Indoors data communications using airborne ultrasound

We have tested an indoor data communications system based on ultrasound which is the core of an indoor positioning system. Unlike most other such systems, it relies on ultrasound alone. No radio or infrared channel nor any tether is used for communication. The main hardware components are a tag and a detector. The detector has an Ethernet interface and 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, we find that the range predictions are consistent with our experience. The channel efficiency of the system is found to be somewhat less than for human speech which also has to deal with a similar environment. This comparison is done by using the Shannon channel capacity theorem.