This paper investigates the signal-to-interference ratio and the achievable rates of underwater acoustic (UA) OFDM systems over channels where time and frequency dispersion are high enough that (i) neither the transmitter nor the receiver can have a priori knowledge of the channel state information and (ii) intersymbol/intercarrier interference (ISI/ICI) cannot be neglected in the information-theoretic treatment. The goal of this study is to obtain a better understanding of the interplay between interference and the achievable transmission rates. Expressions for these rates take into account the “cross-channels” established by the ISI/ICI and are based on lower bounds on mutual information that assume independent and identically distributed input data symbols. In agreement with recent statistical analyses of experimental shallow-water data, the channel is modeled as a multivariate Rician fading process with a slowly time-varying mean and with potentially correlated scatterers, which is more general than the common wide-sense stationary uncorrelated scattering model. Numerical assessments on real UA channels with spread factors around 10−1show that reliable OFDM transmissions at 2 to 4 bits/sec/Hz are achievable provided an average signal-to-noise ratio of 15 to 20 dB.
International audienceThis paper presents an underwater acoustic channel simulation methodology that combines parametric modeling with stochastic replay of at-sea measured channel impulse responses. The motivation behind this approach is to extend the scope of use of replay-based methods by allowing some parameterization of the channel properties while complying with some level of realism. Such an approach is beneficial for extensive testing of communication links. The key idea is to deliberately distort the statistics of the experimental channel in order to meet some user-specified constraints. Our approach is based on a relative entropy minimization between the original time-varying channel impulse response and the simulated one. A particular attention is given to constraints on the channel Doppler spread and on the level of covariance between channel taps. The testing capabilities provided by parametric replay-based simulations are illustrated with real data collected in the bay of Brest, France
Based on a method of inductive inference known as the principle of maximum entropy, a time-varying underwater acoustic channel model is derived. The resulting model is proved to be consistent so that it only relies on the available knowledge of the environment to model. While requiring only a few parameters (e.g. channel average power and Doppler spread), it is shown through fading statistics and bit error rates measurements that accurate channel impulse responses can be obtained for communication applications. The Matlab code of the proposed model is available at http://perso.telecombretagne.eu/fxsocheleau/software.
The resolving power of passive SAS is addressed by NB signal, no coherence loss due to acoustic propagation, computing and plotting its ambiguity function in the array shape and location perfectly known and corrected when frequency/bearing plane. It is proven that, even in ideal necessary), the apparently better spatial resolution of passive conditions, the apparently better spatial resolution of passive SAS SAS is due only to the better frequency resolution which is due only to the better frequency resolution which results from results from the longer processing time and to the coupling the longer processing time. In other respects the computation of between bearing and frequency measurements due to the array Cramer-Rao Lower Bounds (CRLB) for joint source bearing and . . l v . v frequency estimation is detailed. It is shown that, in realistic motion. The same improvement would have been obtained situations, i.e. when the source frequency is not known, the with conventional beamforming, providing narrow band bearing accuracy (standard deviation) does not depend upon the filtering with the equivalent bandwidth (inverse of the whole own speed (and hence on the length of the synthetic aperture). In processing time). Moreover this resolution gain is available fact the true, and maybe the sole, practical interest of the many only when NB signals incoming from the different sources are proposed passive SAS algorithms 12, 41 is that they may appear as received at different frequencies on the hydrophones of the manners to perform a coherent very long time integration, thus physical array. It is also necessary that the transmitted possibly allowing detection of weak coherent narrow band frequency and the Doppler shift due to source motion were sources.both perfectly known, otherwise the bearing frequency ambiguity would result in bearing measurement biases.
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