I. Introduction
Mobile devices such as smart phones are gaining an ever-increasing popularity. These devices are equipped with various sensors such as camera, microphone, accelerometer, GPS, etc. Mobile sensing exploits the data contributed by mobile users (via the mobile devices they carry) to make sophisticated inferences about people (e.g., health, activity, social event) and their surrounding (e.g., noise, pollution, weather), and thus can help improve people's health as well as life. Applications of mobile sensing include traffic monitoring [1], environmental monitoring [2], healthcare [3], [4], etc.