I. Introduction
Today, location-based services (LBS) are an indispensable feature of mobile devices like smartphones or tablets. While taking the user's location into account, they are mainly used to enable the search for location-based information (e.g. surrounding shops, nearby friends, available rental cars). Thereby, a LBS is being queried and the relevant information is being presented by a mobile application only on request by the user. In other words, the requested location-specific information is being pulled by rather than being pushed to the user [1]. Recently, all major mobile operating systems introduced build-in support for proactive LBS, better known as Geofencing, In this case, the mobile device is able to notify the user about location-specific information in case the user enters or leaves a dedicated zone, called geofence. This feature is mainly used by location-based reminder applications as a convenient way for users to be reminded about personal to-do's at particular locations [2]. Another very promising application area deals with location-based advertisements [3]–[6]. Mobile device users that are walking or driving close to a commercial district are considered to be potential customers and will be proactively notified about sales or coupons.