Patent Publication Number: US-2022229169-A1

Title: Antenna selection in a radar system based on multiple detected objects and multi-step planning

Description:
The subject disclosure relates to antenna selection in a radar system based on multiple detected objects and multi-step planning. 
     Vehicles (e.g., automobiles, trucks, construction equipment, farm equipment, automated factory equipment) increasingly include sensors to obtain information about the vehicle and its surroundings. The information facilitates semi-autonomous and autonomous operation of the vehicle, for example. Exemplary sensors include cameras, light detection and ranging (lidar) systems, and radio detection and ranging (radar) systems. A multi-input multi-output (MIMO) radar system has multiple transmit antenna elements and multiple receive antenna elements. The number of transmit and receive channels may be fewer than the number of transmit and receive antenna elements. Accordingly, it is desirable to provide antenna selection in a radar system based on multiple detected objects and multi-step planning. 
     SUMMARY 
     In one exemplary embodiment, a method includes configuring an adaptive switch to couple a plurality of receive channels to a subset of a plurality of antenna elements as selected antenna elements and receiving reflected signals with the selected antenna elements. The method also includes processing each of the reflected signals received at each of the selected antenna elements at a corresponding one of the plurality of receive channels to obtain a digital signal at each of the plurality of receive channels, and processing the digital signal from each of the plurality of receive channels to estimate a direction of arrival (DOA) to each object that is detected based on the reflected signals. A set of candidate configurations of the adaptive switch is generated to be assessed based on the DOA to each object. Assessing the set of candidate configurations of the adaptive switch includes performing a multi-step assessment using a decision tree with each of the set of candidate configurations as a root and examining accuracy of an output at a last step in the decision tree. A selected candidate configuration is selected from the set of candidate configurations of the adaptive switch based on the accuracy obtained with each of the set of candidate configurations as the root of the decision tree. The adaptive switch is configured according to the selected candidate configuration among the set of candidate configurations of the adaptive switch prior to receiving the reflected signals for a next iteration. 
     In addition to one or more of the features described herein, the processing each of the reflected signals includes amplifying, mixing, and performing analog-to-digital conversion on each of the reflected signals. 
     In addition to one or more of the features described herein, estimating the DOA to each object includes performing an iterative process to detect two or more objects. 
     In addition to one or more of the features described herein, the performing the iterative process includes detecting and removing an object among the two or more objects that results in a strongest reflection then removing the object prior to the next iteration. 
     In addition to one or more of the features described herein, the method also includes generating the set of candidate configurations from a set of initial configurations. The assessing the set of initial configurations of the adaptive switch to obtain the candidate configurations is based on the DOA to each object includes determining a Bobrovski-Zakai bound (BZB) to each object. 
     In addition to one or more of the features described herein, the determining the BZB to each object is based on implementing a Metropolis-Hastings (MH) algorithm to simulate each object. 
     In addition to one or more of the features described herein, the method also includes fixing all but one of the plurality of receive channels coupled to all but one of the subset of the plurality of antenna elements to leave one unfixed receive channel and generating the set of initial configurations by coupling, in turn, the one unfixed receive channel to each of the plurality of antenna elements that is not coupled to all but one of the plurality of receive channels. 
     In addition to one or more of the features described herein, assessing the candidate configurations by performing the multi-step assessment using the decision tree includes using each of the candidate configurations as a root of the decision tree to grow sub-trees at each step of the multi-step assessment. 
     In addition to one or more of the features described herein, the using the decision tree includes growing each sub-tree based on random switching. 
     In addition to one or more of the features described herein, the selecting the selected candidate configuration is based on aggregating error statistics and calculating root mean square error (RMSE) at a last step of the multi-step assessment to select a corresponding root as the selected candidate configuration. 
     In another exemplary embodiment, a radar system includes a plurality of antenna elements and a plurality of receive channels. Each receive channel includes an analog-to-digital converter (ADC) to output a digital signal. The radar system includes an adaptive switch to couple the plurality of receive channels to a subset of the plurality of antenna elements as selected antenna elements. The selected antenna elements receive reflected signals that result from reflection by a plurality of objects of transmitted signals emitted by the radar system and each of the plurality of receive channels outputs the digital signal based on the reflected signal from the selected antenna element coupled thereto. A controller processes the digital signal from each of the plurality of receive channels to estimate a direction of arrival (DOA) to each object that is detected based on the reflected signals and generates a set of candidate configurations of the adaptive switch to be assessed based on the DOA to each object. Assessing the set of candidate configurations of the adaptive switch includes performing a multi-step assessment using a decision tree with each of the set of candidate configurations as a root and examining accuracy of an output at a last step in the decision tree. The controller also selects a selected candidate configuration from the set of candidate configurations of the adaptive switch based on the accuracy obtained with each of the set of candidate configurations as the root of the decision tree and configures the adaptive switch according to the selected candidate configuration among the set of candidate configurations of the adaptive switch prior to receiving the reflected signals for a next iteration. 
     In addition to one or more of the features described herein, each of the plurality of receive channels also includes an amplifier and a mixer. 
     In addition to one or more of the features described herein, the controller estimates the DOA to each object based on performing an iterative process to detect two or more objects. 
     In addition to one or more of the features described herein, the iterative process includes detecting and removing an object among the two or more objects that results in a strongest reflection then removing the object prior to the next iteration. 
     In addition to one or more of the features described herein, the controller generates the set of candidate configurations from a set of initial configurations and assesses the set of initial configurations of the adaptive switch to obtain the set of candidate configurations based on the DOA to each object by determining a Bobrovski-Zakai bound (BZB) to each object. 
     In addition to one or more of the features described herein, the controller determines the BZB to each object based on implementing a Metropolis-Hastings (MET) algorithm to simulate each object. 
     In addition to one or more of the features described herein, the controller fixes all but one of the plurality of receive channels coupled to all but one of the subset of the plurality of antenna elements to leave one unfixed receive channel and to generate the set of initial configurations by coupling, in turn, the one unfixed receive channel to each of the plurality of antenna elements that is not coupled to all but one of the plurality of receive channels. 
     In addition to one or more of the features described herein, the controller assesses the set of candidate configurations by performing the multi-step assessment using the decision tree by using each of the set of candidate configurations as a root of the decision tree to grow sub-trees at each step of the multi-step assessment. 
     In addition to one or more of the features described herein, the controller uses the decision tree by growing each sub-tree based on random switching. 
     In addition to one or more of the features described herein, the controller selects the selected candidate configuration based on aggregating error statistics and calculating root mean square error (RMSE) at a last step of the multi-step assessment to select a corresponding root as the selected candidate configuration. 
     The above features and advantages, and other features and advantages of the disclosure are readily apparent from the following detailed description when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features, advantages and details appear, by way of example only, in the following detailed description, the detailed description referring to the drawings in which: 
         FIG. 1  is a block diagram of a vehicle that employs antenna selection in a radar system based on multiple detected objects and multi-step planning according to one or more embodiments; 
         FIG. 2  is a block diagram of an exemplary receive portion of the radar system that performs antenna selection based on multiple detected objects and multi-step planning according to one or more embodiments; and 
         FIG. 3  is a process flow of a method of determining the switching matrix G for a given transmit and receive iteration of a radar system according to one or more embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, its application or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. 
     As previously noted, a number of types of sensors may be used in a vehicle. Certain applications call for a higher degree of angular resolution than others. Angular resolution refers to the accuracy of the angle to an object from some point of the vehicle (i.e., the direction of arrival (DOA)). An autonomous driving application, for example, requires higher angular resolution than other applications, because information obtained by the sensors regarding objects around the autonomous vehicle is critical in performing correct operation of the autonomous vehicle. The density of data provided by a lidar system facilitates the requisite angular resolution. However, lidar systems are more expensive and can be cost-prohibitive in consumer applications as compared with radar systems, for example. A suite of several (e.g., three to seven) radar systems may also achieve the requisite angular resolution but can also prove cost prohibitive. 
     Embodiments of the systems and methods detailed herein relate to antenna selection in a radar system based on multiple detected objects and multi-step planning. A radar system with a large aperture array (i.e., a large number of antenna elements) is desirable for a high-resolution imaging radar, but the transmit and receive channels that obtain and initially process the signals received by the antenna elements can be cost-prohibitive. Thus, as detailed, the number of transmit and receive channels is fewer than the full set of antenna elements, and a switching scheme is used to couple a set of transmit and receive channels to a subset of the available antenna elements. The switching, according to one or more embodiments, is based on multi-step planning and also considers a scenario in which multiple objects are detected. The multi-step planning means that a decision tree is used to consider the accuracy in DOA obtained with each switching scenario after more than one transmit and receive cycle. Thus, a given switching scenario selected for the next step (i.e., next transmit and receive cycle) is based on accuracy resulting after multiple steps of the decision tree. 
     In accordance with an exemplary embodiment,  FIG. 1  is a block diagram of a vehicle  100  that employs antenna selection in a radar system  110  based on multiple detected objects  150   a,    150   b  (generally referred to as  150 ) and multi-step planning. The exemplary vehicle  100  shown in  FIG. 1  is an automobile  101 . The vehicle  100  includes a radar system  110  with a transmit portion  105 , a receive portion  115 , and a radar controller  120 . The radar system  110  may be a MIMO radar system, for example. The vehicle  100  may include one or more other sensors  140  (e.g., cameras, lidar system) to obtain information about objects  150  or the vehicle  100  itself The transmit portion  105  of the radar system  110  emits transmit signals  106  and the receive portion  115  of the radar system  110  receives reflected signals  116 . The reflected signals  116  are portions of the transmit signals  106  that are reflected by objects  150  in the field of view of the radar system  110 . The receive portion  115  is further detailed with reference to  FIG. 2 . 
     A controller  130  may obtain information from the radar system  110  as well as from one or more other sensors  140 . The controller  130  may control semi-autonomous or autonomous operation of the vehicle  100  based on the information. The antenna selection detailed herein may be performed by the radar controller  120  alone or in combination with the controller  130 . The radar controller  120  and the controller  130  may include processing circuitry that may include an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
       FIG. 2  is a block diagram of an exemplary receive portion  115  of the radar system  110  that performs antenna selection based on multiple detected objects  150  and multi-step planning according to one or more embodiments. The receive portion  115  includes receive antenna elements  210 - 1  through  210 -N (generally referred to as  210 ). The receive portion  115  also includes receive channels  230 - 1  through  230 -M (generally referred to as  230 ). As shown for receive channel  230 - 1 , each receive channel  230  generally includes an RF amplifier  231  that amplifies the received radio frequency (RF) signal (i.e., the portion of the received reflections  116  detected by the corresponding receive antenna element  210 ). A mixer  232  converts the received RF signal to an intermediate frequency (IF) signal that is amplified by an IF amplifier  233 . An analog-to-digital converter (ADC)  234  provides a digitized signal to the controller (i.e., radar controller  120  and/or controller  130 ) for processing. 
     The number N of receive antenna elements  210  is greater than the number M of receive channels  230 . Thus, an adaptive switch  220  couples some or all of the M receive channels  230  to a corresponding subset of the N receive antenna elements  210  at a given time. For explanatory purposes, M receive antenna elements  210  corresponding with the M receive channels  230 , rather than fewer than M, are assumed to be selected at each transmit and receive iteration of the radar system  110 . The selection of the receive antenna elements  210  to be operated is based on the adaptive switch  220 , which may be implemented in the form of a switching matrix G, for example. The switching matrix G may be updated through processing by the radar controller  120  or controller  130 , as further detailed with reference to  FIG. 3 . As previously noted, the switching matrix G is provided in consideration of any number of detected objects  150  and according to multi-step planning. 
       FIG. 3  is a process flow of a method  300  of determining the switching matrix G for a given transmit and receive iteration of a radar system  110  according to one or more embodiments. At block  310 , the processes include obtaining and sampling reflected signals  116  with receive antenna elements  210  selected according to a switching matrix G k-1  set for a latest transmit and receive iteration K-1, where the iterations are k=1, . . . , K-1. At block  320 , the DOA to each object  150  that is detected based on the reflected signals  116  received at iteration K-1 is estimated. 
     At block  330 , a process is performed to generate a set of candidate switching matrices {G K } from which the switching matrix G K  to be used in the next transmit and receive iteration K will be selected. At block  340 , multi-step planning is undertaken with the set of candidate switching matrices {G K }. For example, a decision tree is developed with each candidate in the set of candidate switching matrices {G K } acting as a root. At block  350 , the root (i.e., candidate among the set of candidate switching matrices {G K }) that is associated with the best sub-tree is selected as the switching matrix GK for the next transmit and receive iteration K. The best sub-tree refers to the one that results in the most accurate DOA estimate at the last level of the decision tree. Each of these processes is further detailed. 
     At block  310 , the processes include obtaining and sampling reflected signals  116  at the (K-1) th  iteration. Each receive antenna element  210  that is selected according to the switching matrix G K-1  at the (K-1) th  iteration obtains a portion of the reflected signals  116  (i.e., a received signal) and provides the received signal to the corresponding receive channel  230 . The baseband model or vector of the received signals using M of the N receive antenna elements  210  and obtaining J samples is given by: 
         x   jk   =G   k   A (φ) S   jk +υ jk   [EQ. 1]
 
     The index k refers to the transmit and receive iteration (e.g., (K-1)), and the index j refers to the samples 1 through J. In EQ. 1, A is an array of steering vectors [α(φ 1 ), . . . , α(φ M )]. Each steering vector corresponds with a direction of the received signal to the receive antenna element  210  that obtains that portion of the reflected signals  116 . Steering vectors with higher power suggest an estimate for the DOA. A known multiple signal classification (MUSIC) algorithm, which is a frequency estimation method to distinguish closely spaced objects  150 , uses the steering vectors in DOA estimation as detailed with reference to block  320 . 
     The signal sequence S jk  and the noise sequence D jk  in EQ. 1 are mutually independent and have covariance matrices: 
         cov ( S   jk )= R   s   [EQ. 2]
 
         cov (υ jk )=σ v   2   I   M   [EQ. 3]
 
     The signal covariance matrix is R S  in EQ. 2. In EQ. 3, the identity matrix I M  is of size M, where M is the number of receive channels  230 , and the square of noise, σ v   2 , is assumed to be known. The vectors x jk  for the k th  iteration (e.g., K-1 iteration) make up the observation matrix X k =[x 1k  . . . , x jk ]. Thus, the current and past observation matrices [X 1 , . . . , X K-1 ] are available at block  310 . 
     At block  320 , estimating the angle φ (e.g., DOA) to each detected object  150  involves an iterative process in which the object  150  associated with the current strongest signal is detected and then removed to facilitate detection of the object  150  associated with the next strongest signal. This is referred to as a CLEAN algorithm. As previously noted, DOA estimation also uses the MUSIC algorithm for classification of detections. Thus, MUSIC-CLEAN is used such that the MUSIC algorithm is used to estimate DOA for the object  150  associated with the strongest signal at each iteration according to the CLEAN algorithm. 
     At each iteration k the J samples are aggregated to construct the covariance matrix: 
     
       
         
           
             
               
                 
                   
                     
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     In EQ. 4, H indicates a Hermitian transpose. The DOA {circumflex over (φ)} 1  of the object  150  associated with the strongest signal is estimated as a mode of the cumulative MUSIC 1,k=1,  . . . ,K-1 response. That is, the MUSIC algorithm is used cumulatively over K-1 iterations to estimate DOA {circumflex over (φ)} 1  of the object  150  with the strongest signal. In the exemplary case of only two objects  150 , the DOA estimate of the weak object  150  is obtained by subtracting the contribution of the strong object  150  from the received signal spectrum, given by: 
         S=svd ( {circumflex over (R)}   xk )  [EQ. 5]
 
     The singular value decomposition (svd) in EQ. 5 results in a vector of singular values. 
         {circumflex over (R)}   xk,Δ   G   k α({circumflex over (φ)} 1 )α H ({circumflex over (φ)} 1 ) G   k   H   [EQ. 6]
 
         S   Δ   =svd ( {circumflex over (R)}   xk,Δ)   [EQ. 7]
 
     
       
         
           
             
               
                 
                   
                     
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     The DOA {circumflex over (φ)} 2  of the object  150  associated with the weak signal is estimated as a mode of the cumulative MUSIC 2,k=1,  . . . ,K-1 response. That is, the MUSIC algorithm is used cumulatively over K-1 iterations to estimate DOA {circumflex over (φ)} 2  of the object  150  with the weaker signal, which is the only other object  150  according to the exemplary case. 
     At block  330 , the processes include generating a set of candidate switching matrices {G k }. The full set of candidates would include every possible combination of M receive antenna elements  210  out of the full set of N receive antenna elements  210 . However, instead of evaluating every combination of M out of the N receive antenna elements  210 , a set of initial combinations is first evaluated, as detailed, to select the set of candidate switching matrices {G K } at block  330 . For the candidate switching matrices {G K }, a decision tree is generated (at block  340 ) to ultimately select the next M receive antenna elements  210  (at block  350 ) that will be defined by the switching matrix G K . 
     At block  330 , the initial combinations are generated by fixing the receive antenna element  210  corresponding with all but one of the M receive channels  230  (i.e., for M-1 receive channels  230 ). Then each of the (N-M-1) remaining receive antenna elements  210  is added to the fixed set, in turn, to generate each initial combination. According to an exemplary embodiment, the fixing may mean retaining (i.e., the receive antenna element  210  corresponding with M-1 receive channels  230  are retained from the previous iteration). According to an example for explanatory purposes, M=3 and N=5 such that there were three receive channels  230  and five receive antenna elements  210  A1, A2, A3, A4, and A5. Assuming that A3 and A5 are fixed as the receive antenna elements  210  for two of the receive channels  230 , then each of A1, A2, and A4 is added to A3 and A5 to generate an initial combination. That is A1, A3, and A5 is one initial combination of receive antenna elements  210 , A2, A3, A5 is a second initial combination of receive antenna elements  210 , and A4, A3, and A5 is a third initial combination of receive antenna elements  210 . 
     Continuing with the processes at block  330 , after the initial combination of receive antenna elements  210  is generated, a set of candidate switching matrices {G K } representing a subset of the initial combination of receive antenna elements  210  is generated. The metric for evaluation of the initial candidates is a Bobrovski-Zakai bound (BZB). A Metropolis-Hastings (MH) algorithm is used to simulate objects  150  for detection. Specifically, the MH algorithm is used twice to generate single-target two-dimensional samples [φ i ,, SNR i ], where i=1, 2 in the exemplary case of two objects  150 . When i=1, the parameters of the strong object  150  and the covariance matrix of EQ. 9 (below) are used. When i=2, the parameters of the weak object  150  and the covariance matrix of EQ. 10 (below) are used. 
     For each n, where n is an index for the number of objects  150 , the BZB on the DOA estimation error is evaluated. The BZB is combined with the CLEAN algorithm for a two-step BZB-CLEAN bound estimation for the exemplary case of two targets. At the first step, the BZB is derived for the strong object  150  (i.e., the object  150  that corresponds with the stronger signal) by subtracting the contribution of the weak object  150 . Then, at the second step, the BZB is derived for the weak object  150  by subtracting the contribution of the strong object  150 . At each step, the covariance matrix is obtained: 
         {circumflex over (R)}   xm,1   ={circumflex over (R)}   xn   −S{circumflex over (N)}R   2   G   n α({circumflex over (φ)} 2 )α H ({circumflex over (φ)} 2 ) G   k   H   [EQ. 9]
 
         {circumflex over (R)}   xn,2   ={circumflex over (R)}   xn   −S{circumflex over (N)}R   1   G   n α({circumflex over (φ)} 1 )α H ({circumflex over (φ)} 1 ) G   k   H   [EQ. 10]
 
     That is, the covariance matrices per EQS. 9 and 10 are used to obtain BZB1 and BZB2, respectively, for each initial combination of receive antenna elements  210 . BZB1 and BZB2 are independently assessed such that only candidates with high scores for both BZB1 and BZB2 are of interest. A predefined number of the initial combination with the highest {BZB1, BZB2} may be retained or all of the initial combinations with {BZB1, BZB2} over a predefined threshold may be retained as the candidate switching matrices {G K }. 
     The process at block  330  may be repeated by fixing the receive antenna element  210  corresponding with all but a different one of the M receive channels  230  for each iteration. Once the candidate switching matrices {G K } specifying a subset of the initial candidates of receive antenna elements  210  are obtained, a multi-step planning process is performed, at block  340 . Each candidate switching matrix from the set of candidate switching matrices {G k } is at the root level of the decision tree. The sub-trees (i.e., additional levels) are grown through random switching. That is, random combinations of M receive antenna elements  210  are the next level of the decision tree. After a predefined depth of the decision tree is developed, the last level is evaluated based on aggregating error statistics and calculating root mean square error (RMSE), which is the average of the error per combination, for each sub-tree. The sub-tree with the minimum RMSE is traced back to its root to select one of the candidate switching matrices {G K } at block  350 . 
     While the above disclosure has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from its scope. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the essential scope thereof Therefore, it is intended that the present disclosure not be limited to the particular embodiments disclosed, but will include all embodiments falling within the scope thereof.