Abstract-We propose a novel method for detection, synchronization, and Doppler scale estimation for underwater acoustic communication using orthogonal frequency division multiplex (OFDM) waveforms. The method involves transmitting two identical OFDM symbols together with a cyclic prefix, while the receiver uses a bank of parallel self-correlators matched to different Doppler scaling factors on waveform dilation or compression. We characterize the receiver operating characteristic in terms of probability of false alarm and probability of detection, and analyze the impact of Doppler scale estimation accuracy on the data transmission performance. We have tested the proposed method with real data from an experiment at Buzzards Bay, MA, Dec. 15, 2006. Using only one OFDM preamble, the proposed method achieves performance similar to an existing method that uses two linearly-frequency-modulated (LFM) waveforms, one as a preamble and the other as a postamble. Avoiding the need of buffering the whole data packet before data demodulation, the proposed method enables online receiver operation.

Abstract-We propose a novel method for detection, synchronization, and Doppler scale estimation for underwater acoustic communication using orthogonal frequency division multiplex (OFDM) waveforms. The method involves transmitting two identical OFDM symbols together with a cyclic prefix, while the receiver uses a bank of parallel self-correlators matched to different Doppler scaling factors on waveform dilation or compression. We characterize the receiver operating characteristic in terms of probability of false alarm and probability of detection, and analyze the impact of Doppler scale estimation accuracy on the data transmission performance. We have tested the proposed method with real data from an experiment at Buzzards Bay, MA, Dec. 15, 2006. Using only one OFDM preamble, the proposed method achieves performance similar to an existing method that uses two linearly-frequency-modulated (LFM) waveforms, one as a preamble and the other as a postamble. Avoiding the need of buffering the whole data packet before data demodulation, the proposed method enables online receiver operation.

In this paper, we study the performance of orthogonal frequency division multiplexing (OFDM) over underwater acoustic multipath channels with different Doppler scales on different paths. We first derive an exact inter-carrierinterference (ICI) expression after incorporating the compensation of nonuniform Doppler shifts across OFDM subcarriers. Based on the assumption that the residual ICI is dominantly from immediate neighbors, we suggest a practical design that divides subcarriers into groups, where each group of eight subcarriers consists of three contiguous data subcarriers, one pilot subcarrier, and five carefully spaced null subcarriers. We use the orthogonal matching pursuit (OMP) algorithm for sparse channel estimation that identifies distinct physical paths with different Doppler scales. System performance is evaluated using data recorded from the GLINT08 and SPACE08 experiments. Relative to the receiver that ignores the residual ICI, we observe that explicitly suppressing the residual ICI induced by Doppler spread leads to improved performance for the SPACE08 data, while not for the GLINT08 data.

Abstract-Underwater acoustic channels induce large Doppler drifts that render intercarrier interference (ICI) for OFDM transmissions. Assuming that after proper Doppler compensation the residual ICI is limited to only direct neighbors, we propose an OFDM signal design that decouples channel estimation and data demodulation. We investigate eight receivers that are categorized into three groups: (i) three receivers that ignore the residual ICI, (ii) three receivers that are based on a basis expansion model (BEM) and pursue channel estimation independently along each basis, and (iii) two receivers that are based on discrete-path modeling. The receiver performance is compared based on data from the SPACE experiment conducted off the coast of Martha's Vineyard, Massachusetts, October 2008. The receiver based on the discrete-path modeling and a basis pursuit algorithm achieves the best performance while the receiver based on BEM and leastsquares channel estimation performs the worst. The performance differences among different receivers drastically increase as the channel's Doppler spread and the signal constellation size increase. Interestingly, the BEM based receivers are often inferior to the ICI-ignorant counterparts, implying that the ability of ICI compensation could be limited by the estimation accuracy of the much increased number of model parameters. I. INTRODUCTIONFast variation of underwater acoustic (UWA) channels introduces intercarrier interference (ICI) for underwater multicarrier transmissions. The receivers in [1], [2] are constructed assuming that all the propagation paths have a similar path variation rate and the residual ICI can be ignored after proper Doppler compensation. On the other hand, basis expansion models (BEM) have been used to approximate doubly (timeand frequency-) selective UWA channels in [3]- [5] so that the ICI is limited to within neighboring subcarriers.In this paper, we assume that the residual ICI of an orthogonal frequency division multiplexing (OFDM) system is limited to only direct neighbors after proper Doppler compensation. We then propose an OFDM signal design that decouples channel estimation and data demodulation. Specifically, pilot and data subcarriers are separated by at least two null subcarriers so that (presumably) they do not interfere with each other. For this system, we investigate eight receivers that are categorized into three groups:• Three receivers that ignore the residual ICI, where leastsquares (LS) as in [1], orthogonal matching pursuit

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