Mobile Acoustic Communications: Real Data Analysis of Partial FFT Demodulation with Coherent Detection

We address the use of orthogonal frequency division multiplexing (OFDM) for high-rate communication over a mobile acoustic channel. To counteract the frequency offset and time-variability of the broadband mobile channel, we employ a dedicated method for synchronization and partial FFT (P-FFT) demodulation, cast into the framework of multichannel diversity combining. Unlike conventional receivers, where the signal is demodulated using a single FFT operating over the full OFDM block interval, P-FFT employs multiple FFT operations to demodulate the signal over several partial intervals. The partial demodulator outputs are subsequently combined, and the combined signal is fed to a second stage, where refined channel estimation and data detection take place. We investigate both coherent and differentially coherent detection, and test refined channel estimation with least squares (LS) and matching pursuit (MP) algorithms. Partial interval demodulation offers an additional degree of freedom by allowing for suppression of time-variation before inter-carrier interference has been created in the process of demodulation. The result is an improved quality of data detection, which we demonstrate using real data recorded during the 2010 Mobile Acoustic Communications Experiment MACE’10. Results of experimental data processing show excellent performance with up to 2048 QPSK-modulated carriers operating in the 10.5 kHz -15.5 kHz acoustic band over varying distance (3-7 km) and speeds up to 1.5 m/s.