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Section: New Results

Underwater communications

Participants : C. Siclet [ contact person ] , C. Canudas-de-Wit, M. Mossi Idrissa, G. Gomez.

CONNECT (CONtrol of NEtworked Cooperative sysTems) is a project granted by the ANR. In collaboration with IFREMER, GIPSA-lab, PROLEXIA, and PGES, CONNECT aims at studying the problem of multi-agent control (AUVs) and coordination with heterogeneous networks, including underwater communication. In this context, we will interest to underwater acoustic communication.

Current underwater acoustic modems are based on very classical single-carrier modulation with a very low bit rate. In the same time, wireless radio-communications have been significantly improved during the latest ten years, in particular thanks to multi-carrier modulations. These modulations are indeed now used in several high rate applications (ADSL, DVB-T [57] , IEEE 802.11a/g, ...). These applications are all based on the same modulation, OFDM (Orthogonal Frequency Division Multiplex). Thanks to the use of a guard time [53] (or cyclic prefix), it is possible under certain conditions to simplify considerably the equalization step, so that OFDM is particularly performing. That is why OFDM has also recently been considered for underwater communications : the underwater acoustic channel [44] , [65] is indeed particularly frequency selective, so that OFDM is a potential interesting solution [31] .

In spite of its advantages, OFDM also suffers from several drawbacks : the guard time induces a spectral efficiency loss, and, what is more, the pulse shape used for each carrier is rectangular, and therefore badly frequency located. This spectral efficiency loss remains small if we use long duration symbols, but it may cause inter-carrier interferences if the transmission channel is not stationary during a symbol interval. Compensation techniques have then to be studied [40] . Lastly, the modulated signal has an amplitude which may by very high which is problematic for linear amplification.

Several alternatives have been proposed to solve these problems, among them oversampled OFDM/QAM modulations [62] and OFDM/OQAM modulations [41] , [64] , and their bi-orthogonal extensions (BFDM) [61] . For each of these types of modulation, it is possible to use non rectangular pulse shapes, optimized according to a given criterion (time-frequency localization, frequency localization...). They appear then to be promising and give more freedom degrees in their conception than classical OFDM. Nevertheless, the equalization is more complex (in the OQAM case) and their implementation more expensive (in terms of computational complexity).

Our objective is to study multi-carrier modulation systems (OFDM, oversampled OFDM/QAM, OFDM/OQAM as well as their bi-orthogonal extensions) and to determine the corresponding receptors, in the framework of underwater acoustic communications. We will then have develop a complete communication chain using and comparing these different modulations to classical underwater communication systems. We will thus interest to various problems : channel estimation, equalization, synchronization, non-linearity of digital to analog and analog to digital converters.

Further investigations are currently conducted in order to extend this results in the case of more sophisticated multi-carrier modulations and more realistic UWA communication channel (OFDM/QAM and OFDM/OQAM).


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