Advancing Wireless Link Signatures for Location Distinction

Mobicom 2008Junxing Zhang, Mohammad H. Firooz Neal

Patwari, Sneha K. KaseraUniversity of Utah Salt Lake City, USA

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

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.

Location Distinction Localization

Can thrive in multipath channel Suffer from inaccuracies by multipath

More sensitive to motion (less than 1 meter)

May not be able to determine within a meter of accuracy

Less coverage Larger coverage

Multipath-Based Link Signatures• Two existing multipath-base link signatures and metrics

– Multiple tone Probing– Temporal CIR Signature

• Comparison and discussion

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 frequency response

Channel impulse response (CIR)

Multipath Channel Response

• Recover CIR from received signal

Ps : the power of the sent signal inside the band

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

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

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

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

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

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

Issue: Phase Changes

• Given 2 complex temporal link signatures h and g

– Represent the shift-removed difference with a new Φ2 difference

– The Φ2 difference, which minimizes the random phase shift between two measurements before calculating distance, can be efficiently and explicitly calculated using simple vector operations.

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.

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.

=

Normalized Metric

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

Framework for location distinction

N-1 link signatures Metric

Location changed

Threshold

Include it in Discard the oldest

Performance EvaluationPFA : the probability of false alarmPD : the probability of detection

γ

Comparison (1)

• Multiple Tone v.s. Temporal Link Signatures

K increases -> improvementCoherence bandwidth-> not separate enough-> correlation

Comparison (2)

• Three methods

Multiple Receiver Performance

Multiple tone

Complex temporal

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

Observation

• Isomap 2D embedding coordinates– Non-linear dimensionality reduction to reduce the 100 dimension

vectors to just 1-2 dimension

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

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

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

Results

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

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