Processing method for coherent MIMO radar using DDMA waveforms

Abstract

A method for processing coherent MIMO radar processing DDMA waveforms includes: generating waveforms on transmitters, the waveforms, modulo the pulse repetition frequency, being identical from one transmitter to the next, to within a phase ramp specific to each transmit path; generating, for at least one receiver, a Range-Doppler representation of echoes of transmitted waveforms, where, for each receiver, echoes of a transmitter occupy at least one frequency cell in the Doppler spectrum, each signal band specific to a transmitter, placement of the signal bands in the Doppler spectrum being determined by phase ramp applied to each transmitter, the waveforms generated to leave a portion of Doppler spectrum between two signal bands unoccupied; identifying the transmitter corresponding to each signal band, due to Range-Doppler representation of echoes of transmitted waveforms. The method is suitable for the millimetre band, automotive or aircraft radar, for detection of target relative to the carrier.

Claims

1. A method for processing coherent MIMO radar processing DDMA waveforms and comprising NT.sub.X transmitters and NR.sub.X receivers, comprising the steps: a) generating waveforms one or more of the NT.sub.X transmitters, the waveforms, modulo the pulse repetition frequency F.sub.R, being identical from one transmitter to the next, to within a phase ramp specific to each transmitter; b) generating, for at least one receiver, a range-Doppler representation of the echoes of the transmitted waveforms, where, for each receiver, the echoes of a transmitter over a plurality of range cells occupy at least one frequency cell in the Doppler spectrum, called signal band, each signal band being specific to one of the transmitters, the placement of the signal bands in the Doppler spectrum being determined according to the phase ramp applied to each transmitter, the waveforms being generated so as to leave a portion of the Doppler spectrum between two signal bands unoccupied; c) identifying the transmitter corresponding to each signal band, on the basis of the Range-Doppler representation of the echoes of the transmitted waveforms.

2. The method according to claim 1, wherein step c) comprises the sub-steps of: calculating a signal referred to as an echo signal obtained by summing the power of the echoes of each signal band over all of the range cells; generating a signal referred to as a pattern signal, over the same number of frequency cells as the Range-Doppler representation of the echoes of the transmitted waveforms, the transmitters being numbered in ascending order according to the phase ramp applied to the transmitter, the pattern signal being generated such that the transmitter are sorted, according to the ascending order, while taking into account the unoccupied portion of the Doppler spectrum; calculating a cross-correlation signal between the echo signal and the pattern signal; identifying the transmitter corresponding to each signal band on the basis of the maximum of the cross-correlation signal.

3. The method according to claim 2, wherein, in the identification step, the position of the maximum of the cross-correlation signal corresponds to the echoes of the first transmitter in the Doppler spectrum, the first transmitter being defined as being that for which the phase ramp is zero.

4. The method according to claim 2, wherein the unoccupied portion of the Doppler spectrum is obtained by switching off one of the NT.sub.X transmitters, the pattern signal being non-zero for each frequency cell k*Fi/NT.sub.X, where k={0, . . . , NT.sub.X−1}, and zero in the other frequency cells.

5. The method according to claim 2, wherein the unoccupied portion of the Doppler spectrum is obtained by determining the phase ramps such that the signal bands are spaced apart by F.sub.R/NT.sub.x2 frequency cells in the Doppler spectrum, where NT.sub.X2 is an integer such that NT.sub.X2>NT.sub.X, the pattern signal being non-zero for each frequency cell k*F.sub.R/NT.sub.x2, for k={0, . . . , NT.sub.X−1}, and zero in the other frequency cells.

6. The method according to claim 1, wherein the Range-Doppler representation of the echoes is obtained by means of a fast Fourier transform (FFT) on a plurality of acquisition points, the number of acquisition points being supplemented by adding zeros in order to obtain N.sub.FFT points in total, N.sub.FFT being determined such that it is an integer multiple of the number of transmitters NT.sub.X, if NT.sub.X is not a power of two.

7. The method according to claim 1, wherein the Range-Doppler representation of the echoes is obtained by means of a fast Fourier transform (FFT) on pulse sequences coded over a coherent processing interval (CPI), the number of pulses per coherent processing interval (CPI) being adjusted so as to make it a multiple of the number of transmitters.

8. The method according to claim 1, comprising an additional step of digital forming processing, in which the waveforms are combined, in a weighted manner, with the echoes from the various receivers.

9. A MIMO radar, configured to implement the method according to claim 1.

10. The method for processing coherent MIMO radar processing DDMA waveforms and comprising NT.sub.X transmitters and NR.sub.X receivers, comprising the steps of a) generating waveforms one or more of the NT.sub.X transmitters, the waveforms, modulo the pulse repetition frequency F.sub.R being identical from one transmitter to the next, to within a phase ramp specific to each transmitter; b) generating, for at least one receiver, a Range-Doppler representation of the echoes of the transmitted waveforms, where, for each receiver, the echoes of a transmitter over a plurality of range cells occupy at least one frequency cell in the Doppler spectrum, called signal band, each signal band being specific to one of the transmitters, the placement of the signal bands in the Doppler spectrum being determined according to the phase ramp applied to each transmitter, the waveforms being generated such that the spacing between at least two consecutive signal bands is different from the spacing between the other consecutive signal bands; c) identifying the transmitter corresponding to each signal band, on the basis of the Range-Doppler representation of the echoes of the transmitted waveforms.

11. The method according to claim 10, wherein the spacings between two consecutive signal bands are all different.

12. The method according to claim 10, wherein the signal bands are spaced apart according to a predetermined spacing law.

13. The method according to claim 10, wherein the spacing between each of the signal bands is determined such that the average of the spacings is equal to F.sub.R/NT.sub.X.

14. The method according to claim 10, wherein the Range-Doppler representation of the echoes is obtained by means of a fast Fourier transform (FFT) on a plurality of acquisition points, the number of acquisition points being supplemented by adding zeros in order to obtain N.sub.FFT points in total, N.sub.FFT being determined such that the spacing between two consecutive bands is an integer multiple of F.sub.R/N.sub.FFT.

15. The method according to claim 10, wherein the Range-Doppler representation of the echoes is obtained by means of a fast Fourier transform (FFT) on pulse sequences coded over a coherent processing interval (CPI), the number of pulses N.sub.REC per coherent processing interval (CPI) being determined such that the spacing between two consecutive bands is an integer multiple of F.sub.R/N.sub.REC.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features, details and advantages of the invention will become apparent upon reading the description provided with reference to the appended drawings, which are given by way of example and in which, respectively:

(2) FIG. 1, described above, shows a DDMA waveform according to the prior art;

(3) FIG. 2, described above, shows the carrier vector of the radar, with respect to a target;

(4) FIG. 3 shows the diagrams illustrating the MIMO radar processing method according to a first embodiment of the invention, in which one of the transmitter is switched off;

(5) FIG. 4 shows the diagrams illustrating the MIMO radar processing method according to a second embodiment of the invention, in which the spacing between the bands is modified;

(6) FIG. 5 shows the diagrams illustrating the MIMO radar processing method according to a third embodiment of the invention, in which the spacing between the bands is non-uniform.

DETAILED DESCRIPTION

(7) According to the first and second embodiments of the invention, the method comprises three steps.

(8) In a first step, the radar, which comprises NT.sub.X transmitters, generates waveforms on at most NT.sub.X transmitters. According to a first embodiment, described in detail below, one of the NT.sub.X transmitter is switched off. According to the second embodiment, also described in detail below, all of the transmitters are activated. The principle of the DDMA waveform, as explained above, consists in transmitting, in each pulse repetition interval, radar pulses that are identical from one transmitter to the next, to within a phase ramp specific to each transmitter. The phase ramp is created by a phase-shifter, which is a relatively straightforward component to integrate. Next, each of the received echoes is processed by a matched filter bank, namely one filter per waveform.

(9) The processing method according to the invention may be implemented at the level of one of the receiver, or of a plurality thereof, or even of all of them. Application to digital beamforming specifically requires associating all of the receivers with the transmitters, but this application is not limiting. In general, the method according to the invention makes it possible, in a MIMO context, to associate, even for just one receiver, each signal band with one of the transmitters.

(10) For this, in the second step b), a Range-Doppler representation of the echoes of the transmitted waveforms is generated. As illustrated by FIG. 1, described above, the Range-Doppler representation comprises the frequency cells (or the number of FFT, fast Fourier transform, points), on the abscissa and the range cells on the ordinate. Each signal band is centred on the frequency cell of the Doppler shift of each transmitter. The signal bands are thus spaced apart by F.sub.R/NT.sub.X, where F.sub.R is the pulse repetition frequency (inverse of the pulse repetition interval).

(11) Because of the very nature of the DDMA waveform, which introduces a phase shift for each of the transmitter, this phase shift can be found in the Doppler spectrum, in the spacing of the signal bands. Each of the received echoes is processed by a matched filter bank, namely one filter per waveform. On reception, the processor of the MIMO radar removes the phase shift which was applied on transmission.

(12) The particularity of the method according to the invention is that the waveforms are generated such that a portion of the Doppler spectrum is left unoccupied between two signal bands. For a DDMA MIMO radar of the prior art, the phase shifts applied are determined such that the signal bands are all uniformly distributed in the Doppler spectrum. However, by leaving a portion of the spectrum unoccupied, identification of the transmitter corresponding to each of the signal bands is facilitated, as demonstrated below (step c) of the method).

(13) Advantageously, step c) of the processing method comprises a first sub-step of calculating a signal referred to as an echo signal. The echo signal is obtained by summing, for each signal band, the power of the echoes over all of the range cells. Thus, a signal representative of the energy per signal band is obtained. The echo signal is thus obtained directly and easily (namely without computational complexity) from the Range-Doppler representation.

(14) Next, in a second sub-step, a signal referred to as a pattern signal is generated over the same number of frequency cells as the Range-Doppler representation of the echoes of the transmitted waveforms. The pattern signal is a representation, in the order of the transmitters, of the shift caused by the phase ramp applied to each transmitter. Thus, in the MIMO radar, the phase ramp which is applied is associated with each transmitter. Next, a number is assigned to the transmitter: no 1 for the transmitter that has a zero phase ramp, no 2 for the first phase ramp, and so on. Thus, a number from 1 to NT.sub.X is assigned to each transmitter. The pattern signal is a binary signal, the value of which is 1 (or any other value other than zero, as long as this value is the same for all of the transmitters) for each frequency cell corresponding to the shift caused by the slope of the various phase laws and 0 elsewhere. Additionally, the pattern signal is also set to 0 in the unoccupied portion of the Doppler spectrum.

(15) A cross-correlation signal between the echo signal and the pattern signal is then calculated. The cross-correlation signal corresponds to the cross-correlation function between the echo signal and the pattern signal. Each of the two signals may be considered as being a “comb”-type signal. The cross-correlation signal is maximum when the two combs encounter one another.

(16) The position of the maximum of the cross-correlation signal indicates the position, in the Doppler spectrum, of the first transmitter. The position of the other transmitters is deduced by shifting according to the theoretical spacing between the signal bands, modulo the pulse repetition frequency, or modulo the number of FFT points.

(17) The processing described is performed systematically, on each burst of the radar signal.

(18) FIG. 3 illustrates a first embodiment of the processing method according to the invention. According to the first embodiment, the portion of the spectrum that is unoccupied is produced by switching off one of the transmitter. Specifically, MIMO radar generally has lots of transmitters, for example ten or so, and switching off one of the transmitter affects the radar picture only by ten or so percent, in the case that the radar comprises ten transmitters.

(19) In the example of FIG. 3, the MIMO radar has NT.sub.X=6 transmitters, and the fifth transmitter is switched off.

(20) The switching off of one of the transmitter may be performed in various ways. For example, amplitude control, used in active antennas of radar for amplitude and phase control, may apply a zero amplitude to the transmitter to be switched off. This solution has the advantage of being able to be reconfigured on mission, by modifying the amplitude control command. As an alternative, the output of the power transmitters of the transmitter to be reached is not connected, which constitutes a permanent solution, and one which can be modified only between two missions. Another possibility would be to add a controlled switch upstream of each power transmitter.

(21) The choice of the transmitter to be switched off may be completely random, when the number of the transmitter that is switched off is known. The choice of path has no effect on the outcome of the process.

(22) The echo signal is obtained by summing the powers of each range cell for each signal band. In the Doppler spectrum, all of the echoes of the transmitters are spaced apart by F.sub.R/NT.sub.X, except, by definition, in the band where the transmitter has been switched off. In FIG. 3, the echoes are received over 120 FFT points, without this value limiting the scope of the present invention.

(23) The pattern signal is represented over the same number of frequency cells as the Doppler spectrum of the received echoes. The pattern signal is non-zero (it has a value of 1 for example) for each frequency cell k*F.sub.R/NT.sub.X, where k={0, . . . , NT.sub.X−1}, corresponding to a signal band, with the exception of the signal band of the switched-off transmitter. The pattern signal is also zero in the other frequency cells. In the example of FIG. 3, the fifth transmitter (from among the six transmitters) is switched off. Thus, the pattern signal is a comb that has a value other than 0 in the 0, F.sub.R/6, 2*F.sub.R/6, 3*F.sub.R/6 and 5*F.sub.R/6 cells, and zero elsewhere.

(24) Lastly, the cross-correlation signal between the echo signal and the pattern signal is calculated. In FIG. 3, the cross-correlation signal, which should be read in conjunction with the Doppler spectrum of the received echoes, is maximum at a position corresponding, in the Doppler spectrum, to the signal band of the first transmitter (namely the second signal band from the right of the spectrum in the example of FIG. 3). Thus, the second signal band from the right corresponds to the echoes from the first transmitter. Next, the position of the other transmitters may be deduced by simply shifting from left to right, circularly, in relation to the phase ramp. In the example of FIG. 3, the first signal band from the right corresponds to the echoes of the second transmitter, the first band on the left corresponds to the echoes of the third transmitter, and so on.

(25) Thus, the signals from each of the transmitters is associated in the Doppler spectrum, without using information on the radial velocity of the carrier, which corresponds to the projection of the speed of the carrier onto the aiming axis.

(26) The first embodiment described above is particularly suitable for DDMA MIMO radar having a high number of transmitters, ten or so for example. Thus, the radar efficiency is affected little by the switching off of one transmitter.

(27) Switching off one of the transmitter may however prove to be detrimental from the point of view of radar efficiency when the MIMO radar has few transmitters. For example, if the MIMO radar comprises only four transmitters, switching off one of the transmitter affects the radar picture by about 25%, which does not represent optimal use of the radar.

(28) The second embodiment of the processing method according to the invention also makes it possible to associate the transmitters with each of the signal bands. For this, the phase ramps are generated such that the signal bands are spaced apart by F.sub.R/NT.sub.X2 frequency cells in the Doppler spectrum, where NT.sub.X2 is an integer such that NT.sub.X2>NT.sub.X. For example, NT.sub.X2=NT.sub.X+1. In the example of FIG. 4, NT.sub.X2=NT.sub.X+2, with NT.sub.X=6 and NT.sub.X2=8.

(29) The spacing thus generated also creates an empty space in the Doppler space, like for the first embodiment. The Doppler spectrum illustrated in FIG. 4 shows six signal bands spaced apart by F.sub.R/8 in the Range-Doppler representation of the echoes of the transmitted waveforms.

(30) Like for the first embodiment, an echo signal is created. In the second embodiment, the value of the echo signal is the sum of the powers of the echoes received over all of the range cells of a signal band. The echo signal therefore has NT.sub.X positive values, spaced apart by F.sub.R/NT.sub.X2, over N.sub.FFT frequency cells.

(31) Additionally, the pattern signal is non-zero (it has a value of 1 for example) for each frequency cell k*F.sub.R/NT.sub.X2, for k={0, . . . , NT.sub.X−1}, and zero in the other frequency cells. The pattern signal is represented over the same number of frequency cells as the Doppler spectrum of the received echoes. In the example of FIG. 4, the pattern signal takes the value “1” for 0 on the abscissa, and for each frequency cell spaced apart by F.sub.R/NT.sub.X2, and the value “0” elsewhere.

(32) The cross-correlation signal corresponds to the cross-correlation function between the echo signal and the pattern signal. Its maximum indicates the first transmitter, namely that for which the phase ramp applied on transmission is zero.

(33) In the example of FIG. 4, by superposing the cross-correlation signal and the Doppler spectrum, it can be seen that the maximum of the cross-correlation signal corresponds to the leftmost signal band in the Doppler spectrum. The second leftmost band corresponds to the second transmitter path, and so on. Like for the first embodiment, the empty space created in the Doppler spectrum makes it possible to identify the relative positioning of the signals, namely which transmitter they are from, without knowing the radial velocity of the carrier with respect to the target.

(34) The spacing, although uniform, between the DDMA signal bands (having a value of F.sub.R/NT.sub.X or F.sub.R/NT.sub.X2, according to the first or second embodiment), is not always equal to a whole number of frequency cells. This situation arises if NT.sub.X or NT.sub.X2 (according to the second embodiment) is not a power of two, while the number of points on which the FFT is performed is generally a power of two. In this case, one option would be to take the closest frequency cell, but one and the same echo from various transmitters would be sampled differently from one signal band to the next, which would introduce unwanted amplitude modulation, and therefore a worse DBF result. The other solution would be to interpolate the signals so as to put the signals onto a sampling grid common to the various DDMA channels, or equivalently, to perform an inverse Fourier transform, numerically apply the phase ramp opposite that used on transmission and then redo the direct Fourier transform so as to select the samples around the zero frequency cell. All of these operations result in obtaining defect-free DBF, but at the cost of high computational complexity.

(35) To overcome this, it is proposed to supplement the number of acquisition points by adding zeros in the time domain (the “zero padding” technique), until the number of FFT points N.sub.FFT is an integer multiple of the number of transmitters NT.sub.X.

(36) For example, if there are NT.sub.X=12 transmitters and N.sub.REC=512 points on which FFT is initially envisaged (it is recalled that T.sub.E=1/c.sub.F=N.sub.REC/F.sub.R), an FFT of size N.sub.FFT=540 must be performed, which is a multiple of NT.sub.X=12.

(37) As an alternative, the number of pulses per coherent processing interval (CPI) is adjusted to make it a multiple of the number of transmitters.

(38) For example, with NT.sub.X=12, instead of using N.sub.REC=512, N.sub.REC=480 or N.sub.REC=540 will be taken, which makes it possible to perform an FFT without zero padding while remaining a multiple of NT.sub.X.

(39) FIG. 5 illustrates a third embodiment of the invention.

(40) Step a) of the method according to the third embodiment of the invention is identical to step a) of the first two embodiments.

(41) In step b), a Range-Doppler representation of the echoes of the transmitted waveforms is generated for at least one receive path. For each receiver, the echoes of a transmitter over a plurality of range cells occupy at least one frequency cell in the Doppler spectrum (signal band). Each signal band is specific to one of the transmitter. The placement of the signal bands in the Doppler spectrum is determined according to the phase ramp applied to each transmitter. What more particularly characterizes the third embodiment is the spacing between each of the signal bands: the spacing between each of the signal bands is non-uniform: there are at least two signal bands whose spacing (between these two bands) is different from the spacing between the other signal bands. In particular, all of the spacings between two consecutive signal bands are different, but this is not a required condition. This non-uniform spacing makes it possible to easily identify to which transmitter the echoes correspond, without it being necessary to switch off one of the transmitters. This embodiment therefore does not affect the radar efficiency.

(42) The third embodiment comprises a third step c) of identifying the transmitter corresponding to each signal band, on the basis of the Range-Doppler representation of the echoes of the transmitted waveforms. This step is identical to step c) of the first two embodiments.

(43) The Doppler spectrum illustrated in FIG. 5 shows six signal bands, the spacing of which is non-uniform in the Range-Doppler representation of the echoes of the transmitted waveforms. In FIG. 5, the spacing varies from F.sub.R/8 to F.sub.R/6; these values are given only by way of example.

(44) In the third embodiment, the value of the echo signal is the sum of the powers of the echoes received over all of the range cells of a signal band. The echo signal therefore has NT.sub.X positive values.

(45) Additionally, the pattern signal is non-zero (it has a value of 1 for example) for each frequency cell f(k)*F.sub.R/NT.sub.X, for k={0, . . . , NT.sub.X−1}, and zero in the other frequency cells, and f( ) is a function representative of the non-uniform spacing. The function f( ) may therefore define a spacing law. For example, the spacing law may be linear or logarithmic. The pattern signal is represented over the same number of frequency cells as the Doppler spectrum of the received echoes.

(46) Advantageously, a spacing between each of the signal bands is determined such that the average of the spacings is equal to F.sub.R/NT.sub.X.

(47) The cross-correlation signal corresponds to the cross-correlation function between the echo signal and the pattern signal. Its maximum indicates the first transmitter, namely that for which the phase ramp applied on transmission is zero.

(48) In the example of FIG. 5, by superposing the cross-correlation signal and the Doppler spectrum, it can be seen that the maximum of the cross-correlation signal corresponds to the leftmost signal band in the Doppler spectrum. The second leftmost band corresponds to the second transmitter, and so on.

(49) Like for the first two embodiments, it is also possible to supplement the number of FFT acquisition points by adding zeros (“zero padding”) up to N.sub.FFT points, or to adjust the number of pulses N.sub.REC per coherent processing interval (CPI). In this third embodiment with non-uniform spacing, le number of FFT points N.sub.FFT, or, respectively, the number of pulses N.sub.REC, should be such that the frequency difference between two consecutive bands, related to the phase ramps of the DDMA modulation, is a whole number of frequency cells, i.e. integer multiple of F.sub.R/N.sub.FFT or of F.sub.R/N.sub.REC, respectively.

(50) The method according to the invention is particularly suitable for the millimetre band (W band), for automotive radar applications or applications of radar on board aeroplanes or drones, for the detection of fixed or moving target relative to the carrier.