Frequency-domain channel estimation and equalization for shallow-water acoustic communications

A new frequency-domain channel estimation and equalization (FDE) scheme, combined with a new group-wise phase correction scheme, is proposed for single-carrier (SC) underwater acoustic communications systems employing single transducer and multiple hydrophones. The proposed SC-FDE scheme employs a 2N-point Fast Fourier Transform (FFT) to estimate and equalize the channel in frequency domain, where N is the number of symbols in a data block. Both the frequency-domain channel estimation and equalization are designed by the linear minimum mean square error criterion. Initial channel estimation is performed by a pilot signal block and later updates are achieved using the detected data blocks. The proposed phase correction scheme utilizes a few pilot symbols in each data block to estimate the initial phase shift and then correct it for the block to combat the large phase rotation due to the instantaneous Doppler drifts in the acoustic channels. Time-varying instantaneous phase drifts are re-estimated and compensated adaptively by averaging the phase variation across a group of symbols. The proposed SC-FDE and phase correction method is applied to the AUVFest’07 experimental data measured off the coast of Panama City, Florida, USA, June 2007. With the Quadrature Phase Shift Keying (QPSK) modulation and a symbol rate of 4 ksps, the proposed scheme achieves an average uncoded bit error rate on the order of 1×10^?4 for fixed-to-fixed channels with the source–receiver range of 5.06 km. For the moving-to-fixed source–receiver channels where the source–receiver range is 1–3 km, the multipath delay spread is 5 ms, the average Doppler shifts are ±20 Hz, and the maximum instantaneous Doppler drifts from the mean is ±4 Hz, the proposed scheme achieves an average uncoded bit error rate on the order of 1×10^?3.     

Nanotubes Made from Deeply Undercooled Cryolite/Alumina Melts

The rapid‐solidification processing (by a cooling rate of 10⁵–10⁶ K/s) was used for the preparation of deeply undercooled cryolite/alumina (Na₃AlF₆/Al₂O₃) melts. We found a mass of nanotubes on the surface of these undercooled melts. The nanotubes were preferentially located on the defect places of the surface with the following approximate dimensions: base≈100×100 nm, length≈1000 nm. The solidified samples with the nanotubes on the surface were analyzed by scanning electron microscopy (SEM), energy‐dispersive X‐ray spectroscopy (EDX), X‐ray diffraction (XRD), and infrared spectroscopy (IR).     

Enantioselective epoxidations of alkenes catalyzed by (salen)Mn(III) in aqueous surfactant systems

An aqueous reaction medium, based on a surfactant solution of diethyltetradecylamine N-oxide (AOE-14), was developed for the enantioselective epoxidation of 1,2-dihydronaphthalene and of various cis-β-alkyl styrenes with increasing hydrophobicity, using bleach as oxidant and the Jacobsen chiral (salen)Mn(III) as catalyst. AOE-14 is able to both solubilize all reactants in water and bind the metal of the salen complex acting as coligand. Its use leads to good yields (>75%) and to ee values ranging from 75% up to 91% even in the case of cis-β-alkyl styrenes where lower cis/trans epoxide ratios are observed. The ratio of surfactant/substrate used is 1:1 or 4:1, much lower than those generally used in the literature.     

Element neighbourhoods in twofold triple systems

The method of differences is used to establish that every 2-regular multigraph onv– 10,2 (mod 3) points occurs as the neighbourhood graph of an element in a twofold triple system of orderv, with two exceptions: C₂C₃and C₃C₃.Dedicated to Professor Hanfried Lenz on the occasion of his seventieth birthday     

Classification of incidence scrolls (I)

The aim of this paper is to obtain a classification of scrolls of genus 0 and 1, which are defined by a one-dimensional family of lines meeting a certain set of linear spaces in p ⁿ . These ruled surfaces will be called incidence scrolls and such a set will be the base of the incidence scroll. Unless otherwise stated, we assume that the base spaces are in general position. Received: 1 December 2000     

A note on statistical solutions of the three-dimensional Navier–Stokes equations: The stationary case

Stationary statistical solutions of the three-dimensional Navier–Stokes equations for incompressible fluids are considered. They are a mathematical formalization of the notion of ensemble average for turbulent flows in statistical equilibrium in time. They are also a generalization of the notion of invariant measure to the case of the three-dimensional Navier–Stokes equations, for which a global uniqueness result is not known to exist and a semigroup may not be well-defined in the classical sense. The two classical definitions of stationary statistical solutions are considered and compared, one of them being a particular case of the other and possessing a number of useful properties. Furthermore, the so-called time-average stationary statistical solutions, obtained as generalized limits of time averages of weak solutions as the averaging time goes to infinity are shown to belong to this more restrictive class. A recurrent type result is also obtained for statistical solutions satisfying an accretion condition. Finally, the weak global attractor of the three-dimensional Navier–Stokes equations is considered, and in particular it is shown that there exists a topologically large subset of the weak global attractor which is of full measure with respect to that particular class of stationary statistical solutions and which has a certain regularity property.