On Mitigating Channel Time Variation Effect in Acoustic OFDM Systems

OFDM systems are challenged by the time variability of mobile acoustic channels where the Doppler effect can be high enough that the received signal experiences non-negligible residual frequency offset, as well as channel time variability, which cause intercarrier interference (ICI). To mitigate these effects, we introduce an efficient yet computationally manageable two-stage algorithm that counteracts the frequency offset as well as the time selectivity of mobile acoustic channels. In the first stage, frequency offset is compensated using a practical approach based on differentially coherent detection which keeps the receiver complexity at a minimum and requires only a small pilot overhead. In the second stage, a method referred to as partial FFT demodulation is used to tone down the time variability of the broadband acoustic channel. Towards this goal, the time interval of one OFDM block is divided into several partial intervals, giving the channel less chance to change over each shorter interval, and demodulation is performed in each interval separately. The partial demodulator outputs are then combined before applying a coherent detection algorithm where refined channel estimation and data detection take place. Using the experimental data transmitted over a 3–7 km shallow water channel in the 10.515.5 kHz acoustic band, we study the receiver performance in terms of data detection mean squared error (MSE), and show that the proposed algorithm provides excellent performance, surpassing all our previously tested approaches.