This paper deals with a receiver scheme where adaptive equalization and channel decoding are jointly optimized in an iterative process. This receiver scheme is well suited for transmissions over a frequency-selective channel with large delay spread and for high spectral efficiency modulations. A low-complexity soft-input soft-output-ary channel decoder is proposed. Turbo equalization allows intersymbol interference to be reduced drastically. For most time-invariant discrete channels, the turbo-equalizer performance is close to the coded Gaussian channel performance, even for low signal-to-noise ratios. Finally, results over time-varying frequency-selective channel proves the excellent behavior of the turbo equalizer.

This paper presents a novel unsupervised (blind) adaptive decision feedback equalizer (DFE). It can be thought of as the cascade of four devices, whose main components are a purely recursive filter (R) and a transversal filter (T): Its major feature is the ability to deal with severe quickly timevarying channels, unlike the conventional adaptive DFE. This result is obtained by allowing the new equalizer to modify, in a reversible way, both its structure and its adaptation according to some measure of performance such as the mean-square error (MSE). In the starting mode, R comes first and whitens its own output by means of a prediction principle, while T removes the remaining intersymbol interference (ISI) thanks to the Godard (or Shalvi-Weinstein) algorithm. In the tracking mode the equalizer becomes the classical DFE controlled by the decision-directed (DD) least-mean-square (LMS) algorithm. With the same computational complexity, the new unsupervised equalizer exhibits the same convergence speed, steady-state MSE, and bit-error rate (BER) as the trained conventional DFE, but it requires no training. It has been implemented on a digital signal processor (DSP) and tested on underwater communications signals-its performances are really convincing.

This paper introduces a novel blind adaptive multiple-input decision-feedback equalizer (MI-DFE) which is basically characterized by its ability to self-optimize its configuration, in terms of both structure and criteria, according to the severity of the transmission medium. In the first running mode, the novel equalizer is recursive, linear and "blindly" adapted by criteria leading to a solution closely related to the minimum MSE solution. In the second running mode, it becomes the conventional MI-DFE. From the viewpoints of both robustness and spectral efficiency, this equalizer proves to be very attractive since it avoids pathological behaviors, often encountered with the conventional trained MI-DFE, while requiring no training sequence. Furthermore, its very high speed of convergence renders it competitive in various standard applications, even in the case of burst mode transmission systems. Finally, the novel blind MI-DFE has been successfully tested on underwater acoustic communications signals, in a very severe context. The results are clearly convincing.

There is no doubt about the growing interest for the underwater acoustic communications. Among all existing applications, the objective of the Groupe d'Etudes Sous-Marines de I'Atlantique (GESMA) is to develop a sufficiently robust high data rate acoustic link, named TRIDENT. For that purpose, different kinds of information (texts, images ...) could be periodically transmitted through the acoustic channel. A realtime receiver, based on the spatio-temporal blind adaptive decisiun feedback equalizer, developed and patented by ENST Bretagne [I], was designed to cope with all perturbations induced by such harsh channels. Some sea trials have been carried out in June 2002. The first results are clearly convincing since most of the 48 sequences of 5 minutes are successfully detected by the DSP-based real-time receiver. This acoustic system allows transmission at data rates ranging from 8 to 25 kbps in horizontal configuration.

Achieving reliable high speed digital communications over underwater channel is a challenge that many scientists are trying to take up. For various applications such as remote control and data exchange, for example between an autonomous underwater vehicle and a surface vessel, the need is getting more and more important in designing reliable communications systems by means of acoustic waves. In this paper we focus on a coherent receiver where synchronization (timing recovery and carrier recovery) and equalization are jointly optimized.

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andre.goalic@enst-bretagne. fi dbstruct: Current needs in underwater acoustic communicafion, both for civil and military applications, are eontinuouslj increasing. During ihese lost years, GESMA (Groupe d'Efudes Sous-Marines de I'AiIantique) and ENST-Bretagne developed the TRWENT system (TRLDENT : TRansmisswn de Donndes EN Temps reel This is the first real time acoustic (ink able io transmit images, tea? and data based on multichannel blind egualizaiion. In 1994, a low bit rate speech coder (5.45 kbiuj) was designed and validated for speech acoustic wave transmission in Bresi bay [lj. More recenti'y, a speech transmission was also successfully realized in July 2003 at sea, using the new GESMA-ENST-Bretagne acouslic link This paper provides Qt4 overview of this projed The c o n t d of the study and the implementation of a very low speech acoustic link are presented Last Brest harbor speech hammission euperiments (2003) are presentedThe real time horizontal acoustic link performances are evaluated Two carriers frequencies are available (20, 35 kH@. Acousiic communications for bit rule ranging fiom 6 io 20 kbps and range from 500 to 4000 m have been conducied successfuUy over several hours in shallow water environments