Y. Ding, H. Schmidtke, and M. Beigl. Proceedings of the 12th ACM international conference adjunct papers on Ubiquitous computing - Adjunct, page 401--402. New York, NY, USA, ACM, (2010)
D. Korn, R. Huang, D. Beavers, T. Bolioli, and M. Walker. Electronics and the Environment, 2004. Conference Record. 2004 IEEE International Symposium on, page 128 - 131. (May 2004)