Broadband Processing Research Articles

Broadband localization and deconvolution

An algorithm is presented for detecting and localizing a passive broadband signal in an underwater acoustic waveguide. The processor estimates the modal spectrum of a broadband signal for source localization; as a consequence, the source spectrum and signature (time domain waveform) are estimated. (Broadband modal decomposition is the equivalent of time domain deconvolution for source signature extraction.) In addition to the usual lofargram, a frequency-depth plot is proposed to display a broadband signal. Source localization is conducted in the frequency domain by summing the (range and depth) ambiguity functions over the signal bandwidth. Sidelobes are suppressed in broadband processing if the signal has sufficient bandwidth. Coherent summation of the ambiguity functions yields the expected time-bandwidth gain if source signature is known (the extended matched filter processing). Numerical simulations are shown for an Arctic waveguide. The processor works for transient as well as continuous broadband signals.

Broadband source localization and signature estimation

The temporal signature and source location of a broadband impulse propagating in an ocean waveguide are estimated using matched mode processing and illustrated using simulated data in an Arctic sound channel. Matched mode processing provides a simple method for broadband signal detection and localization; the peak-to-sidelobe ratio is significantly improved using broadband data compared with the narrow-band case. The range and depth ambiguity function evaluated at the source location yields directly the source spectrum, which is inverse Fourier transformed to estimate the original signal waveform. For the simulated data, the reconstructed signal, compensated for spreading loss and array signal gain, agrees rather well with the original waveform. The estimated spectrum/waveform is weaker than the original spectrum/waveform due predominantly to the fact that the Arctic wave guide is lossy. A method for coherent broadband processing is proposed to improve signal localization and detection when the original source spectrum is known. Multiplying (correlating) the original spectrum with the range and depth ambiguity function, the product can be coherently summed over the frequency band of the signal. Near theoretical processing gain is achieved for the simulated data. Broadband signal detection is proposed using a frequency versus depth plot in addition to the commonly used frequency versus bearing plot.

Nulling performance of a broadband linearly constrained array

In this paper, the nulling performance of a 2-element broadband linearly constrained array employing tapped delay line processing is investigated by using a first order small bandwidth analysis as well as by using computer simulation. It is shown that even-tap arrays have better nulling performance than odd-tap arrays, and for even-tap arrays, the average null depth is given approximately by (fractional bandwidth/2) number of taps. The results obtained can be used to determine the important design parameters in broadband arrays implemented using either tapped delay line processing or several narrowband real-and-quadrature arrays in parallel. Specifically, it is found that the former broadband processing method requires considerable less adjustable weights than the latter one.

Complex signal detection with broad spectral band processing

A technique of generating a broad spectral band matched filter for complex signal detection with a white-light optical processor is described. Since the filter construction and signal detection utilize primarily a white-light source, the technique is very suitable for color image correlation. Experimental results are also provided.

Harmonic level reduction in arrays reflecting magnetostatic forward volume waves

Reflection of magnetostatic forward volume waves (MSFVW) by ion implanted arrays in a RAC configuration has been investigated as broad-band microwave signal processing components. Harmonics reflected by the 45° oblique incidence array greatly limit the useful bandwidth. They are mainly attributed to the ion implanted bar profile. By improving this profile, the space-harmonic levels have been lowered. Bar width adjustment has been used to reduce the level of even harmonics. A two step-shaped profile realized by two patterns has reduced the level of both amplitudes of odd and even harmonics. A level of 14 dB below the fundamental has been obtained for the second harmonic reflected by periodic gratings. The relative harmonic level reaches - 26 dB for the third one and - 18 dB for the fourth one.

A study of the statistical properties of broadband signals

A common assumption in the processing of broadband acoustic signals is that the underlying processes are Gaussian. Although Gaussian processes arise naturally in the ocean acoustic environment, another motivation for the Gaussian assumption is the desire for analytically tractable results and simple algorithms. In this paper, a broadband propagation model is used to analyze the statistical properties of random signals propagated in a deep ocean environment. In particular, measurements of the first‐order probability density functions associated with the source and received signals will be presented for several source‐receiver configurations. These measurements provide insight into the appropriateness of a Gaussian assumption in broadband processing. They also illustrate the effects of acoustic propagation mechanisms on the statistical structure of a signal. Measurements of correlation functions are presented to show the effects of multipath propagation. [Work supported by the Office of Naval Research.]

Broad spectral band color image deblurring

A broadband white-light processing technique for smeared color photographic image deblurring is described. The technique utilizes a diffraction grating method to disperse the smeared image spectra in the Fourier plane so that the entire spectral band of the white-light source can be utilized for the deblurring. In this paper the technique of synthesizing a fan-shape type complex deblurring filter to accommodate wavelength variation is presented. Experimental results showed that this broad spectral band processing technique offers an excellent coherent artifact noise suppression, and the technique is particularly suitable for color image deblurring. Experimental demonstrations and comparisons with the narrowband and coherent deblurring are also provided.