Implementation of a Software-Defined Acoustic Modem for MBPS Wireless Underwater Communication

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