Abstract:
An adaptive signal processing system utilizes a a Multistage Weiner Filter having an analysis section and a synthesis section, which includes a main input channel for receiving a main input signal, an auxiliary input channel for receiving an auxiliary input signal, and a processor. The processor includes an algorithm for generating a data adaptive linear transformation, followed by computing an adaptive weighting w med  of the data, and then applying the computed adaptive weighting w med  to a function of the main input signal and the auxiliary input signal to generate an output signal. The system includes a plurality of building blocks arranged in a Gram-Schmidt cascaded canceller-type configuration for sequentially decorrelating input signals from each other to thereby yield a single filtered output signal. Each building block includes a local main input channel which receives a local main input signal, a local auxiliary input channel which receives a local auxiliary input signal, and a local output channel which sends a local filtered output signal. Each building block generates an adaptive weight w med  that preferably uses just the sample median value of the real part of a complex weight w med , with the imaginary part set to zero, of the ratio of local main input weight training data to local auxiliary input weight training data. Each building block generates a local output signal utilizing the adaptive weight w med . The effect of non-Gaussian noise contamination on the convergence MOE of the system is negligible. In addition, when desired signal components are included in weight training data they cause little loss of noise cancellation.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates in general to an adaptive signal processing system. More specifically, the invention relates to an adaptive signal processing system that utilizes a multistage median cascaded canceller to formulate an efficient data transformation followed by the calculation of adaptive weights and filter out undesired noise.  
         BACKGROUND OF THE INVENTION  
         [0002]    Adaptive signal processing systems have many applications including radar reception, cellular telephones, communications systems, and biomedical imaging. Adaptive signal processing systems utilize adaptive filtering to differentiate between the desired signal and the combination of interference and noise, i.e. thermal or receiver noise. An adaptive filter is defined by four aspects: the type of signals being processed, the structure that defines how the output signal of the filter is computed from its input signal, the parameters within this structure that can be iteratively changed to alter the filter&#39;s input-output relationship, and the adaptive algorithm that describes how the parameters are adjusted from one time instant to the next.  
           [0003]    Common applications of adaptive signal processing include: an adaptive radar reception antenna array, an adaptive antenna array for adaptive communications, and adaptive sonar. In these systems, desired signal detection and estimation is hindered by noise and interference. Interference may be intentional jamming and or unintentional received radiation. Noise is usually described as ever present receiver thermal noise, generally at a low power level. In these applications antenna arrays may change their multidimensional reception patterns automatically in response to the signal environment in a way that optimizes the ratio of signal power to the combination of interference power plus noise power (abbreviated as SINR). The array pattern is easily controlled by weighting the amplitude and phase of the signal from each element before combining (adding) the signals. In general, multidimensional samples may be collected, e.g. over antenna elements, over time, over polarization, etc., where each sample is a separate input channel to the adaptive processor. Adaptive arrays are especially useful to protect radar and communication systems from interference when the directions of the interference are unknown or changing while attempting to receive a desired signal of known form. Adaptive arrays are capable of operating even when the antenna elements have arbitrary patterns, polarizations, and spacings. This feature is especially advantageous when an antenna array operates on an irregularly shaped surface such as an aircraft or ship.  
           [0004]    Adaptive signal processing systems are required to filter out undesirable interference and noise. Due to the lack of a priori knowledge of an external environment, adaptive signal processing systems require a certain amount of statistically independent weight training data samples to effectively estimate the input noise and interference statistics.  
           [0005]    “Ideal” weight training data has a Gaussian probability distribution for both its real and imaginary baseband components. However, real-world weight training data may be contaminated by undesirable impulse noise outliers, resulting in a non-Gaussian distribution of real and imaginary components.  
           [0006]    The number of weight training data samples required for SINR performance of the adaptive processor to be within 3 dB of the optimum on average is called the convergence measure of effectiveness (MOE) of the processor. A signal is stationary if its statistical probability distribution is independent of time. For the pure statistically stationary Gaussian noise case, the convergence MOE of the conventional Sample Matrix Inversion (SMI) adaptive linear technique can be attained using approximately 2N samples for adaptive weight estimation, regardless of the input noise covariance matrix, where N is the number of degrees of freedom in the processor (i.e., the number of antenna elements or subarrays) for a spatially adaptive array processor, or N is the number of space-time channels in a space-time adaptive processing (STAP) processor). Referred to as the SMI convergence MOE, convergence within 3 dB of the optimum using approximately 2N samples for adaptive weight estimation has become a benchmark used to assess convergence rates of full rank adaptive processors. General information regarding SMI convergence MOE may be found in Reed, I. S., Mallet, J. D., Brennan, L. E., “Rapid Convergence Rate in Adaptive Arrays”, IEEE Trans. Aerospace and Electronic Systems, Vol. AES-10, No. 6, November, 1974, pp. 853-863, the disclosure of which is incorporated herein by reference.  
           [0007]    Conventional sample matrix inversion (SMI) adaptive signal processing systems are capable of meeting this benchmark for the pure statistically stationary Gaussian noise case. If, however, the weight training data contains non-Gaussian noise outliers, the convergence MOE of the system increases to require an unworkably large number of weight training data samples. The performance degradation of the SMI algorithm in the presence of non-Gaussian distributions (outliers) can be attributed to the highly sensitive nature of input noise covariance matrix estimates to even small amounts of impulsive non-Gaussian noise that may be corrupting the dominant Gaussian noise distribution. General information regarding the sensitivity of the SMI algorithm may be found in Antonik, P. Schuman, H. Melvin, W., Wicks, M., “Implementation of Knowledge-Based Control for Space-Time Adaptive Processing”, IEEE Radar 97 Conference, Oct. 14-16, 1997, p. 478-482, the disclosure of which is incorporated herein by reference.  
           [0008]    Thus, for contaminated weight training data, convergence rate may slow significantly with conventional systems. Fast convergence rates are important for several practical reasons including limited amounts of weight training data due to non-stationary interference and computational complexity involved in generating adaptive weights. In other words, the time which elapses while a conventional system is acquiring weight training data and generating adaptive weights may exceed the stationary component of a given non-stationary noise environment, and an adaptive weight thus generated has become obsolete prior to completion of its computation.  
           [0009]    Most real world data does not have a purely Gaussian probability distribution due to contamination by non-Gaussian outliers and/or desired signal components. Conventional signal processors assume that the weight training data has a Gaussian distribution, and therefore they do not perform as well as theory would predict when operating with real world data. If weight training data contains desired signals that appear to be outliers, the performance is similarly degraded. In an effort to compensate for these performance problems, conventional systems employ subjective data screening techniques to remove perceived outliers from the data prior to processing. However, subjective screening is undesirable because the process is ad-hoc in nature, requires many extra processing steps, and may even degrade system performance.  
           [0010]    Optimal, reduced rank, adaptive processors are derived primarily to combat the problem of non-stationary data conditions (i.e. low sample support) often encountered in general applications. However, they still have convergence MOE&#39;s that are degraded by outliers. For radar applications, these provide better SINR output than full rank methods, typically through the use of localized training data to improve statistical similarity with the range cell under test (CUT). An exemplary system is described in U.S. patent application Ser. No. 09/933,004, “System and Method For Adaptive Filtering”, Goldstein et al., incorporated herein by reference.  
           [0011]    Also, full rank, robust, adaptive processor research has resulted in novel open loop processors capable of accommodating an amount of non-Gaussian/outlier contaminated and nonstationary data, while still producing an SMI-like convergence MOE. An exemplary system is described in U.S. patent application Ser. No. 09/835,127, “Pseudo-Median Cascaded Canceller”, Picciolo et al., incorporated herein by reference, hereinafter referred to as a median cascaded canceller.  
           [0012]    Reduced rank processors have a convergence MOE typically on the order of 2r, where r is the effective rank of the interference covariance matrix. Effective rank refers to that value of r which is associated with the dominant eigenvalues of the interference and noise covariance matrix. General information regarding “effective rank” and general trends in the convergence trends MOE of reduced rank processors may be found in “Principal Components, Covariance Matrix Tapers, and the Subspace Leakage Problem”, J. R. Guerci and J. S. Bergin,  IEEE Transactions on Aerospace and Electronic Systems,  Vol. 38, No. 1, January 2002.  
           [0013]    Given the demonstrated capabilities of these processors, it would be desirable to provide an adaptive signal processing system that utilizes features of both these types of systems.  
         SUMMARY OF THE INVENTION  
         [0014]    The present invention provides an adaptive signal processing system that utilizes an MWF filter in combination with a median cascaded canceller to compute a non-adaptive data transformation, forming a main channel and a set of auxiliary channels, followed by a data adaptive transformation and a set of adaptive weights to generate a filtered output signal. The effect of non-Gaussian non-stationary noise contamination on the convergence MOE of the system is significantly reduced. In addition, when desired signal components are included in weight training data they cause reduced deleterious effects on noise cancellation.  
           [0015]    In a preferred embodiment, the system and method are directed to an adaptive signal processing system that includes an analysis section of a Multistage Weiner Filter (MWF), a modified synthesis section of the MWF, which includes a main input channel for receiving a global main input signal, a set of global auxiliary input channels for receiving a corresponding set of auxiliary input signals, and a processor. The global main and the global auxiliary channels may be formed by a non-adaptive linear transformation of the original data channels. The processor includes an algorithm for generating a non-adaptive linear transformation producing a global main channel and global auxiliary channels, a data adaptive linear transformation, followed by an adaptive weighting and subtraction of the auxiliary data channels from the global main channel. The system includes a plurality of building blocks arranged in a Gram-Schmidt cascaded canceller-type configuration for sequentially decorrelating input signals from each other to thereby yield a single filtered output signal. Each building block includes a local main input channel which receives a local main input signal, a local auxiliary input channel which receives a local auxiliary input signal, and a local output channel which sends a local filtered output signal. Each building block generates an adaptive weight that preferably uses just the real parts of a set of complex values determined from the ratios of the local main channel samples to the local auxiliary channel samples, w med . Each building block generates a local output signal utilizing the adaptive weight.  
           [0016]    The effect of non-Gaussian noise contamination on the convergence MOE of the system is negligible. In addition, when desired signal components are included in weight training data they cause little loss of noise cancellation.  
           [0017]    It would advantageously adaptively process non-stationary data conditions via reduced rank processing while also accommodating outlier contaminated weight training data, ideally producing a convergence MOE comparable to the above referenced benchmark for reduced rank processers, and potentially reducing desired signal cancellation when weight training data includes desired signal components.  
           [0018]    Other advantages and features of the invention will become apparent from the following detailed description of the preferred embodiments and the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is a schematic diagram of an MMCC according to the invention.  
         [0020]    [0020]FIG. 2 is a schematic diagram of an MMCC according to the invention.  
         [0021]    [0021]FIG. 3 is a schematic diagram of an MMCC according to the invention.  
         [0022]    [0022]FIG. 4 is a block diagram of a conventional sample matrix inversion processor shown as an equivalent Gram-Schmidt cascaded canceller processor.  
         [0023]    [0023]FIG. 5 is a single block of a conventional Gram-Schmidt cascaded canceller processor.  
         [0024]    [0024]FIG. 6 is a block diagram of a cascaded canceller processor component of an MMCC according to the present invention.  
         [0025]    [0025]FIG. 7 is a single building block of a cascaded canceller processor component of an MMCC according to the present invention.  
         [0026]    [0026]FIGS. 8 a - b  are graphs showing the adapted pattern produced by an MWF as in the prior art.  
         [0027]    [0027]FIGS. 9 a - b  are graphs showing the adapted pattern produced by an MMCC according to the invention.  
         [0028]    [0028]FIG. 10 is a graph as in FIG. 8( a ) for an MWF having a different configuration.  
         [0029]    [0029]FIG. 11 is a graph is in FIG. 9( a ) for an MMCC having a different configuration in accordance with the invention.  
         [0030]    [0030]FIGS. 12 a - b  show the clutter spectrum for measured airborne radar data illustrating the input statistics input to an MMCC according to the invention.  
         [0031]    [0031]FIGS. 13 a - b  are graphs as in FIGS. 8 a - b  for an MWF having a different configuration and having input data statistics as shown in FIGS. 12 a - b.    
         [0032]    [0032]FIGS. 14 a - b  are graphs as in FIGS. 9 a - b  for an MMCC having a different configuration and having input data statistics as shown in FIGS. 12 a - b  in accordance with the invention.  
         [0033]    [0033]FIG. 15 is a comparison graph of a prior art system and an MMCC according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]    Referring now to FIGS.  1 - 3 , a Multistage Median Cascaded Filter (MMCC)  10  receives input data x(k). The x(k) data stream is applied to a steering vector s  12 , where processing begins with non-adaptive calculations. First, the data vectors are projected onto the steering vector to form an initial estimate of the desired signal: 
           d   0 ( k )= s   H   x ( k ).  (1) 
         [0035]    For example, d 0 (k) may be the output of a conventional beamformer or a matched filter. In general, d 0 (k) contains interference that comes in through the sidelobes of s. To prepare for a statistical analysis that will identify and subtract this interference, the rest of the data may be isolated by writing 
           x   0 ( k )= Bx ( k ),  (2) 
         [0036]    where B is a “blocking matrix”  14  that finds the projection of the data x(k) orthogonal to s, i.e., the projection onto the nullspace of s.  
         [0037]    The projection operation in (2) is uniquely defined. However, the projection can be accounted for in at least two ways. In earlier approaches, the blocking matrix was rectangular to account for the result being an N−1 dimensional vector (i.e., a vector with length N−1). The other possibility is to consider the square blocking matrix: 
           B=I−ss   H . 
         [0038]    The square matrix accounts for the same projection, but the projection may be viewed as a subtraction in the fixed perspective of the original N-dimensional coordinate system (i.e., where all vectors have length N). The choice between these two blocking matrices is relevant to the numerical efficiency realized in the present invention. By choosing the square blocking matrix in preferred embodiments of the present invention, we can re-write (2) as 
           x   0 ( k )=[ I−ss   H   ]x ( k )= x ( k )− sd   0 ( k ) 
         [0039]    By accounting for the projection in preferred embodiments of the present invention with the subtraction in the preceding equation, the computational cost for a block is O(NK). This gives us a significant savings over the matrix multiplication shown in (2), which would cost O(N 2 K).  
         [0040]    Given d 0 (k) and x 0 (k) as new inputs, the processing continues with a recursion of adaptive stages. The index i identifies the adaptive stage number (where i=1 is the first adaptive stage, and so on). To support stage i, the block-averaged correlation between the two inputs to that stage is calculated according to 
           r   i−1   =&lt;x   i−1 ( k ) d*   i−1 ( k )&gt; K .  (3) 
         [0041]    Stage i will use this correlation in terms of the correlation magnitude δ i  and the correlation direction vector h i    16 , 
         δ i   =∥r   i−1 ∥=( r   i−1   H   r   i−1 ) 1/2 ,  (4) 
         and  h   i   =r   i−1   /∥r   i−1 ∥.  (5) 
         [0042]    While the correlation direction vector can be calculated by using block averaging with uniform weighting over the block, other averaging techniques known to those skilled in the art may be used, based on the requirement of the particular application (e.g., time constraints, accuracy requirements) and the availability of processing resources. The projections of the data along this direction and orthogonal to this direction can account for this projection as a subtraction, 
           d   i ( k )= h   i   H   x   i−1 ( k )  (6) 
         and  x   i ( k )=[ I−h   i   h   i   H   ]x   i−1 ( k )= x   i−1 ( k )− h   i   d   i ( k ).  (7) 
         [0043]    It should be noted that Equations (6) and (7) pertain to FIG. 1 in that these particularly apply to an embodiment of the invention that employs a square blocking matrix, such as is described in the above-referenced U.S. Ser. No. 09/933,004. For other embodiments using non-square matrices, x i (k)=B i x i−1 (k) and Equation (6) apply in that form but with different dimensions. Either method may be considered a specific implementation used to generate a Krylov subspace representation of the original data, such as is described in  The Theory of Matrices,  P. Lancaster and M. Tismenstsky, Academic Press (1985), incorporated herein by reference. Any other method used to generate such a Krylov subspace representation of the data may be equivalently substituted. In the follow-on synthesis stage of the present invention, median-based filtering (denoted by the use of w med  weights in FIGS. 1 and 2) is then applied to the Krylov subspace. This recursive analysis may be terminated in several ways. If the block of data X has full rank, then the projections may continue until all the data is projected onto a set of orthogonal unit vectors [s, h 1 , h 2 , . . . , h N−1 ]. This analysis would use N−1 stages (because s accounted for one direction already). Or, if the block of data X is not full rank (for example, if K&lt;N is intentionally selected), then the rank of the data will “underflow” in the analysis. In that case, x i (k) would contain only zeros for the highest stages  
         [0044]    Referring also now to FIG. 3, alternatively, the filtering may be intentionally truncated to some smaller number of stages s (where 1≦s≦N−1). Once the analysis is finished, the initialization ε S (k)=d S (k) begins the synthesis along the lower chain of FIG. 1 from right to left.  
         [0045]    The synthesis may be better understood by considering a conventional general adaptive array having a sidelobe canceller form and a conventional Gram-Schmidt cascaded canceller. The adaptive implementation of a conventional Gram-Schmidt cascaded canceller provides a cascaded set of operationally identical, two-input canceller Gram-Schmidt L 2  building blocks, as is well known in the art. In accordance with the MMCC of the present invention, these Gram-Schmidt L 2  building blocks are replaced with new L med  building blocks. Each L med  building block generates an adaptive weight w med  that in one form is a complex adaptive weight whose real and imaginary parts are found, as is further described below, by taking the sample median value of the ratio of local main input weight training data z to local auxiliary input weight training data x for the real and imaginary parts separately, applies the computed weight to a function of a local main input signal and a local auxiliary input signal, and generates a filtered output signal.  
         [0046]    Theoretically, the L med  building blocks produce the same optimal weight as the Gram-Schmidt L 2  building blocks if the number of samples of training data is very large (K→∞), and the two inputs each have symmetric probability density functions about the mean for their real and imaginary components (an example is the Gaussian pdf). In addition, the convergence MOE of the median cascaded canceller offers a significant improvement upon Gram-Schmidt cascaded cancellers in the presence of non-Gaussian noise outliers or equivalently, for some cases, when desired signal components are present in the training data.  
         [0047]    A sidelobe canceller is a specific form of the general adaptive array where the N×1 desired signal (steering) vector is set to [100 . . . 0] T , the “main” or first channel weight is fixed to unity, and the (N−1)×1 auxiliary channel adaptive weight vector estimate ŵ a  ({circumflex over ( )} denotes ‘estimate of’) solves the adaptive Wiener-Hopf matrix equation, 
           ŵ   a   ={circumflex over (R)}   a   −1   {circumflex over (r)}   am , 
         [0048]    where {circumflex over (R)} a  is the (N−1)×(N−1) auxiliary channel input noise and interference estimate, and r{circumflex over ( )} am  is the (N−1)×1 cross-correlation vector estimate between the auxiliary channels and the main channel. For Gaussian statistics, the maximum likelihood estimates of these quantities are  
           R   ^     a     =       1   K            ∑     k   =   1     K                         u   a          (   k   )                u   a          (   k   )       H                 and             r   ^     am     =       1   K            ∑     k   =   1     K                         u   a          (   k   )                u   m          (   k   )       *     ,                               
 
         [0049]    where K is the number of weight training data samples used in the averages and * denotes complex conjugate operation. When employing equation (2) and equation (3), the sidelobe canceller uses the SMI algorithm, and its scalar output, g, is (using vector partitioning)  
       g   =         w   sic   H        u     =         [         1           ⋯             -       w   ^     a             ]     H          u   .                               
 
         [0050]    The conventional SMI-based sidelobe canceller in this last equation has an equivalent Gram-Schmidt cascaded canceller form in the steady state and in the transient state with numerically identical outputs, where transient state refers to the case where weights are estimated from finite length data vectors. Infinite numerical accuracy is assumed, i.e., the cascaded weights correspond to a numerically equivalent set of linear weights equation (1) that can be applied directly to the transformed array input data vector u via equation (4), if so desired.  
         [0051]    [0051]FIG. 4 illustrates a conventional Gram-Schmidt canceller  20  for the N=4 channel case, comprised of six identical two-input canceller Gram-Schmidt L 2  building blocks  22 . In each building block  22 , the optimal weight estimate for that building block is calculated by minimizing the square of the L 2 -norm of the residual output vector from that building block, over some specified number of K weight training data samples.  
         [0052]    [0052]FIG. 5 illustrates a single Gram-Schmidt L 2  building block  22 . For notational simplicity, the left input of any single building block is relabeled z, the right input is relabeled x, and the output is relabeled r. Each building block  22  serves to have the component of z which is correlated with x, subsequently subtracted from z. This is accomplished by choosing an optimum weight estimate ŵ opt  such that the residual r is statistically uncorrelated with x. The least-squares method is used to estimate w opt  by minimizing over the set of complex weights w, the square of the L 2  norm of the residual output r=z−w*x where r=r(k), z=z(k), and x=x(k), for k=1, . . . , K; i.e., for any single building block,  
           w   ^     opt     =           arg                 min     w          [         1   K          ∑     k   =   1     K                  |       z        (   k   )       -       w   *          x        (   k   )                |   2       ]                  .                           
 
         [0053]    This results in the scalar adaptive Wiener-Hopf equation, 
           ŵ   opt   ={circumflex over (R)}   xx   −1   {circumflex over (r)}   xz , 
         [0054]    where maximum likelihood estimates again may be used for the scalar input noise covariance estimates, assuming Gaussian statistics,  
           R   ^     xx     =       1   K            ∑     k   =   1     K                       x        (   k   )              x        (   k   )       *     ,               and             r   ^     xz     =       1   K            ∑     k   =   1     K                       x        (   k   )                z        (   k   )       *     .                                 
 
         [0055]    Note that these last two equations are sensitive to non-Gaussian noise contaminants just like their matrix counterparts.  
         [0056]    As set forth above the conventional general adaptive array processor may be equivalently transformed in terms of SINR into its conventional Gram-Schmidt cascaded canceller form. The adaptive implementation of a conventional Gram-Schmidt cascaded canceller provides a cascaded set of operationally identical, two-input canceller L 2  building blocks.  
         [0057]    Replacing the L 2  building blocks  22  with L med  building blocks throughout a cascaded canceller  30  as illustrated in FIG. 6 results in the MMCC configuration of the present invention. FIG. 7 illustrates a single L med  building block canceller  32 . Each of the L med  building blocks  32  computes an adaptive weight w med  that in one form is a complex adaptive weight whose real and imaginary parts are found by taking the sample median value of the ratio of local main input weight training data to local auxiliary input weight training data for the real and imaginary parts separately. The preferred embodiment uses just the median of the real parts and sets the imaginary part to zero. Each of the L med  building blocks  32  then applies the complex conjugate of the computed weight w* med  to a function of a local main input signal and a local auxiliary input signal, and generates a filtered local output signal, r, by solving the following equation: r=z−w* med x. Although the cascaded canceller illustrated has N=4 inputs, any number N of inputs may be utilized.  
         [0058]    In the first level of processing, N input signals are input into N−1 L med  building blocks  32  to generate N−1 local output signals. In the next level of processing, N−1 local input signals are input into N−2 L med  building blocks  32  to generate N−2 local output signals. This process is repeated until one final output signal remains.  
         [0059]    The MMCC  10  of the present invention may be implemented by a set of program instructions on an arithmetical processing device such as a general-purpose digital signal processor (DSP) or microprocessor to carry out in real time the computational steps presented above. Alternatively, custom-built application specific integrated circuits (ASIC), field-programmable gate arrays (FPGA), or firmware can be fabricated to perform the same computations as a set of logic instructions in hardware. These implementations are interchangeable.  
         [0060]    The L med  building block  32  computes adaptive weight, w k , as the sum of a real part and an imaginary part, which are found by taking the sample median value of the ratio of local main input weight training data to local auxiliary input weight training data for the real and imaginary parts separately. First, form the set w k  as w k =(z(k)/x(k))*, for k=1,2, . . . , K where K is the number of training samples. The sample median of the real parts of {w k } is taken as the real part of the new optimal weight, and the sample median of the imaginary parts of {w k } is taken as the imaginary part of the new optimal weight. As K→∞, and assuming that, for this analysis, there are no outliers, the resulting adaptive weight,  
               w   med     =         MED     k   =     1                 to                 K              [     real   (         z        (   k   )       *         x        (   k   )       *       )     ]       +     j        {       MED     k   =     1                 to                 K              [     imag   (         z        (   k   )       *         x        (   k   )       *       )     ]       }                 (   8   )                               
 
         [0061]    converges to the same optimal complex weight as a Gram-Schmidt L 2  building block using the same weight training data, and as shown in the above referred to U.S. Ser. No. 09/835,127, this convergence only assumes symmetric densities for the real and imaginary portions of the noise density. For Gaussian noise, the two-input convergence MOE was shown to be E{η}=1+2/(K−1), where E denotes expectation and η denotes output residue power, normalized to the optimum value achievable. This is seen to be commensurate to the L 2  convergence MOE of 1+1/(K−1), which is not nearly as robust as the L med  building block is to outliers in the training data. The two-input L med  algorithm, like the two input L 2  algorithm, convergence rate, is only a function of the number of samples, K, and thus is independent of the two-input, input noise covariance matrix.  
         [0062]    The MMCC  10  is a data-adaptive filter that can be used for signal detection, estimation, and classification. For example, when used with a steering vector s, it can process a stream of one or more blocks of data vectors x(k) to estimate how much the data resembles a replica vector s. In more sophisticated applications, MMCC  10  can satisfy multiple constraints (where a single steering vector s may be replaced by a matrix of steering vectors). For radar applications, these provide improved SINR output compared to full rank methods, typically through the use of localized training data to improve statistical similarity with the range cell under test (CUT).  
         [0063]    FIGS.  8 - 15  demonstrate the major increase in performance obtained from MMCC  10  compared with previous systems. To illustrate the performance improvement of the MMCC over the MWF, data is modeled as received by an adaptive airborne (moving) radar, created using the high fidelity RLSTAP airborne radar model. There are two barrage jammers as well as ground clutter modeled with a 45 degree antenna crab angle. The MWF is used with a rank r=50 (out of 112 total DOF−14 antenna elements times 8 pulses) and K=2r=100 samples are used to train adaptive weights. The chosen steering vector points the mainbeam at zero degrees azimuth angle and at −0.47 normalized Doppler frequency.  
         [0064]    [0064]FIG. 8( a ) shows the adapted pattern produced by the MWF, after a self-normalization such that the peak pattern value is 0 dB. (All pattern plots illustrated in these figures are self-normalized in this manner). The simulated clutter ridge cuts diagonally across the azimuth-Doppler plane and aliases up to the limited angular extent (45 deg.) of the modeled antenna pattern. There are two vertical nulls that indicate the filter response to both barrage jammers. The target energy comes through with unity gain satisfying the MVDR constraint, however due to the self-normalization, it is possible the patterns show the target gain different from 0 dB. The interference is well-nulled as shown by the two lines at the jammer azimuths and along the diagonal clutter ridge, but the azimuth-Doppler sidelobe levels are high, close to the mainbeam peak level, averaging approximately −10 dB. FIG. 8( b ) is a 3-dimensional view further illustrating the pattern. Therefore, any interference not sensed (and therefore not nulled) by the adaptive algorithm can pass with little or no attenuation and even experience gain, consequently masking targets or producing false alarms. The mainbeam peak is actually aliasing across both edges of the Doppler axis as seen by two peaks near −0.5 and +0.5 Doppler frequency.  
         [0065]    For the MMCC  10 , however, as shown in FIGS.  9 ( a ) and ( b ), the interference is nulled, and the average sidelobe level reduces to approximately −25 dB, which is a 15 dB improvement compared to the MWF. This feature improves overall target detection ability when any under-nulled azimuth-Doppler sidelobe interference energy may be present in the CUT. Neither extra training samples nor diagonal loading was required to obtain this effect. In addition, the steering vector sidelobes are markedly obvious for the MMCC processor, further illustrating the reduction in depth of the adapted azimuth-Doppler sidelobe levels.  
         [0066]    In FIGS. 10 and 11, the MWF and MMCC patterns are shown for the same simulated data as in FIGS. 8 and 9, except that rank r=60, and K=120 samples. There is good interference mitigation along with the same trend of substantially lower adapted sidelobe levels. The mainbeam is positioned near the clutter ridge (az., +0.23 Dop.) to simulate looking for targets at the MDV. The MMCC has a better adapted mainbeam pattern in this region, indicating an improved ‘slow speed’ target detection performance. The MWF does not have a well-split antenna mainbeam like the MMCC over top of the strong interference (clutter), and the MWF mainbeam peak is distorted and misaligned with the chosen steering vector due to its proximity to, and influence by, the strong clutter null. The MMCC mainbeam resilience near strong interference is a valuable characteristic of the MMCC processor, as well as of the MCC processor (not illustrated).  
         [0067]    FIGS.  12 ( a ) and  12 ( b ) illustrate the clutter spectrum for actual airborne radar data using a linear array at L-band. The STAP-configured, multichannel radar and data collection system used to collect the data is termed MCARM. Data was collected during a flight. For MCARM STAP data processing shown here, N=11 elements and M=13 pulses were used such that full processor rank=143. A target of opportunity was found embedded in the clutter data at range cell 299, approximately at 0° azimuth and +0.16 normalized Doppler frequency.  
         [0068]    The adapted patterns formed by the MWF and MMCC processors for the MCARM data are shown in FIGS.  13 - 14 , respectively, for a reduced rank r=83 and using K=2r=166 samples. Both processors keep high gain on the assumed target direction, and both null the clutter energy. However, the MMCC processor has an order 15 dB lower average adapted azimuth-Doppler sidelobe level compared to the MWF, as seen visually in the plot. These plots illustrate how, even with measured data, the low sidelobe level feature of the MMCC remains intact.  
         [0069]    [0069]FIG. 15 shows that the MCARM target of opportunity is clearly indicated by both the MWF and the MMCC processor outputs. The range cell under test and three guard cells on either side were excluded from the K=166 measured data training samples. Both the MWF and MMCC processors have a peak to peak-noise level ratio (i.e., instantaneous SINR) of approximately 14.5 dB. However, at the same time, the MMCC adapted azimuth-Doppler sidelobe levels are also generally significantly less than those of the MWF.  
         [0070]    Accordingly, MMCC  10  exhibits significantly reduced adaptive azimuth-Doppler sidelobe levels not characteristic of prior art devices. This significantly aids sidelobe blanking and constant false alarm functions.  
         [0071]    The invention has been described with reference to certain preferred embodiments thereof. It will be understood, however, that modification and variations are possible within the scope of the appended claims.