I. Introduction
Inspired by the availability of positioning technique and mobile Internet, Location-based services which bring so much convenience to our daily life have been intensively studied and applied over the years [1]. For example, Foursquare [2] is a location-based social network service that allows users to search for their friends, share information with each other, and provide check-in data. OnStar [3] service is a comprehensive LB S system that provides many services to its users such as emergency, navigation, tracking stolen vehicles, and POI searching.