Time and frequency synchronization for spread radar systems

Abstract

An automotive spread MIMO-configured radar system has a plurality of transceiver antenna units for transmitting mutually orthogonal radar waves. Each transceiver antenna unit has a plurality of range gates to indicate a range detected by the transceiver antenna unit. At least one specific transceiver antenna unit (TRx.sub.1) is configured to transmit a reference signal received directly by at least one transceiver antenna unit (TRx.sub.2) that is separated by an a priori known distance from the specific transceiver antenna unit (TRx.sub.1). An evaluation and control unit is configured for reading out the plurality of range gates for the transceiver antenna unit (TRx.sub.2), and, based on the read-out range gate that indicates the received reference signal and based on the a priori known distance, for synchronizing the specific transceiver antenna unit (TRx.sub.1) and the transceiver antenna unit (TRx.sub.2) that received the reference signal and/or for correcting a measured Doppler shift.

Claims

1. An automotive spread multiple-input multiple-output configured radar system, comprising: a plurality of transceiver antenna units that are configured to transmit mutually orthogonal radar waves, for each transceiver antenna unit of the plurality of transceiver antenna units, a plurality of range gates that are configured to indicate a range detected by the transceiver antenna unit, wherein at least one specific transceiver antenna unit (TRx1) of the plurality of transceiver antenna units is configured to transmit, synchronized with the radar waves, a reference signal that is to be received directly by at least one transceiver antenna unit (TRx2) that is separated by an a priori known distance from the specific transceiver antenna unit (TRx1), and wherein the distance is substantially larger than a radar carrier wavelength, an evaluation and control unit that is configured to: read out the plurality of range gates for the transceiver antenna unit (TRx2) that received the reference signal, based on the read-out range gate of the plurality of range gates that indicates the received reference signal and based on the a priori known distance, synchronize the specific transceiver antenna unit (TRx1) and the transceiver antenna unit (TRx2) that received the reference signal.

2. The automotive spread radar system as claimed in claim 1, wherein the at least one specific transceiver antenna unit (TRx1) of the plurality of transceiver antenna units is configured to transmit the reference signal with a predetermined time delay relative to the transmitted radar wave, wherein the predetermined time delay is larger than zero.

3. The automotive spread radar system as claimed in claim 1, further comprising means for determining a carrier frequency of the specific transceiver antenna unit (TRx1) from a signal of the transceiver antenna unit (TRx2) that received the reference signal, wherein the evaluation and control unit is configured to correct, based on the determined carrier frequency, a Doppler shift measured at a target by the specific transceiver antenna unit (TRx1).

4. The automotive spread radar system as claimed in claim 1, wherein the transceiver antenna units of the plurality of transceiver antenna units are located at a priori known positions at the surrounding of a vehicle.

5. The automotive spread radar system as claimed in claim 1, wherein the evaluation and control unit comprises a processor unit and a digital data memory unit to which the processor unit has data access.

6. The automotive spread radar system as claimed in claim 1, further comprising modulation means to operate the plurality of transceiver antenna units in a phase-modulated continuous wave mode.

7. A method of operating an automotive spread multiple-input multiple-output configured radar system comprising a plurality of transceiver antenna units that are configured to transmit mutually orthogonal radar signals, and for each transceiver antenna unit of the plurality of transceiver antenna units a plurality of range gates for indicating a range detected by the transceiver antenna unit, the method comprising steps of: transmitting modulated, mutually orthogonal radar waves by the plurality of transceiver antenna units in a continuous-wave manner, by at least one specific transceiver antenna unit (TRx1) of the plurality of transceiver antenna units, directly transmitting a reference signal that is synchronized with the radar wave transmitted by the at least one specific transceiver antenna unit (TRx1), to at least one transceiver antenna unit (TRx2) that is separated by an a priori known distance from the specific transceiver antenna unit (TRx1), reading out the plurality of range gates for the at least one transceiver antenna unit (TRx2) that received the reference signal, and based on the read-out range gate of the plurality of range gates that indicates the received reference signal and the a priori known distance, synchronizing the specific transceiver antenna unit (TRx1) and the transceiver antenna unit (TRx2) that received the reference signal.

8. The method as claimed in claim 7, wherein the step of directly transmitting a reference signal that is synchronized with the radar wave comprises transmitting the reference signal with a predetermined time delay relative to the transmitted radar wave, wherein the predetermined time delay is larger than zero.

9. The method as claimed in claim 7, further comprising steps of: determining a carrier frequency of the specific transceiver antenna unit (TRx1) from a signal of the transceiver antenna unit (TRx2) that received the reference signal, and based on the determined carrier frequency, correcting a Doppler shift measured at a target by the specific transceiver antenna unit (TRx1).

10. The method as claimed in claim 7, wherein the steps are repeated for pairs of transceiver antenna units (TRx.sub.1, TRx.sub.2) out of the plurality of transceiver antenna units such that each transceiver antenna unit has directly transmitted a reference signal to at least one other transceiver antenna unit and has directly received a reference signal from at least one other transceiver antenna unit.

11. A non-transitory computer-readable medium for controlling automatic execution of steps of the method as claimed in claim 7, wherein the method steps are stored on the computer-readable medium as a program code, wherein the computer-readable medium comprises a part of the automotive spread radar system or a separate control unit and the program code is executable by a processor unit of the automotive spread radar system or a separate control unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the drawings:

(2) FIG. 1 illustrates a possible embodiment of the automotive spread radar system in accordance with the invention in a state of being installed in a vehicle,

(3) FIG. 2 schematically shows the plurality of transceiver antenna units of the automotive spread radar system pursuant to FIG. 1 and an example of contents of the plurality range gates of a transceiver antenna unit receiving a reference signal,

(4) FIG. 3 shows another example of contents of the plurality range gates of a transceiver antenna unit receiving a reference signal,

(5) FIG. 4 shows a phase-modulated continuous wave for operating the transceiver antenna units pursuant to FIG. 2,

(6) FIG. 5 shows a sequence to be transmitted by the transceiver antenna units, coded by a Hadamard matrix,

(7) FIG. 6 is a schematic layout of a signal processing unit of the automotive spread radar system, and

(8) FIG. 7 is a schematic depicting a receiver that receives transmissions reflected off a target from a plurality of transmitters.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

(9) FIG. 1 shows a possible embodiment of an automotive spread radar system 10 in accordance with the invention. The automotive spread radar system 10 is configured for detecting reflecting targets 16 in its field of view, for unambiguously measuring a range to and a relative radial velocity of each of the detected targets 16, and for measuring an angle of arrival a of the reflected radar signal of each of the detected targets 16.

(10) Means and methods to determine the above-mentioned quantities from radar signals received after having been reflected by a target 16 in the field of view of the radar system are well known in the art, for instance in the prior art cited herein, and shall therefore not be described in detail herein.

(11) The automotive spread radar system 10 is installed in a vehicle 14 formed by a passenger car to provide information that is to be used as an input for a collision avoidance system of the vehicle 14. The automotive spread radar system 10 comprises a plurality of four transceiver antenna units TRx.sub.k, k=1-4, that are arranged at a priori known positions at a front region of the vehicle 14.

(12) FIG. 2 schematically illustrates two transceiver antenna units TRx.sub.1, TRx.sub.2 of the plurality of four transceiver antenna units TRx.sub.k, k=1-4, of the automotive spread radar system 10 pursuant to FIG. 1. In FIG. 2, the transceiver antenna units TRx.sub.1, TRx.sub.2 are shown to be arranged at a priori known positions to form a one-dimensioned linear array, wherein the transceiver antenna units TRx.sub.k are evenly spaced by a distance d, which in this specific embodiment is 0.5 m. For reasons of simplicity of the considerations to follow, this arrangement differs from the arrangement shown in FIG. 1. However, those skilled in the art will appreciate that similar geometrical considerations apply.

(13) In this specific embodiment, the transceiver antenna units TRx.sub.k are identically designed. Each transceiver antenna unit TRx.sub.k includes a plurality of n≥3 patches. In other embodiments, the number of patches may be different for some or for all of the transceiver antenna units TRx.sub.k.

(14) A carrier frequency f of the radar waves transmitted by the transceiver antenna units TRx.sub.k is about 80 GHz, meaning a wavelength λ.sub.c of 37.5 mm. Thus, the distance d between adjacent transceiver antenna units TRx.sub.k, TRx.sub.k+1 is substantially larger than the radar carrier wavelength λ.sub.c, namely by a factor of more than 10.

(15) The plurality of transceiver antenna units TRx.sub.k is configured to work in a multiple-input and multiple-output (MIMO) configuration. The individual transceiver antenna units TRx.sub.k transmit mutually orthogonal radar waves, i.e. each transceiver antenna unit TRx.sub.k can decode its own echo and the echoes generated by other transceiver antenna units TRx.sub.k without cross talk disturbances. To this end, the automotive spread radar system 10 comprises modulation means to operate the plurality of transceiver antenna units TRx.sub.k in a phase-modulated continuous wave (PMCW) mode.

(16) In this embodiment, PMCW is based on bi-phase modulation, which means φ(t)=0° or φ(t)=180° for an emitted signal u(t)=A cos(ft+φ(t)). The phase code c=(c.sub.k)∈{−1,1}.sup.L.sup.c, which is meant to be sent out could be properly chosen, for example as an Almost Perfect Auto-Correlation Sequence (APAS) or a Maximum Length-Sequence (m-sequence), depending on the requirements. The phase can then be modelled via

(17) ${\phi }_c\left(t\right):=\{\begin{array}{c}0°\mathrm{if}{c}_{\frac{t}{T_c}\mathrm{mod}{L}_c}=1\\ 180°\mathrm{if}{c}_{\frac{t}{T_c}\mathrm{mod}{L}_c}=-1\end{array}$
with chirp duration T.sub.C and code (sequence) length L.sub.c. A potential embodiment of a phase-modulated radar waveform is illustrated in FIG. 4.

(18) For the PMCW automotive spread radar system 10 the MIMO concept can be realized via Hadamard coding to provide orthogonal signals. This per se known technique (for instance from Bell, D. A.: “Walsh functions and Hadamard matrixes”, Electronics letters 9.2 (1966), 340-341) is called Outer Code MIMO Concept. For that purpose a matrix can be taken from the Walsh-Hadamard family (exists for all lengths in multiples of 2, ranging from 4 to 664) in which all rows are orthogonal. The length of needed outer code is equal to the number of transceiver antenna units TRx.sub.k. For this specific embodiment with four transceiver antenna units TRx.sub.k, one Hadamard matrix is given by

(19) $H=\left[\begin{array}{cccc}1& 1& 1& 1\\ 1& -1& 1& -1\\ 1& 1& -1& -1\\ 1& -1& -1& 1\end{array}\right]$

(20) Then the sequence S, which is sent out will be coded (multiplied) by such Hadamard matrices as shown in FIG. 5.

(21) Each transceiver antenna unit TRx.sub.k is configured to transmit and to receive radar waves. In FIG. 2, the main lobe 18 and two side lobes 20 are shown for the transceiver antenna units TRx.sub.1, TRx.sub.2.

(22) In the following, an embodiment of a method of operating an automotive spread MIMO radar system 10 in accordance with the invention will be described.

(23) Each transceiver antenna unit TRx.sub.k transmits a reference signal 22 that is synchronized with the transmitted radar waves. The reference signal 22 is formed by the radar waves transmitted by the side lobes 20 of the transmitting transceiver antenna unit TRx.sub.1. In this way, a field of view provided by the main lobes 18 of the transceiver antenna units TRx.sub.k is not affected by any one of the other transceiver antenna units TRx.sub.j, j≠k. The reference signal 22 of each transceiver antenna unit TRx.sub.k is directly received by another transceiver antenna unit TRx.sub.1, j≠k of the plurality of transceiver antenna units. In FIG. 2, this is illustrated for transceiver antenna unit TRx.sub.1 transmitting the reference signal 22 and transceiver antenna unit TRx.sub.2 directly receiving the reference signal 22.

(24) As shown in FIG. 2, for each specific transceiver antenna unit TRx.sub.k of the plurality of transceiver antenna units TRx.sub.k, the automotive spread radar system 10 includes a plurality of range gates 12 that are configured to indicate a range of a target 16 detected by the specific transceiver antenna unit TRx.sub.k. FIG. 2 illustrates the range gates 12 of transceiver antenna unit TRx.sub.2.

(25) The reference signal 22 transmitted by transceiver antenna unit TRx.sub.1 comprising a sequence with outer Hadamard coding, is received by transceiver antenna unit TRx.sub.2.

(26) The received reference signal 22 is then decoded and correlated to determine a distance r between the transceiver antenna units TRx.sub.1, TRx.sub.2 by reading out the plurality of range gates 12 of transceiver antenna unit TRx.sub.2. If the two transceiver antenna units TRx.sub.1, TRx.sub.2 were perfectly synchronized, the measured distance will correspond to the a priori known geometrical distance d, i.e. d=r.

(27) If the transceiver antenna units TRx.sub.1, TRx.sub.2 have a (positive or negative) relative time delay r, the received reference signal 22 will be identified in a shifted range gate 12 corresponding to r.sub.τ=d+c.Math.τ (c: speed of light). If r.sub.τ ≥0, the time delay identification is unique, because the reference signal 22 from the transmitting transceiver antenna unit TRx.sub.1 is always in one of the first range gates 12 which is activated at the receiving transceiver antenna unit TRx.sub.2 (each detour over a target will provide a larger time shift and will occur in a later range gate). The time delay r in this case can be calculated by

(28) $\tau =\frac{r_{\tau }-d}{c}$

(29) For illustration purposes, the following scenario is considered. A PMCW radar sequence is sent from transceiver antenna unit TRx.sub.1 with a chirp duration T.sub.c=0.5 ns, and is received at transceiver antenna unit TRx.sub.2 with the a priori known distance of d=0.5 m. The range resolution R.sub.res of such configuration is given by

(30) $R_{\mathrm{res}}=\frac{{\mathrm{cT}}_c}{2}\approx 0.075m.$
At perfect synchronization, the reference signal 22 takes

(31) $\frac{d}{c}\approx 1.67\mathrm{ns}$
to reach transceiver antenna unit TRx.sub.2 from transceiver antenna unit TRx.sub.1 This means a time delay τ≥−1.67 ns can be corrected, which corresponds to

(32) $\frac{d}{{\mathrm{cT}}_c}=\frac{d}{2R_{\mathrm{res}}}=3$
range gates. In a next step, based on the read-out range gate 12 of the plurality of range gates that indicates the received reference signal 22 and the a priori known distance d, the transceiver antenna unit TRx.sub.1 and the transceiver antenna unit TRx.sub.2 that received the reference signal 22 are synchronized. The larger the distance d and the finer the range resolution R.sub.res, the larger the range of unambiguity.

(33) If transceiver antenna unit TRx.sub.1 sends the reference signal 22 too early, i.e.

(34) $\tau <-\frac{d}{c},$
the activated range gate 12 at transceiver antenna unit TRx.sub.2 will jump to the end of the list of range gates. As a result the reference signal 22 will appear as a faraway target, and the reference signal 22 could no longer be identified via the described first-in uniqueness.

(35) The range of unambiguity arising from the transmitted reference signal 22 being sent out too early to be detected in one of the first range gates 12 of the receiving transceiver antenna unit TRx.sub.2 is avoided for a larger range of time, and the time synchronization is made more robust by a step in which the transceiver antenna unit TRx.sub.1 transmits the reference signal 22 with a predetermined time delay σ relative to the transmitted radar wave. In this way, the transmitting transceiver antenna unit TRx.sub.1 acts as a master transceiver antenna unit and the receiving transceiver antenna unit TRx.sub.2 acts as a slave transceiver antenna unit. The predetermined time delay σ is larger than zero, and namely is σ=10 ns.

(36) In the case of perfect synchronization between the transceiver antenna units TRx.sub.1 and TRx.sub.2, the reference signal 22 is then received by the slave transceiver antenna unit TRx.sub.2 in the range gate corresponding to {tilde over (r)}.sub.0=d+c.Math.σ. In case of a non-zero relative time delay τ, the received signal will be located in the range gate 12 corresponding to
{tilde over (r)}.sub.τ=d+c.Math.σ+c.Math.τ

(37) This means that the range of uniqueness has shifted to {tilde over (r)}.sub.τ≥0, which in turn means that a relative time shift τ can be unambiguously detected if

(38) $\tau \le \frac{{\tilde{r}}_{\tau }-d-c.Math.\sigma }{c}.$

(39) The predetermined time delay σ of the master transceiver antenna unit TRx.sub.1 leads to a reduction of the range of unambiguity from master transceiver antenna unit TRx.sub.1 to slave transceiver antenna unit TRx.sub.2 of

(40) ${\tilde{R}}_{\max }=\frac{c.Math.\left(L_c-\frac{\sigma }{T_c}\right)}{2R_c},$
wherein L.sub.c denotes the sequence length, T.sub.c the chirp duration and R.sub.c the chirp rate. It is noted that this reduction only affects the cross talk of the receiving transceiver antenna units. The delay will not lead to a reduction of the master transceiver antenna unit TRx.sub.1.

(41) It follows that the predetermined time delay σ of the master transceiver antenna unit TRx.sub.1 of 10 ns results in a unique correction time delay of τ≥−11.67 ns, corresponding to 23 range gates. The predetermined time delay σ leads to an additional loss in the range of unambiguity from master transceiver antenna unit TRx.sub.1 to slave transceiver antenna unit TRx.sub.2 with range of unambiguity of R.sub.max=75 m:

(42) 0${\tilde{R}}_{\max }=\frac{c.Math.\left(L_c-\frac{\sigma }{T_c}\right)}{2R_c}=73.5m$
i.e. a loss in the range of unambiguity of ΔR.sub.σ=1.5 m (L.sub.c=1000 and

(43) $R_c=\frac{1}{T_c}=2\mathrm{GHz}).$
As a result, the distance of unambiguity is reduced to 36.75 m.

(44) In another step of the method, the carrier frequency f of the master transceiver antenna unit TRx.sub.1 is determined from the signal of the slave transceiver antenna unit TRx.sub.2 that is generated upon receiving the reference signal 22.

(45) The method relies on the insight that a difference in the radar carrier frequencies results in a measurable Doppler shift. Without loss of generality, the local oscillator at transceiver antenna unit TRx.sub.1 has radar carrier frequency f and the local oscillator of transceiver antenna unit TRx.sub.2 has radar carrier frequency f+f.sub.ε, wherein f.sub.ε<<f.

(46) From the reference signal 22 received by transceiver antenna unit TRx.sub.2 the transceiver antenna unit TRx.sub.1 appears to have a Doppler shift f.sub.D, which differs from an expected Doppler shift f.sub.exp, which in case of the vehicle 14 driving straight ahead is zero. With the method steps described above the direct path reference signal 22 transmitted by transceiver antenna unit TRx.sub.1 can uniquely be identified.

(47) For example, the Doppler frequency resolution of the PMCW radar system 10 is given by

(48) $f_{\mathrm{res}}=\frac{1}{2T_d},$
where T.sub.d=T.sub.c.Math.L.sub.c.Math.M.Math.N is the dwell time, M is a number of accumulations and N is a number of FFT points to extract the Doppler information, as is per se known in the art. The range f.sub.max of maximum unambiguity Doppler frequency can be computed as

(49) $f_{\max }=\frac{N}{2}{f}_{\mathrm{res}}=\frac{N}{4T_d}=\frac{1}{4}\frac{1}{T_c{L}_{c}M}.$

(50) This shows that the number of FFT points has no influence on the range f.sub.max of the maximum unambiguity Doppler frequency, and the smaller the chirp duration T.sub.c, sequence length L.sub.c or the number of accumulations M, the larger the frequency shift which can be detected.

(51) With the local oscillator (carrier) frequency f of transceiver antenna unit TRx.sub.1 of f=80 GHz and the local oscillator (carrier) frequency f′ of transceiver antenna unit TRx.sub.2 of frequency f+f.sub.ε, the Doppler frequency resolution is

(52) $f_{\mathrm{res}}=\frac{1}{2T_d}=52.0833\mathrm{Hz}$
wherein T.sub.d=T.sub.c.Math.L.sub.c.Math.M.Math.N=9.6 ms is the dwell time for M=150, N=128, T.sub.c=0.5 ns and the sequence length L.sub.c=1000. The range of the maximum unambiguity Doppler frequency which could be resolved and corrected for is

(53) $f_{\max }=\frac{N}{2}{f}_{\mathrm{res}}=\frac{N}{4T_d}=\frac{1}{4}\frac{1}{T_c{L}_{c}M}=3.33\mathrm{kHz}.$

(54) With reference to FIG. 1, the automotive spread radar system 10 comprises an evaluation and control unit 24 that is configured for reading out the plurality of range gates 12 for all transceiver antenna units TRx.sub.k that received a reference signal 22, and for synchronizing the transceiver antenna unit TRx.sub.k, which sent out the reference signal 22, and the transceiver antenna unit TRx.sub.j, j≠k, that received the reference signal 22, based on the read-out range gate 12 of the plurality of range gates that indicates the received reference signal 22, and based on the a priori known distance d between the reference signal-transmitting transceiver antenna unit TRx.sub.k and the reference signal-receiving transceiver antenna unit TRx.sub.j, j≠k.

(55) The evaluation and control unit 24 can be located at any place within the vehicle 14 that appears suitable to those skilled in the art.

(56) In order to be able to execute the disclosed steps, the evaluation and control unit 24 is equipped with a processor unit and a digital data memory unit to which the processor unit has data access, and a signal processing unit 26 (FIG. 6) whose function will be described later on. The evaluation and control unit 24 is furnished with a software module for controlling automatic execution of steps of the method.

(57) Method steps to be conducted are converted into a program code of the software module. The program code is implemented in the digital data memory unit of the evaluation and control unit and is executable by the processor unit of the evaluation and control unit.

(58) Signals received by the transceiver antenna units TRx.sub.k are processed by the signal processing unit 26 that forms part of the automotive spread radar system 10. The signal processing unit 26 is known per se and described herein for the sake of completeness.

(59) The layout of the signal processing unit 26 is illustrated in FIG. 6. Correlators 28 will perform ranging, similar as known from global positioning systems (GPS). The number of parallel correlators 28 is equal to the sequence length L.sub.e to provide the range processing in one step. The coherent accumulator 30 will increase the signal-to-noise ratio (SNR) via M accumulations, and at least a Fast Fourier Transform (FFT) to extract the Doppler information will be performed.

(60) At the receiver side a multiplication of a signal v with each row of the Hadamard matrix H..sub.i is necessary to distinguish between each transmitter (after correlation, see FIG. 7).

(61) While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

(62) Other variations to be disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality, which is meant to express a quantity of at least two. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting scope.