A DFnT-Based Orthogonal CSS Method for Underwater Acoustic Communication

Chirp signals are widely used in the field of underwater acoustic (UWA) communication. Chirp-Cyclic Shift Keying (Chirp-CSK) modulation utilizes the auto-correlation property of Chirp signals to achieve significant spread spectrum gain and anti-noise performance. However, the complex multi-path effect of UWA channels may cause severe demodulation interference and result in poor communication performance, which needs to be suppressed by channel equalization. This paper introduces the Orthogonal Chirp Division Multiplexing (OCDM) framework and proposes a CSK demodulation scheme based on the Discrete Fresnel Transform (DFnT). Based on the correlation between OCDM subcarriers and Chirp-CSK symbols, our method extracts multipath information of the current channel through DFnT and applies it to optimize the demodulation. The proposed demodulator shares a similar principle and performance with traditional Virtual Time Reversal Mirror (VTRM) equalization technology, but with significantly reduced computational complexity. Simulation and experimental results show that the DFnT-based demodulator outperform existing conventional demodulators.