Short Paper: The Softwater Modem – A Software Modem for Underwater Acoustic Communication

The Softwater Modem is a software modem for underwater acoustic communication that enables users to run applications on the familiar sockets interface without any additional hardware except for transducers and associated amplification. A standard TCP or UDP transport protocol runs on top of IP, which via the Linux TUN driver, runs on top of custom datalink and PHY layers tailored specifically to the underwater channel. The datalink and PHY layers, written entirely in Java, use frequency-division multiple access (FDMA) with binary and 4-FSK (frequency shift keying) in any frequency band supported by the computer’s sound card and can run at any bit rate supplied by the user. The transmitter sends a per-packet linear frequency modulated (LFM) chirp signal that the receiver uses for packet synchronization as well as channel estimation, with the option of applying impulse response estimates to channel equalization. Frames can contain up to 255 bytes and are encoded with Reed-Solomon (R-S) codes, for which the user can specify the number of parity bytes. The paper describes the architecture and performance of this system, which currently demonstrates two-way communication as well as realtime channel estimation.