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With myriad augmented reality, social networking, and retail shopping applications all on the horizon for the mobile handheld, a fast and accurate location technology will become key to a rich user experience. When roam-ing outdoors, users can usually count on a clear GPS sig-nal for accurate location, but indoors, GPS often fades, and so up until recently, mobiles have had to rely mainly on rather coarse-grained signal strength readings. What has changed this status quo is the recent trend of dramat-ically increasing numbers of antennas at the indoor ac-cess point, mainly to bolster capacity and coverage with multiple-input, multiple-output (MIMO) techniques. We thus observe an opportunity to revisit the important prob-lem of localization with a fresh perspective. This paper presents the design and experimental evaluation of Ar-rayTrack, an indoor location system that uses MIMO-based techniques to track wireless clients at a very fine granularity in real time, as they roam about a building. With a combination of FPGA and general purpose com-puting, we have built a prototype of the ArrayTrack sys-tem. Our results show that the techniques we propose can pinpoint 41 clients spread out over an indoor office envi-ronment to within 23 centimeters median accuracy, with the system incurring just 100 milliseconds latency, mak-ing for the first time ubiquitous real-time, fine-grained location available on the mobile handset. 1