-channel Reader-on-tag Interference
  • Readers with accurate or phase-locked references
    can effectively eliminate beat frequency in
    co-channel operation, but other problems remain.
  • With little or no beat frequency, co-channel
    interfering signal appears identical to desired
    signal
  • Modulating interference appears as a desired
    modulating signal, and without a beat frequency,
    tags cant discriminate between commands from the
    desired and interfering readers.
  • Unmodulated (CW) interference limits the
    modulation depth and corrupts tags ability to
    decode commands. For example, a CIRR higher than
    -13dB is difficult if the minimum voltage
    modulation depth is 80

21
Conclusions (for Backup Slides)
  • To prevent reader-on-tag interference for Gen2
    tags, the difference of readers carrier
    frequencies should be more than 800kHz
  • Tags need significantly (13-20dB) more desired
    signal power if the difference in readers
    carrier frequencies is less than 800kHzHz.
  • Tags need nominally (6dB) more desired signal
    power if the difference in readers carrier
    frequencies is more than 800kHz
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