Hi all !
For our localization project, there’s an urgent need to find practical solutions to transmit ultrasonic frames from a point (emitting beacon) to multiple points (terminals) that are using this service. The most important question we raised after last time we met to deal with the project was the following:
-How to represent binary frames in the physical medium ?
R: we agreed on a 20Khz frequency wave as precised in an earlier post.
However, the chosen physical medium (air) presents some anisotropic properties that we should take into consideration somehow : echo, noise, …
According to this paper , an easy way to handle echo problems would be as follows : consider that we wish to represent our numeric message (succession of 0-1 bits) with a succession of periodic ultrasound impulsions (1:20kHz, 0:silence), and each period would last for about 1 ms. Then we should make sure that the 1 bit impulsion would last shorter enough (say 0,3ms for example) to make sure that the resulting reflected waves would not overlap with the representation of the next sent symbol.
There will be some signal processing ahead to generate ultrasonic waves from transmitter’s point of vue, filter the resulting sound and detect rising and falling edges from the receiver’s one.
This been said, for the medium to be sufficiently reliable, the ultrasonic sent frames must hold error detection and correction information.
 Moo-Kwang Byeon, Jun-Ho Jeon and Sung-Joon Park, “Point-to-Multipoint Ultrasonic Communication Modem for Ubiquitous Underwater Wireless Sensor Networks”, 2008 IEEE/IFIP International Conference on Embedded and Ubiquitous Computing