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Evaluating next-cell predictors with extensive Wi-Fi mobility data
Libo Song, David Kotz, Ravi Jain, Xiaoning He
Dartmouth TR2004-491

Abstract: Location is an important feature for many applications, and wireless networks can better serve their clients by anticipating client mobility. As a result, many location predictors have been proposed in the literature, though few have been evaluated with empirical evidence. This paper reports on the results of the first extensive empirical evaluation of location predictors, using a two-year trace of the mobility patterns of over 6,000 users on Dartmouth's campus-wide Wi-Fi wireless network. We implemented and compared the prediction accuracy of several location predictors drawn from four major families of domain-independent predictors, namely Markov-based, compression-based, PPM, and SPM predictors. We found that low-order Markov predictors performed as well or better than the more complex and more space-consuming compression-based predictors. Predictors of both families fail to make a prediction when the recent context has not been previously seen. To overcome this drawback, we added a simple fallback feature to each predictor and found that it significantly enhanced its accuracy in exchange for modest effort. Thus the Order-2 Markov predictor with fallback was the best predictor we studied, obtaining a median accuracy of about 72\% for users with long trace lengths. We also investigated a simplification of the Markov predictors, where the prediction is based not on the most frequently seen context in the past, but the most recent, resulting in significant space and computational savings. We found that Markov predictors with this recency semantics can rival the accuracy of standard Markov predictors in some cases. Finally, we considered several seemingly obvious enhancements, such as smarter tie-breaking and aging of context information, and discovered that they had little effect on accuracy. The paper ends with a discussion and suggestions for further work.

Note: PDF removed because this version was superceded by the later journal version, and per IEEE rules we can only post the final version. A revised version of this paper appeared in IEEE TMC 5(12), pp. 1633-1649, December 2006. An earlier (and shorter) version of this paper appeared at IEEE Infocom, March 7-11, 2004.

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   Libo Song, David Kotz, Ravi Jain, and Xiaoning He, "Evaluating next-cell predictors with extensive Wi-Fi mobility data." Dartmouth Computer Science Technical Report TR2004-491, February 2004.

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