{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2025,10,12]],"date-time":"2025-10-12T03:39:41Z","timestamp":1760240381470,"version":"build-2065373602"},"reference-count":31,"publisher":"MDPI AG","issue":"11","license":[{"start":{"date-parts":[[2019,5,29]],"date-time":"2019-05-29T00:00:00Z","timestamp":1559088000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100002858","name":"China Postdoctoral Science Foundation","doi-asserted-by":"publisher","award":["2017M620293"],"award-info":[{"award-number":["2017M620293"]}],"id":[{"id":"10.13039\/501100002858","id-type":"DOI","asserted-by":"publisher"}]},{"name":"National Key R&amp;D Program","award":["2016YFC1401900"],"award-info":[{"award-number":["2016YFC1401900"]}]},{"name":"Qingdao National Labor for Marine Science and Technology Open Research Project","award":["QNLM2016ORP0405"],"award-info":[{"award-number":["QNLM2016ORP0405"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Indoor pedestrian tracking has been identified as a key technology for indoor location-based services such as emergency locating, advertising, and gaming. However, existing smartphone-based approaches to pedestrian tracking in indoor environments have various limitations including a high cost of infrastructure constructing, labor-intensive fingerprint collection, and a vulnerability to moving obstacles. Moreover, our empirical study reveals that the accuracy of indoor locations estimated by a smartphone Inertial Measurement Unit (IMU) decreases severely when the pedestrian is arbitrarily wandering with an unstable speed. To improve the indoor tracking performance by enhancing the location estimation accuracy, we exploit smartphone-based acoustic techniques and propose an infrastructure-free indoor pedestrian tracking approach, called iIPT. The novelty of iIPT lies in the pedestrian speed reliability metric, which characterizes the reliability of the pedestrian speed provided by the smartphone IMU, and in a speed enhancing method, where we adjust a relatively less reliable pedestrian speed to the more reliable speed of a passing by \u201cenhancer\u201d based on the acoustic Doppler effect. iIPT thus changes the encountered pedestrians from an\u201cobstacle\u201d into an \u201cenhancer.\u201d Extensive real-world experiments in indoor scenarios have been conducted to verify the feasibility of realizing the acoustic Doppler effect between smartphones and to identify the applicable acoustic frequency range and transmission distance while reducing battery consumption. The experiment results demonstrate that iIPT can largely improve the tracking accuracy and decrease the average error compared with a conventional IMU-based method.<\/jats:p>","DOI":"10.3390\/s19112458","type":"journal-article","created":{"date-parts":[[2019,5,29]],"date-time":"2019-05-29T11:31:28Z","timestamp":1559129488000},"page":"2458","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":6,"title":["Infrastructure-Free Indoor Pedestrian Tracking with Smartphone Acoustic-Based Enhancement"],"prefix":"10.3390","volume":"19","author":[{"given":"Chao","family":"Liu","sequence":"first","affiliation":[{"name":"Department of Information Science and Engineering, Ocean University of China, Qingdao 266100, China"}]},{"given":"Sining","family":"Jiang","sequence":"additional","affiliation":[{"name":"Department of Information Science and Engineering, Ocean University of China, Qingdao 266100, China"}]},{"given":"Shuo","family":"Zhao","sequence":"additional","affiliation":[{"name":"Department of Information Science and Engineering, Ocean University of China, Qingdao 266100, China"}]},{"given":"Zhongwen","family":"Guo","sequence":"additional","affiliation":[{"name":"Department of Information Science and Engineering, Ocean University of China, Qingdao 266100, China"}]}],"member":"1968","published-online":{"date-parts":[[2019,5,29]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"1","DOI":"10.1016\/j.comcom.2015.03.001","article-title":"A survey of calibration-free indoor positioning systems","volume":"66","author":"Hossain","year":"2015","journal-title":"Comput. 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