Patent Publication Number: US-2022236370-A1

Title: Radar System with Paired One-Dimensional and Two-Dimensional Antenna Arrays

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application No. 63/142,434, filed Jan. 27, 2021, the disclosure of which is incorporated in its entirety by reference herein. 
    
    
     BACKGROUND 
     Radar systems use antennas to transmit and receive electromagnetic (EM) signals for detecting and tracking objects. In automotive applications, radar antennas can include a two-dimensional (2D) array of elements to measure an azimuth angle and elevation angle associated with the objects. The resolution of the azimuth and elevation angles is generally proportional to the aperture size of the array. Realizing a large aperture with a 2D array may require many antenna elements, which increases cost. It is desirable to maintain the angular resolution of radar systems for both azimuth and elevation angle estimates with minimal antenna elements and cost. 
     SUMMARY 
     This document describes techniques and systems of a radar system with paired one-dimensional (1D) and 2D antenna arrays. Even with far fewer antenna elements than a traditional radar system, the paired arrays enable an example radar system to have a comparable angular resolution at a lower cost and lower complexity level. For example, a radar system includes a processor and an antenna that can receive electromagnetic energy reflected by one or more objects. The antenna includes a 1D (e.g., linear) array and a 2D array. The 1D array includes antenna elements positioned in a first direction (e.g., azimuth direction) and spaced apart by a first distance and a second distance in the first direction. The 2D array includes at least four other antenna elements positioned in the first direction and a second direction (e.g., elevation direction) that is orthogonal to the first direction. The other antenna elements are spaced apart by a third distance in the second direction and by the sum of the first direction and the second direction in the first direction. The processor can determine, using electromagnetic energy received by the 1D array, first and second angles in the first direction associated with the one or more objects. The processor can also determine, using EM energy received by the 2D array, third angles and fourth angles associated with the one or more objects. The third angles are in the first direction, and the fourth angles are in the second direction. The processor can then associate, using the second angles and the third angles, the first angles and the fourth angles with respective objects of the one or more objects. 
     This document also describes methods performed by the above-summarized system and other configurations of the radar system set forth herein and means for performing these methods. 
     This Summary introduces simplified concepts related to a radar system with paired 1D and 2D antenna arrays, further described in the Detailed Description and Drawings. This Summary is not intended to identify essential features of the claimed subject matter, nor is it intended for use in determining the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The details of one or more aspects of a radar system with paired 1D and 2D antenna arrays are described in this document with reference to the following figures. The same numbers are often used throughout the drawings to reference like features and components: 
         FIG. 1  illustrates an example environment in which a radar system with paired 1D and 2D antenna arrays can be implemented; 
         FIGS. 2A and 2B  illustrate example antennas with paired 1D and 2D antenna arrays; 
         FIG. 3  illustrates an example conceptual diagram of a radar system with paired 1D and 2D antenna arrays; 
         FIG. 4  illustrates an example conceptual diagram of an angle-finding module to associate, using paired 1D and 2D antenna arrays, azimuth angles and elevation angles to respective objects; 
         FIG. 5  illustrates another example conceptual diagram of a radar system with paired 1D and 2D antenna arrays; 
         FIG. 6  illustrates another example conceptual diagram of an angle-finding module to associate, using paired 1D and 2D antenna arrays, azimuth angles and elevation angles to respective objects; and 
         FIG. 7  illustrates an example method of a radar system with paired 1D and 2D antenna arrays and an angle-finding module. 
     
    
    
     DETAILED DESCRIPTION 
     Overview 
     Radar systems can be configured as an important sensing technology that vehicle-based systems use to acquire information about the surrounding environment. For example, vehicle-based systems can use radar systems to detect objects in or near a roadway and, if necessary, take necessary actions (e.g., reduce speed, change lanes) to avoid a collision. Radar systems generally include at least two antennas to transmit and receive EM radiation. Some radar systems include a receive antenna with a one-dimensional (1D) linear array of antenna elements to measure the azimuth angle or the elevation angle associated with objects. Such radar systems can estimate a single angle (e.g., elevation angle or azimuth angle) associated with objects. A large aperture in the azimuth direction or the elevation direction can also increase the number of antenna elements and the cost to provide sufficient angular resolution. 
     Some other radar systems include a receive antenna with a two-dimensional (2D) planar array of antenna elements to estimate both the azimuth angle and the elevation angle of objects. In such a radar system, the angular position of objects can be estimated using digital beamforming. In digital beamforming, the radar system characterizes the angular information for objects by analyzing the relative phase across the antenna elements using a 2D fast Fourier transform (FFT). The angular resolution of such radar systems generally depends on the aperture size of the 2D planar array. A larger aperture size can improve the angular resolution but requires additional antenna elements and increased costs. 
     Yet other radar systems include a receive antenna with a sparse 2D array of antenna elements. Such radar systems can use the azimuth linear array and the elevation linear array to estimate the azimuth and elevation angles of objects, respectively. These radar systems use matching algorithms to associate the azimuth angle and elevation angle for each object. Although such systems generally include fewer antenna elements than planar 2D arrays, the increased aperture size tends to introduce grating lobes in the radiation pattern, especially if the array spacing is larger than one-half a wavelength of the EM signals. The angle finding for these systems can also be too complicated for many applications, including automotive applications. 
     In contrast, this document describes techniques and systems to provide a receive antenna with paired 1D and 2D arrays to support angle-finding features. For example, a radar system can include an antenna array with a first sparse 1D array and a sparse 2D array. The 1D array is positioned in a first direction (e.g., elevation direction or azimuth direction) and includes multiple antenna elements spaced apart by a first distance and a second distance. The 2D array includes multiple antenna elements spaced apart by the sum of the first distance and the second distance in the first direction and by a third distance and/or a fourth distance in a second direction (e.g., azimuth direction or elevation direction). The second direction is orthogonal to the first direction. In this way, the described systems and techniques can reduce the number of antenna elements while preserving the angular resolution that can otherwise be achieved using a rectangular 2D array with similar aperture sizing. 
     The radar system estimates, using EM energy received by the paired 1D and 2D arrays, elevation and azimuth angles associated with one or more nearby objects. The radar system can then associate the elevation and azimuth angles with respective objects of the one or more objects. In this way, the computational complexity for the described radar system to associate the azimuth angles and elevation angles to respective objects is similar to the computational complexity for a conventional radar system with conventional 2D planar arrays. The described angle-finding technique can be applied to various configurations of the described paired 1D and 2D arrays. 
     This example is just one example of the described techniques and systems of a radar system with paired 1D and 2D antenna arrays. This document describes other examples and implementations. 
     Operating Environment 
       FIG. 1  illustrates an example environment  100  in which a radar system  102  with paired 1D and 2D antenna arrays can be implemented. In the depicted environment  100 , the radar system  102  is mounted to, or integrated within, a vehicle  104 . The radar system  102  can detect one or more objects  120  in the vicinity of the vehicle  104 . Although illustrated as a car, the vehicle  104  can represent other types of motorized vehicles (e.g., an automobile, a motorcycle, a bus, a tractor, a semi-trailer truck), non-motorized vehicles (e.g., a bicycle), railed vehicles (e.g., a train), watercraft (e.g., a boat), aircraft (e.g., an airplane), or spacecraft (e.g., satellite). In general, manufacturers can mount the radar system  102  to any moving platform, including moving machinery or robotic equipment. 
     In the depicted implementation, the radar system  102  is mounted on the front of the vehicle  104  and illuminates the object  120 . The radar system  102  can detect the object  120  from any exterior surface of the vehicle  104 . For example, the radar system  102  can be integrated into a bumper, side mirror, headlights, rear lights, or any other interior or exterior location where the object  120  requires detection. In some cases, the vehicle  104  includes multiple radar systems  102 , such as a first radar system  102  and a second radar system  102 , that provide a larger instrument field-of-view. In general, the radar system  102  can be designed to have parts of the radar system  102  distributed at different locations of the vehicle  104  to provide a particular field-of-view that encompasses a region of interest. Example fields-of-view include a 360-degree field-of-view, one or more 180-degree fields-of-view, one or more 90-degree fields-of-view, and so forth, which can overlap or be combined into a field-of-view of a particular size. 
     The object  120  is composed of one or more materials that reflect radar signals. Depending on the application, the object  120  can represent a target of interest. In some cases, the object  120  can be a moving object (e.g., another vehicle) or a stationary object (e.g., a roadside sign). 
     The radar system  102  emits EM radiation by transmitting EM signals or waveforms via antenna elements. In the environment  100 , the radar system  102  can detect and track the object  120  by transmitting and receiving one or more radar signals. For example, the radar system  102  can transmit EM signals between 100 and 400 gigahertz (GHz), between 4 and 100 GHz, or between approximately 70 and 80 GHz. 
     The radar system  102  can include a transmitter  106  and at least one antenna  110  to transmit EM signals. The radar system  102  can also include a receiver  108  and the at least one antenna  110  to receive reflected versions of the EM signals. The transmitter  106  includes one or more components for emitting the EM signals. The receiver  108  includes one or more components for detecting the reflected EM signals. The transmitter  106  and the receiver  108  can be incorporated together on the same integrated circuit (e.g., a transceiver integrated circuit) or separately on different integrated circuits. In other implementations, the radar system  102  does not include a separate antenna  110 , but the transmitter  106  and the receiver  108  each include an antenna or antenna elements. 
     The radar system  102  also includes one or more processors  112  (e.g., an energy processing unit) and computer-readable storage media (CRM)  114 . The processor  112  can be a microprocessor or a system-on-chip. The processor  112  can execute instructions stored in the CRM  114 . For example, the processor  112  can process EM energy received by the antenna  110  and determine, using an angle-finding module  116 , a location of the object  120  relative to the radar system  102 . The processor  112  can also generate radar data for at least one automotive system. For example, the processor  112  can control, based on processed EM energy from the antenna  110 , an autonomous or semi-autonomous driving system of the vehicle  104 . 
     The angle-finding module  116  obtains EM energy received by the antenna  110  or the receiver  108  and determines azimuth angles and elevation angles associated with the object  120 . The angle-finding module  116  can be implemented as instructions in the CRM  114 , hardware, software, or a combination thereof that is executed by the processor  112 . 
     The radar system  102  can determine a distance to the object  120  based on the time it takes for the EM signals to travel from the radar system  102  to the object  120 , and from the object  120  back to the radar system  102 . The radar system  102  can also determine, using the angle-finding module  116 , a location of the object  120  in terms of an azimuth angle  128  and an elevation angle  130  based on the direction of a maximum-amplitude echo signal received by the radar system  102 . 
     As an example,  FIG. 1  illustrates the vehicle  104  traveling on a road  118 . The radar system  102  detects the object  120  in front of the vehicle  104 . The radar system  102  can define a coordinate system with an x-axis  122  (e.g., in a forward direction along the road  118 ), a y-axis  124  (e.g., perpendicular to the x-axis  122  and along a surface of the road  118 ), and a z-axis  126  (e.g., perpendicular to the surface of the road  118 ). The radar system  102  can locate the object  120  in terms of the azimuth angle  128  and the elevation angle  130 . The azimuth angle  128  can represent a horizontal angle from the x-axis  122  to the object  120 . The elevation angle  130  can represent a vertical angle from the surface of the road  118  (e.g., a plane defined by the x-axis  122  and the y-axis  124 ) to the object  120 . 
     The vehicle  104  can also include at least one automotive system that relies on data from the radar system  102 , such as a driver-assistance system, an autonomous-driving system, or a semi-autonomous-driving system. The radar system  102  can include an interface to an automotive system that relies on the data. For example, the processor  112  outputs, via the interface, a signal based on EM energy received by the receiver  108  or the antenna  110 . 
     Generally, the automotive systems use radar data provided by the radar system  102  to perform a function. For example, the driver-assistance system can provide blind-spot monitoring and generate an alert that indicates a potential collision with the object  120  that is detected by the radar system  102 . The radar data from the radar system  102  indicates when it is safe or unsafe to change lanes in such an implementation. The autonomous-driving system may move the vehicle  104  to a particular location on the road  118  while avoiding collisions with the object  120  detected by the radar system  102 . The radar data provided by the radar system  102  can provide information about a distance to and the location of the object  120  to enable the autonomous-driving system to perform emergency braking, perform a lane change, or adjust the speed of the vehicle  104 . 
       FIGS. 2A and 2B  illustrate example antennas  200  with paired 1D and 2D arrays. The antennas  200  (e.g., antenna  200 - 1  and antenna  200 - 2 ) are examples of the antenna  110  of the radar system  102  in  FIG. 1 , with similar components. The antennas  200  include multiple antenna elements  208 . The antenna elements  208  represent physical locations or phase-center locations of elements of a 1D array  204  and 2D array  206 . The antenna elements  208  can also represent synthetic locations or phase-center locations of elements of the antennas  200  formed by multiple-input and multiple-output (MIMO) techniques. 
     Fixed to a printed circuit board (PCB)  202 , the antennas  200  include the 1D array  204  in a first direction (e.g., an azimuth array) and the 2D array  206  (e.g., 2D array  206 - 1  and 2D array  206 - 2 ) in the first direction (e.g., the azimuth direction) and a second direction orthogonal to the first direction (e.g., the elevation direction). The antennas  200  can be fabricated with different materials, components, and techniques in other implementations. For example, the antennas  200  can include a lens antenna, a metalized plastic antenna, a dish antenna, a horn antenna, or a combination thereof. 
     In operation, the antennas  200  can receive EM energy reflected by one or more objects  120 . In the depicted implementations, the 1D array  204  is positioned in an azimuth direction. In other implementations, the 1D array  204  can be positioned in an elevation direction or another direction. In the depicted implementations, the 1D array  204  is positioned below the 2D array  206 - 1  and  206 - 2 . The 1D array  204  can be positioned above or to the side of the 2D array  206 - 1  or  206 - 2  in other implementations. For example, the 1D array  204  and the 2D array  206 - 1  or the 2D array  206 - 2  can be configured or arranged in an approximately T-shape (e.g., the 1D array  204  positioned above the 2D array  206 - 1 ), an approximately upside-down T-shape (e.g., the 1D array  204  positioned below the 2D array  206 - 1 ), or an approximately cross shape (e.g., the 1D array  204  positioned through the approximate center of the 2D array  206 - 1 ). 
     The 1D array  204  is a sparse linear array that includes multiple antenna elements  208 . The antenna elements  208  are alternatingly spaced apart by a first distance, d 1 ,  210  and a second distance, d 2 ,  212 . For the illustrated implementation of the antenna  200 - 1  in  FIG. 2A  and moving left-to-right, the antenna elements  208  of the 1D array  204  are separated by the first distance  210 , the second distance  212 , the first distance  210 , the second distance  212 , and so forth. Every other antenna element  208  of the 1D array  204  is separated by a third distance, d 3 ,  214 , which represents the sum of the first distance  210  and the second distance  212 . In other words, the 1D array  204  can be formed by two uniform linear arrays, which both include antenna elements  208  spaced apart by the third distance  214  and that are offset by the first distance  210 . 
     The antenna elements  208  of the 2D arrays  206  can be arranged in an approximately rectangular shape, as illustrated in  FIGS. 2A and 2B . These antenna elements  208  can be positioned close to the antenna elements  208  of the 1D array (e.g., as illustrated in  FIGS. 2A and 2B ). In other implementations, the antenna elements  208  of the 2D arrays  206  can overlap with (e.g., the 1D array can be positioned at a lateral end of the 2D arrays  206  or in between the lateral ends of the 2D arrays  206 ) or be separated from the antenna elements  208  of the 1D array  204 . The antenna elements  208  of the 2D arrays  206  can be arranged in a two-dimensional sparse array, as illustrated in  FIGS. 2A and 2B . The specific arrangement of the 1D array  204  and the 2D arrays  206  can be chosen based on the position and arrangement of other components in the radar system  102 . 
     The 2D array  206  is a sparse 2D array that includes at least four antenna elements  208 . The antenna elements  208  are spaced apart by the third distance  214  in the azimuth direction (or the same direction as the 1D array  204 ). The antenna elements  208  of the 2D array  206 - 1  in the antenna  200 - 1  of  FIG. 2A  are spaced apart by a fourth distance, d 4 ,  214  in the elevation direction (or the direction orthogonal to the direction of the 1D array  204 ). The antenna elements  208  of the 2D array  206 - 2  in the antenna  200 - 2  of  FIG. 2B  are alternatingly spaced apart by the fourth distance  216  and a fifth distance, d 5 ,  218  in the elevation direction (or the direction orthogonal to the direction of the 1D array  204 ). For the illustrated implementation of the antenna  200 - 2  in  FIG. 2B  and moving top-to-bottom, the antenna elements  208  of the 2D array  206 - 2  are separated in the elevation direction by the fourth distance  216 , the fifth distance  218 , the fourth distance  216 , the fifth distance  218 , and so forth. Every other antenna element  208  of the 2D array  204  is separated by a sixth distance, d 6 ,  220  in the elevation direction, representing the sum of the fourth distance  216  and the fifth distance  218 . In other words, the 2D array  206 - 2  can be formed by two uniform 2D arrays, which both include antenna elements  208  spaced apart by the sixth distance  220  in the elevation direction and that are offset by the fourth distance  216 . As described with respect to  FIGS. 3 through 6 , the angle-finding module  116  uses the first distance  210 , the second distance  212 , the third distance  214 , the fourth distance  216 , the fifth distance  218 , and/or the sixth distance  220  to associate an elevation angle to an azimuth angle for a respective object  120 . 
     The 1D array  204  and the 2D array  206  include multiple antenna elements  208 . The 1D array  204  can include M antenna elements  208 . The 2D array  206  can include N antenna elements  208  (e.g., at least four) not encompassed by the 1D array  204 . In automotive applications, the number of antenna elements  208  in the 2D array  206  can be greater than an anticipated maximum number of objects  120  to be detected by the radar system  102 . The number, N, of antenna elements  208  in the 2D array  206  is generally less than the product of M and P, where P represents the number of antenna elements  208  in the elevation direction of the 2D array  206 . In some implementations, N is less than half of the product of M and P (e.g., 
     
       
         
           
             
               
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                     M 
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     The total number of antenna elements  208  in the antenna  200  generally equals M+N. The number of antenna elements  208  in the antenna  200  (e.g., M+N) is generally much less than the number of antenna elements  208  in a rectangular array (e.g., M×P) with the same aperture sizing. 
     In the depicted implementation of  FIG. 2A , the 1D array  204  includes 14 antenna elements  208  and the 2D array  206  includes 16 antenna elements  208  not encompassed by the 1D array  204 . The antenna  200 - 1  includes 30 antenna elements  208 , much less than 80 antenna elements included in a rectangular array with the same aperture sizing. In other implementations, the 1D array  204  and the 2D array  206  can include fewer or additional antenna elements  208 . The 2D array  206  generally includes at least four antenna elements  208  not encompassed by the 1D array  204 . 
     The 1D array  204  and 2D array  206  can be planar arrays that provide high gain and low loss. Planar arrays are well-suited for vehicle integration due to their small size. For example, the antenna elements  208  can be slots etched or otherwise formed in a plating material of one surface of the PCB  202  for a substrate-integrated waveguide (SIW) antenna. As another example, the antenna elements  208  can be radiation slots of a waveguide antenna built with metalized plastic and/or metal. The antenna elements  208  can also be part of an aperture antenna, a microstrip antenna, or a dipole antenna. For example, the 1D array  204  and the 2D array  206  can include subarrays of patch elements (e.g., microstrip patch antenna subarrays) or dipole elements. 
       FIG. 3  illustrates an example conceptual diagram  300  of a radar system with a paired 1D array and 2D array and the angle-finding module  116 . The radar system of  FIG. 3  can, for example, be the radar system  102  of  FIG. 1 . The radar system  102  includes a paired 1D array and 2D array. In the depicted implementation, the radar system  102  includes the 1D array  204  and the 2D array  206 - 1  of the antenna  200 - 1 , which can be arranged in a variety of positions, including the arrangement illustrated in  FIG. 2A . 
     At  304 , the angle-finding module  116  obtains EM energy  302  received by the 1D array  204  and determines azimuth angle estimates  306  and  308  associated with one or more objects  120 . For example, the angle-finding module  116  can determine the azimuth angle estimates  306 , θ 1 , θ 2 , . . . , θ N , based on the first distance, d 1 ,  210 , where N represents the estimated number of targets. The angle-finding module  116  can also determine the azimuth angle estimates  308 , φ 1 , φ 2 , . . . , φ N , based on the third distance, d 3 ,  214 , which represents the sum of the first distance  210  and the second distance  212 . 
     At  312 , the angle-finding module  116  obtains EM energy  310  received by the 2D array  206 - 1  and determines azimuth angle estimates  314  and elevation angle estimates  316  associated with the one or more objects  120 . For example, the angle-finding module  116  can determine the azimuth angle estimates  314 , φ 1 , φ 2 , . . . , φ N , based on the third distance, d 1 ,  210 . The angle-finding module  116  can also determine the elevation angle estimates  316 , φ 1 , φ 2 , . . . , φ N , based on the fourth distance  216 , d 4 . 
     The angle-finding module  116  can use various angle-finding functions to determine the azimuth angle estimates  306 , the azimuth angle estimates  308 , the azimuth angle estimates  314 , and the elevation angle estimates  316  from the EM energy  302  and the EM energy  310 . As non-limiting examples, the angle-finding module  116  can use a pseudo-spectrum function, including a Space-Alternating Generalized Expectation-maximization (SAGE), Delay-and-Sum (DS), Minimum Variance Distortionless Response (MVDR), and/or a Multiple Signal Classification (MUSIC) based-function, to calculate the direction of arrival of the EM signals received by the 1D array  204  and the 2D array  206 - 1 . As another example, the angle-finding module  116  can use an Estimation of Signal Parameters via Rotational Invariance Technique (ESPRIT) technique or FFT beamforming to calculate the azimuth angle estimates  306 , the azimuth angle estimates  308 , the azimuth angle estimates  314 , and the elevation angle estimates  316 . The angle-finding module  116  can determine the azimuth angle estimates and the elevation angle estimates with relatively low processing complexity and cost. 
     At  318 , the angle-finding module  116  associates, based on common (e.g., shared) azimuth angle estimates  308  and  314 , the azimuth angle estimates  306  and the elevation angle estimates  316  for the objects  120 . In particular, the angle-finding module  116  determines the azimuth angle estimate  306  and the elevation angle estimate  316  associated with each of the one or more objects  120 . An example of the conceptual diagram  300  is described in greater detail with respect to  FIG. 4 . 
       FIG. 4  illustrates an example conceptual diagram  400  of an angle-finding module to associate azimuth angle estimates  306  and elevation angle estimates  316  to respective objects  120 . The angle-finding module of  FIG. 4  can, for example, be the angle-finding module  116  of  FIGS. 1 through 3 . As described with respect to  FIG. 3 , the angle-finding module  116  determines the azimuth angle estimates  306  and the elevation angle estimates  316  associated with the objects  120 . 
     At  402 , the angle-finding module  116  can form or generate a complex covariance matrix based on the EM energy  302  and the EM energy  310  received by the 1D array  204  and the 2D array  206 - 1 , respectively. For example, the angle-finding module  116  can form the complex covariance matrix, R, using Equation (1): 
       R=xx H   (1)
 
     where x represents the measurements from the EM energy  302  and the EM energy  310 , and H represents a Hermitian matrix (e.g., a self-adjoint matrix) of x. The Hermitian matrix is a complex square matrix that is equal to its own conjugate transpose (e.g., an element in the i-th row and the j-th column is equal to the complex conjugate of the element in the j-th row and the i-th column). 
     At  404 , the angle-finding module  116  can estimate the number of targets, N, (e.g., detected objects  120 ) and the signal subspace, E s , from the complex covariance matrix, R, using eigenvalue decomposition. 
     At  406 , the angle-finding module  116  can select elements of the signal subspace, E s , corresponding to the 1D array  204  and determine the azimuth angle estimates  306 , θ 1 , θ 2 , . . . , θ N , and the azimuth angle estimates  308 , φ 1 , φ 2 , . . . , φ N , where N represents the estimated number of objects  120 . The angle-finding module  116  can, for example, apply two-dimensional unitary Estimation of Signal Parameters via Rotational Invariance Technique (ESPRIT) to estimate the azimuth angles. The azimuth angle estimates  306  are based on the first distance  210 , and the azimuth angle estimates  308  are based on the third distance  214 . In other implementations, the angle-finding module  116  can use other pseudo-spectrum functions, including Space-Alternating Generalized Expectation-maximization (SAGE), Delay-and-Sum (DS), Minimum Variance Distortionless Response (MVDR), and/or a Multiple Signal Classification (MUSIC) based-function, to calculate the direction of arrival of the EM signals received by the 1D array  204 . The angle-finding module  116  can determine the azimuth angle estimates  306  and  308  with relatively low processing complexity and cost. 
     At  408 , the angle-finding module  116  can select elements of the signal subspace corresponding to the 2D array  206 - 1  and determine the azimuth angle estimates  314 , φ 1 , φ 2 , . . . , φ N , and the elevation angle estimates  316 , φ 1 , φ 2 , . . . , φ N , where N represents the estimated number of objects  120 . The angle-finding module  116  can, for example, apply two-dimensional unitary ESPRIT to estimate the azimuth angles and elevation angles. The azimuth angle estimates  314  are based on the third distance  214  in the 2D array  206 - 1 , and the elevation angle estimates  316  are based on the fourth distance  216  in the 2D array  206 - 1 . The angle-finding module  116  can use other pseudo-spectrum functions in other implementations, including SAGE, Delay-and-Sum, MVDR, and/or MUSIC based-function to calculate the direction of arrival of the EM signals received by the 2D array  206 - 1 . The angle-finding module  116  can determine the azimuth angle estimates  314  and the elevation angle estimates  316  with relatively low processing complexity and cost. 
     At  410 , the angle-finding module  116  can unfold azimuth angles from the azimuth angle estimates  306 ,  308 , and  314  and pair unfolded azimuth angle estimates  306  with the elevation angle estimates  316 . The elevation angle estimates  316  can be paired with the azimuth angle estimates  306  using common or shared angles in the azimuth angle estimates  308  and  314 . For example, the angle-finding module  116  can automatically pair multiple angle pairs from among the elevation angle estimates  316  and the azimuth angle estimates  306  estimated using 2D ESPRIT. As described above, the azimuth angle estimates  308  and  314  are both based on the third distance  214  in the azimuth direction. 
       FIG. 5  illustrates another example conceptual diagram  500  of a radar system with a paired 1D array and 2D array and the angle-finding module  116 . The radar system of  FIG. 5  can, for example, be the radar system  102  of  FIG. 1 . The radar system  102  includes a paired 1D array and 2D array. In the depicted implementation, the radar system  102  includes the 1D array  204  and the 2D array  206 - 2  of the antenna  200 - 2 , which can be arranged in a variety of positions, including the arrangement illustrated in  FIG. 2B . 
     At  504 , the angle-finding module  116  obtains EM energy  502  received by the 1D array  204  and determines azimuth angle estimates  506  and  508  associated with one or more objects  120 . For example, the angle-finding module  116  can determine the azimuth angle estimates  506 , θ 1 , θ 2 , . . . , θ N , based on the first distance, d 1 ,  210 , where N represents the estimated number of targets. The angle-finding module  116  can also determine the azimuth angle estimates  508 , φ 1 , φ 2 , . . . , φ N , based on the third distance, d 3 ,  214 , which represents the sum of the first distance  210  and the second distance  212 . 
     At  512 , the angle-finding module  116  obtains EM energy  510  received by the 2D array  206 - 2  and determines elevation angle estimates  514  and elevation angle estimates  516  associated with the one or more objects  120 . For example, the angle-finding module  116  can determine the elevation angle estimates  514 , ϕ 1 , ϕ 2 , . . . , ϕ N , based on the sixth distance, d 6 ,  220 . The angle-finding module  116  can also determine the elevation angle estimates  516 , β 1 , β 2 , . . . , β N , based on the fourth distance  216 , d 4 . 
     The angle-finding module  116  can use various angle-finding functions to determine the azimuth angle estimates  506 , the azimuth angle estimates  508 , the elevation angle estimates  514 , and the elevation angle estimates  516  from the EM energy  502  and the EM energy  510 . As described above, the angle-finding module  116  can use a pseudo-spectrum function, including a SAGE, DS, MVDR, and/or a MUSIC based-function, to calculate the direction of arrival of the EM signals received by the 1D array  204  and the 2D array  206 - 2 . As another example, the angle-finding module  116  can use an ESPRIT technique or FFT beamforming to calculate the azimuth angle estimates  506 , the azimuth angle estimates  508 , the elevation angle estimates  514 , and the elevation angle estimates  516 . The angle-finding module  116  can determine the azimuth angle estimates and the elevation angle estimates with relatively low processing complexity and cost. 
     At  518 , the angle-finding module  116  associates, based on the common azimuth angle estimates  508  and the elevation angle estimate  514 , the azimuth angle estimates  506  and the elevation angle estimates  516  to the objects  120 . In particular, the angle-finding module  116  determines the azimuth angle estimate  506  and the elevation angle estimate  516  associated with each of the one or more objects  120 . An example of the conceptual diagram  500  is described in greater detail with respect to  FIG. 6 . 
       FIG. 6  illustrates another example conceptual diagram  600  of an angle-finding module to associate azimuth angle estimates  506  and elevation angle estimates  516  to respective objects  120 . The angle-finding module of  FIG. 6  can, for example, be the angle-finding module  116  of  FIGS. 1 through 3 . As described with respect to  FIG. 5 , the angle-finding module  116  determines the azimuth angle estimates  506  and the elevation angle estimates  516  associated with the objects  120 . 
     At  602 , the angle-finding module  116  can form or generate a complex covariance matrix based on the EM energy  502  and the EM energy  510  received by the 1D array  204  and the 2D array  206 - 2 , respectively. For example, the angle-finding module  116  can form the complex covariance matrix, R, using Equation (2): 
       R=xx H   (2)
 
     where x represents the measurements from the EM energy  502  and the EM energy  510  and H represents a Hermitian matrix (e.g., a self-adjoint matrix) of x. The Hermitian matrix is a complex square matrix that is equal to its own conjugate transpose (e.g., an element in the i-th row and the j-th column is equal to the complex conjugate of the element in the j-th row and the i-th column). 
     At  604 , the angle-finding module  116  can estimate the number of targets, N, (e.g., detected objects  120 ) and the signal subspace, E s , from the complex covariance matrix, R, using eigenvalue decomposition. 
     At  606 , the angle-finding module  116  can select elements of the signal subspace corresponding to the 1D array  204  and determine the azimuth angle estimates  506 , θ 1 , θ 2 , . . . , θ N , and the azimuth angle estimates  508 , φ 1 , φ 2 , . . . , φ N , where N represents the estimated number of objects  120 . The angle-finding module  116  can, for example, apply two-dimensional unitary ESPRIT to estimate the azimuth angles. The azimuth angle estimates  506  are based on the first distance  210 , and the azimuth angle estimates  508  are based on the third distance  214 . In other implementations, the angle-finding module  116  can use other pseudo-spectrum functions, including SAGE, DS, MVDR, and/or a MUSIC based-function, to calculate the direction of arrival of the EM signals received by the 1D array  204 . The angle-finding module  116  can determine the azimuth angle estimates  506  and  508  with relatively low processing complexity and cost. 
     At  608 , the angle-finding module  116  can select elements of the signal subspace corresponding to the 2D array  206 - 2  and determine the elevation angle estimates  514 , ϕ 1 , ϕ 2 , . . . , ϕ N , and the elevation angle estimates  516 , β 1 , β 2 , . . . , β N , where N represents the estimated number of objects  120 . The angle-finding module  116  can, for example, apply two-dimensional unitary ESPRIT to estimate the elevation angles. The elevation angle estimates  514  are based on the sixth distance  220  in the 2D array  206 - 2 , and the elevation angle estimates  516  are based on the fourth distance  216  in the 2D array  206 - 2 . In other implementations, the angle-finding module  116  can use other pseudo-spectrum functions, including SAGE, DS, MVDR, and/or MUSIC based-functions, to calculate the direction of arrival of the EM signals received by the 2D array  206 - 2 . The angle-finding module  116  can determine the potential elevation angle estimates  514  and the elevation angle estimates  516  with relatively low processing complexity and cost. 
     At  610 , the angle-finding module  116  can unfold azimuth angles from the azimuth angle estimates  506  and  508 . The angle-finding module  116  can also unfold elevation angles from the elevation angle estimates  514  and  516 . The angle-finding module  116  can then pair unfolded azimuth angle estimates  506  with elevation angle estimates  516 . The elevation angle estimates  516  can be paired with the azimuth angle estimates  506  using common or shared angle pairs in the azimuth angle estimates  508  and the elevation angle estimates  514 . 
     Example Method 
       FIG. 7  illustrates an example method  700  of the radar system  102  with paired 1D and 2D antenna arrays and the angle-finding module  116 . Method  700  is shown as sets of operations (or acts) performed, but not necessarily limited to the order or combinations in which the operations are shown herein. Further, any of one or more of the operations may be repeated, combined, or reorganized to provide other methods. In portions of the following discussion, reference may be made to the environment  100  of  FIG. 1 , and entities detailed in  FIGS. 1 through 6 , reference to which is made for example only. The techniques are not limited to performance by one entity or multiple entities. 
     At  702 , an antenna of a radar system receives EM energy at a 1D array reflected by one or more objects. The 1D array includes first antenna elements positioned in a first direction and spaced apart by a first distance and a second distance in the first direction. For example, the antenna  200 - 1  or the antenna  200 - 2  of the radar system  102  can receive the EM energy  302  or  502  at the 1D array  204  reflected by the one or more objects  120 . The 1D array  204  can include multiple antenna elements  208  positioned in the azimuth direction. In other implementations, the antenna elements  208  of the 1D array  204  can be positioned in the elevation direction or another direction. The antenna elements  208  of the 1D array  204  are alternatingly spaced apart by the first distance, d 1 ,  210  and the second distance, d 2 ,  212 . 
     At  704 , an antenna of the radar system receives EM energy at a 2D array reflected by the one or more objects. The 2D array includes at least four second antenna elements not encompassed by the 1D array. The second antenna elements are positioned in the first direction and a second direction, which is orthogonal to the first direction. The second antenna elements are spaced apart in the first direction by a sum of the first distance and a second distance. At least some of the second antenna elements are spaced apart by a third distance in the second direction. For example, the antenna  200 - 1  or the antenna  200 - 2  of the radar system  102  can receive EM energy  310  or  510  at the 2D array  206 - 1  or the 2D array  206 - 2 , respectively, reflected by the one or more objects  120 . The 2D arrays  206  can include multiple antenna elements  208  positioned in the azimuth direction and the elevation direction. In particular, the 2D arrays  206  include at least four antenna elements  208  not encompassed by the 1D array  204 . In other implementations, the antenna elements  208  of the 2D arrays  206  can be positioned in other directions that are orthogonal to one another. The antenna elements  208  of the 2D arrays  206  are spaced apart by the third distance, d 3 ,  214 , in the azimuth direction. The third distance  214  represents the sum of the first distance  210  and the second distance  212 . The antenna elements  208  of the 2D array  206 - 1  are spaced apart by the fourth distance, d 4 ,  216  in the elevation direction. The antenna elements  208  of the 2D array  206 - 2  are alternatingly spaced apart by the fourth distance, d 4 ,  216  and the fifth distance, d 5 ,  218  in the elevation direction. 
     At  706 , first angles and second angles associated with the one or more objects are determined based on EM energy received at the 1D array. The first angles and the second angles are in the first direction. For example, the processor  112  of the radar system  102  can determine, using the angle-finding module  116  and the EM energy  302  received by the 1D array  204 , the azimuth angle estimates  306  and  308  associated with the one or more objects  120 . 
     At  708 , third angles and fourth angles associated with the one or more objects are determined using EM energy received by the 2D array. The third angles are in the first direction, and the fourth angles are in the second direction. For example, the processor  112  can determine, using the angle-finding module  116  and the EM energy  310  received by the 2D array  206 - 1 , the azimuth angle estimates  314  and the elevation angle estimates  316  associated with the one or more objects  120 . 
     At  710 , the first angles and the fourth angles are associated with respective objects of the one or more objects using the second angles and the third angles. For example, the processor  112  can associate, using the angle-finding module  116  and the azimuth angle estimates  308  and the azimuth angle estimates  314 , the azimuth angle estimates  306  and the elevation angle estimates  316  with respective objects of the one or more objects  120 . 
     Examples 
     In the following section, examples are provided. 
     Example 1: A radar system comprising: an antenna configured to receive electromagnetic (EM) energy reflected by one or more objects, the antenna comprising: a one-dimensional (1D) array comprising first antenna elements positioned in a first direction, the first antenna elements spaced apart by a first distance and a second distance in the first direction; and a two-dimensional (2D) array comprising at least four second antenna elements not encompassed by the 1D array and positioned in the first direction and a second direction, the second direction being orthogonal to the first direction, at least some of the second antenna elements spaced apart by a third distance in the second direction and by a sum of the first distance and the second distance in the first direction; and one or more processors configured to: determine, using the EM energy received at the 1D array, first angles and second angles associated with one or more objects, the first angles and the second angles being in the first direction; determine, using the EM energy received at the 2D array, third angles and fourth angles associated with the one or more objects, the third angles being in the first direction and the fourth angles being in the second direction; and associate, using the second angles and the third angles, the first angles and the fourth angles with respective objects of the one or more objects. 
     Example 2: The radar system of example 1, wherein: the first angles are determined based on the first distance; the second angles are determined based on the sum of the first distance and the second distance; and the third angles are determined based on the third distance. 
     Example 3: The radar system of example 1, wherein the second antenna elements are spaced apart by the third distance and a fourth distance in the second direction. 
     Example 4: The radar system of example 3, wherein the one or more processors are further configured to: determine, using the EM energy received by the 2D array, fifth angles associated with the one or more objects, the fifth angles in the second direction, wherein the one or more processors are configured to associate the first angles and the third angles with the respective objects of the one or more objects by at least associating, using the second angles, the fourth angles, and the fifth angles, the first angles and the third angles with respective objects of the one or more objects. 
     Example 5: The radar system of example 4, wherein: the first angles are determined based on the first distance; the second angles and the fourth angles are determined based on the sum of the first distance and the second distance; the third angles are determined based on the third distance; and the fifth angles are determined based on the sum of the third distance and the fourth distance. 
     Example 6: The radar system of example 1, wherein the one or more processors are configured to associate the first angles and the fourth angles with the respective objects of the one or more objects in the following manner: generate, using the EM energy received at the 1D array and the 2D array, a complex covariance matrix; determine, using eigenvalue decomposition of the complex covariance matrix, an estimate of a number of the one or more objects; unfold the first angles and the fourth angles; and pair the unfolded fourth angles with the unfolded first angles using common values of the second angles and the third angles. 
     Example 7: The radar system of example 4, wherein the one or more processors are configured to associate the first angles and the fourth angles with the respective objects of the one or more objects in the following manner: generate, using the EM energy received at the 1D array and the 2D array, a complex covariance matrix; determine, using eigenvalue decomposition of the complex covariance matrix, an estimate of a number of the one or more objects; unfold the first angles, the second angles, the third angles, the fourth angles, and the fifth angles; and pair the unfolded fourth angles with the unfolded first angles using common values of the second angles and the fifth angles. 
     Example 8: The radar system of example 1, wherein the 1D array is positioned in an azimuth direction and the 2D array is positioned in the azimuth direction and an elevation direction. 
     Example 9: The radar system of example 1, wherein the 1D array is a linear array. 
     Example 10: The radar system of example 9, wherein the 1D array and the 2D array are configured in an approximately T-shape, an approximately upside-down T-shape, or an approximately cross shape. 
     Example 11: The radar system of example 1, wherein the 1D array comprises a first number of antenna elements and the 2D array comprises a second number of antenna elements in the first direction, the first number of antenna elements greater than the second number of antenna elements. 
     Example 12: The radar system of example 1, wherein the second antenna elements of the 2D array are configured in an approximately rectangular shape. 
     Example 13: The radar system of example 1, wherein the second antenna elements of the 2D array are positioned in a sparse array. 
     Example 14: The radar system of example 1, wherein the first angles, the second angles, the third angles, and the fourth angles are determined using at least one of an Estimation of Signal Parameters via Rotational Invariance Technique (ESPRIT), Space-Alternating Generalized Expectation-maximization (SAGE), Delay-and-Sum (DS), Minimum Variance Distortionless Response (MVDR), a Multiple Signal Classification (MUSIC), or a fast Fourier transform (FFT) beamforming based-function. 
     Example 15: The radar system of example 1, wherein the radar system is configured to be installed on an automobile. 
     Example 16: A computer-readable storage media comprising computer-executable instructions that, when executed, cause a processor of a radar system to: receive, by an antenna of the radar system, electromagnetic (EM) energy reflected by one or more objects; determine, using the EM energy received by a one-dimensional (1D) array of the antenna, first angles and second associated with one or more objects, the 1D array comprising first antenna elements positioned in a first direction that are spaced apart by a first distance and a second distance in the first direction, the first angles and the second angles being in the first direction; determine, using the EM energy received by a two-dimensional (2D) array of the antenna, third angles and fourth associated with the one or more objects, the 2D array comprising at least four second antenna elements not encompassed by the 1D array and positioned in the first direction and a second direction, the second direction being orthogonal to the first direction, at least some of the second antenna elements spaced apart by a third distance in the second direction and by a sum of the first distance and the second distance in the first direction, the third angles being in the first direction and the fourth angles being in the second direction; and associate, using the second angles and the third angles, the first angles and the fourth angles with respective objects of the one or more objects. 
     Example 17: The computer-readable storage media of example 16, wherein: the first angles are determined based on the first distance; the second angles are determined based on the sum of the first distance and the second distance; and the third angles are determined based on the third distance. 
     Example 18: The computer-readable storage media of example 16, wherein the second antenna elements are spaced apart by the third distance and a fourth distance in the second direction. 
     Example 19: The computer-readable storage media of example 18, the computer-readable storage media comprising further computer-executable instructions that, when executed, cause a processor of a radar system to: determine, using the EM energy received by the 2D array, fifth angles associated with the one or more objects, the fifth angles in the second direction, wherein association of the first angles and the third angles with the respective objects of the one or more objects comprises associate, using the second angles, the fourth angles, and the fifth angles, the first angles and the third angles with respective objects of the one or more objects. 
     Example 20: A method comprising: receiving, by an antenna of a radar system, electromagnetic (EM) energy reflected by one or more objects; determining, using the EM energy received by a one-dimensional (1D) array of the antenna, first angles and second associated with one or more objects, the 1D array comprising first antenna elements positioned in a first direction that are spaced apart by a first distance and a second distance in the first direction, the first angles and the second angles being in the first direction; determining, using the EM energy received by a two-dimensional (2D) array of the antenna, third angles and fourth associated with the one or more objects, the 2D array comprising at least four second antenna elements not encompassed by the 1D array and positioned in the first direction and a second direction, the second direction being orthogonal to the first direction, at least some of the second antenna elements spaced apart by a third distance in the second direction and by a sum of the first distance and the second distance in the first direction, the third angles being in the first direction and the fourth angles being in the second direction; and associating, using the second angles and the third angles, the first angles and the fourth angles with respective objects of the one or more objects. 
     CONCLUSION 
     While various embodiments of the disclosure are described in the foregoing description and shown in the drawings, it is to be understood that this disclosure is not limited thereto but may be variously embodied to practice within the scope of the following claims. From the foregoing description, it will be apparent that various changes may be made without departing from the scope of the disclosure as defined by the following claims.