Underwater acoustic (UWA) channels are one of the historical mobile ultrawideband channels characterized by large delay and Doppler spreads, but reliable UWA communication remains challenging. Here we performed an initial demonstration of the Doppler-resilient orthogonal signal division multiplexing (D-OSDM) technique in an actual sea environment. D-OSDM spreads data symbols in both time and frequency with guardbands to exploit the time and frequency diversity of UWA channels. The experiment was performed in a challenging scenario: the transmitter was fixed on a floating pier, and the receiver was mounted on a moving remote-controlled boat. The harbor UWA channel had a delay spread of 50 ms and a Doppler spread of up to 4.5 Hz, in the presence of additive Gaussian noise, and the combination of two Rayleigh fading models (a two-path model without Doppler spread and a multi-path model with Doppler spread) was able to successfully model the actual environment. Our results also confirmed that a UWA communication link using D-OSDM will deliver excellent reliability even for a harbor UWA channel with a mobile receiver; D-OSDM achieves better communication quality compared to other communication schemes in both simulations and experiments.

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Underwater acoustic channels are characterized by a severe spread in time and frequency. To provide a highly reliable communication environment in doubly spread channels for UWA communication, we have proposed Doppler-resilient orthogonal signal-division multiplexing (D-OSDM). D-OSDM is a combination of the OSDM technique and orthogonal multiple access, and it preserves orthogonality among data streams even after propagation through doubly spread channels. We have conducted test tank experiments and simulations, and have found that D-OSDM could cope well with time- and frequency-spread channels and that it achieves a good communication quality [1]. In this talk, we will show some results of our first sea trial of D-OSDM, that was performed on 21 June, 2016 at Hashirimizu port, Kanagawa, Japan. In this trial, the transmitter is fixed on the harbor quay, and the receiver is fixed on a remotely-operated survey boat. Acoustic communication was performed by moving the boat to generate a severe spread in frequency. The experiments show that D-OSDM achieves a good communication quality even if the boat moves at a speed of about 1.5 kt. [1] Tadashi Ebihara and Geert Leus: Doppler-Resilient Orthogonal Signal-Division Multiplexing for Underwater Acoustic Communication, IEEE J. Ocean. Eng. 41 (2016) 408-427.@en

Underwater acoustic (UWA) channels are characterized by a severe spread in time and frequency, and are usually labeled as “doubly spread channels.” In this paper, we propose Doppler-resilient orthogonal signal-division multiplexing (D-OSDM), to provide a highly reliable communication environment in doubly spread channels for UWA communication. D-OSDM multiplexes several data vectors in addition to a pilot vector, and preserves orthogonality among them even after propagation through doubly spread channels, under the assumption that the channel can be modeled by a basis expansion model (BEM). We describe the signal processing steps at the transmitter and the receiver for D-OSDM, and evaluate its performance by both simulations and experiments. To generate a doubly spread channel, a test tank with a wave generator is employed. The obtained results suggest that D-OSDM can provide low-power and high-quality UWA communications in channels with large delay and Doppler spreads; for example, D-OSDM succeeds to achieve a block error rate (BLER) of 10 -3 while BEM-based orthogonal frequency-division multiplexing (OFDM) has a BLER floor of 10 -2 in the experiments. Equivalently, D-OSDM can reduce the signal power required for communications to achieve the same BER significantly. Overall, it was found that D-OSDM can become a powerful communication tool for underwater operations.@en