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Advancing Wireless Link Signatures for Location Distinction

Advancing Wireless Link Signatures for Location Distinction. Mobicom 2008 Junxing Zhang, Mohammad H. Firooz Neal Patwari , Sneha K. Kasera University of Utah Salt Lake City, USA. Outline. Introduction Multipath-Based Link Signatures Multiple Tone Probing Temporal CIR Signature

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Advancing Wireless Link Signatures for Location Distinction

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  1. Advancing Wireless Link Signatures for Location Distinction Mobicom 2008 Junxing Zhang, Mohammad H. Firooz Neal Patwari, Sneha K. Kasera University of Utah Salt Lake City, USA

  2. Outline • Introduction • Multipath-Based Link Signatures • Multiple Tone Probing • Temporal CIR Signature • Innovative Methods • Refined Metric for multiple Tone Signatures • Complex Temporal Signature • Framework for Location Distinction • Quantitative Comparisons of Link Signatures • Temporal Behavior of Link Signatures • Conclusion

  3. Introduction • Location Distinction v.s. Localization • Proposed wireless link signatures • RSS • Channel Gains of Multi-tonal Probes • Temporal Channel Impulse Response • The RSS-based method has a consistently lower detection rate and a higher false alarm rate.

  4. Multipath-Based Link Signatures • Two existing multipath-base link signatures and metrics • Multiple tone Probing • Temporal CIR Signature • Comparison and discussion

  5. Multipath Channel Response • Multipath caused by • Reflections, diffractions and scattering of the radio waves. • Time-delayed, attenuated and phase-shifted • Impulse response of multi-path fading channel • Time-variant: • Time-invariant: (for a packet duration) • Received signal Path number Phase shift Magnitude gain Channel impulse response (CIR) Channel frequency response

  6. Multipath Channel Response • Recover CIR from received signal Ps : the power of the sent signal inside the band

  7. Multiple Tone Probing • Frequency domain • Measures frequency response of multiple carriers • K carrier waves are simultaneously transmitted to the receiver • The nth recorded multiple tone signature of the link between transmitter i and receiver j is: fK: the carrier frequency of the Kth carrier wave {H(fK)} used as the multiple tone signature

  8. Multiple Tone Probing • Metric • The Nth multiple tone signature h(N)is compared with each previously measured signature in the history Hi,j using average correlation statistic. • Measurement of similarity • Low -> different, high -> similare Correlation of the nth and the Nth measurements Average squared magnitude of the elements of

  9. Temporal CIR Signature • Time domain • Estimation of the impulse responses a function of time delay and magnitude • Metric • Minimum normalized Euclidean distance • Difference: Low -> similar The nth sampled link signature measurement of the link between transmitter I and receiver j Tr the sampling interval at the receiver and S+1 is the number of the samples

  10. Comparison and Discussion • Qualitative comparison • Temporal signature can be more robust against small changes in multipath. • The inclusion of phase information in multiple tone signature effectively increases the richness of the measurement space. • The temporal link signature has the advantage of operation in the time domain which de-correlates multipath at different delays • The multiple tone link signature has the advantage of using a complex-valued signature which preserves phase information

  11. Complex Temporal Signature • Proposed method • Combines the best features of both the temporal link signature method and the multiple tone probing method. • enhanced signature: • The complex link signature retains phase information in a manner similar to the multiple tone link signature. Comparing to temporal link signature, the magnitude of each gain is not taken

  12. Issue: Phase Changes • Random Phase Shift: • Some phase changes in the link signature have nothing to do with any changes in the link • Clock or carrier frequency shifts • h -> Time offset between clocks Different carrier frequency in receiver and transmitter

  13. Issue: Phase Changes • Given 2 complex temporal link signatures h and g • Represent the shift-removed difference with a new Φ2 difference • The Φ2difference, which minimizes the random phase shift between two measurements before calculating distance, can be efficiently and explicitly calculated using simple vector operations.

  14. Quantitative Comparisons of Link Signature • Comparison: • Multiple tone probing • Temporal channel impulse response • Complex temporal link signatures • Obtained from CRAWDAD • 5 measurement: 4 for history, 1 for test • ROC plot: receiver operating characteristic • How the probability of detection varies with the probability of false alarm.

  15. Normalized Metric • Normalized Metric • Normalizing each multiple tone signature to its magnitude before the calculation of the correlation statistic • Each channel frequency response is normalized to the square root of its average power. • Avoiding missed detections, when higher h(N) occurs. =

  16. Normalized Metric • ROC curves of comparing performance of original and the normalized metrics in the multiple tone probing method

  17. Framework for location distinction Location changed N-1 link signatures Metric Threshold Include it in Discard the oldest Performance Evaluation PFA : the probability of false alarm PD : the probability of detection γ

  18. Comparison (1) • Multiple Tone v.s. Temporal Link Signatures K increases -> improvement Coherence bandwidth -> not separate enough -> correlation

  19. Comparison (2) • Three methods

  20. Multiple Receiver Performance Multiple tone Complex temporal

  21. Temporal Behavior of Link Signatures • Temporal changes in link behavior can significantly increase the probability of false alarms. • LOCATION A,B,C,D • Record response vectors comprising 600 complex temporal responses. • Each impulse response is a vector of 100 complex numbers

  22. Observation • Isomap 2D embedding coordinates • Non-linear dimensionality reduction to reduce the 100 dimension vectors to just 1-2 dimension

  23. Markov Model • Use 1-D embedding of the Isomap algorithm • Like an amplitude modulation signal -> use AM demodulator to capture the envelope of the pattern

  24. False alarm case • Different-State False Alarm (DSFA) • A link signature is measured in state I, but no signature previously measured in state I exists in the history. False alarm is raised. • Policies of buffer replacement • Policy 1: The history has a FIFO replacement policy. • Policy 2: The history is subdivided into K separate FIFO buffers. One for each state in the Markov chain

  25. Two-state Markov Chain Model • Policy 1: evaluate the probability of DFSA • (1) given we are entering state 1, the probability that we stayed in state 2 N time units ago • (2) given we are entering state2, the probability that we stayed in state 1 N time units ago

  26. Results • The rate of convergence is very slow • By using Policy 2, we see virtually no DSFA errors.

  27. Conclusion • Compare two existing multipath-based location distinction methods • Improve the multiple tone probing method • Develop a new link signature outperforms the existing two • A measurement campaign to understand and model the temporal behavior of link signatures to reduce the probaility of false alarms

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