Adaptive signal processor for interference cancellation

Claim 1

A processor for use in an echo ranging or communication system in which a signal is received by a directional main antenna with possible interference including multi-path signals, said main antenna signal after conversion to a modulation on a carrier of intermediate frequency being denoted m(t); and in which system at least one auxiliary, substantially less directional antenna (i) is provided for intercepting interference, said auxiliary antenna signal after conversion to a modulation on said carrier being denoted a i (t), said processor generating a predicted interference signal m(t), modulated on said carrier at an appropriate phase and amplitude for interference cancellation, and effecting said cancellation, said processor comprising: A. an optical time integrating correlator for deriving at least one correlation function (or weight) W i (x), where x is proportional to time delay, by correlating a residual signal r(t), approximating said main signal with the interference therein cancelled with each auxiliary signal a i (t) comprising: (1) a source of quasi-coherent optical radiation in the visible or near visible spectrum, (2) a first modulator for modulating said optical radiation with said residual signal r(t), said signal (r(t)) being of prescribed phase and amplitude in relation to said carrier, (3) acoustic wave light modulation means to which said modulated light output is coupled and to which said auxiliary signals (a i (t)) are coupled to excite acoustic waves, said modulation means providing point by point multiplications between said optical and acoustic waves to form a first spatial pattern of products ##EQU8## where x is a coordinate measured in the direction of propagation of said acoustic waves, and v a is the acoustic velocity in said acoustic wave light modulation means, said first pattern retaining prescribed phase and amplitude signal information as modulations on said carrier in a spatial format, (4) time-integrating means coupled to the output of said acoustic wave light modulation means, having a time constant adequate for executing a time integral of each point of said first spatial pattern to obtain at least one complex correlation function W(x), as a second spatial pattern, ##EQU9## where T o is 1/e decay time of integration, and x is the coordinate measured in the direction of propagation of said acoustic waves, transformed to said time integrating optical means, said second spatial pattern retaining phase and amplitude signal information as modulations on said carrier in said spatial format; B. a space-integrating optical correlator for correlating each auxiliary signal (a i (t)) while it traverses an optical aperture as an acoustic wave with said second spatial pattern of correlation functions (or weights) W i (x) to derive the predicted interference signal m(t), comprising: (1) acoustic wave light modulation means to which said optical output of said time-integrating optical means is coupled and to which said auxiliary signals a i (t) are coupled to excite acoustic waves, said modulation means providing point by point multiplications between said optical and acoustic waves to form a third spatial pattern of products ##EQU10## where x is the coordinate measured in the direction of propagation of said acoustic waves, said third pattern retaining prescribed phase and amplitude signal information, as modulations on said carrier in a spatial format, (2) a space integrating optical detector to which said third spatial pattern is coupled, said optical detector having adequate bandwidth to accommodate said modulated spatial carrier, said optical detector producing an output m(t) ##EQU11## where xΔ is the interval of spatial integration in which said predicted interference signal is converted from said spatial modulation back to a temporal modulation on said carrier, said predicted interference signal (m(t)) approximating the interference in said main signal in respect to phase and amplitude, and C. cancellation means in an adaptive feedback path for obtaining said residual signal r(t) as a modulation on said carrier, comprising: (1) a bandpass filter whose input is coupled to the output of said optical detector for restricting the predicted interference signal m(t) to the useful signal band about said carrier, and (2) subtraction means having an input coupled to said bandpass filter for application of said predicted interference signal m(t), said main signal m(t) being coupled to said other input, said subtraction means obtaining the complex difference denoted r(t), between the main carrier-borne signal m(t) and said predicted interference signal m(t), said difference (r(t)) being coupled to the first modulator, as earlier recited, to complete a feedback loop in which said predicted interference signal m(t) is caused to adaptively converge toward equality with the noise present in said main signal, said convergence causing said residual signal r(t) to approach equality to the main signal without said interference.