Characterization of an Unintentional Wi-Fi Interference Device

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Characterization of an Unintentional Wi-Fi
Interference Device The Residential Microwave
Oven

• Tanim M. Taher
• Ayham Z. Al-Banna
• Joseph L. LoCicero
• Donald R. Ucci

Presented by Tanim M. Taher
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Outline

• Motivation
• Experimental Analysis of Microwave Oven (MWO)
  signal
• Frequency Shifting part
• Transients
• AM modulated Envelope of the signal
• Model Developed
• Simulation Results
• Interference Mitigation
• Conclusions
• Ongoing and Future Work

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Motivation (1)
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Motivation (2)

• ISM band is crowded.
• MWOs were designed many years ago and they
  radiate energy in the 2.4 GHz ISM band.
• EM waves radiated by MWO devices interfere with
  ISM devices unintentionally (no intelligence
  signal in the MWO signal).
• Understanding the interference signal helps
  mitigation
• Spectral signatures of MWO signals need to be
  identified.

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Experimental Analysis of MWO Signal (1)

• The Residential MWO signal is synchronized with
  the 60 Hz AC line cycle, and it radiates in the
  positive half cycle.
• The MWO signal has the main characteristics
• An AM-FM signal that is radiated for about 5-6
  ms.
• Transient signals before and after FM signal
  (1ms).

On cycle of MWO
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Experimental Analysis of MWO Signal (2)

• Spectrogram shows FM nature of MWO signal.
• The frequency sweeping is roughly sinusoidal in
  nature.

AM-FM Signal
Figure Courtesy of Stevens Institute of
Technology
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Experimental Analysis of MWO Signal (3)

• The AM-FM signal has a bandwidth of between 15-20
  MHz (depends on MWO).

AM-FM Signal
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Experimental Analysis of MWO Signal (4)

• The MWO emits wideband Transient Signals before
  and after the FM signal. Transient durations are
  around 1 ms each.

• Figure shows wideband nature of transients.
• Observe the high transient energy concentrated in
  frequencies near FM signal.

Transients
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Experimental Analysis of MWO Signal (5)

• Zero Span Measurements show Transient Signal
  durations. Observe, they exist for only about 15
  of the time during the time period of 16.67 ms.

• Zero-span measurements at 2.46 GHz and 2.44 GHz
  over two 60 Hz cycles.
• Transients classified as Turn-on and Turn-off

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Experimental Analysis of MWO Signal (6)

• The amplitude of the FM signal is not constant,
  it varies sinusoidaly!
• The Zero-Span Measurement indicates this. So the
  FM signal is further AM modulated (AM-FM signal
  obtained).

• Zero-span measurement at 2.455 GHz. Note the
  changing amplitude in the middle.
• Transients are also observable before and after
  the AM-FM signal.

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Experimental Analysis of MWO Signal (7)

• Power Spectral Density of MWO Signal

• Most power is concentrated over the narrow
  frequency range (15 MHz) swept by AM-FM signal.
• There is power scattered over entire ISM band.
• The wideband power is due to transients.

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Model for the MWO Signal (1)

• A model for the time-domain MWO signal was
  developed featuring its main characteristics.
• Model Composition
• An FM signal with instantaneous frequency
  proportional to AC line voltage.
• The FM signal is further AM modulated forming an
  AM-FM signal. The AM amplitude, again, is
  proportional to the 60 Hz AC cycle.
• Transient Signals with a wide bandwidth to span
  the entire ISM band.
• Transient Signals with a narrow bandwidth with
  power concentrated in the AM-FM-swept frequency
  band.

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Model for the MWO Signal (2)

• Qualitative representation of the MWO signal
  model.

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Model for the MWO Signal (3)

• Mathematical Representation of model MWO signal.

, where T 1/fac and fac 60 Hz.
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Model for the MWO Signal (4)

• Mathematical Representation of model MWO signal
  (contd.).

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Simulation Results (1)

• The model developed was simulated. Simulated
  Spectrograms and Power Spectral Density plots
  were obtained and compared to the experimental
  plots.
• The simulations were performed at frequencies
  much lower than the ISM band for computational
  convenience. The model is scalable to higher
  frequencies, and the spectral signatures are
  preserved.

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Simulation Results (2)
Power Spectral Densities
Experimental PSD
Simulated at 100 KHz carrier frequency
Simulated at 1 MHz carrier frequency (parameters
different from first one)
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Simulation Results (3)
Spectrograms
Experimental Spectrogram
Simulated at 100 KHz carrier frequency
Simulated at 1 MHz carrier frequency (parameters
different from first one)
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Interference Mitigation (1)

• The transients of the MWO signal interfere with
  all Wi-Fi channels, however, only for 15 of
  time. Since the transients are synchronized to
  the 60 Hz line cycle, we can predict the
  transient times and avoid interference by not
  transmitting at those times.
• The AM-FM signal is narrowband and interferes
  with only some IEEE 802.11 channels (like channel
  11). In such a case, the Wi-Fi channel can be
  changed to another channel outside the AM-FM
  signals frequency band (like channel 1).

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Interference Mitigation (2)

• Data transmission using 802.11 channel 1 (shaded
  areas are transient locations)

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Conclusions

• MWO signal was thoroughly studied and
  characterized.
• A novel model for the MWO signal was developed.
• Simulation and experimental results supported the
  theoretical model.
• Interference mitigation techniques were proposed.

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Ongoing Future Work

• Investigating random variations in the MWO signal
  signature
• Refining the model to include the random aspects
  of MWO signal behavior
• Further research on the proposed interference
  mitigation techniques

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Thank you!Questions?