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

1. 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

2. 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

3. Why can I never connect to the internet during lunch time everyday? Motivation (1)

4. Motivation (2) • ISM band is crowded. • MWO’s 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.

5. 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

6. 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

7. Experimental Analysis of MWO Signal (3) • The AM-FM signal has a bandwidth of between 15-20 MHz (depends on MWO). AM-FM Signal

8. Experimental Analysis of MWO Signal (4) • The MWO emits wideband Transient Signalsbefore 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

9. 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”

10. 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.

11. 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.

12. 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.

13. Model for the MWO Signal (2) • Qualitative representation of the MWO signal model.

14. Model for the MWO Signal (3) • Mathematical Representation of model MWO signal. , where T = 1/fac and fac = 60 Hz.

15. Model for the MWO Signal (4) • Mathematical Representation of model MWO signal (contd.).

16. 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.

17. 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)

18. Simulation Results (3) Spectrograms Experimental Spectrogram Simulated at 100 KHz carrier frequency Simulated at 1 MHz carrier frequency (parameters different from first one)

19. 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 signal’s frequency band (like channel 1).

20. Interference Mitigation (2) • Data transmission using 802.11 channel 1 (shaded areas are transient locations)

21. 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.

22. 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

23. Thank you!Questions?