Fractional Fourier Transform Based Channel Estimation in Underwater Acoustic Communications

Underwater communications have severe channel distortions which can greatly affect its performance. This paper proposes a novel approach for estimating channel parameters (multipath delay and Doppler scaling) for underwater acoustic channels. The method is a Fractional Fourier Transform (FrFT) based approach with linear frequency-modulated (chirp) signals as a pilot. Innovation lies in the receiver design, where a 2D array is formed by performing a scan of FrFTs on the received signal. Key point detection algorithms are exploited to extract distinct X-shaped features in a 2D array, allowing for simultaneous estimation of both channel delay and Doppler parameters for all multipath components. The novel approach avoids the costly iterative process used in existing algorithms, which calculates Doppler scaling for one path per iteration. Simulation results show that the proposed algorithm is capable of estimating channel parameters in a severely time-delayed and Doppler-scaled multipath channel. The algorithm achieved a ±0.1% estimation accuracy with Doppler scaling factors ranging between 0.95 to 1.05. Multipath delay was estimated to within ±2ms for delays ranging from 0 to 0.2 seconds.