The mobile devices basically display whatever they receive within sight of a certain transmitter. Since every sender streams locally adapted information this results in a seeming location awareness of the device without ever dealing with explicit locations on the client side. The granularity of this kind of implicit tracking is determined by the (adjustable) range of the senders. Since human perception structures the surrounding space largely based on its visual appearance and infrared light follows the same physical rules as visible light, the information space is effectively structured in a very adequate way. Information behind a wall cannot be received, since infrared light is blocked by it, but in the vast majority of cases this is just what we want. Radio networks and radio bearing have to deal with this fact explicitely by employing a 3D model of the building and explicit tracking, while in the case of infrared light it is a side effect of the very working principle. Another advantage of optical transmission over radio cells is its sensitivity to orientation in space. A visitor of an exhibition might stand closer to one exhibit, but still look at another one. If a person has to be guided across a large building, the information that has to be shown (such as arrows or floor plans) depends not only on her position, but also on her walking direction. Similarly several senders can be installed in one position streaming different data in different directions. By making the simple assumption that the PDA is always held upright in front of the user we are able to position several senders in such a way, that, depending on where a person is facing from the same position, different information is displayed. The 3D space is thus augmented not only by location specific, but also by orientation specific information.
Finally a unidirectional communication is the most effective way to protect a user's privacy within this kind of system. Since there is no global tracking and since the mobile devices never send back any information or request, there is no way to find out about people's locations within the building. Systems such as the active badge system [6] take the opposite approach and customize the environment based on a badge wearers tracked position. While this is an acceptable approach in homogeneous and cooperative communities such as a company or a research institute, it will be hard to convince people at a fair or exhibition or in public buildings that the digital trail they leave as they use the system will never be used against their personal interest. On the other hand the aforementioned digital trail can very well be collected on the mobile device and a visitor might easily decide at the end of an exhibition day to give out this information and have the fair organizers send him additional material about selected exhibits she has seen on her journey. The important difference is that this decision is made consciously by the user and that there is no way for the system carrier of cheating around it.