JAMMING SUPPRESSION OF SPREAD SPECTRUM ANTENNA/RECEIVER SYSTEMS

Abstract: A jamming suppression system which includes a plurality of receivers for receiving electromagnetic radiations and a separate notch filter couple to each receiver for filtering selected frequencies from electromagnetic radiations received by each of the receivers. In accordance with one embodiment of the invention, a spatial combiner responsive to the outputs of each of the notch filters extracts essentially jam-free information from the electromagnetic radiations. In accordance with a second embodiment of the invention, a plurality of spatial combiners, each responsive to the outputs of each of the notch filters, extracts essentially jam-free information from the electromagnetic radiations. The notch filters are adaptive notch filters for on-line selection of their filtering action. One of the notch filters is a master filter and the remainder of the notch filters are slave filters controlled by the master filter. In both embodiments, spatial filtering action is accomplished using a generalized sidelobe canceler that permits gain (relative to an isotropic antenna) to be preserved in the direction of one or more satellites while casting spatial mulls in the directions of inadvertent friendly or hostile jamming signals.

Description: JAMMING SUPPRESSION OF SPREAD SPECTRUM ANTENNA/RECEIVER SYSTEMS BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to improved j amming suppression techniques and, more specifically, to a technique for improved j amming suppression of spread spectrum antenna/ receiver systems , such as the Global Positioning System (GPS ) . BRIEF DESCRIPTION OF THE PRIOR ART Spread spectrum communication is well known and is a means of communicating by purposely spreading the spectrum (frequency extent or bandwidth) of the communication signal well beyond the bandwidth of the unspread signal. Spread spectrum signals are typically transmitted by electromagnetic waves in free space with usage in both non-military and military systems. Spread spectrum communication and/or navigation receivers have some natural jamming immunity that is inherent to the way that matched filter receivers operate. However, there is a limit to this natural jamming rejection capability. When a jammer (either inadvertently friendly or hostile) produces an amount of jamming power at the antenna of the spread spectrum receiver that exceeds the limit of the receiver, the receiver is then unable to recover the desired information. For example, in the case of a GPS receiver, a 0 dBi antenna receives a signal from the GPS satellites at a level of approximately -133 dBm. There is some spread spectrum processing gain against jamming signals, however, initial acquisition becomes difficult when the jamming signal is approximately 30 dB stronger than the GPS signal, such jamming signal then being only -133 dBm + 30 dBm = -103 dBm. It is therefore not particularly difficult for a hostile or inadvertent friendly jammer to exceed this amount of jamming power in the GPS receiver/antenna system. When this jamming situation occurs, a standard GPS receiver cannot acquire the GPS signals. Since the receiver cannot track GPS signals, the result is an inability to develop a navigation solution based upon GPS. Though the discussion herein will generally be provided using GPS as a principal example, it should be understood that the techniques described are applicable to any spread spectrum communications or navigation system which is required to acquire and track radio signals arriving simultaneously from multiple angles and that is being jammed by electromagnetic radiations. Spread spectrum communication/navigation system receivers, such as GPS, typically operate under conditions where the signal power is substantially lower than the total receiver front end thermal noise power in the spread spectrum bandwidth of the signal . In the specific case of GPS, the GPS signal can easily be 30 dB to 40 dB below the receiver front end noise power level in the 10.23 Mhz chip rate bandwidth. The matched filter processing improves the signal to noise ratio (SNR) by the time-bandwidth product which is 53 dB for GPS P(Y) code and 43 dB for C/A code. The result of this matched filter processing is to increase the net SNR well above 0 dB so that accurate parameter estimates can be made. The fact that the spread spectrum signal power is well below the receiver front end noise power level at the point in the signal processing flow where both the temporal narrowband jammer rejection and spatial adaptation wideband jammer rejection are accomplished is very significant. In many adaptive cancellation applications where the desired signal is at or above the receiver front end noise level, the adaptive algorithm has a tendency to cancel the desired signal. In GPS, for example, the desired signal is so far below the front end noise level of the receiver that the jammer cancellation algorithms cannot sense the desired signal sufficiently well to cancel it through adaptation. Consequently, the jammer suppression algorithms may freely adapt to the frequencies of the jammers with only one exception. This exception is when there are more narrowband jammers than the degrees of freedom provided in the temporal nulling assets can effectively null, in which case some of the narrowband jamming signals will reach the spatial adaptation process where they will be nulled if there are sufficient remaining degrees of freedom, considering the number of broadband jammers present. The spatial adaptation algorithm should spend one of its degrees of freedom protecting the single angle of its designated GPS satellite signal. In general, signal jammers are of two types, one type having a very narrow bandwidth, such as, for example, a sine wave. Such jammers are suppressed by operating in the time domain with notch filters which filter out the jamming frequency or frequencies from the filter output while passing the remainder of the received signal. In this way, only a small part of the GPS signal is removed with a sufficient amount of the GPS signal remaining to permit appropriate operation therewith. The other type of jammer is broadband and generally has a bandwidth substantially the same as the GPS signal. In this case, a notch filter which filters out the jamming frequencies will also filter out most of or all of the GPS signal, with an insufficient portion of the GPS signal remaining to permit appropriate operation therewith. Accordingly, jamming suppression of jamming signals having substantially the same bandwidth as the GPS signal is instead provided by a procedure known as spatial filtering. With spatial filtering, the direction from which the signal is received is taken into account with the filtering action being based upon such direction rather than frequency. Therefore, as long as the jamming signal is not travelling to the receiver in the same direction as the GPS signal, the jamming signal can be filtered out by such well known spatial filtering techniques . While the above described prior art jamming suppression techniques have been successfully employed, there is always a need for even better, jamming suppression techniques to combat improvements in jamming techniques. SUMMARY OF THE INVENTION In accordance with the present invention, there is provided an improved jamming suppression technique and system for spread spectrum antenna/receiver systems. Briefly, according to one feature of the invention, the temporal filters precede the spatial combiner, resulting in the suppression of narrowband jamming signals to a degree that would be much more difficult using spatial techniques. Having the temporal filters precede the spatial combiner results in the suppression of narrowband jamming signals before the spatial adaptation algorithm is invoked. This allows the spatial adaptation algorithm to apply all degrees of freedom to the remaining jammers, all of which are broadband. As an example of what could otherwise happen, assume the strongest jamming signals into the antenna are narrowband. If the spatial adaptation were attempted first, then it would cast spatial nulls in the direction of the strong narrowband jammers. Every null so cast is one less null that is available for broadband jammer suppression. (The temporal finite impulse response (FIR) filter is unable to suppress the broadband jamming signals without also suppressing the GPS signal by the same amount or more.) Having one spatial combiner per GPS satellite signal to be acquired/tracked rather than a single spatial combiner for all GPS satellite signals to be acquired/tracked, is also highly desirable. In some spatial adaptation schemes, one may use spatial degrees of freedom in the following way: one degree of freedom is required per angle where a null is cast, and one.degree of freedom is required per angle required to be protected from nulling (in this case, the GPS satellite signal angle of arrival) . If only a single spatial combiner is used, and if six GPS satellites are being acquired/tracked, then the first six degrees of freedom could be required just to retain gain toward the GPS satellites. If several spatial nulls are required, then a very large number of antenna/receiver channels are required. In contrast, where there is one spatial combiner per GPS satellite signal to be acquired/tracked, by definition, only one spatial degree of freedom is required to protect the angle of its GPS satellite signal. None of the remaining GPS satellite angles need to be protected for this combiner since every GPS satellite being acquired/tracked has its own unique spatial combiner. The combination of having the temporal filters precede the spatial combiner and having a spatial combiner associated with each satellite provides even greater jamming suppression capability since the combined improvements discussed above are obtained. This overall architecture is very efficient in its use of assets. The narrowband jamming signals are attenuated in adaptive FIR filters where this is accomplished with maximum efficiency. Digital signal processing (DSP) chips are well known which are capable of handling this task by attenuating narrowband jamming signals by 60 to 80 dB while leaving a GPS signal relatively undistorted so that the matched filter receiver can still acquire and track the resultant signal. To obtain spatial degrees of freedom, the spatial combiners generally require one antenna/receiver channel per degree of freedom, which is relatively expensive in terms of RF hardware, antenna elements and antenna area on the host platform. Consequently, it is important that these precious resources be used to meet only essential requirements, such as wideband jammer suppression, while retaining gain -on at least one GPS satellite. This is exactly what the architecture of the present invention accomplishes . The antijam architecture generally includes a plurality of channels. In the case of GPS, each channel receives signals from all visible satellites of the GPS system. Each channel has its own antenna and its own receiver, a digitizer in the form of an analog/digital (A/D) converter and an adaptive notch filter, one of the channels having a master notch filter and the other channels having slave notch filters controlled by the master notch filter. In accordance with one embodiment of the invention, all of the notch filters are coupled to a single spatial combiner for all of the receiving channels through which all of the GPS satellites must be tracked. In accordance with a second embodiment of the invention, all of the notch filters are coupled to a plurality of spatial combiners, one for each GPS satellite signal processor. BRIEF DESCRIPTION OF THE DRAWINGS FIGURES la and lb together are a block diagram of an anti-jam architecture in accordance with a first embodiment of the present inven ion; FIGURES 2a and 2b together are a block diagram of an anti-jam architecture in accordance with a second embodiment of the present invention; and FIGURE 3 is a block diagram of the generalized sidelobe canceler algorithm used in both embodiments of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIGURE la, there is shown a plurality of receiving channels (1...N), each channel designed to receive a signal from the GPS satellites. Each channel includes a standard L-band downconverter which translates the L-band GPS signal to a lower intermediate frequency (IF) using well known superheterodyne techniques, as described, for example in The ARRL Handbook for the Radio Amateur, American Relay League, Newington, CT., 1991, the contents of which are incorporated herein by reference. The L-band downconverter includes an RF amplifier and two mixers, for example, and provides an IF output to a twelve bit, 50 MHz analog to digital (A/D) converter, for example. The digitized signal is sent from the A/D converter to a standard DSP adaptive notch filter, the operation of which is described in Li, 1., and Milstein, L. B., "Rejection of Narrow-Band Interference in PN Spread-Spectrum Systems Using Transversal Filters", IEEE Transactions on Communications, Vol. COM-30, No. 5, May, 1982, pp. 925-928, the contents of which are incorporated herein by reference. One of the notch filters is a master and the other notch filters are slaves and are controlled by the master notch filter. The function of the notch filter is to recognizes and filter out the narrowband noise or jamming signals, using, for example, an iterative technique known as the Least Mean Squares (LMS) algorithm as described in the above noted Li and Milstein reference. Since. each receiving channel is subjected to the same set of jamming signals, the filter coefficients computed by the master notch filter can be used in each slave filter to effectively remove the jamming signals in the other receiving channels. Furthermore, by utilizing the same filter coefficients in all receiving channels, the master and slave filters all have the same impulse response, thereby eliminating any channel-to-channel variation to the GPS signals which would arise if each filter were allowed to adapt its coefficients independently. The slave filters are conventional finite impulse response (FIR) filters whose coefficients are obtained from the master notch filter by electronic means such as, for example, a parallel to serial computer data bus. Implicit in this data transfer mechanism is the existence of certain control signals whose purpose, broadly speaking, is to facilitate the movement of the master coefficients to each slave filter. In summary, each of the channels 1...N will thereby provide a digitized and filtered output Zj...Z N which includes essentially the originally transmitted signal and any broadband receiver front-end noise or broadband jamming noise, but no narrowband noise. The digitized and filtered outputs Z —Z N of FIGURE la are fed to like referenced inputs of the spatial combiner in FIGURE lb where these signals are operated upon in conjunction with the weights W^ ^ W,, to provide a spatially filtered output, Y. The output of the spatial filter combiner structure shown in FIGURE lb is the weighted and summed composite of the signals Z X ...Z N . The single output Y . is fed to each of the GPS satellite signal processors where up to M satellites may be tracked simultaneously. Although containing certain unique elements, this structure is consistent with that of the classical phased array antenna described in -W.L. Stutzman and G.A. Thiele, Antenna Theory and Design, John Wiley & Sons, New York, NY, 1981, pp. 108-168, the contents of which are incorporated herein by reference. The weights W^.- ,, shown in FIGURE lb are determined using a standard beamforming technique such as the one described in M.I. Skolnik, Introduction to Radar Systems, McGraw-Hill, New York, NY, 1980, pp. 278-342, the contents of which are incorporated herein by reference. The inputs to the beamforming algorithm are the azimuth and elevation angles (with respect to some reference plane) to the desired GPS satellite. The azimuth and elevation angles are calculated using the orientation of the reference plane and the coordinates of the GPS satellite. The orientation of the reference plane is calculated from knowledge of the host platform's attitude which is obtained from an inertial measurement unit (IMU) and the position of the GPS satellite is learned in the process of demodulating the GPS navigation message. The spatial filtering operation is accomplished using an algorithm based upon what is widely known as the generalized sidelobe canceler algorithm, the background theory and operation of which is described in Frost, III, O.L., "An Algorithm for Linearly Constrained Adaptive Array Processing", Proceedings of the IEEE, Vol. 60, 1972, pp. 926-935 and in Griffiths, L.J. and Jim, C.W., "An Alternative Approach to Linearly Constrained Adaptive Beamforming", IEEE Transactions on Antenna and Propagation, Vol. AP-30, No. 1, Jan. 1982, pp. 27-34, the contents of both of which are incorporated herein by reference. The topology of the generalized sidelobe canceler algorithm used in this preferred embodiment is shown in FIGURE 3. The digital signals Zi...Z N are multiplied by the weights W X ...W N and then summed together. From this output, denoted S+J in FIGURE 3, an estimate J of the jamming signals is subtracted to obtain a signal Y (c.f. FIGURE lb), which is free of both narrowband and wideband jamming signals. The estimate of jamming signals is formed by first arranging the digital signals Zi...Z N into a vector and carrying out a matrix multiplication with a blocking matrix N s . The output of the matrix multiplication X x ... ^ is then weighted and summed together to form the estimate J., The function of the blocking matrix is to effectively place a broad spatial null in the direction of the desired GPS satellite so that only the jamming signals are present at the vector output Xj.-.X,,. The weights that are applied to the intermediate signals Xj.-.X,, are obtained using the LMS algorithm described in the above mentioned Griffiths and Jim reference which requires the difference signal output Y as illustrated in FIGURE 3. The elements of the blocking matrix N s are calculated according to a unique procedure described in Reid, A. and Harding, D., "An Adaptive Sidelobe Canceler for Monopulse Configurations Using Dyadic Wavelet Classes for Constraints", Sept. 15, 1995 (appendix hereto) . By forming the difference signal in the manner shown in the lower portion of FIGURE 3 , an amount of gain relative to an isotropic antenna can be preserved in the direction of a desired GPS satellite while spatial nulls are cast in the direction of inadvertent friendly or hostile jamming signals . Referring now to FIGURE 2a, there is shown a plurality of receiving channels (1...N), each channel designed to receive a signal from all visible GPS satellites (also known as Satellite Vehicles or SV's) as in FIGURE la. Each channel includes a standard L-band downconverter which includes an RF amplifier and mixer and provides an IF output as in FIGURE la to a twelve bit, 50 MHz A/D converter, for example, as in FIGURE la. The digitized signal is sent from the A/D converter to a standard DSP adaptive notch filter as in FIGURE la, one of the notch filters being a master and the other notch filters being slaves and being controlled by the master notch filter. The notch filters recognizes and filter out the narrowband noise or jamming signals. Each of the channels 1...N will thereby provide a digitized and filtered output ' Z t ...Z N which includes essentially the originally transmitted signal and any broadband noise, but no narrowband noise. The digitized and filtered outputs Z t ...Z„ of FIGURE 2a are all fed to like referenced inputs of each of a plurality of spatial combiners, one for each GPS signal processor as shown in FIGURE 2b. These signals are operated upon in conjunction with the weights W L ...W N to provide a filtered output Y l . ..Y M , one for each of the GPS signal processors. In this preferred embodiment, each set of weights Wi...w N is computed as described previously for the first embodiment, except now the azimuth and elevation angles to each of the satellites being tracked is used. That is, the azimuth and elevation angles for SVl are used to compute the weights W L ...W N for channel 1, the azimuth and elevation angles for SV2 are used to compute the weights for channel 2, the azimuth and elevation angles for SVM are used to compute the weights for channel M, etc. The outputs Yi...Y M are derived exactly as described for the first embodiment, except now there is one output for each GPS signal processor. Each spatial combiner implements the same generalized sidelobe canceler algorithm described in the first embodiment, except that in this case each algorithm functions independently of the others. In this embodiment, an amount of gain relative to an isotropic antenna can be preserved in the direction of all GPS satellites being tracked while spatial nulls are cast in the direction of inadvertent friendly or hostile jamming signals. Concisely stated, the second embodiment has the ability to cancel N-2 inadvertent friendly or hostile jamming signals while protecting all GPS signals being tracked. This is an improvement over standard approaches which are able to cancel N-l jamming signals but provide no protection of any GPS signal being tracked. That is, in standard approaches, any GPS satellite that happens to be in the line-of-sight of an inadvertent friendly or hostile jammer will be canceled as well as the jamming signal. The second preferred embodiment avoids this difficulty. Though the invention has been described with respect to specific preferred embodiments thereof, many variations and modifications will immediately become apparent to those skilled in the art . It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications. APPENDIX An Adaptive Sidelobe Canceller for Monopulse Configurations Using Dyadic Wavelet Classes for Constraints Anthony Reid and Dave Harding September 15, 1995 Abstract For many applications for phase array radar the conventional modes of a radar must be preserved. Phased array radar elements are configured to provide directions-of-arrival (DOA) for targets using a technique called monopulse. To preserve the salient features of this technique it is important that sidelobe jammer suppression techniques preserve the "sum" (Σ) and "delta" (Δ) mainbeam channel patterns. This paper applies a form of derivative constraints to the blocking matrix of the Griffiths-Jim constrained adaptive sidelobe cancellation configuration using wavelet transform constraints. Wavelet transform "blocking" matrices provide mainbeam nulling patterns that significantly reduce the attenuation of array gain relative to conventional Haar basis (e.g. Δ channel) thus improving DOA performance. The inherent "zero-mean" property of wavelet transforms allows selection of blocking matrices across classes over the angular extent of the array beampattern. This paper also applies optimization techniques such as least squares to systematically construct blocking matrices composed of linear combinations of wavelet basis matrices. Some results are also shown for other multireβolution wavelet classes such as "coifletβ" . 1 Introduction For many applications for phase array radar the conventional modes of a radar must be preserved. Phased array radar elements are configured to provide direction-of-aπival (DOA) for targets using a technique called monopulse. To preserve the salient features of this technique it is important that sidelobe jammer suppression techniques preserve the "sum" (Σ) and "delta" (Δ) mainbeam channel patterns. This paper applies a form of derivative constraints to the blocking matrix of the Griffiths-Jim constrained adaptive sidelobe cancellation configuration using wavelets constraints. Wavelet transform "blocking" matrices provide mainbeam nulling patterns that significantly reduce the attenuation of array gain relative to conventional Haar basis (e.g. Δ channel). The inherent "zero-mean" property of wavelet transforms across classes allows a systematic construction of blocking matrices over the angular extent of the array beam pattern. This paper also applies optimization techniques such as least squares to systematically construct blocking matrices composed of linear combinations of wavelet basis matrices. This procedure can replace the manual procedure of selecting combinations of wavelet classes to realize a blocking matrix response. Some results are also shown for other multiresolution classes such as "coiflets". Bi-orthonormal wavelets are also an attractive wavelet class because of minimal sidelobe properties [13], [14]. This paper is organized as follows. Section 2 summarizes the conventional adaptive sidelobe cancellation techniques and defines the signal processing chain for the monopulse configuration. Section 3 describes the application of derivative constraints to "blocking matrix" design and the properties of wavelets constraint that are more attractive. Array responses are shown over the angular extent of the array for different dyadic wavelet transform classes of Daubechies. Alternative multiresolution discrete orthonoπnal classes include "coiflets" and bi-orthonormal wavelets. This section also describes the construction of "blocking matrices" using least squares criteria to form blocking matrices composed of linear combinations of wavelet classes. Another approach considered but not investigated was to use the best basis wavelet packets selection technique [15] to jointly design a blocking matrix that extracts the jammer signal for the jammer estimation channel which essentially eliminates the need for the LMS algorithm currently used for jammer signal estimation in most applications. Section 4 shows some simple results for each application described in section 3. Section 5 concludes the paper. 2 Problem Definition and Conventional Approaches The Frost [1] and Griffiths-Jim algorithms [2] are constrained optimization techniques that can be used more effectively when estimating target angles in the presence of sidelobe jamming. A summary of these approaches can be found in [3]. The Griffiths- Jim algorithm decomposes the problem into two processing channels. One channel is used to adaptively estimate the jammer signal for subsequent subtraction from another channel that represents the desired unadapted mainbeam response. The jammer estimation channel has a pre-processing step which uses a signal blocking matrix Ws to null out the desired signal at antenna boresight. This step minimizes the inadvertent estimation of the signal along with the jammer. Any residual signal would be subtracted from the desired signal response in the mainbeam. Figure (1) shows the Griffiths-Jim adaptive sidelobe canceller for a monopulse configuration. The Least-Mean-Squares (LMS) algorithm is used to estimate the jammer signal in the signal blocking channel. When the antenna boresight is 0 deg one of the conditions on the blocking matrix is that each row sum to zero. Mathematically this can be written as 6^1 = 0, m = 1, 2, . . . , M where δ represents the m-th row of an M row signal blocking matrix Ws where and 6^ = [ό tm , b 3m , bim, • ■ ■ , l>Nm} where N is the size of a uniform linear array (ULA) with I = [1 1 1 . . . 1] . Griffiths-Jim [2] show a simple example of two functions that satisfy the constraint. There are other classes of orthogonal functions that satisfy the constraint equation on the row weights. Griffiths- Jim showed variations of the Haar transform and the Haar wavelet basis. A Haar transform is formed by the following recursion 1 1 where Hi and N is the size of a square matrix. For example 1 -1 1 1 1 1 1 -1 1 -1 H 2 = 1 Each row of the Haar matrix - , 1 -1 -1 1 -1 -1 1 H ff could be used in the blocking matrix Ws- However in their examples the rows were not orthogonal. Orthogonal rows provide more degrees of freedom for the LMS algorithm to estimate jammer signals. Figure 2 shows the normalized magnitude response of an 8 element ULA with 7 constraints applied to the array using 7 rows of the Haar Transform. Notice the deep nulls at 0 degrees which is desirable however there are βidelobes associated with the constraints. Although the deep nulls are needed to suppress the signal in the jammer channel there is a non-zero response in the mainbeam which can reduce the array gain in the "sum" (Σ) and "delta" (Δ) rhimnelii of a monopulse radar. A monopulse radar configuration is shown in figure (1) which includes the adaptive sidelobe structure of Griffiths-Jim. The angle-of-arrival for a target in the mainbeam of the Σ, Δ channels is formed by first computing the monopulse ratio This ratio is then used as the index for a look-up table that maps r to angle θ in degrees. The details of monopulse processing are found in [4]. The Σ and Δ channel mainbeam patterns must be preserved in the presence of jammers in order for the monopulse computation to work properly. Any blocking matrix that degrades the mainbeam signal in either channel will reduce the effectiveness of the monopulse computation. It is also important that the jammer signal is not attenuated by the blocking matrix Ws at the Σ, Δ channel sidelobes. Attenuation reduces the signal strength thus reducing the effectiveness of the jammer estimation algorithm. As shown the Haar transform does an excellent job in creating a deep null in the jammer channel degress Figure 2: Haar Transform Blocking Matrix Beamforms at the sacrifice of mainbeam response for the Σ, Δ channels. The blocking matrix must also be robust to either signal angular excursions away from the O deg boresight axis and errors in the actual blocking matrix weights. In either case there is a potential for the signal to "leak" through the blocking matrix in the jammer estimation channel. When the signal "leaks" through the jammer channel it is inadvertently estimated with the jammer signal and subsequently subtracting from the Σ, Δ monopulse channels. This can degrade the overall Direction-of-Arrival (DOA) estimation. Figure (2) also shows array responses that are oscillatory outside of the mainbeam. This is undesirable for purposes of designing the LMS algorithm that is used to estimate the jammer signal(β). LMS design involves the specification of a step size parameter μ for the recursion [16]. This step size depends on the signal energy. Since the jammer channel transfer function modifies the Σ, Δ monopulse channel angular responses the complexity of the LMS design process can be increased if the jammer channel response is complicated (e.g. oscillatory). What is needed is a blocking matrix which has a 0 response in the Σ, Δ monopulse channel mainbeams and has a unity response outside of the mainbeam thus preserving the unadapted Σ, Δ monopulse channel angle responses. A judicious choice of wavelet transform constraints across classes can be used to meet this objective. Another advantage of the wavelet transform classes is the satisfaction of the constraint 5^1 = 0 for the blocking matrix Ws. The wavelet transforms are also orthogonal which implies that fewer rows are needed to estimate the jammer signal relative to a non- orthonormal basis set used for rows in Ws- These properties are discussed and demonstrated in the next section. 3 Dyadic Wavelet Transforms as Derivative Constraints This section reviews the application of wavelet transforms as alternative beamformerβ for signal "blocking matrices" Ws for adaptive sidelobe cancellation in a monopulse radar configuration. A review of the fundamentals of wavelet transforms can be found in [10]. A serious mathematical treatment of wavelet transforms can be found in [11], [12]. Qualitatively there are 2 properties of wavelets being exploited for this application. First the wavelet transforms are "zero-mean" signals thus meeting the requirements being rows of a blocking matrix Ws- Secondly the amount of smoothing of the array response near boresight is controlled by the number of filter coefficients. There are some examples of wavelet transform matrices shown in Appendix A. Figure (3) shows the normalized magnitude response of an 8 element ULA with 7 constraints applied to the array using 7 rows of the Daubechies-4 class of wavelets. Subsets of rows of a wavelet transform basis matrix form subarrays of array elements which have uniformly spaced phase centers. Coarser scale wavelet bases form new effective ULAs with effective element spacing that is some multiple of the element array spacing of λ/2 Unfortunately these array spacings result in array response patterns with grating lobes or aliased patterns. These coarser scaled patterns were not investigated. Wavelet transforms have the property that the sums of the basis functions (i.e. row vectors) are 0. The wavelet transform rows are also mutually orthogonal. Figure (4) shows the mainbeam response for the same array for comparison to the amount of nulling in the mainbeam. Notice the deep nulls at 0 degrees and there are also 3 constraints that give a wider null at 0 degrees. (The magnitude responses super-imposed on one curve in the figure.) The broader constraints are actually derivative constraints inherent in the transform. Broader nulls are desirable to reduce the impact on bias errors or random angular excursions near the antenna boresight. Another potential class of transforms are "coiflets". These wavelets have additional smoothness constraints which equate to broader nulls at 0 degrees for some transform coefficients. Figures (5) and (6) compare the array response for 4 constraints for both Daubechies and coiflets for 6 coefficient filter bases. Figure (5) shows 4 responses for Daubechies-6 wavelet. There are 2 superimposed responses with broad nulls and two responses with deeper nulls and sidelobes. Figure 6 shows the coiflet-6 response which is smoother and has smaller sidelobe structure. This approach can also be compared to the techniques developed that exploit derivative constraints to define adaptive beamforming blocking matrices Ws [6] ,[7]. One disadvantage of derivative constraints is the loss of array gain at the expense of broadening the array response. One hybrid approach [8] devises a beamforming structure that combines both gain and derivative constraints using a design parameter " ?". There is also a class of algorithms (i.e. partially adaptive arrays) that address more practical issues such as limited Figure 3: D-4 Wavelet Transform Blocking Matrix Beamforms N/fcrteaTregpcree Figure 4: Radar Mainbeam Response Daubθc βs-6 wavelet ccnstrairts Figure 5: D-6 Wavelet Transform Blocking Matrix Beamforms channels [9]. These will not be discussed in this correspondence. The best basis wavelet selection [15] can be also used to construct rows in Ws. Figure 13 shows a more general monopulse configuration that allows Ws to also be adaptive. There are numerous techniques that could be be investigated for adaptation of Ws- One technique is to use linear programming to determine the best linear combination of Daubechies wavelet transforms from a given class of Daubechies blocking matrices. Starting with a given desired array response for the blocking matrix, the blocking matrix is assumed Cdtetθ detαΕtrarb Figure 6: "Coiflet-6" Wavelet Transform Blocking Matrix Beamforms to be a linear combination of Daubechies blocking matrices £> 3| - or _V W s = ∑ a i D 2i (1) -=1 where α, can be a scalar or matrix. This decomposition is valid because ^ ■ I = ∑ α, l = 0 (2) 1=1 since Da • 1 = 0, Vt for any Daubechies class of wavelets. Equation (2) also shows that the blocking matrix constraint is true for any a,. What is desired is to determine c*, that minimize some objective function of interest. The objective function could be a blocking matrix response at particular angles. For this problem a linear programming formulation can be used by first defining the desired array responses Ws(θk) and a response constraint at a set of angles 0*, k = 0, ±1, ±2, . . . , ±K. as shown in figure (7). This approach is similar to Finite Impulse Response (FIR) filter design used by Rabiner [20]. As shown the desired blocking matrix response is bounded by a constraint δ \ at angles that are in the sidelobeβ of the ∑, Δ channels. A different constraint δ iS defined for the desired blocking matrix response in the mainbeam of the ∑, Δ channels. A system of constrained linear equations is defined for the blocking matrix response in the sidelobes as JV Ws(* fc ) - δi ≤ ∑ iD 2i {θ k ) < W s (θ k ) + δ k = ±V, ±(V + l), . . . , ±K »=ι where angles Q k . k = ±N, ±(N + 1), . . . , ±K are in the sidelobes. A system of constrained linear equations is defined the blocking matrix response in the mainbeam as _V 0 < ∑ OiDniθ ) < δ 2 , k = 0, ±1, ±2, . . . , ±(V - 1) ts-l where angles 0*, k = 0, ±1, ±2, . . . , ±(V — 1) are in the mainbeam response. The objective is to determine α, by minimizing the objective function J = ∑"- N {Ws{ β k ) - ∑£ x OiDuV.)} or min {W s (θ k ) - ∑£, < D 2i (θ k )} , k = 0,±1,±2, ... ,±K ∑" i αΛ(* < W s (θ k ) + δ lt k = ±V, ±( V + 1), ... , ±K ∑£ι anDtiiθk) > W s (θ k ) -δ k = ±V, ±(V + 1),...,±K subject to ∑Ϊι<*iD 2i (θ k )<δ 2 , A = 0,±1,±2,...,±(V-1) ∑£ j OiDaiθk) ≥ 0, k = 0, ±1, ±2, ... , ±(V - 1) The martrices D 2 i can be selected as subsets of Daubechies basis functions for a particular wavelet transform class. This approach should essentially replace the manual procedure for sellecting individual wavelet basis functions within different Daubechies classes to get a desired array blocking matrix response. A least squares objective function could also be used for determining α,. The objective function J = ∑f .^ {W s (θ k ) - ∑£, α,-D 3 ,( β t)} 2 would be by minimized with respect to α,- or Rewriting equation (3) in matrix-vector notation yields <*ι W s (θ k ) = D 7 (θ k ) D A (θ k ) D 6 {θ k ) D 7N (θ k ) <*3 + θ k ) (4) CtN where Du(θ ) are the array responses for Daubechies blocking matrices and e(0 f c) represents the fit error. The least squares solution for I o_ι a 2 α 3 • • • α / y gives oti α = [D 7 (θ k ) D < (θ k ) D β (θ k ) ••• D 2fr (θ k ) }* W s (θ k ) (5) α/v ^ αδ.S , Figure 7: Ws - Desired Blocking Matrix Response where # denotes the pseudo-inverse operation. The estimate Ws(θ k ) of the desired response is Q 2 W s (θ k ) = [ D 2 (θ k ) D (θ k ) D 6 (θ k ) ■ ■ ■ D 2N (θ k ) α 3 ON Figure (8) shows the channel responses for the Σ and Δ beams for a 16 element uniform linear array (ULA). The first null responses outside of the mainbeam for the Σ channels are w ±8 deg while the null responses for the Δ channel are « ±10 deg Figure (9) shows an example of estimating wavelet class D t (θ k ) using a linear combination of D 2 (θ ), D^(θ ) and Ds(θ ). Five rows of each martrix D i(β k ) are selected which have minimal oscillatory behavior. Row 6 has no oscillations for D 2 (θ k ), D (θ k ), Dβ(θ ) however row 6 has oscillations for D s (θ k ). Row 7 has oscillations for both D 9 (θ k ), D B (θ k ). The objective function is the magnitude response Dβ(0fc) = ∑?- ! 5,- for this example. When α,- axe scalars the estimated constants δ, from equation (5) are αi = 0.1840, a 2 = -1.1417 and α 3 = 1.9580. The adaptive array response in the figure shows a reshaped response that has characteristics of D at angles near θ = 0 deg . At larger angles θ k the array response shows characteristics of D (θ). α,- in the least squares objective can be diagonal matrices which increase the number of degrees of freedom for a least squares fit or A, = diag {βu.βii, - ■ ■ ,βu) where L is the number of rows in steering matrices D 2 i(θ k ). The objective function J = ∑^ _ / ^ {WsCO — ∑ ι AijDa;(0fc)] is minimized with respect to # «j or J WW - , j = l, 2, . . . , L (6) A matrix-vector form of equation (4) can be written for equation (6) in the form by first defining D'(θ k ) t = [ D 2 (θ k )i £> 4 (0 t ). ■ • D Θ i UN-adaptad Array Raaponaa β1 (40 db .40 dag.). *2(30 db .1 6 dag ), Figure 8: Σ, Δ magnitude responses for ULA(16) Blocking Matrix Response Figure 9: Blocking Matrix responses for D 2 ,D f D^ and the W,. , are scalars. ' where D j (θ k )i is the array response to particular Daubechies class wavelet matrix for a row i. The desired array response Ws(θ k ) can now be written in the form where each β = (/3π, 3i2, • ■ • .βirr]- βi are interleaved so that each β are the coefficients for a particular row t of Ws(θ ). D'(θ k )i are the same rows of a wavelet basis matrix.. A least squares solution can be determined for [^ t β^, ^ • • ■ , §_ N ] in equation (7). Figure (10) shows an example of estimating wavelet class D t (θ ) using a linear combin D (θ ). For this case the magnitude response The estimated constants §. 3 j = [0.2097, 0.2097, 0.2097, 0.2097, 0.2097, 0.4661, 0.0023] £, = [-1.2286, -1.2286, -1.2286, -1.2286, -1.2286, -1.6238, -1.1047] , = [2.0193, 2.0193, 2.0193, 2.0193, 2.0193, 2.1594, 2.1020] (8) The adaptive array response in the figure shows a reshaped response that has characteristics of D at angles near θ = 0 deg . At larger angles of θ k the array response shows characteristics of D t (θ). The first 5 channels of the blocking matrix responses are the same since the coefficients for p. are the same in equation (8). The last 2 rows of the blocking matrix have different coefficients. The extra degrees of freedom using a matrix linear sum instead of a scalar linear sum of basis matrices allow better control over how individual rows are adapted into the estimated blocking matrix. Figure (10) shows the response for the adapted blocking matrix Ws(θ) having characteristics of D \ (θ) near θ — deg and a faster rise to the desired magnitude of 1 at array mainbeam sidelobes. Ws(θ) should make the blocking channel design more robust to jammers that are closer to the mainbeam over any individual wavelet basis matrices. However the signal blocking characteristics are not as good as D 6 (θ). Therefore the signal blocking characteristics can be compromised at larger angles off array boresight. Figure (10) also shows that D 2 (θ) has the best jammer signal response close to the mainbeam. Figure (11) shows the array response for row 6 of the estimated blocking matrix for D Λ (Θ). The oscillatory array response originates from the desired signal array response D β (θ). Howeyer none of the basis kernels D 2 (β), Di(θ) or D 6 (θ) is oscillatory therefore the estimate of D 8 (θ) is smooth. The estimate also takes on some of the characteristics of D 2 (θ) near O deg and the smoothed characteristics of D Λ (Θ) at array sidelobes which is desirable. Figure (12) shows the array response for row 7 of the estimated blocking matrix for Ds(θ). In this case the array response is oscillatory for the underlying wavelet basis matrix D 6 (θ) and the desired response D t (θ). The overall estimate seems worse than either D Λ (Θ) and Dβ(θ) however the estimate also takes on some of the characteristics of D 2 (θ) near 0 deg although now of an oscillatory form. The least squares is using the least smooth wavelet bases (i.e. D 2 (θ), D^(θ)) to estimate an oscillatory signal D Λ (Θ). The estimated array response at the sidelobes follows the characteristics of D%(θ). Another objective function could be related to minimizing the amount of signal that leaks through the blocking matrix Ws when the signal of interest is off-boresight. Figure (7) shows the algorithm structure to achieve an adaptive blocking matrix that would be determined to minimize signal leakage. For this configuration the coefficients for both the LMS algorithm and , for the decomposition matrices D 2 { are adaptive. The adaptation is done in two steps. The first step uses an approach similar to alternating projections alternating [21] to perform an adaptation which estimates both signal J, and jammer sj by alternating the estimation of the LMS coefficients c, and wavelet blocking matrix coefficients α t . Since the cascaded structure of the blocking matrix Ws and the LMS coefficients c, results in a nonlinear set of equations c,-,α,- one technique to preserve the LMS algorithm functionality is to alternate the adaptation process. First the LMS coefficients c,- are estimated keeping α, constant. Then c, are estimated keeping α, constant. The procedure repeats until some time. The goal of this step is to do an unconstrained estimate of both signal and jammer which allows the signal through the blocking matrix and estimates the jammer with the LMS algorithm. Once the coefficients α, are determined which allows signal J, through the blocking matrix Ws then the next step would be to reassign the coefficients αc, to D 2 i which would emphasize the selection of £> 2 , which Figure 10: Blocking Matrix responses for D 2 ,D 4 ,Dβ and the W,. α, are diagonal matrices. Figure 11: Blocking Matrix responses for D 2 , D Λ , D 6 and the W t . at, are diagonal matrices. Row 6 of blocking matrix. Figure 12: Blocking Matrix responses for D ,D , D 6 and the W,. α,- are diagonal matrices. Row 7 of blocking matrix. would minimize the leakage through the blocking matrix. For example, letting N = 3, let c*ι > αa > α 3 from the adaptation process. This implies that Daubechies matrix D allows more signal through than D 3 for the unconstrained signal and jammer estimation problem. Since it is desired to minimize signal leakage for the adaptive sidelobe cancellation problem then a new assignment β \ — a 3 ,β 2 = a 2 ,β 3 = aiχ would minimize signal leakage for this configuration. 4 Results This section presents results that demonstrate the concepts developed previously. Three wavelet basis matrices were selected to demonstrate the improvements in DOA estimation by using wavelets with broader nulling patterns along the array boresight. The wavelet bases D 2) D< and D β were selected. The signals of interest are a signal at 1.5 deg off antenna boresight and a jammer impinging at 40 deg off boresight. Letting z— \z , z , . . . , z ff ) τ be defined as a vector of array responses, the signals impinging on the ULA of N elements were generated using the standard simple array model for ULAs [16]. z = [ A(θ l ) A(θ 2 ) } r Ji + n 3 2 (9) The signal and jammer signals sχ, s 2 respectively, are modeled as zero-mean, independent white noise sources with statistics σ^ , σ^. Signals Sι, a are assumed to be uncorrelated thus £ (J I , a 2 ) — 0 where S (•) is the "expectation" operator in statistics. Vector n = . . . ,njγ] represents background array element noise and is also modeled as uncorrelated, additive zero-mean, white noise. The DOAs of each signal are embodied in the steering vectors A(θ 2 ) A(β 2 ) where θχ,θ are the directions of arrival. To model additional signals 3i the vector of s \ , s 2 would be extended and the appropriate steering vectors Λ(0,) would be concatenated to the steering matrix. This model builds planar waves impinging on a ULA at different angles. Figure (14) shows snapshots of signal s \ = SQ and jammer signal s 2 = j 0 . s 0 is a 20 db. signal impinging on the ULA at O deg. j 0 is a 30 db. signal at 40 deg. As an example, decibles (db.) are defined as 20 db = 101og 10 σ a with σ 2 = 100. An array snapshot is generated by selecting random samples of jo, Jo using σ Λ > σ «o - Equation (9) is used to generate array snapshots as shown in figure Figure 13: Monopulse Radar Configuration Adaptive Sidelobe Cancellation - Adaptive Ws (14). The additive background noise statistics were identical σ . = 1. The wavelet bases selected for each case D 2 , £> 4 , Z? β were subsets of each wavelet bases matrix. These rows were selected to provide smooth array responses across the sidelobes of Σ, Δ channels and to minimize oscillatory behavior. Row subsets of blocking matrices were also selected to minimize array response grating lobes. The qualitative performance measure for these 3 cases was DOA. The results show that D$ provides the best DOA performance when the jammer is in one of the far sidelobes of Σ, Δ channels. This was expected since £> 8 has the broadest null pattern in the Σ, Δ channel main- beams thus providing better blockage of signal a 0 in the jammer estimation channel. However there are other examples that show Z?β demostrates worst results when the jammer is in the near sidelobes of Σ, Δ channel mainbeams. This too is expected since the broad nulling response pattern now attenuates the jammer signal in the jammer estimation channel. Thus the motivation for an adaptive blocking matrix design that minimizes jammer signal attenuation while maximizing signal blockage. Reducing jammer signal attenuation might be more critical since errors in jammer signal estimation could have more impact on the overall performance since jammers using have more power than the signal of interest. It is also very important that the LMS algorithm provide very accurate jammer estimates in the near sidelobes to minimize any impact on array gain of Σ, Δ channel mainbeams. The results for D blocking matrix are discussed first. Seven of the 16 rows of the matrix are used to form the output channels for the blocking matrix for each Σ, Δ channel. These 7 channels are used by the LMS algorithm to estimate the jammer signal in each Σ, Δ channel. Figure (15) shows the adapted array response for the Griffiths- Jim algorithm for both the Σ, Δ channels. The unadapted array responses are shown in figure (8). There is a null in the adapted array response for both Σ, Δ channels. However there is some perturbation to the unadapted array response near 20 deg. This can be explained by looking at figure (16). This figure shows the array response of the filter coefficients of the LMS algorithm and the blocking matrix Ws after 100 array snapshots. Each LMS filter has 7 taps which are complex numbers. As shown the largest array responses are near 40 deg in each Σ, Δ channel. (The amplitudes are different because the array responses are not the same for the unadapted Σ, Δ channels.) The next largest array responses are near — 20 deg. There are also other less significant extrema in the array response. These additional array response extrema near —20 deg. are a result of grating Array Snapshot Figure 14: Jammer j 0 at 40 deg, 40 db. Signal Λ 0 at 1.5 deg, 30 db. 30 Angle (Degrees), Array Snapshot = 100 , Monte Carlo = 3 Figure 15: Adapted array response Σ, Δ channels. D 2 blocking matrix. lobes in the beams formed at the output of the blocking matrix Ws which have effective spacing of A wavelengths, λ/2 is the maximum array spacing to avoid grating lobes. The figure also shows the array response pattern near 0 deg which reflects the blocking matrix response of D as shown in figure (9). The figure also shows secondary array responses that get larger in amplitude at angles further from the O deg. This also reflects the array response of the blocking at angles further from 0 deg as shown in figure (9). Notice how the secondary extrema are relatively flat at angles less than —40 deg which also agrees with the flat response of the blocking matrix in figure (9). Figures (17) and (18) show the zeros found for the LMS algorithm for the Angle (Degrees), Array Snapshot = 100 , Monte Carlo = 3 Figure 16: LMS array response Σ, Δ channels. D blocking matrix. fil'.er coefficients. Letting the 7 tap filter be defined as h = [α 0 , αι , . . . , α 6 ]. (The coefficients α, axe estimates of the true coefficients α,- estimated by the LMS algorithm.) Solving for the of polynomial p(z) = oo + ιz ~2 + α z ~ * + ■ ■ ■ + αβz -12 = 0 ( 10) the roots are just the phase shifts expected between array elements for z = e jβi . These phase shifts should be equally spaced on the unit circle and this trend is shown. The zeros are closer to the unit circle for the zeros of the Δ channel. This behavior is discussed further when the LMS coefficients are 21 show the adapted array response output for Σ, Δ channels respectively. Both the /-channel (imaginary) and Λ-channels (real) parts of the output are shown versus the original input signal (so)- There is no apparent jammer signal in the adapted array response in either channel since the relative amplitudes of both channels are the same as the input signal. Otherwise there would be a much larger response as shown in figure (14). The LMS algorithm is doing a reasonably good job in estimating the jammers which are subtracted from the unadopted Σ, Δ channels. The Σ channel response in figure (21) shows very close agreement between the I, Q signal components and the input signal. The Δ channel response in figure (20) shows a close agreement between the J, Q signal components and the input signal - except for a phase shift - after 60 array snapshots. Some of the response behavior can be attributed to the LMS coefficients for Σ, Δ channels as shown in figures (22) and (23). The LMS roots for the Σ channel are inherently "noisier" than the Δ channel. This output of unadapted Δ channel is the jammer and an attenuated version of the signal. The amount of attenuation is greatest at boresight. The Δ channel gain increases thus signal attenuation decreases away from boresight. Therefore D2(7.16), J_DOA = 40, SLC = 1 , Sum zeros. DOA sig = 1 5, SNR (db) sig = 30 Array Snapshot = 100 , Monte Carlo = 3 Figure 17: ∑ channel LMS algorithm zero patterns. D 2 blocking matrix. D2(7,16), J_DOA = 40, SLC = 1 , Delta zeros, DOA sig = 1 5, SNR (db) sig = 30 Array Snapshot = 100 , Monte Carlo = 3 Figure 18: Δ channel LMS algorithm zero patterns. D 2 blocking matrix. Figure 19: Roots of polynomial for zeros of LMS algorithm for Σ, Δ channels D 2 blocking matrix. Array Snapshot, Monte Carlo = 3 Figure 20: /, Q channels of adaptive Δ channel. Input signal s 0 at different array snapshots. Array Snapshot, Monte Carlo = 3 Figure 21: I, Q channels of adaptive Σ channel. Input signal s 0 at different array snapshots. at small angles away from boresight the desired signal for the LMS algorithm is essentially the jammer. This output of the unadapted Σ channel - which is the desired signal for the LMS algorithm - has both signal and jammer. The signal becomes a noise source with respect to the adaptation processes because the blocking matrix removes the signal from the LMS channel. The LMS coefficients are "noisier" in the unadapted Σ channel relative to the unadapted Δ channel because of the additional noise source in the unadapted Σ channel. The coefficients for both channels seems to have converged to steady state values after 60 array snapshots. The LMS roots shown in figure (18) of the Δ channel LMS coefficients are less noisy than Σ channel LMS coefficients in figure (17). This is because the Δ channel LMS coefficients are less noisy. The additional noise due to the signal "leaking" through the unadapted Σ channel result in noisier coefficients. Figures (25) and (26) summarize the DOA performance for D 2 . Figure (25) shows the monopulse ratio r at each snapshot where r = Re j^j and Δ, Σ are the angle responses of the adapted channels. The unsettled behavior of monopulse ratio for the first 60 snapshots represents the settling time of the LMS coefficients of the Δ channel. Figure (26) shows DOAs computed at each snapshot for r at each. Figure (24) shows the monopulse function that maps monopulse ratio r = Re { jj π j t} to DOA θ. The summary statistics showed the median and truth (1.5 deg) coinciding over 100 snapshots despite the long transient interval. There are many outliers before and after 60 snapshots relative to D , D 6 which will discussed. The ±1 MAD intervals were used to estimate the "spread" of DOA estimates. The MAD statistic is defined as the "median absolute difference" statistic and is computed as MAD — median |/j — median ( χ, l , . . . , lp) | , i = 1, 2, . . . , P where /< are random samples of a sequence of P i.i.d. random variable /. These test statistics were chosen because of the "heavy-tailed" outliers expected in a r a quotient of 2 random variables. The spread is « 2 deg. Figures (27) - (31) represent the array responses for blocking matrix D 4 . Figures (32) - (37) represent the array signal outputs at different processing steps for blocking matrix D . The trends are similar to D 2 however figure (37) shows DOA estimates converge more quickly, with fewer outliers and the "spread" is « 1 deg. The median and truth coincide at ±l.δdeg. Figures (38) - (42) represent the array responses for blocking matrix D Λ . Array Snapshot, Monte Carlo = 3 Figure 22: LMS coefficients for /, Q signal components of adaptive Σ channel. sig = 30 Array Snapshot. Monte Carlo = 3 Figure 23: LMS coefficients for I, Q signal components of adaptive Δ channel. Monopulse Ratio Table Angle (degrees) Figure 24: Monpulse ratio r function which maps r to DOA θ D2(7,16), J_DOA = 40. SLC = 1 , Monopulse Ratio : DOA sig = 1 .5. SNR (db) sig = 30 Array Snapshot, Monte Carlo = 3 Figure 25: Monopulse ratio. D 2 Figures (43) - (48) represent the array signal outputs at different processing steps for blocking matrix D - The trends are similar to D 2 , D* however figure (48) shows DOA estimates converge more quickly than either D 2 , D , with no outliers after 20 snapshots and the "spread" is « ±0.75 deg. The median and truth coincide at 1.5 deg. The LMS algorithm converges in less than 20 snapshots for Σ, Δ channels. For the case of a jammer impinging on a far sidelobe as the blocking matrix null becomes broader (e.g. Z) β ) the signal blockage in the jammer channel improves the overall quality of the estimated DOA using the monopulse algorithm. D2(7,16). J_DOA = 40, SLC = 1 , DOA Monopulse. DOA sig = 1.5, SNR (db) sig = 30 Array Snapshot, Monte Carlo = 3, low = 1 high = 100 Figure 26: Direction-of-Arrival (DOA). D 2 0 Figure 27: Adapted array response Σ, Δ channels. D blocking matrix. D4(7,16), J_DOA = 40, SLC = 1. LMS Response, DOA sig = 1.5, SNR (db) sig = 30 Angle (Degrees), Array Snapshot = 100 , Monte Carlo = 3 Figure 28: LMS array response Σ, Δ channels. Z? 4 blocking matrix. D4(7,16), J_DOA = 40, SLC = 1 , Sum zeros. DOA sig = 1.5. SNR (db) sig = 30 Figure 29: ∑ channel LMS algorithm zero patterns. D* blocking matrix. D4(7,16). J_DOA = 40, SLC = 1 , Delta zeros, DOA sig = 1 5. SNR (db) sig = 30 Array Snapshot = 100 , Monte Carlo = 3 Figure 30: Δ channel LMS algorithm zero patterns. D blocking matrix. D4(7.16). J_DOA = 40, SLC = 1 , Sum/Delta zeros (LMS), DOA sig = 1.5. SNR (db) sig = 30 Array Snapshot = 100 , Monte Carlo = 3 Figure 31: Roots of polynomial for zeros of LMS algorithm for Σ, Δ channels. D 2 blocking matrix. Array Snapshot, Monte Carlo = 3 Figure 32: I, Q channels of adaptive Δ channel. Input signal s 0 at different array snapshots. D \ Array Snapshot, Monte Carlo = 3 Figure 33: /, Q channels of adaptive Σ channel. Input signal s 0 at different array snapshots. D 4 D4(7,16), J_DOA = 40, SLC = 1 , 1/Q - Sum channel, LMS coeffldents, DOA sig = 1.5, SNR (db) sig = 30 Array Snapshot, Monte Carlo = 3 Figure 34: LMS coefficients for J, Q signal components of adaptive Σ channel. D4(7,16), J_DOA = 40, SLC = 1 , 1/Q - Delta channel, LMS coeffldents, DOA sig = 1.5, SNR (db) sig = 30 Array Snapshot, Monte Carlo = 3 Figure 35: LMS coefficients for /, Q signal components of adaptive Δ channel. D4(7.16), J_DOA = 40, SLC = 1 , Monopulse Ratio : DOA sig = 1.5, SNR (db) sig = 30 Array Snapshot, Monte Carlo = 3 Figure 36: Monopulse ratio. D A Array Snapshot, Monte Carlo = 3, low = 1 Ngh = 100 Figure 37: Direction-of-Arrival (DOA). D 30 Angle (Degrees), Array Snapshot = 100 , Monte Carlo = 3 Figure 38. Adapted array response Σ, Δ channels. D blocking matrix. D8(5,16), J_DOA = 40, SLC = 1 , LMS Response, DOA sig = 1.5, SNR (db) sig = 30 Angle (Degrees), Array Snapshot = 100 , Monte Carlo = 3 Figure 39: LMS array response Σ, Δ channels. D B blocking matrix. 08(5,16), J_DOA = 40, SLC = 1 , Sum zeros, DOA sig = 1.5, SNR (db) sig = 30 Figure 40: ∑ channel LMS algorithm zero patterns. _Dg blocking matrix. D8(5,16). J_DOA = 40. SLC = 1. Delta zeros, DOA sig = 1 5, SNR (db) sig = 30 -1 -0.5 0 0.5 1 Array Snapshot = 100 , Monte Carlo = 3 Figure 41: Δ channel LMS algorithm zero patterns. D a blocking matrix. D8(5,16), J_DOA = 40, SLC = 1 , Sum/Delta zeros (LMS), DOA sig = 1.5, SNR (db) sig = 30 Array Snapshot = 100 , Monte Carlo = 3 Figure 42: Roots of polynomial for zeros of LMS algorithm for Σ, Δ channels. D B blocking matrix. 5 Summary A Wavelet Basis Matrices Some wavelet bases are described in this section. The examples assume a 16 element ULA so there are 16 columns in each matrix. Each wavelet basis matrix is of size (N — 1) x N where N is the number of elements in a ULA. Each row of a wavelet basis matrix represents a beamforming pattern Any subset of rows can be used to form a set of beams. For example a 7 x 16 matrix would form 7 manifolded array channels using 16 array elements. The simplest example is the Haar basis. The first 12 rows of wavelet basis D 2 or the Haar basis has the form 1 -1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 -1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 -1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 -1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 -1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 1 -1 0 0 0 0 D 2 — ~Z 2 0 0 0 0 0 0 0 0 0 0 0 0 1 - 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 — 1 1 -1 -1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 -1 -1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 -1 -1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 ■ 1 - 1 Each row is shifted by two elements when looking at rows 1-8. For rows 8-12 a new pattern is shifted by four elements. Before describing D and other classes of wavelets a brief review is presented. The wavelet transform of a square-integrable signal x(t) is defined as where g( - ) represents the a wavelet function and * denotes the complex conjugate of g(- ). s represents a scale factor for the function g(t). The transform operator is a kernel of the time-scaled operator that transforms a 1-Dimensional (1-D) signal into a 2-Dimensional (2-D) representation. The transform output is a function of time and scale. The plot of W g x(τ, a) is called a "scalogram" where scale parameter 5 is the analog to frequency in a spectrogram. In order for the wavelet transform to be invertible g(t) must at least be zero-mean. Furthermore, g(t) must satisfy the following requirement dω \g(ω)\ 2 — < oo -oo lωi where g (ω) is the Fourier transform of g (t) and C ω is a constant that guarantees invertibility of the wavelet transform. An example is the Gabor wavelet [17] which has been used to extract chirp signal parameters. The Gabor wavelet h the form mechanism to estimate the parameters ( , / 0 ) of the chirp signal. This is an example of a wavelet transform related to the signal of interest. In many cases signal parameterization is not as simple. The Daubechies wavelets are a class of discrete wavelet transforms meeting general requirements without any signal specific parameterization. The Daubechies wavelet transforms are formed by solving a 2-scale discrete difference equation for φ(t) of the form 3,-1 ¥>(*) = ∑ c k ψ(2t - k), q ≥ l (11) fc=o where φ(t) is called a scaling function. The wavelet g(t) is formed as 2,-1 9(t) = ∑ d k ψ(2t - k) (12) fc=o where = (-l)*c 2 ,_ι-* (13) Th re are additional constraints on c k to satisfy convergence requirements, namely Σ f c C fc = 2 ∑ k (-l) k k n c k = 0, n = 0, . . . , - l (14) ∑ k CfcCfc+2/ = 25|,o where δ^o denotes the Kronecker delta. The condition ∑ c c k+2 ι = 25j , o expresses the orthonormality of wavelet transform coefficients across scales and within scale. This is essentially the whitening property. The coefficients d are the filter taps for an FIR filter used to construct a wavelet transform of a signal at different resolutions. For this problem the highest resolution wavelet transform coefficients. For example D 4 , d k has 4 filter taps. For a signal with 16 samples the wavelet transform matrix would be a (15 x 16) matrix. The first 6 rows of wavelet basis D has the form 0.3415 -0.5915 0.1585 0.0915 0 0 0 0 0 0 0 0 0 0.3415 -0.5915 0.1585 0.0915 0 0 0 0 0 0 0 0 0 0.3415 -0.5915 0.1585 0.0915 • • • 0 0 0 where the first 6 rows are shifted versions of row 1. The next 2 rows 7 and 8 have the form 0 0 0 ■ • • 0 0 0 0 0.3415 -0.5915 0.1585 0.0915 0.1585 0.0915 0 • • • 0 0 0 0 0 0 0.3415 -0.5915 The last row has the coefficients "wrapped- around" to represent the periodic extension assumption for signals of interest. This assumption is not true for the ULA's. The first 6 rows of wavelet basis D$ have the form 0.2352 -0.5706 0.3252 0.0955 -0.0604 -0.0249 0 0 0 0 0 0 0 0.2352 -0.5706 0.3252 0.0955 -0.0604 -0.0249 ■ • ■ 0 0 0 where the first 6 rows are shifted versions of row 1. The next 2 rows 6 and 7 have the form 0 0 0 ■ • • 0 0 0.2352 -0.5706 0.3252 0.0955 -0.0604 -0.0249 -0.0604 -0.0249 0 • ■ • 0 0 0 0 0.2352 -0.5706 0.3252 0.0955 but the row 7 has the coefficients "wrapped-around" to represent the assumption of periodic extensions of the signals of interest. B Simple Performance Metrics The performance metrics are defined by defining the input signal as s 0 , j 0 . the input to any arbitrary array is defined as r = As + n w here 5 = [θo, Jo]- The output of the Σ, Δ channels are defined as y = TE IΔ Γ = T ∑Λ As + T Ω n The output of adapted Σ, Δ channels is defined as a. The performance metrics are defined as follows. • Σ/Δ ,SIR = 101og 10 where = £ (a 0 y) = E {s 0 (T τ , Δ Aa + r E , Δ ny)} and σ 2 = £(joy) = ε {jo (T ∑t ^As + T ∑t ^ny)}. This metric measures the signal-to-interference ratio (SIR) at the output of the unadapted Σ and Δ channels which can differ from the input βignal-to-interference at the array elements. By doing a correlation with the input signal and input jammer the individual signal and jammer components of the additive signals at the output of the Σ and Δ channels. Now £ {joT ∑ ^ny} — 0, because of independence of signal and noise therefore σ = £ (j Q y) = T E , Δ Λ and σ| = £ (a Q y) = T ∑ ^A ' «0 Λ 0 In summary = T Ztt (15) A - = r E , Output [∑/Δ] 5 7 to Σ/Δ or SIR = 10 £ (5 2 )and σ 2 is defined in equation (15). This metric measures t e signal-to- interference between the estimated signal after adaptive beamforming to the input jammer power before adaptation. is measured by form- ing £ so ) w σ which extract signal only form the estimate. If the jammer signal "leaks" into the adapted output channel this would give an erroneous indicator of signal strength. Output [Σ/Δ] SJtc to Σ/Δ or 5/SΛ = 101og 10 where σ si. — £ (.f 2 )and σ 2 , are defined in equation (15). This metric measures the "signal-to-interference plus signal" between the estimated signal after adaptive beamforming to the input jammer power before adaptation. Output [∑/Δ] SX/C to Σ/Δ or SSR = 10 log 10 where σ = £ (? 2 )and σ| is defined in equation ( 15). This metric measures the signal power loss before and after the adaptation process. This metric determines the effectiveness of the blocking matrix Ws to suppress the signal in the jammer channel at off-boresight targets. The Σ channel can be effected for any targets within the mainbeam including targets at O deg DOA. Array Snapshot, Monte Carlo = 3 Figure 43: J, Q channels of adaptive Δ channel. Input signal s 0 at different array snapshots. D β D8(5,16), J_DOA = 40, SLC = 1 , 1/Q - Sum channel, DOA sig = 1.5, SNR (db) sig = 30 Aπ-ay Snapshot, Monte Carlo = 3 Figure 44: I, Q channels of adaptive Σ channel. Input signal Jo at different array snapshots. D Aπay Snapshot, Monte Carlo = 3 Figure 45: LMS coefficients for /, Q signal components of adaptive Σ channel. D8(5,16), J_DOA = 40, SLC = 1 , l/Q - Delta channel, LMS coefficients, DOA sig = 1.5. SNR (db) sig = 30 Array Snapshot, Monte Carlo = 3 Figure 46: LMS coefficients for /, Q signal components of adaptive Δ channel. Array Snapshot, Monte Carlo = 3 Figure 47: Monopulse ratio. D 8 Array Snapshot. Monte Carlo = 3, tow = 1 high = 100 Figure 48: Direction-of-Arrival (DOA). D 6 References [I] O.L. Frost III, "An Algorithm for Linearly Constrained Adaptive Array Processing," IEEE Proceedings, vol. 60, No. 8, pp. 926-935, August 1972 [2] L.J. Griffiths and C.W. Jim, "An Alternative Approach to Linearly Constrained Adaptive Beamforming," IEEE T ansactions on Antennas and Propagation, Vol. AP-30, No. 1, January 1982. [3] N.L. Owβley, "A Recent Trend in Adaptive Spatial Processing for Sensor Arrays : Constrained Adaptation," Signal Processing, Academic Press, pp. 591-604, 1973. [4] D. K. Barton, Modern Radar Signal Processing , Artech House, Inc., 19XX. [5] S. M. Sherman, Monopulse Principles and Techniques, ARxtech House, Inc., 1984. [6] K.M. Buckley and L.J. Griffiths, "An Adaptive Generalized Sidelobe Canceller with Derivative Constraints," IEEE Transactions on Antennas and Propagation, Vol. AP-34, No. 3, March 1986. [7] M.H. Er and A. Cantoni, "Derivative Constraints for Broad-Band element Space Antenna Array Processors," IEEE Transactions on Acoustics, Speech and Signal Processing, Vol. ASSP-31, No. 6, December 1983. [8] Y. Chen and C. Chen, "A New Structure for Adaptive Broad-Band Beamforming," IEEE Transactions on Antennas and Propagation, Vol. 39, No. 4, April 1991 [9] D.R. Morgan, "Partially Adaptive Array Techniques," IEEE Transactions on Antennas and Propagation, Vol. AP-26, No. 6, November 1978. [10] M. Vetterli and O. Rioul, "Wavelets and Signal Processing," IEEE Signal Processing Magazine, pp. 14-38, 1991. [II] I. Daubechies, Ten Lectures on Wavelets. SIAM, CBMS series, 1992. [12] I. Daubechies, S. Mallat and A.S. Willsky, (Editors), "Special Issue on Wavelet Transforms and Multiresolution Signal Analysis," IEEE Transactions on Information Theory, Vol 38, No. 2, March 1992. [13] C.K. Chui, An Introduction to Wavelets. Academic Press Inc., 1992. [14] C.K. Chui, Wavelets : A Tutorial in Theory and Applications. Academic Press, Inc., 1992. [15] R. Cofiman and M. Wickerhauser, "Entropy-based Algorithms for Best Basis Selection," IEEE Transactions on Information Theory, Vol. 38, pp. 713-718, March 1992. [16] S.H. Haykin, (1986), Adaptive Filter Theory , Prentice Hall, Englewood Cliffs, N.J. [17] V.C. Chen, "Signal Detection and Estimation by Wavelet Transforms," Proceedings of ICASSP, 1992. [18] C. Herley and M. Vetterli, "Orthogonal Time- Varying Filter Banks and Wavelet Packets," IEEE Transactions on Signal Processing, Vol. 42, No. 10, October 1994. [19] R.L. de Queiroz, "Subband Processing of Finite Length Signals Without Border Distortions," Proceedings of IEEE International Conference on Acoustics, Speech and Signal Processing (ASSP), San Francisco, CA, pp. 613-616, May 1992. [20] L. Rabiner, Gold, Digital Filtering , Prentice Hall, XXX [21] I. Zisking and M. Wax, "Maximum Likelihood Localization of Multiple Sources by Alternating Projection," IEEE Transactions on Acoustic, Speech and Signal Processing, Vol. 36, No. 10, October 1988.

Claims: CLAIMS 1. A jamming suppression system which comprises: (a) a plurality of receivers for receiving electromagnetic radiations; (b) a separate notch filter coupled to each said receiver for filtering selected frequencies from electromagnetic radiations received by each said receiver; and (c) a spatial combiner responsive to the outputs of each of said notch filters for extracting essentially jam-free information from said electromagnetic radiations . 2. The system of claim 1 wherein said notch filters are adaptive notch filters for on-line selection of their filtering action. 3. The system of claim 1 wherein one of said notch filters is a master filter and the remainder of said notch filters are slave filters controlled by said master filter. 4. The system of claim 2 wherein one of said notch filters is a master filter and the remainder of said notch filters are slave filters controlled by said master filter. 5. The system of claim 1 wherein said spatial combiner provides spatial filtering action using a generalized sidelobe canceler that permits gain (relative to an isotropic antenna) to be preserved in the direction of one or more satellites while casting spatial mulls in the directions of inadvertent friendly or hostile jamming signals. 6. The system of claim 2 wherein said spatial filter provides spatial filtering action using a generalized sidelobe canceler that permits gain (relative to an isotropic antenna) to be preserved in the direction of one or more satellites while casting spatial mulls in the directions of inadvertent friendly or hostile jamming signals . 7. The system of claim 3 wherein said spatial filter provides spatial filtering action using a generalized sidelobe canceler that permits gain (relative to an isotropic antenna) to be preserved in the direction of one or more satellites while casting spatial mulls in the directions of inadvertent friendly or hostile jamming signals . 8. The system of claim 4 wherein said spatial filter provides spatial filtering action using a generalized sidelobe canceler that permits gain (relative to an isotropic antenna) to be preserved in the direction of one or more satellites while casting spatial mulls in the directions of inadvertent friendly or hostile jamming signals . 9. A jamming suppression system which comprises: (a) a plurality of receivers for receiving electromagnetic radiations; (b) a separate notch filter coupled to each said receiver for filtering selected frequencies from electromagnetic radiations received by each said receiver; and (c) a plurality of spatial combiners, each said spatial combiner responsive to the outputs of each of said notch filters for extracting essentially jam-free information from said electromagnetic radiations. 10. The system of claim 9 wherein said notch filters are adaptive notch filters for on-line selection of their filtering action. 11. The system of claim 9 wherein one of said notch filters is a master filter and the remainder of said notch filters are slave filters controlled by said master filter. 12. The system of claim 10 wherein one of said notch filters is a master filter and the remainder of said notch filters are slave filters controlled by said master filter. 13. The system of claim 9 wherein said spatial combiner provides spatial filtering action using a generalized sidelobe canceler that permits gain (relative to an isotropic antenna) to be preserved in the direction of one or more satellites while casting spatial mulls in the directions of inadvertent friendly or hostile jamming signals. 14. The system of claim 10 wherein said spatial combiner provides spatial filtering action using a generalized sidelobe canceler that permits gain (relative to an isotropic antenna) to be preserved in the direction of one or more satellites while casting spatial mulls in the directions of inadvertent friendly or hostile jamming signals. 15. The system of claim 11 wherein said spatial combiner provides spatial filtering action using a generalized sidelobe canceler that permits gain (relative to an isotropic antenna) to be preserved in the direction of one or more satellites while casting spatial mulls in the directions of inadvertent friendly or hostile jamming signals. 16. The system of claim 12 wherein said spatial combiner provides spatial filtering action using a generalized sidelobe canceler that, permits gain (relative to an isotropic antenna) to be preserved in the direction of one or more satellites while casting spatial mulls in the directions of inadvertent friendly or hostile jamming signals.

Inventors: BARRON KENNETH S, HOLMES JERRY D, REID ANTHONY, BARRON, KENNETH, S., HOLMES, JERRY, D., REID, ANTHONY
Applicants: RAYTHEON TI SYST INC, RAYTHEON TI SYSTEMS, INC.
Application year: 1997
Main classification: H04B1
IPC codes: G01S1, G01S5, H01Q3, H04B1, H04K3