Abstract
An adaptive iterative receiver structure for the shallow underwater acoustic channel (UAC) is proposed using a decision feedback equalizer (DFE) and employing bit-interleaved coded modulation with iterative decoding (BICM-ID) in conjunction with adaptive Doppler compensation. Experimental results obtained from a sea trial demonstrate that the proposed receiver not only reduces inherent problem of error propagation in the DFE but also improves its convergence, carrier phase tracking, and Doppler estimation. Furthermore, simulation results are carried out on UAC, modelled by utilizing geometrical modelling of the water column that exhibits Rician statistics and a long multipath spread resulting in severe frequency selective fading and intersymbol interference (ISI). It has been demonstrated that there is a practical limit on the number of feedback taps that can be employed in the DFE and data recovery is possible even in cases where the channel impulse response (CIR) is longer than the span of the DFE. The performance of the proposed receiver is approximately within 1 dB of a similar system employing DFE and turbo code, however, at a significantly reduced computational complexity and memory requirements, making our system attractive for real-time implementation.
Highlights
The underwater acoustic channel (UAC) is considered to be one of the most difficult and challenging physical communications media in use today
Unlike in Radio Frequency- (RF-) based communications systems, the electromagnetic waves do not propagate over long distances through the water, and acoustic waves are employed in order to carry the information signal through a UAC instead
It is assumed that the Doppler shift due to relative motion between Tx and Rx is estimated correctly and the resampling operation does not introduce any significant distortion, which leads to the simplified received signal model of (6)
Summary
The UAC is considered to be one of the most difficult and challenging physical communications media in use today. The Doppler effect caused by the relative motion between transmitter and receiver plays an important role due to the wideband nature of the transmitted signal, which results in time expansion or compression of the symbol duration, depending on the direction of motion, and requires compensation in order to establish carrier phase and timing synchronization. The combination of these effects poses many challenges to the realization of robust, high data rate communications. These velocities can cause an excessive rate of equalizer tap rotation, and the required convergence rate may lead to instability of the adaptive receiver algorithms
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