Application-Layer Clock Synchronization for Wearables Using Skin Electric Potentials Induced by Powerline Radiation Zhenyu Yan School of Computer Science and Engineering Nanyang Technological University Singapore zyan006@ntu.edu.sg Yang Li Advanced Digital Sciences Center Illinois at Singapore Singapore yang.li@adsc.com.sg Rui Tan ∗ School of Computer Science and Engineering Nanyang Technological University Singapore tanrui@ntu.edu.sg Jun Huang Center for Energy Eicient Computing and Applications Peking University Haidian, Beijing, China jun.huang@pku.edu.cn ABSTRACT Design of clock synchronization for networked nodes faces a fun- damental trade-of between synchronization accuracy and univer- sality for heterogeneous platforms, because a high synchronization accuracy generally requires platform-dependent hardware-level network packet timestamping. This paper presents TouchSync,a new indoor clock synchronization approach for wearables that achieves millisecond accuracy while preserving universality in that it uses standard system calls only, such as reading system clock, sampling sensors, and sending/receiving network messages. The design of TouchSync is driven by a key inding from our extensive measurements that the skin electric potentials (SEPs) induced by powerline radiation are salient, periodic, and synchronous on a same wearer and even across diferent wearers. TouchSync inte- grates the SEP signal into the universal principle of Network Time Protocol and solves an integer ambiguity problem by fusing the am- biguous results in multiple synchronization rounds to conclude an accurate clock ofset between two synchronizing wearables. With our shared code, TouchSync can be readily integrated into any wearable applications. Extensive evaluation based on our Arduino and TinyOS implementations shows that TouchSync’s synchroniza- tion errors are below 3 and 7 milliseconds on the same wearer and between two wearers 10 kilometers apart, respectively. CCS CONCEPTS · Networks → Time synchronization protocols; · Human- centered computing → Ubiquitous and mobile devices; ∗ Corresponding author. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for proit or commercial advantage and that copies bear this notice and the full citation on the irst page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior speciic permission and/or a fee. Request permissions from permissions@acm.org. SenSys ’17, November 6ś8, 2017, Delft, Netherlands © 2017 Association for Computing Machinery. ACM ISBN 978-1-4503-5459-2/17/11. . . $15.00 https://doi.org/10.1145/3131672.3131681 KEYWORDS Clock synchronization, wearables, skin electric potential ACM Reference Format: Zhenyu Yan, Yang Li, Rui Tan, and Jun Huang. 2017. Application-Layer Clock Synchronization for Wearables Using Skin Electric Potentials Induced by Powerline Radiation. In Proceedings of SenSys ’17, Delft, Netherlands, November 6ś8, 2017, 14 pages. https://doi.org/10.1145/3131672.3131681 1 INTRODUCTION The annual worldwide shipments of consumer wearables (e.g., smart watches, wristbands, eyewears, clothing, etc) have grown by 29% in 2016 [11]. This rapid growth is expected to continue, projecting to 213 million units shipped in 2020 [11]. Along with the proliferation of consumer wearables, specialized domains such as clinical/home healthcare [1] and exercise/sport analysis [25] are also increasingly adopting smart wearable apparatuses. In the body-area networks formed by these wearables, a variety of system functions and applications depend on tight clock synchronization among the nodes. For instance, two earbuds of a wireless headphone need to be synchronized mutually and/or with a master device (e.g., a smartphone) to control the playback positions in their bufers to deliver audio synchronously [3]. Motion analysis [22] and muscle activity monitoring [24, 25] require sensory data from multiple tightly synchronized nodes. While current wearable systems adopt customized, proprietary clock synchronization approaches [8], we envisage a wide spectrum of interoperable wearables that can synchronize with each other to enable more novel applications. For instance, in body sensor as- sisted multi-user gaming that may need to decide which participant performs an action or gesture irst, tight clock synchronization among the body sensors and/or the handheld game consoles is needed. In the envisaged scheme, an application developer can readily synchronize any two communicating wearables using high- level and standard system calls provided by their operating systems (OSes), such as reading system clock, transmitting and receiving network messages. However, the design of clock synchronization approaches faces a fundamental trade-of between the synchroniza- tion accuracy and the universality for heterogeneous platforms.