Frequency-Fresnel Domain Equalization for Underwater Acoustic OCDM Systems

Orthogonal Chirp Division Multiplexing (OCDM) is a promising multi-carrier modulation for Underwater Acoustic (UWA) systems, as several low complexity OCDM receivers capable of exploiting the channel diversity have already been proposed. However, OCDM receivers are often designed and studied over time-invariant channels. This paper proposes a hybrid Frequency-Fresnel domain equal-izer to enhance the system performance over the doubly selective UWA channel. Our method first estimates the Channel Impulse Response (CIR) using pilot blocks and the Least Square (LS) criterion. The CIR is then sparsified by removing coefficients below a threshold, and the receiver performs equalization in the frequency domain. To address residual InterSymbol Interference (ISI) in doubly selective UWA channels, we propose to equalize again the obtained symbols in the Fresnel domain and perform jointly phase synchronization. Simulations both on synthetic channels and a real underwater channel show that the proposed algorithm improves significantly the BER performance compared to existing methods.