Holmquist, L.E., Falk J. and Wigström, J. Supporting Group Collaboration with Inter-Personal Awareness Devices. Journal of Personal Technologies, 3(1-2), Springer, 1999.
————————————

This paper describes one of the initial attempt to build an inter–personal awareness device. The system known as Hummingbird is in essence an active RF-id tag that emit a sound when a colleague or teammate happen to be close by.