Volume 117, Issue 3, March 2005
Index of content:
- ACOUSTIC SIGNAL PROCESSING 
117(2005); http://dx.doi.org/10.1121/1.1856411View Description Hide Description
High rate underwater communications have traditionally relied on equalization methods to overcome the intersymbol interference (ISI) caused by multipath propagation. An alternative technique has emerged in the form of time-reversal, which comes at virtually no cost in computational complexity, but sacrifices the data rate and relies on the use of large arrays to reduce ISI. In this paper, spatiotemporal processing for optimal multipath suppression is addressed analytically. A communication link between a single element and an array is considered in several scenarios: uplink and downlink transmission, with and without channel state information and varying implementation complexity. Transmit/receive techniques are designed which simultaneously maximize the data detection signal-to-noise ratio and minimize the residual ISI, while maintaining maximal data rate in a given bandwidth and satisfying a constraint on transmitted energy. The performance of so-obtained focusing techniques is compared to the standard ones on a shallow water channel operating in a 5 kHz bandwidth around a 15 kHz center frequency. Results demonstrate benefits of focusing techniques whose performance is not conditioned on the array size. Optimal configurations are intended as a basis for adaptive system implementation in which channel estimates will replace the actual values.
117(2005); http://dx.doi.org/10.1121/1.1850231View Description Hide Description
The feasibility is considered of synthesizing a spatially correlated random pressure field having specified statistical properties. Of particular interest is the use of a near-field array of acoustic sources to synthesize a pressure field whose statistical properties are similar to either a diffuse acoustic sound field or to that generated by a turbulent boundary layer (TBL). A formulation based on least-squares filter design is presented. Initially, the more fundamental question is addressed of how many uncorrelated signal components are required to approximate the pressure field. A one-dimensional analysis suggests that two uncorrelated components per acoustic wavelength are required to approximate a diffuse pressure field. Similarly, for a TBL pressure field, about one uncorrelated component per correlation length is required in the spanwise direction and about two uncorrelated components per correlation length are required in the streamwise direction. These estimates are in good agreement with theoretical predictions for an infinite array, based on the Fourier transform of the spatial correlation function. When a full simulation is performed, including the acoustic effect of an appropriately positioned array of monopole sources, it is found that the number of acoustic sources required to reasonably approximate the diffuse or TBL pressure field is only slightly greater than the lower bound on this number, set by the number of uncorrelated components required.
Efficiency parameters in time reversal acoustics: Applications to dispersive media and multimode wave propagation117(2005); http://dx.doi.org/10.1121/1.1856272View Description Hide Description
This paper develops a theoretical background for the interpretation of the time reversal (TR) processes applied to the refocalization of acoustic waves in dispersive media with multimode propagation of waves. Two parameters are introduced in order to measure the efficiency of signal recompression in TR processes: spatial and temporal efficiencies. It is demonstrated that the signal recompression enhances when the medium is highly dispersive, when there are several modes involved in the process, and when the angular aperture of the TR mirror is appreciable. These results are applied to Lamb waves recompression in thin plates and focalization efficiencies in longitudinal and transversal directions are deduced. Finally, these theoretical results are compared with the experimental ones obtained for different plate thicknesses.