Patent Publication Number: US-2021181303-A1

Title: Calibrating array antennas based on signal energy distribution as a function of angle

Description:
FIELD 
     This application relates generally to frequency-modulated continuous wave (FMCW) radar systems, and more particularly to systems that employ antenna calibration based on target detection. 
     BACKGROUND 
     In the quest for ever-safer and more convenient transportation options, many car manufacturers are developing self-driving cars which require an impressive number and variety of sensors, often including arrays of acoustic and/or electromagnetic sensors to monitor the distance between the car and any nearby persons, pets, vehicles, or obstacles. Attempts to calibrate sensors based on one or more detected targets (e.g., vehicles) have not been wholly satisfactory. Thus, there is room for improvement in the art. 
     SUMMARY 
     In accordance with at least one example of this disclosure, a radar detection method with receive antenna calibration comprises: forming a detection matrix from signals detected by an arrangement of receive antennas in response to chirps transmitted by an arrangement of transmit antennas, the detection matrix having multiple rows each corresponding to one of said chirps, multiple columns each corresponding to a sample of said signals, and multiple planes each corresponding to one of said receive antennas; deriving a range matrix by performing a frequency transform on a portion of each row of the detection matrix; deriving a velocity matrix by performing a frequency transform on a portion of each column of the range matrix; deriving a direction-of-arrival matrix by performing a frequency transform on a portion of one or more layers of the velocity matrix; analyzing the direction-of-arrival matrix to determine a current peak width; and adjusting, based on the current peak width, phase shifts associated with one or more of the receive antennas. 
     In accordance with at least one other example of this disclosure, a radar detection method with transmit antenna calibration comprises: forming a detection matrix from signals detected by an arrangement of receive antennas in response to chirps transmitted by an arrangement of transmit antennas, the detection matrix having multiple rows each corresponding to one of said chirps, multiple columns each corresponding to a sample of said signals, and multiple layers each corresponding to one of said receive antennas; deriving a range matrix by performing a frequency transform on each row of the detection matrix; deriving a velocity matrix by performing a frequency transform on a portion of each column of the range matrix; deriving a direction-of-arrival matrix by performing a frequency transform on a portion of one or more layers of the velocity matrix; analyzing the direction-of-arrival matrix to determine a peak width; and adjusting phase shifts associated with one or more of the transmit antennas to minimize the peak width. 
     In accordance with at least one other example of this disclosure, a radar transceiver comprises: one or more transmitter circuits configured to drive a transmit antenna arrangement to emit a signal towards one or more objects; an arrangement of receivers, wherein each receiver is configured to detect a signal from a receive antenna during a detection period, each receive antenna having an associated phase shift that is adjustable relative to the other receiver antennas, and wherein each of the detected signals corresponds to the emitted signal; a processor coupled to the one or more transmitter circuits and the arrangement of receivers, wherein the processor is configured to perform a method comprising: forming a detection matrix from signals detected by an arrangement of receive antennas in response to chirps transmitted by an arrangement of transmit antennas, the detection matrix having multiple rows each corresponding to one of said chirps, multiple columns each corresponding to a sample of said signals, and multiple planes each corresponding to one of said receive antennas; deriving a range matrix by performing a frequency transform on a portion of each row of the detection matrix; deriving a velocity matrix by performing a frequency transform on a portion of each column of the range matrix; deriving a direction-of-arrival matrix by performing a frequency transform on a portion of one or more layers of the velocity matrix; analyzing the direction-of-arrival matrix to determine a current peak width; and adjusting, based on the current peak width, phase shifts associated with one or more of the receive antennas. 
     In accordance with at least one other example of this disclosure, a radar transceiver comprises: one or more transmitter circuits configured to drive a transmit antenna arrangement to emit a signal towards one or more objects, each transmit antenna in the arrangement having an associated phase shift that is adjustable relative to the other transmit antennas, the emitted signal comprising a plurality of chirps; an arrangement of receivers, wherein each receiver is configured to detect a signal from a receive antenna during a detection period, and wherein each of the detected signals corresponds to the emitted signal; a processor coupled to the one or more transmitter circuits and the arrangement of receivers, wherein the processor is configured to perform a method comprising: forming a detection matrix from signals detected by an arrangement of receive antennas in response to chirps transmitted by an arrangement of transmit antennas, the detection matrix having multiple rows each corresponding to one of said chirps, multiple columns each corresponding to a sample of said signals, and multiple layers each corresponding to one of said receive antennas; deriving a range matrix by performing a frequency transform on each row of the detection matrix; deriving a velocity matrix by performing a frequency transform on a portion of each column of the range matrix; deriving a direction-of-arrival matrix by performing a frequency transform on a portion of one or more layers of the velocity matrix; analyzing the direction-of-arrival matrix to determine a current peak width; and adjusting, based on the current peak width, phase shifts associated with one or more of the transmit antennas. 
     In accordance with at least one other example of this disclosure, a system for calibrating antennas comprises a non-transitory computer readable medium storing instructions executable by a processor, wherein the instructions comprise instructions to: form a detection matrix from signals detected by an arrangement of antennas in response to chirps transmitted by an arrangement of transmit antennas, the detection matrix having multiple rows each corresponding to one of said chirps, multiple columns each corresponding to a sample of said signals, and multiple planes each corresponding to one of said antennas; derive a range matrix by performing a frequency transform on a portion of each row of the detection matrix; derive a velocity matrix by performing a frequency transform on a portion of each column of the range matrix; derive a direction-of-arrival matrix by performing a frequency transform on a portion of one or more layers of the velocity matrix; analyze the direction-of-arrival matrix to determine a current peak width; and adjust, based on the current peak width, phase shifts associated with one or more of the antennas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overhead view of an illustrative vehicle equipped with sensors. 
         FIG. 2  is a block diagram of a driver-assistance system, in accordance with an example of this disclosure. 
         FIG. 3  is a schematic of a reconfigurable multiple input multiple output radar system, in accordance with an example of this disclosure. 
         FIG. 4  illustrates a radar system in block diagram form, in accordance with an example of this disclosure. 
         FIG. 5  illustrates a chirp signal profile  502 , in accordance with an example of this disclosure. 
         FIGS. 6A-B  illustrate antenna array radiation patterns in accordance with examples of this disclosure. 
         FIG. 7A  illustrates a radar system, in accordance with an example of this disclosure. 
         FIG. 7B  illustrates a plot of the results of a method of using a radar system, in accordance with an example of this disclosure. 
         FIGS. 8A-D  illustrate a radar detection method, in accordance with an example of this disclosure. 
         FIGS. 9A-9D  illustrate another radar detection method, in accordance with an example of this disclosure. 
         FIG. 9E  illustrates a plot corresponding to the peak widths associated with chirps detected according to the method illustrated in  FIGS. 9A-D . 
     
    
    
     DETAILED DESCRIPTION 
     The accompanying drawings and following detailed description do not limit the disclosure, but on the contrary, they provide the foundation for understanding all modifications, equivalents, and alternatives falling within the scope of the appended claims. Specific configurations, parameter values, and operation examples are provided for the purposes of explanation rather than for circumscribing any scope of disclosure. 
       FIG. 1  shows an illustrative vehicle  102  equipped with an array of radar antennas, including antennas  104  for short range sensing (e.g., for park assist), antennas  106  for mid-range sensing (e.g., for monitoring stop &amp; go traffic and cut-in events), antennas  108  for long range sensing (e.g., for adaptive cruise control and collision warning), each of which may be placed behind the front bumper cover. Antennas  110  for short range sensing (e.g., for back-up assist) and antennas  112  for mid-range sensing (e.g., for rear collision warning) may be placed behind the back bumper cover. Antennas  114  for short range sensing (e.g., for blind spot monitoring and side obstacle detection) may be placed behind the car fenders. Each antenna and each set of antennas may be grouped in one or more arrays. Each array may be controlled by a radar array controller ( 205 ). Each set of antennas may perform multiple-input multiple-output (MIMO) radar sensing. The type, number, and configuration of sensors in the sensor arrangement for vehicles having driver-assist and self-driving features varies. The vehicle may employ the sensor arrangement for detecting and measuring distances/directions to objects in the various detection zones to enable the vehicle to navigate while avoiding other vehicles and obstacles. 
       FIG. 2  shows an electronic control unit (ECU)  202  coupled to the various ultrasonic sensors  204  and a radar array controller  205  as the center of a star topology. Other topologies including serial, parallel, and hierarchical (tree) topologies, are also suitable and contemplated for use in accordance with the principles disclosed herein. The radar array controller  205  couples to the transmit and receive antennas in the radar antenna array  106  to transmit electromagnetic waves, receive reflections, and determine a spatial relationship of the vehicle to its surroundings. The radar array controller  205  couples to carrier signal generators ( 404 ). In at least one example, the radar array controller  205  controls the timing and order of actuation of a plurality of carrier signal generators ( 404 ). 
     To provide automated parking assistance, the ECU  202  may further connect to a set of actuators such as a turn-signal actuator  208 , a steering actuator  210 , a braking actuator  212 , and throttle actuator  214 . ECU  202  may further couple to a user-interactive interface  216  to accept user input and provide a display of the various measurements and system status. 
     Using the interface, sensors, and actuators, ECU  202  may provide automated parking, assisted parking, lane-change assistance, obstacle and blind-spot detection, autonomous driving, and other desirable features. In an automobile, the various sensor measurements are acquired by one or more ECU  202 , and may be used by the ECU  202  to determine the automobile&#39;s status. The ECU  202  may further act on the status and incoming information to actuate various signaling and control transducers to adjust and maintain the automobile&#39;s operation. Among the operations that may be provided by the ECU  202  are various driver-assist features including automatic parking, lane following, automatic braking, and self-driving. 
     To gather the necessary measurements, the ECU  202  may employ a MIMO radar system. Radar systems operate by emitting electromagnetic waves which travel outward from the transmit antenna before being reflected towards a receive antenna. The reflector can be any moderately reflective object in the path of the emitted electromagnetic waves. By measuring the travel time of the electromagnetic waves from the transmit antenna to the reflector and back to the receive antenna, the radar system can determine the distance to the reflector and its velocity relative to the vehicle. If multiple transmit or receive antennas are used, or if multiple measurements are made at different positions, the radar system can determine the direction to the reflector and hence track the location of the reflector relative to the vehicle. With more sophisticated processing, multiple reflectors can be tracked. At least some radar systems employ array processing to “scan” a directional beam of electromagnetic waves and construct an image of the vehicle&#39;s surroundings. Both pulsed and continuous-wave implementations of radar systems can be implemented. 
       FIG. 3  shows an illustrative system having a MIMO configuration, in which J transmitters are collectively coupled to M transmit antennas  301  to send transmit signals  307 . The M possible signals  307  may variously reflect from one or more targets to be received as receive signals  309  via N receive antennas  302  coupled to P receivers. Each receiver may extract the amplitude and phase or travel delay associated with each of the M transmit signals  307 , thereby enabling the system to obtain N*M measurements (though only J*P of the measurements may be obtained concurrently). The processing requirements associated with each receiver extracting J measurements can be reduced via the use of time division multiplexing and/or orthogonal coding. The available antennas are systematically multiplexed to the available transmitters and receivers to collect the full set of measurements for radar imaging. 
       FIG. 4  illustrates a radar transceiver circuit  402  in block diagram form, in accordance with an example of this disclosure. In at least one example, the radar transceiver circuit  402  is implemented as an integrated circuit in a packaged chip. Radar transceiver circuit  402  includes a carrier signal generator  404 , a transmission filter  420 , an amplifier  412 , and transmit antennas  301  which can transmit signals  307  (e.g., chirps  409 ) based on the output of the carrier signal generator  404 . Radar transceiver circuit  402  also includes receiver antennas  302 , a low noise amplifier  413 , and a mixer  407 . Mixer  407  mixes signals (e.g.,  411 ) detected by antennas  302  with the signal from the carrier signal generator  404 . Low noise amplifier  413  is used to amplify signals  411  detected by antennas  302 . Radar transceiver circuit  402  also includes a sensitivity time controller and equalizer  413 , a broadband filter  415 , an analog-to-digital converter  417  and a digital signal processor  419 . The digital signal processor  419  and low noise amplifier  413  can be coupled for bi-directional communication as shown. 
     In examples of this disclosure, carrier signal generator  404  is coupled to the radar array controller  205 . Carrier signal generator  404  includes a chirp generator to create a frequency-modulated continuous-wave (FMCW) signal. The chip rate of the carrier signal generator  404  may be controlled by the radar array controller  205 . In at least one example, the carrier signal generator  404  can be deactivated by the radar array controller  205  to provide an unmodulated carrier signal. The carrier signal generator  404  may be implemented as a local oscillation (LO) signal generator as a fractional-N phase lock loop (PLL) with a ΣΔ controller, or as a direct-digital synthesis (DDS) generator. 
     Carrier signal generator  404  is connected to transmitter (TX)  301  through transmission filter  410  and amplifier  412 . Carrier signal generator  404  is connected to receiver (RX) through mixer  407  and low noise amplifier  413 . Carrier signal generator  404  generates a signal (e.g., a chirp signal). Amplifier  412  receives the signal from carrier signal generator  404  and the signal  307  is transmitted by transmit antennas  301 . 
       FIG. 5  illustrates a chirp signal profile  502 , in accordance with an example of this disclosure. FMCW radar (e.g.,  402 ) transmits chirp signals in order to detect a target&#39;s  305  range and relative velocity. A chirp signal is a frequency-modulated radio frequency (RF) signal whose frequency increases or decreases linearly over a time period. A chirp signal is composed of a series of individual chirps (e.g.,  503 ). A chirp  503  is characterized by, among other things, its carrier frequency, chirp bandwidth  504  and chirp duration  506 . A chirp sequence  508  is a portion of a chirp signal, and is defined by the number of chirps N C    510  and by a chirp period T C    512 .  FIG. 5  shows four chirps  503  of duration  506  T. Hence, the value of N C  is 4 in  FIG. 5 . 
     During operation, the signals (e.g.,  309 ) that are reflected off different objects (targets)  305  are received by the radar  402  and mixed with the transmitted signal to generate an intermediate frequency (IF) signal whose frequency depends on the target range and relative velocity. An IF is a frequency to which a carrier wave is shifted as an intermediate step in transmission or reception. The IF signal is sampled by an analog-to-digital converter (ADC) (e.g.,  417 ) at a sampling frequency f s  and processed by a processor (e.g., ECU  202 , DSP  419 ). 
       FIG. 7A  illustrates a radar transceiver  700  (e.g.,  402 ) in block diagram form, in accordance with an example of this disclosure. Radar transceiver  700  includes a carrier signal generator  404 , amplifiers  412 , and transmit antennas  301  which can transmit signals (e.g.,  307 ) based on the output of the carrier signal generator  404 . Radar transceiver  700  also includes receiver antennas  302  and mixers  407 . Mixers  407  mix signals (e.g.,  309 ) detected by antennas  302  with the signal from the carrier signal generator  404 . The detected signals are converted to digital signals by analog-to-digital converter(s)  417 . Processor  419  applies a frequency transform (e.g., a Fast Fourier transform—FFT1) to the digital output of analog-to-digital converter(s)  417  for each receive antenna  302 , creating a three-dimensional matrix  707  in which each row stores reflected signal energy as a function of range to the target, each column stores reflected signal energy corresponding to the different chirps, and each layer corresponds to the transformed data from each receive antenna  302 . The number of layers for transceiver  700  would be four. In examples of this disclosure, processor  419  applies  709  a second frequency transform, (FFT2) to columns of the matrix  707  to obtain a three dimensional matrix in which the columns store reflected signal energy as a function of target velocity. The described operations of transceiver  700  can be repeated, e.g., over multiple measurement cycles on a given target  305 , to adaptively calibrate the phase ϕ of each of the receive antennas  302 , as explained further below. 
       FIG. 7B  illustrates a plot  711  of the results of the second frequency transform with respect to target  305 . When target  305  is initially detected by receive antennas, the transformed data may show reflected signal energy somewhat dispersed over multiple velocity bins (as indicated by rectangle  704 ). As the receiver antennas  302  are tuned, the reflected signal energy becomes concentrated in a smaller number of velocity bins (as indicated by rectangle  714 ). The energy distribution  704  peaks at a local maximum value. The peak width of the distribution may be defined as the difference between the FT2 bins (apparent velocities or, as discussed further below, between apparent angles) at which the energy distribution is half of the local maximum value. The energy distribution  714  is more concentrated, as can be determined by its smaller peak width and/or its higher local maximum value. At least one technical advantage of transceiver  700  is that receiver antennas  302  can be quickly calibrated “on the fly”, minimizing the need for dedicated calibration intervals. 
       FIGS. 6A-B  illustrate antenna array radiation patterns in accordance with examples of this disclosure.  FIG. 6A  illustrates a collective radiation pattern  602  of an antenna array, which is associated with a transmit circuit or a receive circuit that is not well calibrated. Individual signal amplitudes are not equal and/or relative phases are not equal.  FIG. 6B  illustrates a collective radiation pattern  612  of an antenna array, which is associated with a transmit circuit or a receive circuit that is either ideal or well calibrated. Notably, the radiation pattern  602  of  FIG. 6A  has a beam width  604  which is wider than the beam width  614  of pattern  612  in  FIG. 6B . Beam width  604  is measured between the points at which the gain is half of the local maximum  606 , and similarly beam width  614  is measured between the points at which the gain is half of the maximum  616 . If the energy distribution in  FIG. 7B  becomes more concentrated in response to adjustments of relative amplitudes and phases, the calibration process may be safely assumed to be enhancing the antenna array&#39;s collective radiation pattern by narrowing its effective beam width. 
       FIGS. 8A-D  illustrate a radar detection method  500  with receive antenna calibration based on signal energy distribution as a function of velocity. As illustrated in  FIG. 8A , the method  500  includes forming a detection matrix  505  from signals detected by an arrangement of receive antennas in response to a sequence of chirps transmitted by an arrangement of transmit antennas  301 . The detection matrix  505  has N rows  507 , with each row  507  corresponding to one of the chirps. The detection matrix  505  has M columns  509 , with each column  509  corresponding to a sample within each chirp. The detection matrix  505  has K planes  511 . Each plane  511  corresponds to one receive antenna  302 . As illustrated in  FIG. 8B , the method  500  further includes deriving a three-dimensional range matrix  513  by performing a frequency transform (FT1) on some or all of each row  507  of the detection matrix  505 , and further includes extracting  515  a two-dimensional slice  517  of the range matrix  513 , with different rows  507  of the slice  517  being associated with different with different receive antennas  302  and optionally with different chirps. Optionally, one or more targets with high signal amplitude are identified at this stage. As illustrated in  FIG. 8C , the method  500  further includes deriving  519  a velocity matrix  521  from the extracted slice  517  by performing a frequency transform (e.g., FT2) on some or all of each column  509  of the extracted slice  517 . (Those columns not having reflected signal energy from the identified targets may be excluded to reduce processing requirements.) The velocity matrix  521  is analyzed to determine a peak width  523  and the phase shifts of the receive antennas  302  are adjusted so as to narrow the peak width. 
       FIGS. 9A-9D  show an illustrative radar detection method  900  with transmit and/or receive antenna array calibration based on reflected signal energy distribution as a function of angle rather than velocity. As illustrated in  FIG. 9A , the method  900  comprises forming a three-dimensional detection matrix  909  (using e.g., processor  202 ,  419 ) from signals  309  detected by an arrangement of receive antennas ( 302 ) in response to chirps ( 503 ) transmitted by an arrangement of transmit antennas ( 301 ). The detection matrix  909  has multiple rows N C . Each row  911  corresponds to one chirp ( 503 ). The detection matrix  909  formed by the processor has multiple columns  913 , each of which corresponds to a respective sample within each chirp signal. The detection matrix  909  formed by the processor also has multiple planes  915 , each of which corresponds to one receive antenna ( 302 ) which detected the chirp signals. As illustrated in  FIG. 9B , the method  900  further includes deriving a range matrix by performing a frequency transform (e.g., FFT1) on a portion of each row  911  of the detection matrix  909 . Optionally, one or more targets with high signal amplitude are identified at this stage. As illustrated in  FIG. 9C , the method  900  further includes deriving a velocity matrix  919  by performing a frequency transform (e.g., FFT2) on a portion of each column  913  (or only those columns having signal energy from the identified targets) of the range matrix  917 . Optionally, one or more targets with high signal amplitude are identified at this stage. As illustrated in  FIG. 9D , the method  900  further includes deriving a direction-of-arrival matrix  921  by performing a frequency transform (e.g., FT3) on a portion of one or more layers  915  of the velocity matrix  919 . According to the method  900 , the direction-of-arrival matrix  921  is analyzed using the processor to determine the width of one or more peaks in the reflected signal energy distribution as a function of angle. In one example of the method  900 , the phase shifts ϕ of one or more receive antennas ( 302 ) are adjusted based on the current peak width. In another example of the method  900 , the phase shifts ϕ of one or more transmit antennas ( 301 ) are adjusted based on the current peak width. 
       FIG. 9E  illustrates a plot  922  corresponding to the reflected signal energy distribution from a target detected by the receive antennas ( 302 ). By adjusting the phase shifts ϕ of the antennas ( 301  and/or  302 ), the peak width of the FFT3 data can be narrowed from a first value  604  to a second value  614 . In accordance with at least one example of this disclosure, the information of plot  922  can be displayed by a display device. 
     Though the operations described herein may be set forth sequentially for explanatory purposes, in practice the method may be carried out by multiple components operating concurrently and perhaps even speculatively to enable out-of-order operations. The sequential discussion is not meant to be limiting. Moreover, the focus of the foregoing discussions has been radar sensors, but the principles are applicable to any pulse-echo or continuous-wave travel time measurement systems. These and numerous other modifications, equivalents, and alternatives, will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such modifications, equivalents, and alternatives where applicable.