Patent Publication Number: US-2022236369-A1

Title: Dual-polarized mimo radar

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims the filing benefits of U.S. provisional application, Ser. No. 63/141,020, filed Jan. 25, 2021, which is hereby incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to radar systems, and more particularly to radar systems for vehicles and robotics. 
     BACKGROUND OF THE INVENTION 
     The use of radar to determine range, velocity, and angle (elevation or azimuth) of objects in an environment is important in a number of applications including automotive radar and gesture detection. Radar systems typically transmit a radio frequency (RF) signal and listen for the reflection of the radio signal from objects in the environment. A radar system estimates the location of objects, also called targets, in the environment by correlating delayed versions of the received radio signal with the transmitted radio signal. A radar system can also estimate the velocity of the target by Doppler processing. A radar system with multiple transmitters and multiple receivers can also determine the angular position of a target. Depending on antenna scanning and/or the number of antenna/receiver channels and their geometry, different angles (e.g., azimuth or elevation) can be determined. 
     A radar system consists of transmitters and receivers. The transmitters generate a baseband signal which is upconverted to a radio frequency (RF) signal that propagates according to an antenna pattern. The transmitted signal is reflected off of object or targets in the environment. The received signal at each receiver is the totality of the reflected signal from all targets in the environment. The receiver down converts the received signal to baseband and compares the baseband received signal to the baseband signal at one or more transmitters. This is used to determine the range, velocity, and angle of targets in the environment. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide for a radar system that provides for greater immunity to interference from other radar systems, particularly from chirp radars. Exemplary embodiments also provide “good citizen” measures that help to reduce interference that might be caused to other radar systems. An exemplary radar system will include dual polarization receive channels in the expectation that interference will be a different polarization than the desired radio signals transmitted by own transmitters and reflected from targets in the environment. The radar system provides improved signal handling dynamic range to avoid receive channels saturating at the A-to-D converter stage before the radio signal has reached the digital signal processing domain. 
     In an aspect of the present invention, a radar system includes a transmit pipeline that includes a plurality of transmitters. The radar system also includes a receive pipeline that includes a plurality of receivers. The transmitters are configured to transmit radio signals. The receivers are configured to receive radio signals that include the transmitted radio signals transmitted by the transmitters and reflected from objects in the environment. The receive pipeline is configured to provide interference immunity from interfering radio signals transmitted by other radar systems. 
     In an aspect of the present invention, the interfering radar systems may be chirp radars. 
     In another aspect of the present invention, the transmit pipeline and/or the receive pipeline is configured to avoid transmitting radio signals that interfere with the other radar systems. 
     In a further aspect of the present invention, the receive pipeline comprises exemplary dual polarization receive channels. The interfering radio signals are a different polarization than the radio signals transmitted by the transmitters and reflected from targets in the environment. 
     In yet another aspect of the present invention, the receive pipeline is configured to provide improved signal handling dynamic range to avoid receive channels saturating at the A-to-D converter stage before the radio signal has reached the digital signal processing domain. 
     These and other objects, advantages, purposes and features of the present invention will become apparent upon review of the following specification in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of an automobile equipped with a radar system in accordance with the present invention; 
         FIG. 2A  and  FIG. 2B  are block diagrams of radar systems in accordance with the present invention; 
         FIG. 3  is a block diagram illustrating a radar with a plurality of receivers and a plurality of transmitters (MIMO radar) in accordance with the present invention; and 
         FIG. 4  is a block diagram of an exemplary dual-polarized MIMO radar system with dual polarization receive channels in accordance with the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings and the illustrative embodiments depicted therein, wherein numbered elements in the following written description correspond to like-numbered elements in the figures, a radar system provides for greater immunity to interference from other radar systems, particularly chirp radars. The exemplary radar system also provides “good citizen” measures that help to reduce interference that might be caused to other radar systems. The radar system will include exemplary dual polarization receive channels in the expectation that interference will be a different polarization than the desired radio signals transmitted by own transmitters and reflected from targets in the environment. The radar system also provides improved signal handling dynamic range to avoid receive channels saturating at the A-to-D converter stage before the radio signal has reached the digital signal processing domain. 
       FIG. 1  illustrates an exemplary radar system  100  configured for use in a vehicle  150 . In an aspect of the present invention, a vehicle  150  may be an automobile, truck, or bus, etc. The radar system  100  may utilize multiple radar systems (e.g.,  104   a - 104   d ) embedded in the vehicle  150  (see  FIG. 1 ). Each of these radar systems may employ multiple transmitters, receivers, and antennas (see  FIG. 3 ). These signals are reflected from objects (also known as targets) in the environment and received by one or more receivers of the radar system. A transmitter-receiver pair is called a virtual radar (or sometimes a virtual receiver). As illustrated in  FIG. 1 , the radar system  100  may comprise one or more transmitters and one or more receivers ( 104   a - 104   d ) for a plurality of virtual radars. Other configurations are also possible.  FIG. 1  illustrates the receivers/transmitters  104   a - 104   d  placed to acquire and provide data for object detection and adaptive cruise control. As illustrated in  FIG. 1 , a controller  102  receives and then analyzes position information received from the receivers  104   a - 104   d  and forwards processed information (e.g., position information) to, for example, an indicator  106  or other similar devices, as well as to other automotive systems. The radar system  100  (providing such object detection and adaptive cruise control or the like) may be part of an Advanced Driver Assistance System (ADAS) for the automobile  150 . 
     An exemplary radar system operates by transmitting one or more signals from one or more transmitters and then listening for reflections of those signals from objects in the environment by one or more receivers. By comparing the transmitted signals and the received signals, estimates of the range, velocity, and angle (azimuth and/or elevation) of the objects can be estimated. 
     There are several ways to implement a radar system. One way, illustrated in  FIG. 2A , uses a single antenna  202  for transmitting and receiving. The antenna  202  is connected to a duplexer  204  that routes the appropriate signal from the antenna  202  to a receiver  208  or routes the signal from a transmitter  206  to the antenna  202 . A control processor  210  controls the operation of the transmitter  206  and the receiver  208  and estimates the range and velocity of objects in the environment. A second way to implement a radar system is shown in  FIG. 2B . In this system, there are separate antennas for transmitting ( 202 A) and receiving ( 202 B). A control processor  210  performs the same basic functions as in  FIG. 2A . In each case, there may be a display  212  to visualize the location of objects in the environment. 
     A radar system with multiple antennas, multiple transmitters, and multiple receivers is shown in  FIG. 3 . Using multiple antennas  302 ,  304  allows an exemplary radar system  300  to determine the angle (azimuth or elevation or both) of targets in the environment. Depending on the geometry of the antenna system, different angles (e.g., azimuth or elevation) can be determined. 
     The radar system  300  may be connected to a network via an Ethernet connection or other types of network connections  314 , such as, for example, CAN-FD and FlexRay. The radar system  300  may also have memory ( 310 ,  312 ) to store software used for processing the signals in order to determine range, velocity, and location of objects. Memory  310 ,  312  may also be used to store information about targets in the environment. There may also be processing capability contained in the ASIC  208  apart from the transmitters  203  and receivers  204 . 
     The description herein includes an exemplary radar system in which there are N T  transmitters and N R  receivers for N T ×N R  virtual radars, one for each transmitter-receiver pair. For example, a radar system with eight transmitters and eight receivers will have 64 pairs or 64 virtual radars (with 64 virtual receivers). When three transmitters (Tx 1 , Tx 2 , Tx 3 ) generate signals that are being received by three receivers (Rx 1 , Rx 2 , Rx 3 ), each of the receivers is receiving the transmission from each of the transmitters reflected by objects in the environment. Each receiver can attempt to determine the range and Doppler of objects by correlating with delayed replicas of the signal from each of the transmitters. The physical receivers may then be “divided” into three separate virtual receivers, each virtual receiver correlating with delay replicas of one of the transmitted signals. 
     There are several different types of signals that transmitters in radar systems employ. A radar system may transmit a pulsed signal or a continuous signal. In a pulsed radar system, the signal is transmitted for a short time and then no signal is transmitted. This is repeated over and over. When the signal is not being transmitted, the receiver listens for echoes or reflections from objects in the environment. Often a single antenna is used for both the transmitter and receiver and the radar transmits on the antenna and then listens to the received signal on the same antenna. This process is then repeated. In a continuous wave radar system, the signal is continuously transmitted. There may be an antenna for transmitting and a separate antenna for receiving. 
     Another classification of radar systems is in the modulation of the signal being transmitted. A first type of continuous wave radar signal is known as a frequency modulated continuous wave (FMCW) radar signal. In an FMCW radar system, the transmitted signal is a continuous sinusoidal signal with a varying frequency. By measuring a time difference between when a certain frequency was transmitted and when the received signal contained that frequency, the range to an object can be determined. By measuring several different time differences between a transmitted signal and a received signal, velocity information can be obtained. 
     A second type of continuous wave signal used in radar systems is known as a phase modulated continuous wave (PMCW) radar signal. In a PMCW radar system, the transmitted signal from a single transmitter is a continuous sinusoidal signal in which the phase of the sinusoidal signal varies. Typically, the phase during a given time-period (called a chip period or chip duration) is one of a finite number of possible phases. A spreading code consisting of a sequence of chips, (e.g., +1, +1, −1, +1, −1 . . . ) is mapped (e.g., +1→0 radians, −1→π radians, where +1 corresponds to a phase of 0 radians and −1 corresponds to a phase of n radians) into a sequence of phases (e.g., 0, 0, π, 0, π . . . ) that is used to modulate a carrier signal to generate the radio frequency (RF) signal. The spreading code could be a periodic sequence or could be a pseudo-random sequence with a very large period, so it appears to be a nearly random sequence. The spreading code could be a binary code (e.g., +1 or −1). The resulting signal has a bandwidth that is proportional to the rate at which the phases change, called the chip rate R c , which is the inverse of the chip duration T c =1/R c . By comparing the return signal to the transmitted signal, the receiver can determine the range and the velocity of reflected objects. 
     In some radar systems, the signal (e.g., a PMCW signal) is transmitted over a short time-period (e.g., 1 microsecond) and then turned off for a similar time-period. The receiver is only turned on during the time-period where the transmitter is turned off. In this approach, reflections of the transmitted signal from very close targets will not be completely available because the receiver is not active during a large fraction of the time when the reflected signals are being received. This is called pulse mode. 
     The radar sensing system of the present invention may utilize aspects of the radar systems described in U.S. Pat. Nos. 10,261,179; 9,971,020; 9,954,955; 9,945,935; 9,869,762; 9,846,228; 9,806,914; 9,791,564; 9,791,551; 9,772,397; 9,753,121; 9,599,702; 9,575,160, and/or 9,689,967, and/or U.S. Publication Nos. 2018/0231656, 2018/0231652, 2018/0231636, and 2017/0309997, and/or U.S. provisional applications, Ser. No. 62/486,732, filed Apr. 18, 2017, Ser. No. 62/528,789, filed Jul. 5, 2017, Ser. No. 62/573,880, filed Oct. 18, 2017, Ser. No. 62/598,563, filed Dec. 14, 2017, Ser. No. 62/623,092, filed Jan. 29, 2018, and/or Ser. No. 62/659,204, filed Apr. 18, 2018, which are all hereby incorporated by reference herein in their entireties. 
     Digital frequency modulated continuous wave (FMCW) and phase modulated continuous wave (PMCW) are techniques in which a carrier signal is frequency or phase modulated, respectively, with digital codes using, for example, GMSK (Gaussian minimum shift keying). Digital FMCW radar lends itself to be constructed in a MIMO variant in which multiple transmitters transmitting multiple codes are received by multiple receivers that decode all codes. The advantage of the MIMO digital FMCW radar is that the angular resolution is that of a virtual antenna array having an equivalent number of elements equal to the product of the number of transmitters and the number of receivers. Digital FMCW MIMO radar techniques are described in U.S. Pat. Nos. 9,989,627; 9,945,935; 9,846,228; and 9,791,551, which are all hereby incorporated by reference herein in their entireties. 
     Dual-Polarized Multiple-Input, Multiple-Output (MIMO) Radar: 
       FIG. 4  is a block diagram of an exemplary radar system that provides for greater immunity to interference from other radar systems, particularly chirp radars. The exemplary radar system also provides “good citizen” measures that help to reduce interference (from the radar system) that might be caused to other radar systems. The radar system will include exemplary dual polarization receive channels  400  in the expectation that interference will be a different polarization than the desired radio signals transmitted by own transmitters and reflected from targets in the environment. The radar system also provides an improved signal handling dynamic range to avoid the receive channels saturating at the A-to-D converter stage  470  before the radio signal has reached the digital signal processing domain  480 ,  550 . 
     The exemplary radar system of  FIG. 4  improves the dynamic range up to and including the analog-to-digital (A-to-D) converters  470  by using the results of digital radar signal analysis to date from the digital signal processing domain  550  in a digital signal prediction step  540  to construct a digital prediction of the receiver channel signals to be received at a future time, probably on 1 μS into the future. The dynamic range may also be improved by digital-to-analog (D-to-A) converting the digital predictions into the analog domain using, for example, coarse digital-to-analog converters  460  and to then subtract the analog interference prediction signals from the corresponding receive channel signals in a summing or subtracting junction (e.g., the analog interference cancellation junction  450 ), such that the residual signals presented to the A-to-D converters  470  are of a reduced amplitude but still filling the dynamic range of the A-to-D converters  470 . Thus, the total dynamic range for signal handling is equal to the dynamic range of the A-to-D converters  470  enhanced by the amount by which interference subtraction lowered the residuals. One way to think of it is that the coarse D-to-A converters  460  might slice off the top 4 bits of signal dynamic range leaving the bottom 6 bits only to be converted by A-to-D converters  470 , and thereby achieving the equivalent of a 10-bit conversion. 
     To achieve the accuracy of, for example, a 10-bit conversion, the amount of interference subtracted in the analog domain has to be added back in the digital interference re-addition module  480  with high accuracy. The method envisaged to do this is that each level (perhaps 16 to 64 levels) of each of the coarse D-to-A converters  460  will have an auto-learned digital word to describe it, which will be adaptively learned to a high accuracy so that when that level is subtracted in the analog domain (in the analog interference cancellation junction module  450 ), an accurate digital value will be added back in the digital interference re-addition module  480 . 
     It is assumed that, after the A-to-D converters  470  of limited word length, the digital signal processing thereafter (e.g., via the digital radar signal analysis module  550  and/or subsequent data processors) can have whatever word length is needed to avoid digital saturation. 
     The analog interference subtraction (via the analog interference cancellation junction module  450 ) should occur as early as possible in the analog path. In one exemplary embodiment, the analog interference subtraction is performed after downconversion to the (I,Q) baseband. It is seen that the subtraction is often complex and power consuming to convert the predictions to 80 GHz for subtraction in the RF domain; and moreover, causes significant noise factor degradation. To do that, measures must be taken to avoid local transmit-receive spillover, such as separating transmit and receive chips, separation between transmit and receive antennas arrays, and use of a balanced RF I/O. For dual-polarization receivers (e.g., containing dual polarized receiver channels  400 ), the balanced dual-polarization antenna (V,H) 
     
       
         
         
             
             
         
       
     
     connection to the dual polarization antenna element  410  can comprise four ball-bonds in a square as shown below: 
     When arranged in the above way, the signals are nominally spatially orthogonal and any residual coupling between them can hardly be important given that the dual-polarization antenna is also crossed-dipoles for example. 
     With the availability of the dual-polarization signals from Nrx receivers and both polarizations, the digital radar signal analysis can comprise, as previously seen, of an FFT-based scheme for pulse-by-pulse correlation of the received signal in each channel with the known transmitter codes. If this is done, note that transmitting with a GMSK.UMSK modulation using the GSM 90-degree per bit pre-rotation coding reduces the correlation to correlating a complex received signal with a real template, rather than a full complex* complex correlation. There might however be even faster and less power consuming correlation methods that need no multiplies, which can be when the same received signal is to be correlated with many binary codes (many shifts of many different codes is a large number of binary correlations). These are based on the fact that the number of possible bit patterns of finite length, such as 8, is 256 times however many codes are used to correlate with, and since the same 256-bit patterns will reoccur many times in many codes, 8 signal samples need be combined only once in all 256 ways, and by doing it in Gray code order, only one new addition is required for each combination. (The latter alone is an 8:1 speed up). 
     There is an advantage in the per-pulse FFT correlation method. When the signal is temporarily available in the frequency domain, narrow-band interference stands out and can be clipped, nulled or otherwise mitigated. 
     An advance on interference nulling in the spectral domain only is to perform a rough beamforming over all antenna channels for each FFT component. A rough beamforming over, for example, 16 receive channels can be a 16-point FFT. Whatever is used, it should be an easily invertible, information lossless transform, but not necessarily an orthogonal transform like the FFT. 
     The combination of a 256-pt FFT for correlation with a 16-pt FFT over corresponding spectral components of the 256-pt FFT is in fact a 256×16 2-D FFT, which is a 2,048-pt Walsh-Fourier transform. The difference between a 2,048-pt Fourier transform and a Walsh-Fourier transform is that the former has twiddles at each stage while the latter omits twiddles between certain stages corresponding to the “Walsh” part. So, there are no twiddles between the 256-pt correlation FFT and the 16-pt beamforming FFT. 
     After a rough beamforming of each FFT component, the signal is in the 3D domain of spectrum and space. Nulling out large components at particular frequencies and in particular spatial directions removes less of the wanted signal energy. Moreover, the directions from which other-radar interference is received are likely to be long-term stable. Therefore, the directions in which to null-out spectral components of the interference can be determined over many pulses or even scans and do not change even if the interference spectrum changes. 
     Polarization can be added as yet another dimension in which to segregate interference. Since the polarization of other radar interference is also likely to be long-term stable, it can be determined solidly and then the following algorithm can be used to annul it to great advantage even though the polarization domain has only two points. 
     For example, for each spatio-spectral component to be cleaned up, the equation αV+βH is formed, where V and H are the horizontal and vertical components (or other cross-polarized components such as +/−45). The resulting polarization, which is determined by the ratio of α to β, is orthogonal to the interferer&#39;s polarization. However, but the scaling of α and β is chosen to leave the signal component unattenuated. Unattenuated within reason—if the polarizations of the signal and interferer were close, α and β would become large, magnifying noise. Thus, in at least that case, there is a compromise between noise magnification and signal loss that is known from many other similar problems. 
     So, using all 3 domains (i.e., spectral, spatial and polarization), substantial reduction of interference from other radars can already be obtained at the per-pulse stage. It is possible that only a single polarization, the above weighted combination of V and H, need be passed on to be accumulated over all pulses. 
     Inverting the rough beamforming needs to be done only once on the accumulated array just before the correlation IFFTs are performed. 
     The additional complexity introduced for spatio-spectral-polarization interference nulling is a doubling of the number of correlation FFTs to be performed, as there is one for each polarization. This is needed on the assumption that different interferers in different parts of the spectrum (or lying in different directions) might have different polarizations. Thus, even though their polarizations may be known in advance, if they are not the same, a polarization combination cannot be performed ahead of the FFT and rough beamforming FFT. Instead, the interferences need to be separated by spectrum and direction in order to apply a polarization nulling adapted to each one. 
     In addition, the exemplary embodiment illustrated in  FIG. 4  includes the rough beamforming FFTs. This includes 256, 16-point beamforming FFTs after sixteen 16×16 point correlation FFTs (assuming 16 receivers, one FFT per RX (and per TX)). This is less than a 50% increase in processing, as the FFT has twiddles between its two stages of 16pt FFTs, while no twiddles are needed between the FFTs and the beamforming. 
     Thus, the exemplary embodiments discussed herein include an exemplary radar system that provides for greater immunity to interference from other radar systems, particularly from chirp radars. The exemplary radar system also provides “good citizen” measures that help to reduce interference (by the radar system) that might be caused to other radar systems. The radar system includes exemplary dual polarization receive channels in the expectation that interference will be a different polarization than the desired radio signals transmitted by own transmitters and reflected from targets in the environment. The radar system also provides an improved signal handling dynamic range to avoid receive channels saturating at the A-to-D converter stage before the radio signal has reached the digital signal processing domain. 
     Changes and modifications in the specifically described embodiments can be carried out without departing from the principles of the present invention, which is intended to be limited only by the scope of the appended claims, as interpreted according to the principles of patent law including the doctrine of equivalents.