TX beamformer in MIMO radar

Abstract

A method (100) for transmit beamforming in a multiple-input-multiple-output (MIMO) antenna for a radar system having N transmit antennas (16), the method (100) comprising: Acquiring a coding matrix (G) defining a desired field of view; Generating a transmission signal matrix (S) based on a singular value decomposition (SVD) of the coding matrix (G), wherein the columns of the transmission signal matrix (S) are transmission signal vectors formed from the singular vectors corresponding to the maximum singular values ​​of the coding matrix (G) based on the SVD, wherein the transmission signal vectors define spatial codewords; and Transmitting signals in sequences via the N transmit antennas (16) according to the transmit signal matrix (S), the sequences corresponding to the spatial codewords from the transmit signal vectors, each sequence being defined by a number of spatial codewords transmitted in a single repeat sequence interval, and wherein transmitting the spatial codewords according to the transmit signal matrix (S) enables beamforming of the transmitted signals into the desired field of view, wherein the coding matrix (G) includes a diagonal masking matrix (A) configured to mask an angular range of the field of view of the radar (10) to exclude spatial regions outside the desired field of view.

Description

INITIATIONThe present invention relates generally to radar systems, and more particularly to a multi-input multiple-output (MIMO) radar and method for implementing a coding scheme that enables transmit beamforming.Modern radar systems in use today use a multiple-input multiple-output (MIMO) concept that uses multiple antennas at the transmitter to transmit independent (orthogonal) waveforms and multiple antennas at the receiver to receive the radar echoes. In a "collated" MIMO radar configuration, the antennas in both the transmitter and the receiver are arranged so close together that each antenna views the same aspect of an object so that a point target is assumed. In the MIMO receiver, an adjusted filter bank is used to extract the orthogonal waveform components. When the orthogonal signals are transmitted from different antennas, the echoes of the individual signals carry independent information about the detected objects and the different propagation paths. Phase differences caused by different transmit antennas, as well as phase differences caused by different receive antennas mathematically form a virtual antenna array that enables a larger virtual aperture with fewer antenna elements. Conceptually, the virtual array is created by interleaving between each of the transmitter T x- and receiver R x- antenna elements, so that the elements in the virtual array represent T x- R x- pairs for each of the transmitter T x and receiver R x- antennas in the MIMO array. In the case of referenced MIMO antennas, a transmission arrangement with N transmitter antennas and a reception arrangement with M receiver antennas generates a virtual arrangement with MxN virtual receiver elements. In other words, the orthogonal waveforms are extracted by the matched filters at the receiver so that a total of MxN extracted signals are present in a virtual array.In addition to generating and transmitting individual orthogonal waveforms from each transmitter antenna, the transmitted signals may be orthogonalized using different encoding techniques. For example, each transmitter antenna may be configured to transmit a waveform with a different code. Thus, a transmit array transmits n orthogonal spatial codewords spanning the entire field of view of the radar, which generally ranges from 120° (+ / - 60° from the field of view) to 180° (+ / - 90° from the field of view). The codewords are transmitted across each of the n transmit antennas as a sequence of symbols. Since the received signal vector is a sum of the echo signals transmitted by all n transmit antennas, the number of symbols in a sequence (i.e., the number of transmitted codewords) must be equal to the number of transmit antennas, n, in order to achieve separation of the n transmission channels at the receiver. Consequently, as the number of transmit antennas increases, the length of the codewords and the repetition interval of each sequence also increase. However, as the repetition interval increases, there is ambiguity in the velocity estimate (i.e., Doppler frequency).U.S. Pat. No. 9,118,372 B2 discloses a method for MIMO transmission in which a base station in a wireless network transmits to a plurality of relay stations using a beamforming matrix, wherein the beamforming matrix is created by firstly extracting a column vector from the beamforming matrix of each relay station. An intermediate matrix is then created from these column vectors and a zero forcing method is used to process the intermediate matrix and generate the final beamforming matrix. DE 11 2015 006 626 T5 teaches a method for determining a precoding matrix for a MIMO transmitter, in which a first matrix expression is transformed into a second matrix expression, wherein the first matrix expression comprises a matrix inversion operation of a quadratic matrix having a rank equal to a number of antennas of the MIMO transmitter and the second matrix expression comprises a matrix inversion operation of a quadratic matrix having a rank equal to a number of receivers scheduled for the MIMO transmitter.SUMMARYIt is an object of the invention to provide an improved method for transmit beamforming in a MIMO antenna.This object is achieved by the method according to claim 1.According to one embodiment of the invention, a method for transmit beamforming in a multiple-input multiple-output (MIMO) antenna is provided for a radar system having N transmit antennas. The method includes detecting an encoding matrix defining a desired field of view, generating a transmit signal matrix based on singular value decomposition (SVD) of the encoding matrix, wherein the columns of the transmit signal matrix are transmit signal vectors formed from the singular vectors corresponding to the maximum singular values of the encoding matrix based on the SVD, wherein the transmit signal vectors define spatial codewords, and transmitting signals in sequences via the N transmit antennas according to the transmit signal matrix, wherein the sequences correspond to the spatial codewords from the transmit signal vectors, wherein each sequence is defined by a number of spatial codewords transmitted in a single repetition sequence interval, and wherein transmitting the spatial codewords according to the transmit signal matrix enables beamforming the transmitted signals into the desired field of view. The transmission of the spatial codewords spans the desired field of view.In one example, the number of columns in the transmit signal matrix is less than the N transmit antennas, where the number of columns in the transmit signal matrix corresponds to the number of spatial codewords transmitted in the single repetition sequence interval.In another example, the number of spatial codewords transmitted in a single repetition sequence interval is less than the N transmit antennas, wherein each spatial codeword in the transmit signal vectors has N symbols, and wherein the sequence of codewords transmitted via the N antennas is orthogonal, allowing separation of the transmit channels in the desired field of view.In another example, the transmit signal vectors are eigenvectors corresponding to the maximum eigenvalues of the encoding matrix, whereby the maximum energy from the transmitted signals can be directed to the desired field of view, wherein the transmit signal vectors have a dimension of N x 1.The encoding matrix further includes a diagonal masking matrix configured to mask an angular range of the field of view of the radar to exclude spatial regions outside of the desired field of view. In another example, the diagonal masking matrix is an N x N matrix with diagonal elements corresponding to N equally spaced regions in the field of view of the radar, where diagonal elements having a value of 1 indicate spatial regions in the desired field of view and diagonal elements having a value of 0 indicate spatial regions outside the desired field of view.In another example, the SVD of the encoding matrix is the product of an orthogonal matrix, a diagonal matrix, and a transposon matrix of the orthogonal matrix, wherein the orthogonal matrix is an N x N matrix with columns representing left singular vectors of the encoding matrix and diagonal elements in the diagonal matrix represent the singular values of the encoding matrix, wherein the columns of the transmit signal matrix formed from singular vectors of the encoding matrix are the singular vectors in the orthogonal matrix corresponding to the maximum singular values in the diagonal matrix.According to another embodiment of the invention, a method for transmit beamforming in a multiple-input multiple-output (MIMO) antenna is provided for a radar system having N transmit antennas. The method includes computing an encoding matrix as a function of a desired field of view, decomposing the encoding matrix to determine eigenvectors of the encoding matrix corresponding to the maximum eigenvalues of the encoding matrix, generating a transmit signal matrix based on the decomposition of the encoding matrix, wherein the transmit signal matrix has a dimension of n x q, wherein the q columns of the transmit signal matrix are the eigenvectors of the encoding matrix, and wherein each column defines a spatial codeword having n symbols that transmits signals in sequences via the n transmit antennas according to the transmit signal matrix, the sequences corresponding to the spatial codewords from the columns of the transmit signal matrix, wherein q spatial codewords are transmitted for each sequence, and wherein transmitting the q spatial codewords according to the transmit signal matrix enables beamforming of the transmitted signals into the desired field of view.According to yet another embodiment of the invention, a multiple-input multiple-output (MIMO) antenna is provided for a radar system. The antenna includes an antenna array having n transmit antennas and a transmitter configured to acquire an encoding matrix defining a desired field of view, a transmit signal matrix based on singular value decomposition (SVD) of the encoding matrix, wherein the columns of the transmit signal matrix are transmit signal vectors formed from the singular vectors corresponding to the maximum singular values of the encoding matrix based on the SVD, the transmit signal vectors defining spatial codewords, and transmit signals in sequences via the N transmit antennas according to the transmit signal matrix, wherein the sequences correspond to the spatial codewords from the transmit signal vectors, wherein each sequence corresponds to a number of spatial codewords transmitted in a single repetition sequence interval, and wherein transmitting the spatial codewords according to the transmit signal matrix achieves beamforming of the transmitted signals into the desired field of view.In one example, the number of spatial codewords transmitted in a single repetition sequence interval is less than the n transmit antennas, and wherein the encoding matrix includes a diagonal masking matrix configured to mask an angular range of the field of view of the radar to exclude spatial ranges outside the desired field of view.BRIEF DESCRIPTION OF THE DRAWINGSOne or more embodiments of the invention will be described below in connection with the accompanying drawings, wherein like numerals denote like elements, and wherein: FIG. 1 illustrates an exemplary architecture of a MIMO radar system according to an exemplary embodiment of the present disclosure; and FIG. 2 illustrates a flow diagram illustrating an example method for implementing the disclosed encoding scheme, according to an embodiment of the present disclosure.DETAILED DESCRIPTIONAs needed, embodiments are broadly disclosed herein. It should be understood, however, that the disclosed embodiments are merely illustrative of various and alternative forms and combinations thereof. As used herein, the term "exemplary" indicates an expansive use of embodiments as representations, copies, models, or patterns. The figures are not necessarily to scale, and some features may be larger or smaller in size to illustrate the details of particular components. In other instances, components, systems, materials, or methods will not be described in detail in order not to obscure the present disclosure. Accordingly, specific details disclosed herein are not to be interpreted as limiting the construction and function, but merely as a basis for the claims, as well as a representative basis for teaching one skilled in the art. While the approach and methodology are described below in terms of vehicles in some cases, it will be understood by those skilled in the art that the application in the vehicle industry is merely exemplary and that the concepts disclosed herein may also be applied to any other suitable radar detection systems, such as flight protection, nautic navigation, and weapon guidance systems, to name a few. The use of the term "vehicle" is also to be broadly construed and includes not only passenger cars but all other vehicles, including, but not limited to, motorbikes, trucks, SUVs, recreational vehicles, watercraft and aircraft.The system and method described below is directed to a multiple-input multi-output (MIMO) radar and method for implementing a coding scheme that enables transmit beamforming. FIG. 1 illustrates an example architecture for a MIMO radar 10 with which the disclosed method may be implemented. While the approach and methodology described herein relate to the radar configuration illustrated in FIG. 1, those skilled in the art will appreciate that the radar 10 is merely exemplary and is provided in many respects for ease of explanation. It should also be noted that the transmitter, receiver and antenna arrangements shown in FIG. 1 are too much simplified for ease of explanation.The MIMO radar 10 includes a transmitter 12 communicatively coupled to a transmit array 14 having N transmit antenna elements 16 configured to generate a field of view of the sensor that monitors a particular zone of interest. The transmitting arrangement 14 is configured to transmit electromagnetic signals 18 reflected from remote objects or targets 20 within the field of view of the radar sensor. In one embodiment, the MIMO radar 10 is configured to transmit and detect radar signals in a direction perpendicular to the view of the MIMO antenna array. That is, the line of sight of each MIMO antenna is generally normal to a surface of the antenna array.The transmitter 12 may be a unitary module or may include a plurality of other modules or sub-modules configured to generate electromagnetic signals for transmission via the transmit antennas 16 according to the method and algorithms disclosed herein. In one embodiment, transmitter 12 includes without limitation waveform generators, oscillators, amplifiers, mixers, combiners, filters, converters, and processors. For example only, the waveform generator may be configured to generate pulses having a different pulse width, a different waveform type, and different pulse sequence intervals (PRI) within a given coherent processing interval (CPI). The pulses may then be digitized by a digital-to-analog (D / A) converter and converted to a radio frequency carrier with a boost converter. The upconverter consists of intermediate frequency (IF) and / or high frequency (RF) oscillators, filters and synchronizing circuits. A transmit amplifier generates a transmit signal that is provided to a circulator coupled to one or more of the transmit antennas 16.Return signals 22 reflected from the objects 20 are received as echoes by a receiver arrangement 24 having M receiving antenna elements 26. The return signals 22 are processed by a signal processing module 28 configured to extract information from the echoes relating to the detected targets, such as range, azimuth angle, and range rate data including Doppler measurements. The signal processing module 28 may be a unitary module, or may include a plurality of other modules or sub-modules configured to receive and process radar echo signals. In one embodiment, the signal processing module 28 includes, without limitation, amplifiers, mixers, oscillators, combiners, filters, and converters. The functions of the signal processing module 28 may vary, but generally include performing various filtering, amplifying, converting and digitizing functions, as well as analyzing various features of the signal to determine signal characteristics such as phase, frequency and amplitude. As will be understood by those skilled in the art, the techniques used to obtain this information from the signals may vary, including, without limitation, in-phase, quadrature, and Fourier transform frequency domain analyses. In one embodiment, the signal processing module 28 may also include, without limitation, pulse compression and noise suppression (e.g., Doppler filtering) components.In a particular embodiment, the MIMO radar 10 may be implemented on a vehicle, wherein the transmitter 12 and the signal processing module 28 may be implemented as a vehicle control module. The control module may include any of a variety of electronic processing devices, memory devices, input / output (I / O) devices, and / or other known components, and may perform various control and / or communication related functions. Depending on the embodiment, the control module may be a stand-alone vehicle electronics module (e.g., object detection controller, safety controller, etc.), it may be integrated or incorporated into another vehicle electronics module (e.g., park assist module, brake control module, etc.), or it may be part of a larger network or system (e.g., traction control [TCS], electronic stability control [ESC], anti-lock braking system [ABS], driver assistance system, adaptive cruise control system, lane keeping warning system, etc.) to cite only a few possibilities. The control module is not limited to any particular embodiment or arrangement.FIG. 2 illustrates a flow diagram illustrating an example method 100 for implementing an encoding scheme that enables transmit beamforming with the MIMO radar 10 described above compared to FIG. 1. It should be appreciated that operations of the method 100 need not necessarily be presented in a particular order, and that performance of some or all of the operations may be possible and contemplated in a different order. The method 100 begins at step 102 with acquiring an encoding matrix G defining a desired field of view (FOV). When implemented on a vehicle, the disclosed MIMO radar may be a long-range radar, wherein the desired FOV is relatively narrow, which may be less than 30° (+ / -15°) in a non-limiting example. The encoding matrix G is a function of the desired FOV and may be preconfigured or generated during operation of the radar 10. In any event, the encoding matrix is known to transmitter 14.In one embodiment, the encoding matrix is G=F H ΛF, where F is a beamforming matrix of dimension NxN corresponding to N transmit antenna elements (e.g., a Bartlett beamforming matrix or discrete Fourier transform (DFT) matrix), F H is a conjugate transpose of F, and A is an NxN diagonal matrix with diagonal elements having a value of 1 or 0. The diagonal elements correspond to N evenly spaced areas throughout the FOV of about + / -90 degrees. The diagonal elements having a value of 1 indicate the desired FOV (i.e., the desired FOV). The diagonal elements with a value of 0 indicate a spatial range that is outside the desired FOV. Thus, diagonal matrix A is a diagonal masking matrix configured to mask an angular range of the field of view of the radar to exclude spatial regions outside of the desired field of view. In another embodiment, diagonal matrix A may have binary values (i.e., 0 / 1), where diagonal matrix A may generally have floating values representing high values in the FOV of interest and low values (falling values) in the range that is outside the FOV of interest.In one embodiment, acquiring the encoding matrix G may include computing the encoding matrix as a function of the desired FOV and decomposing the encoding matrix G at the transmitter 12 using known decomposition techniques such as eigenvalue or singular value decomposition (SVD). Using the SVD technique, the decomposition of G = UΓU is H, where U is a NxN matrix with columns that are the left singular vectors of G, where U H is a conjugated transpose of U and Γ is a diagonal matrix with singular values of G. The SVD decomposition of the coding matrix G generates a set of eigenvectors of G from which a smaller subset is selected, such that the smaller subset corresponds to the maximum eigenvalues of G.At step 104, transmitter 12 generates a transmit signal matrix S based on the decomposition of encoding matrix G at step 102. In one embodiment, the transmit signal matrix comprises S=[S 0, S i,... S Q] has a dimension of NxQ, where Q is the number of codewords transmitted via the N transmit antennas in a single repetition interval, S i is a transmit signal vector of dimension Nx1 representing a spatial codeword transmitted at symbol time index i, where i = 0 - Q. The columns of the transmit signal matrix S are set as Q single vectors corresponding to the maximum singular Q values of the encoding matrix G of step 102, and in particular the individual Q vectors in U corresponding to the maximum singular Q values in Γ. In other words, the columns of the transmit signal matrix S are the eigenvectors corresponding to the maximum eigenvalues of the encoding matrix G. By using the maximum eigenvectors corresponding to the maximum eigenvalues, the maximum energy from the transmitted signals can be directed to the desired FOV. The transmit signal matrix S is configured such that Q is less than the N transmit antennas. In other words, the repetition sequence interval of the spatial codewords is smaller than the number of the N transmit antennas, wherein each spatial codeword in the transmit signal vectors has N symbols. In this way, the time sequence of each N transmit antenna is shorter (i.e., the number of spatial codewords in a period is shorter), resulting in less ambiguity in the target Doppler frequency estimate. The Doppler frequency ambiguity is proportional to the code sequence period. As the sequence is shorter, ambiguity in the Doppler estimate decreasesAt step 106, signals are transmitted in sequences via the N transmit antennas according to transmit signal matrix S, wherein the sequences correspond to the spatial codewords from the transmit signal vectors (i.e., the column vectors of S), wherein each sequence is defined by the number of spatial codewords transmitted in a single repetition sequence interval, and wherein transmitting the spatial codewords according to transmit signal matrix S enables beamforming of the transmitted signals into the desired FOV. The sequence of codewords transmitted over the N antennas is orthogonal and allows separation of transmission channels in the desired field of view.The transmission signals transmitted according to the transmission signal matrix S are received as echoes by the receiver arrangement 24 and processed by the signal processing module 28. The noise free receive signal vector Y = HθGS is an MxQ matrix, where Hθis a channel coefficient matrix of dimension MxNthat represents all channel coefficients (i.e., channel response) between the transmit and receive antenna elements. More specifically, where a Tx( θ) is a vector of length Nx1 which is the channel complex coefficient (i.e., the channel response) between each transmit antenna element and the reflection at angle θ, where H represents the conjugate transpose, and a Rx( θ) is a vector of length Mx1 which is the channel complex coefficient (i.e., the channel response) between each receive antenna element and the reflection at angle θ.One skilled in the art may further understand that in combination with the embodiments, units, and algorithm steps disclosed in this specification may be implemented by electronic hardware, computer software, or a combination thereof. In order to clearly describe interoperability between the hardware and the software, the compositions and steps of the individual examples according to functions have generally been described above. Whether the functions are executed by hardware or software is dependent on specific applications and design boundary conditions of the technical solutions. One skilled in the art may use various methods for implementing the described functions for each individual application, but it should not be taken into account that the implementation exceeds the scope of the present patent application.Steps of methods or algorithms described in the embodiments disclosed in this specification may be implemented by hardware, a software module executed by a processor, or a combination thereof. The software module may reside in random access memory (RAM), memory, read only memory (ROM), electrically programmable ROM, electrically erasable programmable ROM, register, hard disk, removable disk, CD-ROM, or any other form of storage medium known in the art.It is understood that the foregoing is a description of one or more embodiments of the invention. The invention is not limited to the particular embodiment(s) disclosed herein, but is defined solely by the following claims. Moreover, the statements made in the foregoing description relate to particular embodiments and are not to be construed as limitations on the scope of the invention or on the definition of the terms used in the claims, except where a term or term has been expressly defined above. Various other embodiments and various changes and modifications to the identified embodiment(s) will be apparent to those skilled in the art. All such other embodiments, changes and modifications should be understood within the scope of the appended claims.As used in this specification and claims, the terms "for example," "for example," "e.g., "like," and "like," and the verbs "comprise," "have," "include," and their other verb forms, when used in connection with a listing of one or more components or other items, are each to be construed as open-ended, meaning that the listing is not considered to exclude other, additional components or elements. Other terms are to be interpreted in their broadest reasonable sense unless they are used in a context that requires a different interpretation.

Claims

A method (100) for transmit beamforming in a multiple-input multiple-output (MIMO) antenna for a radar system having N transmit antennas (16), the method (100) comprising: acquiring a coding matrix (G) defining a desired field of view; generating a transmit signal matrix (S) based on singular value decomposition (SVD) of the coding matrix (G), wherein the columns of the transmit signal matrix (S) are transmit signal vectors formed from the singular vectors corresponding to the maximum singular values of the coding matrix (G) based on the SVD, wherein the transmit signal vectors define spatial codewords; transmitting signals in sequences via the N transmit antennas (16) according to the transmit signal matrix (S), the sequences corresponding to the spatial codewords from the transmit signal vectors, each sequence being defined by a number of spatial codewords transmitted in a single repetition sequence interval, and wherein transmitting the spatial codewords according to the transmit signal matrix (S) enables beamforming of the transmitted signals into the desired field of view, wherein the encoding matrix (G) includes a diagonal masking matrix (A) configured to mask an angular range of the field of view of the radar (10) to exclude spatial ranges outside the desired field of view.The method (100) of claim 1, wherein transmitting the spatial codewords spans the desired field of view.The method (100) of claim 1, wherein the number of columns in the transmit signal matrix (S) is less than the N transmit antennas (16).The method (100) of claim 1, wherein the number of columns in the transmit signal matrix (S) corresponds to the number of spatial codewords transmitted in the single repetition sequence interval.The method (100) of claim 1, wherein the number of spatial codewords transmitted in a single repetition sequence interval is less than the N transmit antennas (16).The method (100) of claim 1, wherein each spatial codeword in the transmit signal vectors comprises N symbols.The method (100) of claim 1, wherein the diagonal masking matrix is an N x N matrix having diagonal elements corresponding to N equally spaced regions in the radar field of view, wherein diagonal elements having a value of 1 indicate spatial regions in the desired field of view and diagonal elements having a value of 0 indicate spatial regions outside the desired field of view.The method (100) of claim 1, wherein the SVD of the coding matrix (G) is the product of an orthogonal matrix, a diagonal matrix, and a transposon matrix of the orthogonal matrix, wherein the orthogonal matrix is an N x N matrix with columns representing left singular vectors of the coding matrix (G), and diagonal elements in the diagonal matrix represent the singular values of the coding matrix (G).The method (100) of claim 8, wherein the columns of the transmit signal matrix (S) formed of singular vectors of the encoding matrix (G) are the singular vectors in the orthogonal matrix corresponding to the maximum singular values in the diagonal matrix.

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