Low Frequency Mobile Communications in Underwater Networks Abdel-Mehsen Ahmad 1 , Michel Barbeau 2[0000−0003−3531−4926] , Joaquin Garcia-Alfaro 3[0000−0002−7453−4393] , Jamil Kassem 1 , Evangelos Kranakis 2[0000−0002−8959−4428] , and Steven Porretta 2 1 School of Engineering, Lebanese International University, Bekaa, Lebanon. 2 School of Computer Science, Carleton University, Ottawa, ON, Canada, K1S 5B6. 3 Telecom SudParis, CNRS Samovar, UMR 5157, Evry, France. Abstract. We present a receiver for low frequency underwater acous- tic communications addressing the Doppler shift that occurs during the transmission of frames at a very low data rate. The receiver handles constant or variable (linearly and nonlinearly) Doppler shift patterns. The waveform supported by the receiver is adapted to difficult underwa- ter channel conditions, such as the ones present in long range under-ice Arctic communications. The bandwidth is extremely narrow (less than six Hz). Redundancy is very high (300%). Our main contributions are in an aspect of the receiver that handles arbitrary types of Doppler shifts. We use the idea of signal tracking function. It follows the progression of a carrier during the reception of a frame. Evaluation results are reported using our GNU Radio implementation. 1 Introduction Underwater data communications and networking have applications in moni- toring and surveillance of coastal waters [1], submarine activity sensors [2], au- tonomous undersea vehicles [3] and submerged airplane locator beacons [4]. We focus on low frequency mobile communications [5,6], i.e., in the range 0.3 to 3 kHz. Relative to higher frequencies, Stojanovic stressed that attenuation is lower [7]. Hence, there is potential for long distance contacts [8]. However, be- cause of the narrow half-power bandwidth of low frequency and long distance operation, only extremely low data rates are possible. Furthermore, the relative mobility of a transmitter and a receiver affects the acoustic waves used for un- derwater communications. This is the Doppler effect. Contrasted with classical electromagnetic communications, it has a significant impact. In this paper, we consider the Doppler shift that occurs during the reception of low-data rate frames in low frequency and long distance acoustic underwater communications. There are three cases: constant, linearly variable and nonlin- early variable Doppler shift. In background research [9,10], we concluded that in the case of transmitter-receiver collateral motions, in the zero to eight knot range, we have constant relative velocity and constant Doppler shift within zero to eight Hz. For transverse motions, the Doppler effect is nonlinearly variable in