This article describes a novel approach for robust and reliable underwater acoustic communication targeting low data rate applications in very challenging channels. The well-known binary frequency shift keying (BFSK) modulation is extended with multiple pair-wise BFSK subcarriers forming a multicarrier (MC) waveform referred to as MC-BFSK. Time offsets between neighboring subcarriers combined with windowing and guard periods are employed to increase the robustness against Doppler-induced frequency spread/shift and reverberation together with novel methods for synchronization, peak-to-average-power ratio reduction, and soft-value extraction. For the design of the parameters of the waveform, a heuristic approach is utilized. A noncoherent matched filter receiver with spatial diversity is adopted. The MC-BFSK transmission scheme is evaluated by means of computer simulations as well as sea-trial measurements. The obtained results are compared with competing modulation schemes where the proposed MC-BFSK shows better performance in challenging channels. In particular, we are able to decode frames in a sea-trial scenario where the direct path between the transmitter and the receiver is blocked by an island.

In this paper we propose and analyze a new approach for synchronization of OFDM based underwater communication systems with significant Doppler spread. In order to keep cyclic prefix overhead low - long OFDM symbols are needed which in turn leads to significant inter-carrier interference (ICI). For signal acquisition and synchronization we propose the use of a special OFDM symbol which has regions of null subcarriers i.e. subcarriers where no power is transmitted but also still has subcarriers for channel estimation and payload data. By estimating the signal to noise ratio on candidate demodulation positions we can calculate a “would-be” signal to noise ratio by adding up the power on the active subcarriers and dividing it by the power in the null subcarriers. By only using the inner subcarriers in a region of null subcarreries resistance against Doppler spread is achieved. We optimize and compare the proposed solution against a base-line solution which uses a pre-amble with a hyperbolic frequency modulated signal. We find our solution to compare well and present bit error rate and frame error rate results on a complete system which uses our proposed acquisition and synchronization scheme.