Abstract:
A system and method for phased-array radar that is capable of accurate phase-only steering at any unambiguous angle is provided. In various embodiments, system and method that compensates for the varying effective element spacing of phased array radar, which occurs as a result of transmitting a wideband signal with phase shifters operating at a fixed phase, by interpolating and resampling across all elements, per frequency, to generate a desired effective spacing between the elements.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is directed generally to systems and methods for wideband beamsteering and more particularly to a system and method to permit phase-only steering of a wideband signal. 
       BACKGROUND 
       [0002]    Phased array radar systems employ a bank of antennas, arranged in a particular orientation, each emitting a signal that is phase-adjusted to construct a radiation pattern in a desired direction. Phased arrays often use antennas having a fixed phase-relationship to generate a signal in one direction. Other phased arrays adjust the phase of each antenna to steer the beam in different directions. 
         [0003]    Wideband radar systems are highly desirable, as they offer the increased ability to discriminate and identify a target. However, current phased arrays cannot be steered, using only phase, over wide bandwidths. This is because the phase shifters at a fixed phase will only be accurate for one frequency within the frequency spectrum transmitted. In other words, if the phase is fixed, the direction of the beam will shift over frequency and even small changes in frequency can effectively mispoint the phased array. Further, for very narrow wideband pulses, the returns across the array will not align in time and so cannot be added coherently. Ideally, wideband beam steering of phased arrays could be accomplished through time delay steering, instead of phase shifters. Instead of adjusting for phase difference, the time delay units adjust for the difference in time of arrival at each element. But time delay units are still too large and expensive to be practically implemented. Accordingly, there is a need in the art for phased-array radar that is capable of phase-only steering, at any unambiguous angle (i.e. out to the first grating lobe) for wideband signals. 
       SUMMARY OF THE INVENTION 
       [0004]    The present disclosure is directed to systems and methods for phased-array radar that is capable of accurate phase-only steering at any unambiguous angle. In various embodiments, the disclosure provides a system and method that compensates for the varying effective element spacing as a fraction of wavelength, which occurs as a result of transmitting a wideband signal with phase shifters operating at a fixed phase, by interpolating and resampling across all elements, per frequency, to generate a desired effective spacing between the elements. The resulting data may be then be transformed to the time-domain and used for phase-only beam steering, without the effects of beam broadening that results from the wideband signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The various embodiments of the invention will be better understood when read in conjunction with the following drawings: 
           [0006]      FIG. 1  shows a schematic of a system that permits phase-only steering of a wideband signal according to an embodiment; 
           [0007]      FIG. 2  shows a flowchart of a method that permits phase-only steering of a wideband signal according to an embodiment; 
           [0008]      FIG. 3  shows a chart according to an embodiment; 
           [0009]      FIG. 4  shows a graph according to an embodiment; 
           [0010]      FIG. 5A  shows a graph according to an embodiment; 
           [0011]      FIG. 5B  shows a graph according to an embodiment; 
           [0012]      FIG. 6  shows a graph according to an embodiment; 
           [0013]      FIG. 7A  shows a graph according to an embodiment; and 
           [0014]      FIG. 7B  shows a graph according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0015]    Referring now to the drawings wherein like reference numerals refer to like parts throughout, there is seen in  FIG. 1 , a system  100  capable of phase-only steering of a wideband signal. In an exemplary embodiment, system  100  comprises a plurality of antenna elements  102  arranged in a predetermined orientation to form a phased-array antenna. Elements  102  may be configured to receive wideband signals, which exhibit an effective spacing (in units of wavelength) between elements  102  that varies across frequency. One of ordinary skill in the art will appreciate that, although not shown, each element may have an associated phase shifter for adjusting the outputted phase to steer the beam in a desired direction. Each element may further have an associated receiver to amplify the signal received at each element  102 . System  100  may further comprise a plurality of A/D converters  104 , configured to digitize the signals received by elements  102 . The received signals, once amplified and digitized, may be stored for further processing. 
         [0016]    System  100  may further comprise a plurality of Digital Fourier Transform (DFT) modules  106  (shown in  FIG. 1 , in an exemplary embodiment, as Fast Fourier Transform (FFT) modules) configured to receive a predetermined portion of the digitized signals. In an exemplary embodiment, the portion received by the DFT modules  106  may be a pulse repetition interval; however other intervals, such as a fixed time interval, may be used. 
         [0017]    System  100  may further comprise a plurality of correlators (not shown), configured to compress the signal to recover an unmodulated pulse signal, with some bandwidth. In an alternate embodiment the correlators may be located elsewhere, such as after the inverse DFT modules  108 . 
         [0018]    System  100  may further comprise a frequency-dependent interpolator  108 . Frequency dependent interpolator  108  may receive the collective output from DFT modules  106 . This collective output may be thought of as a frequency-by-element matrix. Frequency-dependent interpolator  108  may be configured to interpolate and resample, per frequency, across the elements, to recreate a desired effective spacing as a fraction of a wavelength between elements  102 . Frequencies located higher than the center frequency will exhibit an equivalent element spacing that is closer together, while frequencies lower than the center frequency will be spaced farther apart. Accordingly, across the DFT module  106  outputs, the frequency bins higher than the reference frequency of the received signal will be interpolated and resampled at a faster rate to “spread the elements out” and the frequency bins lower than the center frequency will be interpolated and resampled at a slower rate to shrink the effective distance between the elements. In this way, each frequency bin across the DFT  106  outputs may be interpolated to correct the effective element spacing as a fraction of a wavelength. One of ordinary skill will appreciate that any effective element spacing may be selected, such as a half a wavelength. 
         [0019]    To illustrate, a particular frequency bin, say the frequency bins located at 500 MHz of each DFT module  106  output, will be interpolated to create a set of data points “between” the elements. These created data points represent estimated frequency bin values at those points spatially between the elements. From these created data points, data points at certain locations will be selected, according to the frequency bins&#39; relative distance from the reference frequency, to virtually resample the signal at appropriate points between the actual elements. These points between the elements are specifically selected to return the effective element spacing back to a desired distance at that frequency. The points are selected according to the following equation: 
         [0000]    
       
         
           
             x 
             = 
             
               
                 ( 
                 
                   
                     f 
                     o 
                   
                   
                     f 
                     + 
                     
                       f 
                       o 
                     
                   
                 
                 ) 
               
                
               
                 x 
                 ′ 
               
             
           
         
       
     
         [0020]    Where x is the actual element spacing (the equations here assume a one-dimensional array, but one of ordinary skill will understand that it may be extended to two-dimensions), x′ represents the interpolated and resampled element desired spacing, f 0  is the user-defined reference frequency of the signal and f is a selected frequency from each Fourier transform output. 
         [0021]    One of ordinary skill in the art will also appreciate that any number of interpolation algorithms may be used to estimate values between the received samples. For example, in an exemplary embodiment, cubic spline interpolation may be used. 
         [0022]    System  100  may further comprise a plurality of inverse DFT modules  110  (FFT in the embodiment shown in  FIG. 1 ) to convert the output of the frequency-dependent interpolator  108  back into the time domain. In an alternate embodiment, system  100  may not have any inverse DFT modules  110  (or any DFT modules located after interpolation but prior to beamforming) and any beamforming may be performed in the frequency domain. 
         [0023]    System  100  may further comprise a phase-steered digital beamformer  112  which is configured to receive the output from frequency-dependent interpolator  108 , or in alternate embodiments, the output of inverse DFT modules  110 , and process phase-only beamsteering according to methods known in the art, using the element spacing corrected data received from frequency-dependent interpolator  108  or DFT modules  110 . 
         [0024]      FIG. 2  shows a method to permit phase-only steering of wideband signals. As shown, in step  200 , signals from each antenna element  102  of a phased array are received, digitized by A/D converters  104 , and stored for further processing. The stored result of step  100  can be thought of as a matrix of data: time-by-element. 
         [0025]    In step  202 , an interval of data from each element may be Fourier transformed, by DFT modules  106 , into the frequency domain. The results of this process may be thought of as a matrix of data: frequency-by-element. In exemplary embodiment, this interval may be one pulse repetition interval; however one of ordinary skill will appreciate that other intervals, including fixed time intervals, may be used. 
         [0026]    In step  204 , the data output in step  202  may be interpolated and resampled across elements at each frequency to obtain an effective element spacing that is some fraction of a wavelength at each frequency, according to the process performed by frequency-dependent interpolator  108 . In exemplary embodiment, cubic spline interpolation may be used; however, in alternate embodiments, different interpolation algorithms, such as linear interpolation, that are sufficient for interpolating the data as described in step  204  may be used. 
         [0027]      FIG. 3  shows a chart of the of actual element spacing against the interpolated and resampled element spacing, versus fast frequency. As shown, the actual element spacing versus frequency is depicted as a set of data points connected by parallel lines, because the elements are spatially fixed in the array and do not, of course, change in frequency. The effective element spacing, as a result of the interpolation and resampling, is depicted as the darker data points deviating from the position of the actual elements. Note, at lower frequencies, the data points are interpolated to spread them out and have a larger apparent physical spacing (since lower frequencies have a longer wavelength). At higher frequencies, the data points are interpolated to have a smaller effective spacing. Relative to the equation above, f 0  is the highest frequency in this example.  FIG. 4  shows the effective aperture length of the array versus frequency without the processing—a rectangle, overlaid over the effective aperture spacing following the interpolation and resampling, which forms the trapezoidal shape shown. 
         [0028]      FIG. 5A  shows the time-domain of each element (of a 20 element array) following the analog to digital conversion and correlation (correlation, here, refers to compression such that a simple, unmodulated pulsed signal, with some bandwidth, is recovered).  FIG. 5B  shows the time domain of each element following the interpolation and resampling according to frequency. Notice that the time dispersion of the signal in  FIG. 5A  is eliminated by the interpolation/resampling step, which is key to wideband beamsteering and equivalent to a time-delay-steered beamformer. Similarly,  FIG. 6  shows an example of the frequency domain support following the interpolation and resampling across each element. 
         [0029]    Returning to  FIG. 2 , in step  206 , the interpolated and resampled points are used in place of the original points and inverse Fourier transformed back to the time domain. And finally, in step  208 , beams can now be formed with phase-only steering according to known methods in the art. In an alternate embodiment, step  206  may be skipped entirely and beamforming may be conducted in the frequency domain. 
         [0030]    One of ordinary skill in the art will appreciate that this process can be reversed for transmitting a wideband signal, i.e. the signal is generated, phase steered, and distributed to each element. The signal is Fourier transformed, and interpolated and resampled using the inverse of the transformation described above. The resampled data are then inverse Fourier transformed per element and transmitted to form a wideband antenna beam. 
         [0031]    System  100  may be further advantageously employed for multiple simultaneous beamforming or adaptive beamforming.  FIGS. 7A and 7B  show two signals coming from different angles in the time domain following correlation and following interpolation and resampling. The process has collapsed the time dispersion of both signals simultaneously, despite the fact that they arrive from different directions. The difference between this process and time-delay steering is that this process simultaneously collapses the dispersion of all signals coming from all unambiguous angles simultaneously, allowing subsequent wideband digital beamforming in any and all unambiguous directions. 
         [0032]    A “module,” as may be used herein, can include, among other things, the identification of specific functionality represented by specific computer software code of a software program. A software program may contain code representing one or more modules, and the code representing a particular module can be represented by consecutive or non-consecutive lines of code. 
         [0033]    As will be appreciated by one skilled in the art, aspects of the present invention may be embodied/implemented as a computer system, method or computer program product. The computer program product can have a computer processor or neural network, for example, that carries out the instructions of a computer program. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment, and entirely firmware embodiment, or an embodiment combining software/firmware and hardware aspects that may all generally be referred to herein as a “circuit,” “module,” “system,” or an “engine.” Furthermore, aspects of the present invention may take the form of a computer program product embodied in one or more computer readable medium(s) having computer readable program code embodied thereon. 
         [0034]    Any combination of one or more computer readable medium(s) may be utilized. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A computer readable storage medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the computer readable storage medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. In the context of this document, a computer readable storage medium may be any tangible medium that can contain, or store a program for use by or in connection with an instruction performance system, apparatus, or device. 
         [0035]    The program code may perform entirely on the user&#39;s computer, partly on the user&#39;s computer, as a stand-alone software package, partly on the user&#39;s computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user&#39;s computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). 
         [0036]    The flowcharts/block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present invention. In this regard, each block in the flowcharts/block diagrams may represent a module, segment, or portion of code, which comprises instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be performed substantially concurrently, or the blocks may sometimes be performed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions.