Wireless communication technologies such as cellular networks, GPS and Wi-Fi
are ubiquitous on land, but underwater, they are inadequate. The radio
waves that these technologies use to carry information wirelessly over land
can only penetrate a few centimeters of saltwater. This paper will discuss
the implementation of an acoustic modem that uses sound waves, as whales
and dolphins do, for sending information and that can support data rates
beyond 1 Mbps, 1000 times faster than existing commercial systems. We developed
and implemented a physical layer protocol specifically tailored to the
needs of a reliable high-speed acoustic communication link. First a custom
hyperbolic chirp waveform is sent for robust receiver synchronization and
initialization, then a stream of QAM symbols follows. Some of these QAM
symbols are known a priori, such that the receiver algorithms can adapt to
the time-varying reverberation and Doppler structure of the acoustic communication
channel. The modem is implemented in software on the BeagleBone
Black (BBB), a cheap single board computer, and an attached analog frontend
(AFE) daughter card is used for signal generation and acquisition. The
signal processing software runs in a non-preemptive operating system (Debian)
on an ARM processor. The programmable real-time units (PRUs) on
the BBB are utilized to interface with the AFE. The PRUs stream signals
from and to the AFE in real time and communicate with the ARM processor
through fast direct memory access transfers.Ope
