For some applications the need for stealth and covert communications is crucial, especially for military purposes. The underwater acoustic channel is very noisy environment. It is subjected to multiple noise sources, reflections and refractions of acoustic waves, which creates multipath. Considering the multipath nature of the underwater acoustic channel and the transmission of acoustic waveforms at very low signal to noise ratio (SNR) as the need for low probability of intercept (LPI) is essential for our application, we propose a transmission scheme based on direct sequence spreading spectrum and code division multiple access (DS-CDMA). We present the receivers structures for this transmission method and we evaluate their performance. We study the performance of the proposed receivers for Rayleigh fading multipath channels in presence of multiuser access interference. The receivers structures allow both the channel parameters estimation by using a bench of matching filters to the spread code sequences of each user and to exploit the spatial diversity of the underwater acoustic channel by the mean of a coherent recombination of the different paths energy contributions, after the channel estimation (MRC : maximum ratio combining). The channel parameters, paths time delays and channel coefficients, are estimated at the output of matched filters at symbol rate. We show that we can potentially improve the system performance by implementing a successive interference cancellation method. The results obtained establish that the proposed system achieves excellent and robust performance at very signal to noise ratio. The convergence of the successive interference cancellation structure is achieved with no more than three iterations. At last, we introduce channel coding that allows us to improve the overall system performance along with its robustness by using error-correcting codes. Some of the developed algorithms were implemented and tested on real data procured by a sea trials campaign conducted off the coast of Brittany at Rade de Brest and in the Mediterranean Sea. ¿¿¿
