Low Complexity Iterative Receiver Design for Shallow Water Acoustic Channels
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Research Article Low Complexity Iterative Receiver Design for Shallow Water Acoustic Channels C. P. Shah, C. C. Tsimenidis, B. S. Sharif, and J. A. Neasham School of Electrical, Electronics and Computer Engineering, Newcastle University, Newcastle upon Tyne NE1 7RU, UK Correspondence should be addressed to C. P. Shah, [email protected] Received 16 July 2009; Revised 3 November 2009; Accepted 3 February 2010 Academic Editor: Frank Ehlers Copyright © 2010 C. P. Shah et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 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 realtime implementation.
1. Introduction The 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 thus, acoustic (pressure) waves are employed in order to carry the information signal through a UAC instead. The acoustic waves propagate at a very low speed of approximately 1500 m/s and the propagation occurs over multiple paths due to reflections from the surface and bottom of the sea. Hence, the UAC is considered and modelled as a highly time varying frequencyselective channel. In practice, the multipath profile of the channel depends on the channel geometry and density of the propagation medium. In the case of vertical channels the multipath spread is very short; however, horizontal channels exhibit a multipath spread of 100 s of symbols. Owing to this long multipath spread, the transmitted signal suffers from ISI that degrades the quality of the received signal which needs to be compensated for befor
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