Airborne ultrasound data communications: the core of an indoor positioning system

This paper gives an analysis of an ultrasonic indoor data communications system. It may be the core of an indoor positioning system which can rely on ultrasound alone, as no radio, no infrared channel nor any tether is required. The main hardware components are a tag (transmitter) and a detector (receiver). The detector uses digital signal processing to cope with the acoustic environment and its noise, reverberations and Doppler shift. The attainable range is 10-20 meters and by making a comparison with the range for speech and for the Silbo whistling language, we find that the range can be predicted well. The channel efficiency of the system is found to approach 0.025 bits per second per Hz. This is somewhat less than speech which also has to deal with a similar environment. This comparison is done by using the Shannon channel capacity theorem. It indicates that there is a potential for improvement which will impact capacity in a positioning system as the channel is time-shared among the transmitters. The output energy of the system is compared to currently accepted exposure limits and found to be safe.