Joel Labat

Adaptive decision feedback equalization: can you skip the training period?

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 (/spl Rscr/) and a transversal filter (/spl Tscr/). Its major feature is the ability to deal with severe quickly time-varying 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, /spl Rscr/ comes first and whitens its own output by means of a prediction principle, while /spl Tscr/ removes the remaining intersymbol interference (ISI) thanks to the Godard (1980) (or Shalvi-Weinstein (1990)) 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.

Toward a digital acoustic underwater phone

The paper deals with the design of a digital acoustic underwater phone prototype. Digital techniques allow to achieve a synthesized speech with a quality close to the telephonic one. The input speech signal is compressed down to 5.45 kbit/s using a CELP coder. The bit rate is 6 kbit/s before channel coding and expected to be about 8 kbits/s after channel coding. A QPSK modulation with differential encoding was chosen to transmit the useful signal. For the receiver the authors use a scheme where synchronization and equalization (FSE+DFE) were jointly optimized. The whole system (unidirectional link) has been implemented on single DSPs (Motorola 56001) and tested successfully in a very difficult environment (IFREMER pool).<<ETX>>

Iterative equalization for underwater acoustic channels potentiality for the TPIDENT system

This paper deals with blind adaptive equalization for short burst communication system. Until now, blind adaptive equalizers have been mainly dedicated to continuous data stream applications because of their supposedly well-known slow convergence. The aim of the paper is twofold. It intends both to combat this latter idea and to stress the relevance of the blind approach, under two mild conditions of sufficient burst length and channel stationarity. In some cases, conventional results of equalization are not adequate. This paper proposes two different techniques for improving the performance of equalization processing. For that purpose, the blind decision feedback equalizer introduced in [J. Labat, et al., 2001] is combined with an iterative procedure and operates on both direct and time reverse sequences. Performances are illustrated with examples of 4-PSK short bursts and underwater acoustic signals. The results are very convincing since, at a low computational cost, the challenge of recovering all the data has now been taken up.

Real time underwater communications

Achieving reliable high speed digital communications over an 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. The authors focus on a coherent receiver where synchronization (timing recovery and carrier recovery) and equalization are jointly optimized.<<ETX>>

Blind decision feedback equalization application to underwater communication systems

This paper deals with a new blind decision feedback equalizer (DFE) introduced in Labat et al. (1995) and illustrates its good behaviour in an underwater acoustic communication system. Over such a severe channel, the results turn out to be really outstanding in the sense that the new equalizer clearly outperforms the conventional trained DFE thanks to its ability to modify, in a reversible way, both its structure and its criteria for adaptation. This is evidenced through real signals originating from experiments carried out in Brest Bay. Moreover this new equalizer has been implemented on a DSP and its behaviour observed for a few hours. The results are fully satisfactory.