We present a novel system that allows small handheld devices to detect other devices in the local proximity and to exchange messages with them. The client devices use WLAN to access the system via a web interface and do not require additional software or modification of the device's operating system. The physical closeness of client devices is estimated based on their signal strengths measured by the system's access points. A major advantage compared to indoor positioning systems is the fact that our system does not require a training phase. We conduct a comprehensive evaluation to demonstrate the practicability of our approach.

In contrast to many applications on the PC and in the Internet which are accessible to visually impaired people today, making electronic music with limited eye-sight is still a challenge. Standard software and hardware are both strongly dominated by graphical output. In order to close this cap, we developed a MIDI (Musical Instrument Digital Interface) sequencer with audio-feedback and a new interaction paradigm which eliminates interaction with the PC's keyboard and screen. The blind musician relies solely on input via the instrument itself. He can both, record and play music via the instrument's black & white keys but at the same time control all functions of a multi-track MIDI sequencer without ever taking the hands off the instrument. We also use the MIDI-connection for coding different kinds of feedback to the user in an efficient way. The software which runs on a PC and is connected to an electronic instrument has been evaluated and improved extensively and is made available free of charge for visually impaired and normal-sighted people.

The presentation of multimedia data and especially of high resolution videos on small mobile devices is still a great challenge today. Both cropping of borders and scaling of frames may result in the removal of essential content of videos or lost details due to the reduced size of the visual content. Another major problem emerges if the aspect ratio of the original video and the display of the mobile device differ. User evaluations indicate that changing the aspect ratio may reduce the visual quality of videos significantly. In this paper, we present the new FSCAV algorithm (Fast Seam Carving for Size Adaptation of Videos) to adapt the size of videos to the limited display resolution and different aspect ratios of handheld mobile devices. The general idea of the seam carving algorithm for still images is to remove seams in images so that the essential content is preserved. We extended this technique which works very well for images to create videos without jitter or visible artifacts. A major feature of our FSCAV algorithm is the low computational complexity which enables an efficient adaptation of videos to small screens. Nevertheless, severe distortions are clearly visible in some shots of the adapted videos. We present a new heuristic to identify shots with such a low visual quality. If the quality drops below a threshold, a different adaptation technique is used for this shot (e.g., scaling or cropping). User evaluations confirm a very high visual quality of our approach.

A challenge for the development of mobile applications is the heterogeneity of the different device classes. For instance, an application which runs well on a personal digital assistant (PDA) might be unsuitable for mobile phones. To facilitate the development of mobile applications we have develope

Location-based services in general require information about the position of certain objects. For instance, for a navigation service the position of the user needs to be known. This position is usually provided by a positioning system. However, it is typical for all positioning systems that they are