Fractional FFT demodulation for differentially coherent detection of acoustic OFDM signals

We propose Fractional FFT (F-FFT) demodulation as a method for compensating the Doppler distortion in an acoustic orthogonal frequency division multiplexing (OFDM) system. F-FFT is based on demodulation with two FFTs, shifted by half the carrier spacing, whose outputs are combined prior to data detection. We consider differentially coherent detection, and develop an algorithm for recursive computation of the combiner weights. The algorithm is cast into a multi-channel framework that takes advantage of spatial diversity. Synthetic data, and experimental data obtained over a mobile underwater acoustic channel, are used to compare the proposed method to conventional coherent and differentially coherent detection, showing significant improvement in performance (several dB), as well as the possibility to increase the bandwidth efficiency.