Introduction
Due to continuing advances in low-power sensors and actuators, we are entering an era of rapid expansion in wearable and pervasive computing. Nowadays, it is common for people to carry smart devices, such as sensor-equipped smartphones, smart watches, and healthcare devices. Those devices periodically measure users' physical activities (e.g., Fitbit) and physiology, such as Samsung Simband with electrocardiogram (ECG) and photoplethysmogram (PPG) sensors embedded, making it possible to continuously track users' contexts including mobility modes, social activities, and health conditions [1]. These devices are normally capable of wireless connectivity, which enables them to upload their sensor readings to their service providers. Examples of these service providers include healthcare providers that own electronic medical record systems for users' data collection and analysis, activity trackers that keep track of users' trajectories or mobility modes, and environment-based applications that offer personalized services based on the operating conditions of users and their surrounding environments.