Abstract-In this paper we propose a novel method for communication over underwater acoustic channels that exhibit simultaneously large delay spread and Doppler spread, such as those found in the surf zone. In particular, we propose a coded pulse-shaped multicarrier scheme that converts the doubly dispersive channel into an inter-carrier interference (ICI) channel with small ICI spread. The resulting ICI is mitigated using a soft noncoherent equalizer that leverages sparsity in the delay-power profile to generate near-optimal bit estimates with low complexity. The noncoherent equalizer uses a delay-power-profile estimate (rather than a channel estimate) which is obtained from pilots. Numerical simulations with surf-zone-like channels demonstrate performance close to genie-aided bounds.

In this paper we propose a novel method for communication over underwater acoustic channels for which the product of delay-spread and Doppler-spread is large, such as those pertaining to the surf zone. In particular, we propose the use of pulse-shaped multicarrier modulation to convert the doubly dispersive channel into an inter-carrier interference (ICI) channel with small ICI spread. We then propose a novel joint ICI-estimation/data-detection strategy which performs near optimally yet at low complexity, due to the use of an efficient tree search algorithm and the ability to leverage sparseness in the delay-power profile. The delay-power profile, which itself varies in time, can be readily tracked via pilots. Numerical simulations show that our technique gives performance close to genie-aided bounds over highly spread channels.

We propose a high-spectral-efficiency multicarrier system for communication over the doubly dispersive (DD) channel which yields very low frame error rate (FER), with quadratic (in the frame length) receiver complexity. To accomplish this, we combine a non-(bi)orthogonal multicarrier modulation (MCM) scheme recently proposed by the authors with novel sequence detection (SD) and channel estimation (CE) algorithms. In particular, our MCM scheme allows us to accurately represent the DD channels otherwise complicated intercarrier interference (ICI) and intersymbol interference (ISI) response with a relatively small number of coefficients. The SD and CE algorithms then leverage this sparse ICI/ISI structure for low-complexity operation. Our SD algorithm combines a novel adaptive breadth-first search procedure with a new fast MMSE-GDFE preprocessor, while our CE algorithm uses a rank-reduced pilot-aided Wiener technique to estimate only the significant ICI/ISI coefficients.

Abstract-We propose a scheme for near-optimal sequence detection (SD) of uncoded block transmissions over unknown doubly dispersive (DD) channels. Starting with a noncoherent maximum likelihood (ML) metric that leverages a basis expansion model (BEM) for the channel's time-variation, we propose an efficient noncoherent SD strategy based on suboptimal tree search with a fast metric update. Our scheme yields performance within a fraction-of-a-dB from ML sequence detection with genieaided channel estimates, and maintains complexity that is only quadratic in the block length.

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