1、Advancing Wireless Link Signatures for Location Distinction,J. Zhang, M. H. Firooz, N. Patwari, S. K. Kasera MobiCom 08 Presenter: Yuan Song,Introduction,What we want to do:Detecting whether a transmitter is changing its location or not. Unlike localization or location estimation, location distincti
2、on does not attempt to determine where a transmitter is Useful in many applications, especially it can enforce physical security by identifying illegal transmitter,Methodology,Basic MethodEstimating Channel Impulse Response (CIR), called Signature of the channel, to checking whether multipath channe
3、l is changing or not. Multipath CIR (time-variant channel),Methodology (cont.),Multipath CIR (time-invariant channel)Sending s(t), received signal r(t) will beIn frequency domain,Methodology (cont.) Estimating methods,Two methods (used in existing papers related to location distinction and this pape
4、r) 12Both need S(f) to be known in receiveris nearly constant within the band,Previous Work in Location Distinction 1 Multi Tone Probing,Signature K carrier waves are simultaneously transmitted to the receiverfk is separated by an amount greater than Channel Coherence Bandwidth, and thus each carrie
5、r wave is attenuated by the channel complex gain.Signature of the Channel (nth recorded signature), in frequency domainBased on the method,Previous Work in Location Distinction (Cont.) 1 Multi Tone Probing,Metric (The paper also proposes a slightly modified version for enhancing detection stability)
6、 the Nth multiple tone signature h(N) is compared with each previously measured signature in the history Hi,j using a measure called the correlation statisticT(n) is the correlation of the nth and the Nth measurements,Previous Work in Location Distinction 2 Temporal CIR Signatures,Signature Using sa
7、mpled CIR, in time domainBased on the method,Previous Work in Location Distinction (Cont.) 2 Temporal CIR Signatures,Metric the difference between the Nth signature h(N) and those in the history Hi,j is given as the minimum normalized Euclidean distance between the new signature and any signature in
8、 the history set.,Comparison Between Two Previous Work,Cons of Multi Tone Probingthe channel frequency response is sensitive to each multipath. An impulse in the time domain is a constant in the frequency domain, and thus a change to a single path may change the entire multiple tone link signature.
9、Temporal CIR signature use a time domain signature, and thus are more robust against small changes of channel. Cons of Temporal CIR SignatureLack of phase information limits its ability in uniquely identifying links.This paper addressed these two cons and made improvement. It proposes a new signatur
10、e called complex temporal link signature,Complex Temporal Signature,Signature Slight modification to Temporal CIR Signature, “without taking the magnitude of each gain”Contrast to Signature of Temporal CIR SignatureSpecial issues: Difficult to discriminate between channel response phase and oscillat
11、or drift, and thus some phase changes in the link signature have nothing to do with any changes in the link. Solution gived.,Complex Temporal Signature,Metric (same as Temporal CIR Signature),Performance Evaluation Framework of Location Distinction,1 For a given transmitter i and a receiver j, a his
12、tory of N-1 link signatures is measured and stored 2 The Nth signature h(N) at j from an unknown transmitter in the neighborhood of j is then taken, and an evaluation criterion ei,j = sigEval(h(N); Hi,j ) is computed. 3 ei,j is compared to a threshold 4 When ei,j does not satisfy certain condional r
13、elationship with threshold, the new signature is determined to be from the same transmitter, i.e. h(N) = h(N)i,j , and we include it in history of H. For constant memory usage, the oldest measurement in H is then discarded. The algorithm returns to step 2 until enough measurements have been collecte
14、d,Performance Evaluation Framework of Location Distinction (Cont.),5 Final Step (Principles used to compare the algorithms) We first define the null and alternate hypotheses:Then we treat ei,j as a random variable and define,Performance Evaluation Multiple Tone and Temporal CIR,Performance Evaluatio
15、n Multiple Tone, Temporal CIR and Complex Temporal CIR,Temporal Behavior of Link Signatures Introduction,The multipath characteristics of a link can change with time. A link can thus be in different distinct states.A location distinction mechanism that does not consider the temporal changes in link
16、behavior can significantly increase the probability of false alarms.The paper propose a Markov Model to further decrease the probability of false alarm,Temporal Behavior of Link Signatures Intuition 1,Using non-linear dimensionality reduction (a method in Statistics to visualize high dimensional dat
17、a) to reduce the 100 dimension vectors to just 1-2 dimensions. Below is a 2-D embedding plotted with one set of 333 complex link signature measurements.,Temporal Behavior of Link Signatures Intuition,Two States With 1D embedding of Isomap algorithm, from the total number of state changes, and the nu
18、mber of times we are in a state, we calculate the state transition probabilities and the limiting probabilities of the Markov chain,Temporal Behavior of Link Signatures Markov Model,Performance Evaluation Further Definitions of Probability of False Alarm,1. Same-State False Alarm (SSFA): A link sign
19、ature is measured in state i while there exists in the history some other signatures of state i, however, the new measurement is far enough away from the measurements in the history that they are detected as different, thus a false alarm is raised.2 Different-State False Alarm (DSFA): A link signatu
20、re is measured in state i, but no signature previously measured in state i exists in the history. Because link signatures from states j i are very different from those measured in state i, this new measurement does not match any in the history, and a false alarm is raised.,Performance Evaluation Further Definitions of Probability of False Alarm (Cont.),Caculation of PDSFA,Performance Evaluation PDSFA,The END,Thank You,
