Patent Publication Number: US-2023161027-A1

Title: Smart-Device-Based Radar System Performing Near-Range Detection

Description:
BACKGROUND 
     Radars are useful devices that can detect objects. Relative to other types of sensors, like a camera, a radar can provide improved performance in the presence of different environmental conditions, such as low lighting and fog, or with moving or overlapping objects. Radar can also detect objects through one or more occlusions, such as a purse or a pocket. While radar has many advantages, there are many challenges associated with integrating radar in consumer devices. These challenges include size and layout constraints of the consumer device, internal interference generated by other components within the consumer device, and external interference generated by other consumer devices. 
     SUMMARY 
     Techniques and apparatuses are described that implement a smart-device-based radar system capable of performing near-range detection. The radar system employs separate modules for detecting objects at different range intervals. In particular, the radar system includes a near-range detection module for detecting objects at near ranges in the presence of interference and a far-range detection module for detecting objects at far ranges. By evaluating separate range intervals, these modules can be designed to achieve a target false-alarm rate and detection performance by tailoring their processing to general characteristics of objects and interference at their respective range intervals. 
     For example, the near-range detection module can distinguish between an interference artifact at near ranges and a near-range object by evaluating differences in range rates, signal-to-noise ratios, or spatial coverages. To identify the near-range object, the near-range detection module processes detections within low-Doppler bins, processes detections with amplitudes that are greater than or equal to a near-range threshold, and/or processes detections associated with a near-range spatial coverage. In this way, the near-range detection module can filter (e.g., not process) an interference artifact that is within high-Doppler bins, has an amplitude that is less than the near-range threshold, and/or is not associated with the near-range spatial coverage. Using these techniques, the near-range detection module can successfully detect the near-range object without generating a false detection associated with the interference artifact. 
     In contrast to the near-range detection module, the far-range detection module processes detections within high-Doppler bins, processes detections with amplitudes that are greater than or equal to a far-range threshold, which is less than the near-range threshold, and/or processes detections associated with a far-range spatial coverage. In some implementations, the near-range detection module can be dynamically enabled or disabled depending on the behavior of objects detected by the far-range detection module or an indication from a proximity sensor (e.g., a camera or an infrared sensor). By utilizing the near-range detection module and the far-range detection module, the radar system can detect objects at both near and far ranges while achieving a target false-alarm rate. 
     Aspects described below include a method performed by a radar system. The method includes transmitting radar transmit signals using an antenna array of the radar system and receiving radar receive signals using the antenna array. The radar receive signals comprise respective reflected versions of the radar transmit signals. The radar transmit signals are reflected by a user. The method also includes generating range-Doppler maps based on the radar receive signals and processing far-range portions of the range-Doppler maps using a far-range detection module of the radar system. The method additionally includes detecting, within the far-range portions of a set of the range-Doppler maps, the user approaching the smart device. The method further includes processing near-range portions of the range-Doppler maps using a near-range detection module of the radar system. The near-range portions of the range-Doppler maps includes at least one interference artifact. The processing is effective to filter the at least one interference artifact from the near-range portions of the range-Doppler maps. The method also includes detecting, within the near-range portions of another set of the range-Doppler maps, the user interacting with the smart device. 
     Aspects described below also include an apparatus with a radar system. The radar system includes an antenna array and a transceiver. The radar system also includes a processor and computer-readable storage media configured to perform any of the methods described herein. 
     Aspects described below also include a radar system with means for performing near-range detection. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Apparatuses for and techniques implementing a smart-device-based radar system capable of performing near-range detection are described with reference to the following drawings. The same numbers are used throughout the drawings to reference like features and components: 
         FIG.  1    illustrates example environments in which a smart-device-based radar system capable of performing near-range detection can be implemented. 
         FIG.  2 - 1    illustrates an example implementation of a radar system as part of a smart device. 
         FIG.  2 - 2    illustrates an example location of a radar system relative to other components within a smartphone. 
         FIG.  3 - 1    illustrates an operation of an example radar system. 
         FIG.  3 - 2    illustrates an example radar framing structure. 
         FIG.  4    illustrates an example antenna array and an example transceiver of a radar system. 
         FIG.  5    illustrates an example scheme implemented by a radar system for performing near-range detection. 
         FIG.  6    illustrates an example portion of a hardware-abstraction module for performing near-range detection. 
         FIG.  7    illustrates an example range-Doppler map for performing near-range detection. 
         FIG.  8 - 1    illustrates an example scheme implemented by a range-windowing module, a far-range detection module, and a near-range detection module. 
         FIG.  8 - 2    illustrates an example implementation of a near-range detection module. 
         FIG.  8 - 3    illustrates an example implementation of a far-range detection module. 
         FIG.  9    illustrates an example method of a radar system for performing near-range detection. 
         FIG.  10    illustrates an example computing system embodying, or in which techniques may be implemented that enable use of, a radar system capable of performing near-range detection. 
     
    
    
     DETAILED DESCRIPTION 
     Overview 
     Integrating a radar system within an electronic device can be challenging. One such challenge involves size or layout constraints of the electronic device, which may limit where the radar system can be placed relative to other components within the electronic device. In some cases, these components generate interference, which can be detected by the radar system. This interference can include, for instance, an audible sound produced by a speaker of the electronic device or a wireless communication signal transmitted by a wireless transceiver of the electronic device. The radar system can also detect interference (e.g., another wireless communication signal or a radar signal) that is generated from another electronic device. 
     These types of interference can generate an interference artifact at near ranges, which can make it challenging for the radar system to detect near-range objects and achieve a target false-alarm rate. To the radar system, the interference artifact can appear to be one or more moving objects within a near-range portion of the radar system&#39;s range-Doppler map. If the interference artifact is not filtered, some radar systems may generate a false detection (or a false alarm) based on the interference artifact. A false detection or false alarm represents an erroneous detection that does not correspond to an object of interest. It can be challenging for the radar system to detect objects of interest (e.g., desired objects) that are in the far-range portion or the near-range portion of the range-Doppler map without generating a false detection based on the interference artifact. As such, evaluating the near-range portion of the range-Doppler map can increase the radar system&#39;s false-alarm rate and degrade the performance of the radar system. 
     To avoid increasing the false-alarm rate, some radar systems may not process the near-range portion of the range-Doppler map. Although this enables the radar system to avoid generating false detections based on the interference artifact, the radar system is unable to detect an object at near ranges. Consequently, this limits the volume of space in which the radar system can detect a desired object and therefore limits effective operation of the radar system. 
     In contrast, this document describes techniques and devices that implement a smart-device-based radar system capable of performing near-range detection. The radar system employs separate modules for detecting objects at different range intervals. In particular, the radar system includes a near-range detection module for detecting objects at near ranges in the presence of interference and a far-range detection module for detecting objects at far ranges. By evaluating separate range intervals, these modules can be designed to achieve a target false-alarm rate and detection performance by tailoring their processing to general characteristics of objects and interference at their respective range intervals. 
     For example, the near-range detection module can distinguish between an interference artifact at near ranges and a near-range object by evaluating differences in range rates, signal-to-noise ratios, or spatial coverages. To identify the near-range object, the near-range detection module processes detections within low-Doppler bins, processes detections with amplitudes that are greater than or equal to a near-range threshold, and/or processes detections associated with a near-range spatial coverage. In this way, the near-range detection module can filter (e.g., not process) an interference artifact that is within high-Doppler bins, has an amplitude that is less than the near-range threshold, and/or is not associated with the near-range spatial coverage. Using these techniques, the near-range detection module can successfully detect the near-range object without generating a false detection associated with the interference artifact. 
     In contrast to the near-range detection module, the far-range detection module processes detections within high-Doppler bins, processes detections with amplitudes that are greater than or equal to a far-range threshold, which is less than the near-range threshold, and/or processes detections associated with a far-range spatial coverage. In some implementations, the near-range detection module can be dynamically enabled or disabled depending on the behavior of objects detected by the far-range detection module or an indication from a proximity sensor (e.g., a camera or an infrared sensor). By utilizing the near-range detection module and the far-range detection module, the radar system can detect objects at both near and far ranges while achieving a target false-alarm rate. 
     For example, a range-Doppler map can comprise an array of cells, each cell being associated with one of a plurality of range bins and with one of a plurality of Doppler bins, each range bin corresponding to a particular range interval, and each Doppler bin corresponding to a particular Doppler frequency interval. 
     Example Environment 
       FIG.  1    is an illustration of example environments  100 - 1  to  100 - 6  in which techniques using, and an apparatus including, a smart-device-based radar system capable of performing near-range detection may be embodied. In the depicted environments  100 - 1  to  100 - 6 , a radar system  102  of a smart device  104  is capable of detecting one or more objects (e.g., users). The smart device  104  is shown to be a smartphone in environments  100 - 1  to  100 - 5  and a smart vehicle in the environment  100 - 6 . In general, the smart device  104  may, e.g., be a user device comprising a computer processor and computer-readable media. 
     In the environments  100 - 1  to  100 - 4 , a user performs different types of gestures, which are detected by the radar system  102 . In some cases, the user performs a gesture using an appendage or body part. Alternatively, the user can also perform a gesture using a stylus, a hand-held object, a ring, or any type of material that can reflect radar signals. 
     In environment  100 - 1 , the user makes a scrolling gesture by moving a hand above the smart device  104  along a horizontal dimension (e.g., from a left side of the smart device  104  to a right side of the smart device  104 ). In the environment  100 - 2 , the user makes a reaching gesture, which decreases a distance between the smart device  104  and the user&#39;s hand. The users in environment  100 - 3  make hand gestures to play a game on the smart device  104 . In one instance, a user makes a pushing gesture by moving a hand above the smart device  104  along a vertical dimension (e.g., from a bottom side of the smart device  104  to a top side of the smart device  104 ). In the environment  100 - 4 , the smart device  104  is stored within a purse, and the radar system  102  provides occluded-gesture recognition by detecting gestures that are occluded by the purse. 
     The radar system  102  can also recognize other types of gestures or motions not shown in  FIG.  1   . Example types of gestures include a knob-turning gesture in which a user curls their fingers to grip an imaginary doorknob and rotate their fingers and hand in a clockwise or counter-clockwise fashion to mimic an action of turning the imaginary doorknob. Another example type of gesture includes a spindle-twisting gesture, which a user performs by rubbing a thumb and at least one other finger together. The gestures can be two-dimensional, such as those used with touch-sensitive displays (e.g., a two-finger pinch, a two-finger spread, or a tap). The gestures can also be three-dimensional, such as many sign-language gestures, e.g., those of American Sign Language (ASL) and other sign languages worldwide. Upon detecting each of these gestures, the smart device  104  can perform an action, such as display new content, move a cursor, activate one or more sensors, open an application, and so forth. In this way, the radar system  102  provides touch-free control of the smart device  104 . 
     In the environment  100 - 5 , the radar system  102  generates a three-dimensional map of a surrounding environment for contextual awareness. The radar system  102  also detects and tracks multiple users to enable both users to interact with the smart device  104 . The radar system  102  can also perform vital-sign detection. In the environment  100 - 6 , the radar system  102  monitors vital signs of a user that drives a vehicle. Example vital signs include a heart rate and a respiration rate. If the radar system  102  determines that the driver is falling asleep, for instance, the radar system  102  can cause the smart device  104  to alert the user. Alternatively, if the radar system  102  detects a life threatening emergency, such as a heart attack, the radar system  102  can cause the smart device  104  to alert a medical professional or emergency services. 
     In the environments  100 - 1  to  100 - 6 , the radar system  102  can detect both near-range objects and far-range objects. This enables the radar system  102  to detect a gesture that traverses from a far range to a near range or a user&#39;s face that is within a near range from the smart device  104 . By detecting objects within the near range, the radar system  102  can reliably control the smart device  104 . For example, the radar system  102  can conserve power or transition to a lock screen responsive to determining that the user is not present within the near range. 
     Some implementations of the radar system  102  are particularly advantageous as applied in the context of smart devices  104 , for which there is a convergence of issues. This can include a need for limitations in a spacing and layout of the radar system  102  and low power. Exemplary overall lateral dimensions of the smart device  104  can be, for example, approximately eight centimeters by approximately fifteen centimeters. Exemplary footprints of the radar system  102  can be even more limited, such as approximately four millimeters by six millimeters with antennas included. Exemplary power consumption of the radar system  102  may be on the order of a few milliwatts to tens of milliwatts (e.g., between approximately two milliwatts and twenty milliwatts). The requirement of such a limited footprint and power consumption for the radar system  102  enables the smart device  104  to include other desirable features in a space-limited package (e.g., a camera sensor, a fingerprint sensor, a display, and so forth). The smart device  104  and the radar system  102  are further described with respect to  FIG.  2 - 1   . 
       FIG.  2 - 1    illustrates the radar system  102  as part of the smart device  104 . The smart device  104  is illustrated with various non-limiting example devices including a desktop computer  104 - 1 , a tablet  104 - 2 , a laptop  104 - 3 , a television  104 - 4 , a computing watch  104 - 5 , computing glasses  104 - 6 , a gaming system  104 - 7 , a microwave  104 - 8 , and a vehicle  104 - 9 . Other devices may also be used, such as a home service device, a smart speaker, a smart thermostat, a security camera, a baby monitor, a router, a drone, a trackpad, a drawing pad, a netbook, an e-reader, a home-automation and control system, a wall display, and another home appliance. Note that the smart device  104  can be wearable, non-wearable but mobile, or relatively immobile (e.g., desktops and appliances). The radar system  102  can be used as a stand-alone radar system or used with, or embedded within, many different smart devices  104  or peripherals, such as in control panels that control home appliances and systems, in automobiles to control internal functions (e.g., volume, cruise control, or even driving of the car), or as an attachment to a laptop computer to control computing applications on the laptop. 
     The smart device  104  includes one or more computer processors  202  and computer-readable media  204 , which includes memory media and storage media. Applications and/or an operating system (not shown) embodied as computer-readable instructions on the computer-readable media  204  can be executed by the computer processor  202  to provide some of the functionalities described herein. The computer-readable media  204  also includes a radar-based application  206 , which uses radar data generated by the radar system  102  to perform a function, such as presence detection, gesture-based touch-free control, collision avoidance for autonomous driving, human vital-sign notification, and so forth. 
     The smart device  104  can also include a network interface  208  for communicating data over wired, wireless, or optical networks. For example, the network interface  208  may communicate data over a local-area-network (LAN), a wireless local-area-network (WLAN), a personal-area-network (PAN), a wire-area-network (WAN), an intranet, the Internet, a peer-to-peer network, point-to-point network, a mesh network, and the like. The smart device  104  may also include a display (not shown). 
     The radar system  102  includes a communication interface  210  to transmit the radar data to a remote device, though this need not be used when the radar system  102  is integrated within the smart device  104 . In general, the radar data provided by the communication interface  210  is in a format usable by the radar-based application  206 . 
     The radar system  102  also includes at least one antenna array  212  and at least one transceiver  214  to transmit and receive radar signals. The antenna array  212  includes at least one transmit antenna element and at least one receive antenna element. In some situations, the antenna array  212  includes multiple transmit antenna elements to implement a multiple-input multiple-output (MIMO) radar capable of transmitting multiple distinct waveforms at a given time (e.g., a different waveform per transmit antenna element). The antenna elements can be circularly polarized, horizontally polarized, vertically polarized, or a combination thereof. 
     In some implementations, the antenna array  212  includes two or more receive antenna elements for digital beamforming. The receive antenna elements of the antenna array  212  can be positioned in a one-dimensional shape (e.g., a line) or a two-dimensional shape (e.g., a rectangular arrangement, a triangular arrangement, or an “L” shape arrangement) for implementations that include three or more receive antenna elements. The one-dimensional shape enables the radar system  102  to measure one angular dimension (e.g., an azimuth or an elevation) while the two-dimensional shape enables the radar system  102  to measure two angular dimensions (e.g., to determine both an azimuth angle and an elevation angle of the object). An element spacing associated with the receive antenna elements can be less than, greater than, or equal to half a center wavelength of the radar signal. 
     Using the antenna array  212 , the radar system  102  can form beams that are steered or un-steered, wide or narrow, or shaped (e.g., hemisphere, cube, fan, cone, cylinder). The steering and shaping can be achieved through analog beamforming or digital beamforming. The one or more transmitting antenna elements can have, for instance, an un-steered omnidirectional radiation pattern or can produce a wide steerable beam to illuminate a large volume of space. To achieve target angular accuracies and angular resolutions, the receiving antenna elements can be used to generate hundreds or thousands of narrow steered beams with digital beamforming. In this way, the radar system  102  can efficiently monitor an external environment and detect one or more users. 
     The transceiver  214  includes circuitry and logic for transmitting and receiving radar signals via the antenna array  212 . Components of the transceiver  214  can include amplifiers, mixers, switches, analog-to-digital converters, or filters for conditioning the radar signals. The transceiver  214  also includes logic to perform in-phase/quadrature (I/Q) operations, such as modulation or demodulation. A variety of modulations can be used, including linear frequency modulations, triangular frequency modulations, stepped frequency modulations, or phase modulations. Alternatively, the transceiver  214  can produce radar signals having a relatively constant frequency or a single tone. The transceiver  214  can be configured to support continuous-wave or pulsed radar operations. 
     A frequency spectrum (e.g., range of frequencies) that the transceiver  214  uses to generate the radar signals can encompass frequencies between 1 and 400 gigahertz (GHz), between 4 and 100 GHz, between 1 and 24 GHz, between 2 and 4 GHz, between 57 and 64 GHz, or at approximately 2.4 GHz. In some cases, the frequency spectrum can be divided into multiple sub-spectrums that have similar or different bandwidths. The bandwidths can be on the order of 500 megahertz (MHz), 1 GHz, 2 GHz, and so forth. Different frequency sub-spectrums may include, for example, frequencies between approximately 57 and 59 GHz, 59 and 61 GHz, or 61 and 63 GHz. Although the example frequency sub-spectrums described above are contiguous, other frequency sub-spectrums may not be contiguous. To achieve coherence, multiple frequency sub-spectrums (contiguous or not) that have a same bandwidth may be used by the transceiver  214  to generate multiple radar signals, which are transmitted simultaneously or separated in time. In some situations, multiple contiguous frequency sub-spectrums may be used to transmit a single radar signal, thereby enabling the radar signal to have a wide bandwidth. 
     The radar system  102  also includes one or more system processors  216  and a system media  218  (e.g., one or more computer-readable storage media). The system media  218  optionally includes a hardware-abstraction module  220 . The system media  218  also includes a near-range detection module  222  and a far-range detection module  224 . The hardware-abstraction module  220 , the near-range detection module  222 , and the far-range detection module  224  can be implemented using hardware, software, firmware, or a combination thereof. In this example, the system processor  216  implements the hardware-abstraction module  220 , the near-range detection module  222 , and the far-range detection module  224 . Together, the hardware-abstraction module  220 , the near-range detection module  222 , and the far-range detection module enable the system processor  216  to process responses from the receive antenna elements in the antenna array  212  to detect a user, determine a position of the object, and/or recognize a gesture performed by the user. 
     In an alternative implementation (not shown), the hardware-abstraction module  220 , the near-range detection module  222 , and the far-range detection module  224  are included within the computer-readable media  204  and implemented by the computer processor  202 . This enables the radar system  102  to provide the smart device  104  raw data via the communication interface  210  such that the computer processor  202  can process the raw data for the radar-based application  206 . 
     The hardware-abstraction module  220  transforms raw data provided by the transceiver  214  into hardware-agnostic radar data, which can be processed by the near-range detection module  222  and the far-range detection module  224 . In particular, the hardware-abstraction module  220  conforms complex radar data from a variety of different types of radar signals to an expected input of the near-range detection module  222  and the far-range detection module  224 . This enables the near-range detection module  222  and the far-range detection module  224  to process different types of radar signals received by the radar system  102 , including those that utilize different modulations schemes for frequency-modulated continuous-wave radar, phase-modulated spread spectrum radar, or impulse radar. The hardware-abstraction module  220  can also normalize complex radar data from radar signals with different center frequencies, bandwidths, transmit power levels, or pulsewidths. 
     Additionally, the hardware-abstraction module  220  conforms complex radar data generated using different hardware architectures. Different hardware architectures can include different antenna arrays  212  positioned on different surfaces of the smart device  104  or different sets of antenna elements within an antenna array  212 . By using the hardware-abstraction module  220 , the near-range detection module  222  and the far-range detection module  224  can process complex radar data generated by different sets of antenna elements with different gains, different sets of antenna elements of various quantities, or different sets of antenna elements with different antenna element spacings. Furthermore, the hardware-abstraction module  220  enables the near-range detection module  222  and the far-range detection module  224  to operate in radar systems  102  with different limitations that affect the available radar modulation schemes, transmission parameters, or types of hardware architectures. The hardware-abstraction module  220  is further described with respect to  FIG.  6   . Both the near-range detection module  222  and the far-range detection module  224  can operate on the hardware-agnostic radar data provided by the hardware-abstraction module  220 . 
     The near-range detection module  222  is designed to detect near-range objects in the presence of interference. To distinguish between an interference artifact at near ranges and a near-range object, the near-range detection module  222  utilizes general characteristics of the near-range object and the interference, which can differ. The near-range object, for example, can have a smaller range-rate relative to an interference artifact generated by the interference. This range rate enables the near-range object to avoid colliding with the smart device  104 . Additionally or alternatively, the near-range object can have a large signal-to-noise ratio compared to the interference artifact. The near-range object can also be spatially large relative to the interference artifact and therefore have a large amplitude across multiple range bins or angular bins. 
     To identify the near-range object, the near-range detection module processes detections within low-Doppler bins, processes detections with amplitudes that are greater than or equal to a near-range threshold, and/or processes detections associated with a near-range spatial coverage. In this way, the near-range detection module can filter (e.g., not process) the interference artifact that is within high-Doppler bins, has an amplitude that is less than the near-range threshold, and/or is not associated with the near-range spatial coverage. Using these techniques, the near-range detection module can successfully detect the near-range object without generating a false detection associated with the interference artifact. The near-range detection module  222  is further described with respect to  FIGS.  8 - 1  and  8 - 2   . 
     The far-range detection module  224  is designed to detect far-range objects. Characteristics of a far-range object can differ from a near-range object. For example, the far-range object can have a larger range rate than the near-range object. Due to the far-range object being observed at farther ranges, the far-range object can also have a smaller signal-to-noise ratio than the near-range object and appear to be spatially smaller than the near-range object. Therefore, the far-range detection module  224  performs one or more different operations relative to the near-range detection module  222  in order to detect the far-range object. For example, the far-range detection module  224  processes detections within high-Doppler bins, processes detections with amplitudes that are greater than a far-range threshold, which is less than the near-range threshold, and/or processes detections associated with a far-range spatial coverage, which is smaller than the near-range spatial coverage. The far-range detection module  224  is further described with respect to  FIGS.  8 - 1  and  8 - 3   . 
     In some implementations, the near-range detection module  222  is dynamically enabled or disabled depending on the behavior of objects detected by the far-range detection module  224  or an indication from a proximity sensor (e.g., a camera or an infrared sensor). For example, the radar system  102  can enable the near-range detection module  222  if the far-range detection module  224  identifies an object that is approaching the near-range interval (e.g., moving towards the radar system  102  or the smart device  104 ). As another example, the proximity sensor enables the near-range detection module  222  if the proximity sensor detects the user. 
     The near-range detection module  222  produces near-range detection data, and the far-range detection module  224  produces far-range detection data, both of which can be further analyzed by the system processor  216 . For example, the system processor  216  can process the near-range detection data and the far-range detection data to generate radar-application data for the radar-based application  206 . Example types of radar-application data include presence of a user, position of the user, movement of the user, a type of gesture performed by the user, a measured vital-sign of the user, a collision alert, or characteristics of an object. 
       FIG.  2 - 2    illustrates an example location of the radar system  102  relative to other components within the smart device  104 . In this example, the smart device  104  is shown to be a smartphone  104 - 10 . An exterior of the smartphone  104 - 10  includes an exterior housing  242  and an exterior viewing panel  226 . As an example, the exterior housing  242  has a vertical height of approximately 147 millimeters (mm), a horizontal length of approximately 69 mm, and a width of approximately 8 mm. The exterior housing  242  can be composed of metal material, for instance. 
     The exterior viewing panel  226  forms an exterior face of the smartphone  104 - 10  and has a vertical height of approximately 139 mm and a horizontal length of approximately 61 mm. The exterior viewing panel  226  includes cut-outs for various components that are positioned within an interior of the smartphone  104 - 10  (e.g., positioned beneath the exterior viewing panel  226 ). These components are further described below. 
     The exterior viewing panel  226  can be formed using various types of glass or plastics that are found within display screens. In some implementations, the exterior viewing panel  226  has a dielectric constant (e.g., a relative permittivity) between approximately four and ten, which attenuates or distorts radar signals. As such, the exterior viewing panel  226  is opaque or semi-transparent to a radar signal and can cause a portion of a transmitted or received radar signal to be reflected. 
     At least a portion of the radar system  102 , such as an integrated circuit that includes the antenna array  212  and the transceiver  214 , is positioned beneath the exterior viewing panel  226  and near an edge of the smartphone  104 - 10 . As an example, the integrated circuit has a vertical height of approximately 5 mm, a horizontal length of approximately 6.5 mm, and a thickness of approximately 0.85 mm (within +/−0.1 mm along each dimension). These dimensions enable the integrated circuit to fit between the exterior housing  242  and a display element  228 . The vertical height of the integrated circuit can be similar to other components that are positioned near the edge of the smartphone  104 - 10  so as to avoid reducing a size of the display element  228 . 
     In this example implementation, the antenna array  212  is oriented towards (e.g., faces) the exterior viewing panel  226 . As such, the integrated circuit radiates through the exterior viewing panel  226  (e.g., transmits and receives the radar signals that propagate through the exterior viewing panel  226 ). If the exterior viewing panel  226  behaves as an attenuator, the radar system  102  can adjust a frequency or a steering angle of a transmitted radar signal to mitigate the effects of the attenuator instead of increasing transmit power. As such, the radar system  102  can realize enhanced accuracy and longer ranges for detecting the user without increasing power consumption. 
     The display element  228  displays images that are viewed through the exterior viewing panel  226 . As shown, the antenna array  212  of the radar system  102  is oriented towards (e.g., faces) a same direction as the display element  228  such that the radar system  102  transmits radar signals towards a user that is looking at the display element  228 . 
     In this example, the radar system  102  transmits and receives radar signals with frequencies between approximately 57 and 64 GHz. This mitigates electromagnetic interference with a wireless communication system of the smartphone  104 - 10 , which uses frequencies below 20 GHz, for instance. Transmitting and receiving radar signals with millimeter wavelengths further enables the integrated circuit of the radar system  102  to realize the above footprint. 
     A depicted interior of the smartphone  104 - 10  includes the integrated circuit of the radar system  102 , the display element  228 , an infrared sensor  230 , a speaker  232 , a proximity sensor  234 , an ambient light sensor  236 , a camera  238 , and another infrared sensor  240 . The integrated circuit of the radar system  102 , the infrared sensor  230 , the speaker  232 , the proximity sensor  234 , the ambient light sensor  236 , the camera  238 , and the infrared sensor  240  are positioned beneath an upper portion of the exterior viewing panel  226 . The display element  228  is positioned beneath the lower portion of the exterior viewing panel  226 . In this example, a distance between a top edge of the display element  228  and a top edge of the exterior viewing panel  226  (D GD ) is approximately 6.2 mm. 
     The infrared sensors  230  and  240  can be used for facial recognition. To conserve power, the infrared sensors  230  and  240  operate in an off-state when not in use. However, a warm-up sequence associated with transitioning the infrared sensors  230  and  240  from the off-state to an on-state can require a significant amount of time, such as a half-second or more. This can cause a delay in execution of the facial recognition. To reduce this time delay, the radar system  102  proactively detects the user reaching towards or approaching the smartphone  104 - 10  and initiates the warm-up sequence prior to the user touching the smartphone  104 - 10 . As such, the infrared sensors  230  and  240  can be in the on-state sooner and reduce a time the user waits for the facial recognition to complete. 
     In this example, the integrated circuit of the radar system  102  is positioned between the infrared sensor  230  and the speaker  232 . A distance between the integrated circuit and the speaker  232  (D SR ) is approximately 0.93 mm or less. As such, the radar system  102  is within close proximity to the speaker  232  and can vibrate while the speaker  232  produces audible sounds. 
     The radar system  102  can detect internal interference  244 , which is generated by components within the smartphone  104 - 10 , or external interference  246 , which is generated by an external device, such as another smartphone. Example types of internal interference  244  include audible sounds produced by the speaker  232 , wireless communication signals generated by a wireless transceiver (not shown) of the smartphone  104 - 10 , or noise that propagates along a power line, which supplies power to the radar system  102 . Another type of internal interference  244  includes the cross-coupling within the antenna array  212  of the radar system  102 . The external interference  246  can include other audible sounds within the external environment, other wireless communication signals transmitted by other devices, or other radar signals generated by radar systems of other devices. Both the internal interference  244  and the external interference  246  can produce interference artifacts that are detected by the radar system  102  within the near range or the far range, as further described with respect to  FIG.  7   . By using separate detection modules that are tailored to different range intervals, the radar system  102  can filter the interference artifacts to achieve the target false alarm rate. 
       FIG.  3 - 1    illustrates an example operation of the radar system  102 . In the depicted configuration, the radar system  102  is implemented as a frequency-modulated continuous-wave radar. However, other types of radar architectures can be implemented, as described above with respect to  FIG.  2 - 1   . In environment  300 , a user  302  is located at a particular slant range  304  from the radar system  102 . To detect the user  302 , the radar system  102  transmits a radar transmit signal  306 . At least a portion of the radar transmit signal  306  is reflected by the user  302 . This reflected portion represents a radar receive signal  308 . The radar system  102  receives the radar receive signal  308  and processes the radar receive signal  308  to extract data for the radar-based application  206 . As depicted, an amplitude of the radar receive signal  308  is smaller than an amplitude of the radar transmit signal  306  due to losses incurred during propagation and reflection. 
     The radar transmit signal  306  includes a sequence of chirps  310 - 1  to  310 -N, where N represents a positive integer greater than one. The radar system  102  can transmit the chirps  310 - 1  to  310 -N in a continuous burst or transmit the chirps  310 - 1  to  310 -N as time-separated pulses, as further described with respect to  FIG.  3 - 2   . A duration of each chirp  310 - 1  to  310 -N can be on the order of tens or thousands of microseconds (e.g., between approximately 30 microseconds (μs) and 5 milliseconds (ms)), for instance. 
     Individual frequencies of the chirps  310 - 1  to  310 -N can increase or decrease over time. In the depicted example, the radar system  102  employs a two-slope cycle (e.g., triangular frequency modulation) to linearly increase and linearly decrease the frequencies of the chirps  310 - 1  to  310 -N over time. The two-slope cycle enables the radar system  102  to measure the Doppler frequency shift caused by motion of the user  302 . In general, transmission characteristics of the chirps  310 - 1  to  310 -N (e.g., bandwidth, center frequency, duration, and transmit power) can be tailored to achieve a particular detection range, range resolution, or doppler sensitivity for detecting one or more characteristics the user  302  or one or more actions performed by the user  302 . 
     At the radar system  102 , the radar receive signal  308  represents a delayed version of the radar transmit signal  306 . The amount of delay is proportional to the slant range  304  (e.g., distance) from the antenna array  212  of the radar system  102  to the user  302 . In particular, this delay represents a summation of a time it takes for the radar transmit signal  306  to propagate from the radar system  102  to the user  302  and a time it takes for the radar receive signal  308  to propagate from the user  302  to the radar system  102 . If the user  302  and/or the radar system  102  is moving, the radar receive signal  308  is shifted in frequency relative to the radar transmit signal  306  due to the Doppler effect. In other words, characteristics of the radar receive signal  308  are dependent upon motion of the hand and/or motion of the radar system  102 . Similar to the radar transmit signal  306 , the radar receive signal  308  is composed of one or more of the chirps  310 - 1  to  310 -N. 
     The multiple chirps  310 - 1  to  310 -N enable the radar system  102  to make multiple observations of the user  302  over a predetermined time period. A radar framing structure determines a timing of the chirps  310 - 1  to  310 -N, as further described with respect to  FIG.  3 - 2   . 
       FIG.  3 - 2    illustrates an example radar framing structure  312  for near-range detection. In the depicted configuration, the radar framing structure  312  includes three different types of frames. At a top level, the radar framing structure  312  includes a sequence of main frames  314 , which can be in the active state or the inactive state. Generally speaking, the active state consumes a larger amount of power relative to the inactive state. At an intermediate level, the radar framing structure  312  includes a sequence of feature frames  316 , which can similarly be in the active state or the inactive state. Different types of feature frames  316  include a pulse-mode feature frame  318  (shown at the bottom-left of  FIG.  3 - 2   ) and a burst-mode feature frame  320  (shown at the bottom-right of  FIG.  3 - 2   ). At a low level, the radar framing structure  312  includes a sequence of radar frames (RF)  322 , which can also be in the active state or the inactive state. 
     The radar system  102  transmits and receives a radar signal during an active radar frame  322 . In some situations, the radar frames  322  are individually analyzed for basic radar operations, such as search and track, clutter map generation, user location determination, and so forth. Radar data collected during each active radar frame  322  can be saved to a buffer after completion of the radar frame  322  or provided directly to the system processor  216  of  FIG.  2   . 
     The radar system  102  analyzes the radar data across multiple radar frames  322  (e.g., across a group of radar frames  322  associated with an active feature frame  316 ) to identify a particular feature. Example types of features include a particular type of motion, a motion associated with a particular appendage (e.g., a hand or individual fingers), and a feature associated with different portions of the gesture. To detect a change in the radar system  102 &#39;s frame of reference or recognize a gesture performed by the user  302  during an active main frame  314 , the radar system  102  analyzes the radar data associated with one or more active feature frames  316 . 
     A duration of the main frame  314  may be on the order of milliseconds or seconds (e.g., between approximately 10 ms and 10 seconds (s)). After active main frames  314 - 1  and  314 - 2  occur, the radar system  102  is inactive, as shown by inactive main frames  314 - 3  and  314 - 4 . A duration of the inactive main frames  314 - 3  and  314 - 4  is characterized by a deep sleep time  324 , which may be on the order of tens of milliseconds or more (e.g., greater than 50 ms). In an example implementation, the radar system  102  turns off all of the active components (e.g., an amplifier, an active filter, a voltage-controlled oscillator (VCO), a voltage-controlled buffer, a multiplexer, an analog-to-digital converter, a phase-lock loop (PLL) or a crystal oscillator) within the transceiver  214  to conserve power during the deep sleep time  324 . 
     In the depicted radar framing structure  312 , each main frame  314  includes K feature frames  316 , where K is a positive integer. If the main frame  314  is in the inactive state, all of the feature frames  316  associated with that main frame  314  are also in the inactive state. In contrast, an active main frame  314  includes J active feature frames  316  and K-J inactive feature frames  316 , where J is a positive integer that is less than or equal to K. A quantity of feature frames  316  can be adjusted based on a complexity of the environment or a complexity of a gesture. For example, a main frame  314  can include a few to a hundred feature frames  316  (e.g., K may equal 2, 10, 30, 60, or 100). A duration of each feature frame  316  may be on the order of milliseconds (e.g., between approximately 1 ms and 50 ms). 
     To conserve power, the active feature frames  316 - 1  to  316 -J occur prior to the inactive feature frames  316 -(J+1) to  316 -K. A duration of the inactive feature frames  316 -(J+1) to  316 -K is characterized by a sleep time  326 . In this way, the inactive feature frames  316 -(J+1) to  316 -K are consecutively executed such that the radar system  102  can be in a powered-down state for a longer duration relative to other techniques that may interleave the inactive feature frames  316 -(J+1) to  316 -K with the active feature frames  316 - 1  to  316 -J. Generally speaking, increasing a duration of the sleep time  326  enables the radar system  102  to turn off components within the transceiver  214  that require longer start-up times. 
     Each feature frame  316  includes L radar frames  322 , where L is a positive integer that may or may not be equal to J or K. In some implementations, a quantity of radar frames  322  may vary across different feature frames  316  and may comprise a few frames or hundreds of frames (e.g., L may equal 5, 15, 30, 100, or 500). A duration of a radar frame  322  may be on the order of tens or thousands of microseconds (e.g., between approximately 30 μs and 5 ms). The radar frames  322  within a particular feature frame  316  can be customized for a predetermined detection range, range resolution, or doppler sensitivity, which facilitates detection of a particular feature or gesture. For example, the radar frames  322  may utilize a particular type of modulation, bandwidth, frequency, transmit power, or timing. If the feature frame  316  is in the inactive state, all of the radar frames  322  associated with that feature frame  316  are also in the inactive state. 
     The pulse-mode feature frame  318  and the burst-mode feature frame  320  include different sequences of radar frames  322 . Generally speaking, the radar frames  322  within an active pulse-mode feature frame  318  transmit pulses that are separated in time by a predetermined amount. This disperses observations over time, which can make it easier for the radar system  102  to recognize a gesture due to larger changes in the observed chirps  310 - 1  to  310 -N within the pulse-mode feature frame  318  relative to the burst-mode feature frame  320 . In contrast, the radar frames  322  within an active burst-mode feature frame  320  transmit pulses continuously across a portion of the burst-mode feature frame  320  (e.g., the pulses are not separated by a predetermined amount of time). This enables an active-burst-mode feature frame  320  to consume less power than the pulse-mode feature frame  318  by turning off a larger quantity of components, including those with longer start-up times, as further described below. 
     Within each active pulse-mode feature frame  318 , the sequence of radar frames  322  alternates between the active state and the inactive state. Each active radar frame  322  transmits a chirp  310  (e.g., a pulse), which is illustrated by a triangle. A duration of the chirp  310  is characterized by an active time  328 . During the active time  328 , components within the transceiver  214  are powered-on. During a short-idle time  330 , which includes the remaining time within the active radar frame  322  and a duration of the following inactive radar frame  322 , the radar system  102  conserves power by turning off one or more active components within the transceiver  214  that have a start-up time within a duration of the short-idle time  330 . 
     An active burst-mode feature frame  320  includes P active radar frames  322  and L-P inactive radar frames  322 , where P is a positive integer that is less than or equal to L. To conserve power, the active radar frames  322 - 1  to  322 -P occur prior to the inactive radar frames  322 -(P+1) to  322 -L. A duration of the inactive radar frames  322 -(P+1) to  322 -L is characterized by a long-idle time  332 . By grouping the inactive radar frames  322 -(P+1) to  322 -L together, the radar system  102  can be in a powered-down state for a longer duration relative to the short-idle time  330  that occurs during the pulse-mode feature frame  318 . Additionally, the radar system  102  can turn off additional components within the transceiver  214  that have start-up times that are longer than the short-idle time  330  and shorter than the long-idle time  332 . 
     Each active radar frame  322  within an active burst-mode feature frame  320  transmits a portion of the chirp  310 . In this example, the active radar frames  322 - 1  to  322 -P alternate between transmitting a portion of the chirp  310  that increases in frequency and a portion of the chirp  310  that decreases in frequency. 
     The radar framing structure  312  enables power to be conserved through adjustable duty cycles within each frame type. A first duty cycle  334  is based on a quantity of active feature frames  316  (J) relative to a total quantity of feature frames  316  (K). A second duty cycle  336  is based on a quantity of active radar frames  322  (e.g., L/2 or P) relative to a total quantity of radar frames  322  (L). A third duty cycle  338  is based on a duration of the chirp  310  relative to a duration of a radar frame  322 . 
     Consider an example radar framing structure  312  for a power state that consumes approximately 2 milliwatts (mW) of power and has a main-frame update rate between approximately 1 and 4 hertz (Hz). In this example, the radar framing structure  312  includes a main frame  314  with a duration between approximately 250 ms and 1 second. The main frame  314  includes thirty-one pulse-mode feature frames  318  (e.g., K is equal to 31). One of the thirty-one pulse-mode feature frames  318  is in the active state. This results in the duty cycle  334  being approximately equal to 3.2%. A duration of each pulse-mode feature frame  318  is between approximately 8 and 32 ms. Each pulse-mode feature frame  318  is composed of eight radar frames  322  (e.g., L is equal to 8). Within the active pulse-mode feature frame  318 , all eight radar frames  322  are in the active state. This results in the duty cycle  336  being equal to 100%. A duration of each radar frame  322  is between approximately 1 and 4 ms. An active time  328  within each of the active radar frames  322  is between approximately 32 and 128 μs. As such, the resulting duty cycle  338  is approximately 3.2%. This example radar framing structure  312  has been found to yield good performance results. These good performance results are in terms of good near-range detection, gesture recognition, and presence detection while also yielding good power efficiency results in the application context of a handheld smartphone in a low-power state. Generation of the radar transmit signal  306  (of  FIG.  3 - 1   ) and the processing of the radar receive signal  308  (of  FIG.  3 - 1   ) are further described with respect to  FIG.  4   . 
       FIG.  4    illustrates an example antenna array  212  and an example transceiver  214  of the radar system  102 . In the depicted configuration, the transceiver  214  includes a transmitter  402  and a receiver  404 . The transmitter  402  includes at least one voltage-controlled oscillator  406  and at least one power amplifier  408 . The receiver  404  includes at least two receive channels  410 - 1  to  410 -M, where M is a positive integer greater than one. Each receive channel  410 - 1  to  410 -M includes at least one low-noise amplifier  412 , at least one mixer  414 , at least one filter  416 , and at least one analog-to-digital converter  418 . 
     The antenna array  212  includes at least one transmit antenna element  420  and at least two receive antenna elements  422 - 1  to  422 -M. The transmit antenna element  420  is coupled to the transmitter  402 . The receive antenna elements  422 - 1  to  422 -M are respectively coupled to the receive channels  410 - 1  to  410 -M. Although the radar system  102  of  FIG.  4    is shown to include multiple receive antenna elements  422 - 1  to  422 -M and multiple receive channels  410 - 1  to  410 -M, the described techniques for near-range detection can also be applied to radar systems  102  that utilize a single receive antenna element  422  and a single receive channel  410 . 
     During transmission, the voltage-controlled oscillator  406  generates a frequency-modulated radar signal  424  at radio frequencies. The power amplifier  408  amplifies the frequency-modulated radar signal  424  for transmission via the transmit antenna element  420 . The transmitted frequency-modulated radar signal  424  is represented by the radar transmit signal  306 , which can include multiple chirps  310 - 1  to  310 -N based on the radar framing structure  312  of  FIG.  3 - 2   . As an example, the radar transmit signal  306  is generated according to the burst-mode feature frame  320  of  FIG.  3 - 2    and includes 16 chirps  310  (e.g., N equals 16). 
     During reception, each receive antenna element  422 - 1  to  422 -M receives a version of the radar receive signal  308 - 1  to  308 -M. In general, relative phase differences between these versions of the radar receive signals  308 - 1  to  308 -M are due to differences in locations of the receive antenna elements  422 - 1  to  422 -M. Within each receive channel  410 - 1  to  410 -M, the low-noise amplifier  412  amplifies the radar receive signal  308 , and the mixer  414  mixes the amplified radar receive signal  308  with the frequency-modulated radar signal  424 . In particular, the mixer performs a beating operation, which downconverts and demodulates the radar receive signal  308  to generate a beat signal  426 . 
     A frequency of the beat signal  426  represents a frequency difference between the frequency-modulated radar signal  424  and the radar receive signal  308 , which is proportional to the slant range  304  of  FIG.  3 - 1   . Although not shown, the beat signal  426  can include multiple frequencies, which represents reflections from different portions of the user  302  (e.g., different fingers, different portions of a hand, or different body parts). In some cases, these different portions move at different speeds, move in different directions, or are positioned at different slant ranges relative to the radar system  102 . 
     The filter  416  filters the beat signal  426 , and the analog-to-digital converter  418  digitizes the filtered beat signal  426 . The receive channels  410 - 1  to  410 -M respectively generate digital beat signals  428 - 1  to  428 -M, which are provided to the system processor  216  for processing. The receive channels  410 - 1  to  410 -M of the transceiver  214  are coupled to the system processor  216 , as shown in  FIG.  5   . 
       FIG.  5    illustrates an example scheme implemented by the radar system  102  for performing near-range detection. In the depicted configuration, the system processor  216  implements the hardware-abstraction module  220 , the near-range detection module  222 , and the far-range detection module  224 . The system processor  216  is connected to the receive channels  410 - 1  to  410 -M. The system processor  216  can also communicate with the computer processor  202  (of  FIG.  2   ). Although not shown, the hardware-abstraction module  220 , the near-range detection module  222 , and/or the far-range detection module  224  can be implemented by the computer processor  202 . 
     In this example, the hardware-abstraction module  220  accepts the digital beat signals  428 - 1  to  428 -M from the receive channels  410 - 1  to  410 -M. The digital beat signals  428 - 1  to  428 -M represent raw or unprocessed complex radar data. The hardware-abstraction module  220  performs one or more operations to generate hardware-agnostic radar data  502 - 1  to  502 -M based on digital beat signals  428 - 1  to  428 -M. The hardware-abstraction module  220  transforms the complex radar data provided by the digital beat signals  428 - 1  to  428 -M into a form that is expected by the near-range detection module  222  and the far-range detection module  224 . In some cases, the hardware-abstraction module  220  normalizes amplitudes associated with different transmit power levels or transforms the complex radar data into a frequency-domain representation. 
     The hardware-agnostic radar data  502 - 1  to  502 -M can include magnitude information or both magnitude and phase information (e.g., in-phase and quadrature components). In some implementations, the hardware-agnostic radar data  502 - 1  to  502 -M includes range-Doppler maps for each receive channel  410 - 1  to  410 -M and for a particular active feature frame  316 , as further described with respect to  FIGS.  6  and  7   . 
     The near-range detection module  222  generates near-range detection data  504  based on the hardware-agnostic radar data  502 - 1  to  502 -M. As an example, the near-range detection data  504  includes measured characteristics associated with one or more objects detected by the near-range detection module  222 . The measured characteristics can include a signal-to-noise ratio, range, range rate, azimuth, or elevation of the one or more objects detected within the near-range interval. 
     The far-range detection module  224  generates far-range detection data  506  based on the hardware-agnostic radar data  502 - 1  to  502 -M. As an example, the far-range detection data  506  includes measured characteristics associated with one or more objects detected by the far-range detection module  224 . The measured characteristics can include a signal-to-noise ratio, range, range rate, azimuth, or elevation of the one or more objects detected within the far-range interval. 
     The near-range detection data  504  and the far-range detection data  506  can be provided to other modules within the radar system  102 , such as a gesture-recognition module, a presence-detection module, a collision-avoidance module, a vital-sign measurement module, a tracking module, and so forth. These modules produce radar-application data  508 , which is provided to the radar-based application  206  of  FIG.  2 - 1   . Generally, the techniques described above can be expanded to implement more than two detection modules for processing more than two different range intervals. Operation of the hardware-abstraction module  220  is further described with respect to  FIG.  6   . 
       FIG.  6    illustrates an example hardware-abstraction module  220  for performing near-range detection. In the depicted configuration, the hardware-abstraction module  220  includes a pre-processing stage  602  and a signal-transformation stage  604 . The pre-processing stage  602  operates on each chirp  310 - 1  to  310 -N within the digital beat signals  428 - 1  to  428 -M. In other words, the pre-processing stage  602  performs an operation for each active radar frame  322 . In this example, the pre-processing stage  602  includes M one-dimensional (1D) Fast-Fourier Transform (FFT) modules, which respectively process the digital beat signals  428 - 1  to  428 -M. Other types of modules that perform similar operations are also possible, such as a Fourier Transform module. 
     For simplicity, the hardware-abstraction module  220  is shown to process a digital beat signal  428 - 1  associated with the receive channel  410 - 1 . The digital beat signal  428 - 1  includes the chirps  310 - 1  to  310 -M, which are time-domain signals. The chirps  310 - 1  to  310 -M are passed to a one-dimensional FFT module  606 - 1  in an order in which they are received and processed by the transceiver  214 . Assuming the radar receive signals  308 - 1  to  308 -M include 16 chirps  310 - 1  to  310 -N (e.g., N equals 16), the one-dimensional FFT module  606 - 1  performs 16 FFT operations to generate pre-processed complex radar data per chirp  612 - 1 . 
     The signal-transformation stage  604  operates on the sequence of chirps  310 - 1  to  310 -M within each of the digital beat signals  428 - 1  to  428 -M. In other words, the signal-transformation stage  604  performs an operation for each active feature frame  316 . In this example, the signal-transformation stage  604  includes M buffers and M two-dimensional (2D) FFT modules. For simplicity, the signal-transformation stage  604  is shown to include a buffer  608 - 1  and a two-dimensional FFT module  610 - 1 . 
     The buffer  608 - 1  stores a first portion of the pre-processed complex radar data  612 - 1 , which is associated with the first chirp  310 - 1 . The one-dimensional FFT module  606 - 1  continues to process subsequent chirps  310 - 2  to  310 -N, and the buffer  608 - 1  continues to store the corresponding portions of the pre-processed complex radar data  612 - 1 . This process continues until the buffer  608 - 1  stores a last portion of the pre-processed complex radar data  612 - 1 , which is associated with the chirp  310 -M. 
     At this point, the buffer  608 - 1  stores pre-processed complex radar data associated with a particular feature frame  614 - 1 . The pre-processed complex radar data per feature frame  614 - 1  represents magnitude information (as shown) and phase information (not shown) across different chirps  310 - 1  to  310 -N and across different range bins  616 - 1  to  616 -A, where A represents a positive integer. 
     The two-dimensional FFT  610 - 1  accepts the pre-processed complex radar data per feature frame  614 - 1  and performs a two-dimensional FFT operation to form the hardware-agnostic radar data  502 - 1 , which represents a range-Doppler map  620 . The range-Doppler map  620  includes complex radar data for the range bins  616 - 1  to  616 -A and Doppler bins  618 - 1  to  618 -B, where B represents a positive integer. In other words, each range bin  616 - 1  to  616 -A and Doppler bin  618 - 1  to  618 -B includes a complex number with real and imaginary parts that together represent magnitude and phase information. Each complex number is represented by a cell  622 , which is associated with a particular range bin  616  and a particular Doppler bin  618 . The quantity of range bins  616 - 1  to  616 -A can be on the order of tens or hundreds, such as 64 or 128 (e.g., A equals 64 or 128). The quantity of Doppler bins can be on the order of tens or hundreds, such as 32, 64, or 124 (e.g., B equals 32, 64, or 124). As described above with respect to  FIGS.  1  and  3 - 1   , the range-Doppler map  620  can include a near-range interference artifact, as further described with respect to  FIG.  7   . 
       FIG.  7    illustrates an example range-Doppler map  620  for performing near-range detection. In this example, the amplitude (or magnitude) information of the hardware-agnostic radar data  502  is illustrated with different patterns. Larger amplitudes are represented with patterns that have a larger percentage of black. Smaller amplitudes are represented with patterns that have a smaller percentage of black (e.g., a higher percentage of white). Although not shown, the range-Doppler map  620  can also include phase information. 
     Each range bin  616  and Doppler bin  618  contains amplitude information for a particular range interval (e.g., slant-range interval or distance interval) and Doppler frequency interval. The range bins  616  are labeled from  1  to A. The Doppler bins  618  are labeled from −B/2 to 0 to B/2. The zero Doppler bin  618  includes amplitude information for objects that have a Doppler frequency of 0 Hz or a Doppler frequency equal to a multiple of the pulse repetition frequency (PRF). The ±B/2 bins include amplitude information for objects that have a Doppler frequency of ±PRF/2. 
     The range-Doppler map  620  includes a near-range portion  702  and a far-range portion  704 . A first threshold range bin  706 - 1  indicates an upper boundary of the near-range portion  702 . In the depicted configuration, the near-range portion  702  includes at least a portion of the range bins  616  between the first range bin  616 - 1  and the first threshold range bin  706 - 1 . In other words, near-range portion  702  includes distances that are less than or equal to a first threshold and the far-range portion  704  includes distances that are greater than the first threshold. 
     As an example, the first threshold range bin  706 - 1  can be associated with a range (e.g., distance) that is on the order of centimeters, such as 10 centimeters, 15 centimeters, 20 centimeters, and so forth. The first threshold range bin  706 - 1  can be determined based on the internal interference  244  or the external interference  246  such that the near-range portion  702  includes distances that are greater than or equal to distances associated with one or more interference artifacts  716  generated by the internal interference  244  or the external interference  246 . In some implementations, the far-range portion  704  includes a larger quantity of range bins  616  than the near-range portion  702 . 
     In some cases, a second threshold range bin  706 - 2  defines a lower boundary of the near-range portion  702 . In other words, the near-range portion  702  includes distances that are between a second threshold and the first threshold. As an example, the second threshold range bin  706 - 2  can be set such that the near-range portion  702  excludes range bins  616  that are associated with an interior of the smart device  104 . For example, the second threshold range bin  706 - 2  can include a distance that is greater than or equal to a distance between the radar system  102  and the exterior viewing panel  226  or exterior housing  242  of  FIG.  2 - 2   . The second threshold range bin  706 - 2  can also be set to exclude range bins  616  associated with cross-coupling between antenna elements of the antenna array  212  (e.g., cross-coupling between the transmit antenna element  420  and one or more of the receive antenna elements  422 - 1  to  422 -M of  FIG.  4   ). 
     In this example, the radar receive signal  308  includes reflections from a near-range object  708  (e.g., a hand of the user  302 ) and reflections from a far-range object  710  (e.g., a body of the user  302 ). The far-range object  710  is moving and appears within high-Doppler bins  712  and the far-range portion  704  of the range-Doppler map  620 . The near-range object  708  appears within low-Doppler bins  714  and the near-range portion  702  of the range-Doppler map  620 . Generally, the low-Doppler bins  714  include Doppler bins  618  associated with a small percentage of the PRF, which correspond to relatively slow or stationary range rates. Example Doppler frequencies of the low-Doppler bins  714  can be less than or equal to 10% of the PRF, such as between approximately 0% and 5% of the PRF. The low-Doppler bins  714  can include the zero, positive one, and negative one Doppler bins  618 , for instance. In some cases, the low-Doppler bins  714  can include additional Doppler bins  618 , such as the positive two and negative two Doppler bins  618 . 
     Due to the internal interference  244  or the external interference  246  detected by the radar system  102  (shown in  FIG.  2 - 2   ), the range-Doppler map  620  also includes interference artifacts  716 - 1  to  716 - 3  within the near-range portion  702 . The interference artifact  716 - 1  contributes to amplitudes within the first range bin  616 - 1  and the second threshold range bin  706 - 2 , as well as amplitudes across the low-Doppler bins  714  and a portion of the high-Doppler bins  712 . The interference artifact  716 - 2  contributes to amplitudes within the near-range portion  702  and a portion of the negative high-Doppler bins  712 . The interference artifact  716 - 3  contributes to amplitudes within the near-range portion  702  and another portion of the positive high-Doppler bins  712 . Both the near-range detection module  222  and the far-range detection module  224  analyze the range-Doppler map  620  to detect the near-range object  708  and the far-range object  710 , respectively, as further described with respect to  FIG.  8 - 1   . 
       FIG.  8 - 1    illustrates an example scheme implemented by a range-windowing module  802 , the near-range detection module  222 , and the far-range detection module  224 . An input of the range-windowing module  802  is coupled to the hardware-abstraction module  220  (of  FIG.  6   ). An input of the near-range detection module  222  and an input of the far-range detection module  224  are coupled to respective outputs of the range-windowing module  802 . Outputs of the near-range detection module  222  and the far-range detection module  224  can be coupled to other modules implemented by the system processor  216 , as described above with respect to  FIG.  2 - 1   . 
     During operation, the range-windowing module  802  accepts the range-Doppler map  620  from the hardware-abstraction module  220 . The range-windowing module  802  provides different portions of the range-Doppler map  620  to the near-range detection module  222  and the far-range detection module  224  based on the first threshold range bin  706 - 1  of  FIG.  7   . In particular, the range-windowing module  802  provides the near-range portion  702  of the range-Doppler map  620  to the near-range detection module  222  and the far-range portion  704  of the range-Doppler map  620  to the far-range detection module  224 . 
     In this example, the near-range portion  702  of the range-Doppler map  620  includes the range bins  616  between the second threshold range bin  706 - 2  and the first threshold range bin  706 - 1 . By setting the second threshold range bin  706 - 2  to be greater than the first range bin  616 - 1 , large amplitudes associated with the interference artifact  716 - 1  within the range bin  616 - 1  are not included in the near-range potion  702 . The near-range portion  702  includes a portion of the interference artifact  716 - 1 , the interference artifact  716 - 2 , the interference artifact  716 - 3 , and the near-range object  708 . 
     The far-range portion  704  of the range-Doppler map  620  includes range bins  616  that are greater than the first threshold range bin  706 - 1 . In other words, the far-range portion  704  of the range-Doppler map  620  includes range bins  616  between the next range bin that is larger than the first threshold range bin  706 - 1  (e.g., range bin ( 706 - 1 )+1) to the range bin  616 -A. 
     As described above with respect to  FIG.  5   , the near-range detection module  222  processes the near-range portion  702  to generate the near-range detection data  504 . The far-range detection module  224  processes the far-range portion  704  to generate the near-range detection data  504 . Operations of the near-range detection module  222  and the far-range detection module  224  are further described with respect to  FIGS.  8 - 2  and  8 - 3   , respectively. To achieve a target false-alarm rate, the near-range detection module  222  can filter remaining portions of the interference artifacts  716 - 1  to  716 - 3  from the near-range portion  702  without attenuating the near-range object  708 , as further described with respect to  FIG.  8 - 2   . 
       FIG.  8 - 2    illustrates an example implementation of the near-range detection module  222 . The near-range detection module  222  includes at least one amplitude-thresholding module  806  and optionally includes at least one Doppler-windowing module  804  or at least one spatial-coverage module  808 . In the depicted configuration, the near-range detection module  222  includes the Doppler-windowing module  804 , the amplitude-thresholding module  806 , and the spatial-coverage module  808 . Each of these modules can operate on the hardware-agnostic radar data  502  provided by the hardware-abstraction module  220  or operate on other data that is provided by another one of the modules. 
     The Doppler-windowing module  804  filters the hardware-agnostic radar data  502  along the Doppler dimension according to a Doppler interval  810 , which specifies a set of Doppler bins  618 , such as the low-Doppler bins  714 . The amplitude-thresholding module  806  identifies cells  622  with amplitudes that are greater than or equal to a near-range detection threshold  812 . The near-range detection threshold  812  can be a constant-false-alarm-rate threshold and set based on a target false-alarm rate. In other words, the near-range detection threshold  812  can be set to be greater than an estimated amplitude of possible interference artifacts  716 . The spatial-coverage module  808  compares a percentage of cells  622  with amplitudes that are greater than or equal to the near-range detection threshold  812  to a near-range spatial threshold  814 . Responsive to the percentage being greater than or equal to the near-range spatial threshold  814 , the spatial-coverage module  808  generates the near-range detection data  504 , which indicates the presence of the near-range object  708 . The near-range spatial threshold  814  can be set based on a size of the windowed far-range portion  832  and an expected size of an object within the near-range portion  702 . As an example, the near-range spatial threshold  814  can be set to 50%. 
     Consider an example operation of the near-range detection module  222  given the range-Doppler map  620  of  FIG.  7   . During operation, the Doppler-windowing module  804  filters the near-range portion  702  of the range-Doppler map  620  to generate a windowed near-range portion  816 , which includes the range bins  706 - 2  to  706 - 1  and the low-Doppler bins  714 . By performing this operation, the Doppler-Windowing module  804  filters the interference artifacts  716 - 2  and  716 - 3  without attenuating the near-range object  708 . A portion of the interference artifact  716 - 1  is also filtered. 
     The amplitude-thresholding module  806  identifies cells  622  with amplitudes that are greater than the near-range detection threshold  812 . In this case, the amplitude-threshold module  806  identifies the cells  622  associated with the near-range object  708 , as indicated by a box. This effectively filters the cells  622  associated with the interference artifact  716 - 1 . The amplitude-threshold module  806  provides amplitude-selected cells  818  to the spatial-coverage module  808 . 
     The spatial-coverage module  808  compares a percentage of the windowed near-range portion  816  that is associated with the amplitude-selected cells  818  to the near-range spatial threshold  814 . If the percentage is greater than or equal to the near-range spatial threshold  814 , the spatial-coverage module  808  generates the near-range detection data  504  based on the information contained within the amplitude-selected cells  818 . In this way, the near-range detection module  222  detects the near-range object  708  and the near-range detection data  504  includes information associated with the near-range object  708 . As an example, the near-range detection data  504  can include the range bin  616  and Doppler bin  618  associated with the highest amplitude within the amplitude-selected cells  818 . While the near-range detection module  222  detects the near-range object  708 , the far-range detection module  224  processes the far-range portion  704  of the range-Doppler map  620  to detect the far-range object  710 , as further described with respect to  FIG.  8 - 3   . 
       FIG.  8 - 3    illustrates an example implementation of the far-range detection module  224 . The far-range detection module  224  includes at least one amplitude-thresholding module  822 . The far-range detection module  224  can also optionally include at least one Doppler-windowing module  820  or at least one spatial-coverage module  824 . In the depicted configuration, the far-range detection module  224  includes the Doppler windowing module  820 , the amplitude-thresholding module  822 , and the spatial-coverage module  824 . Each of these modules can operate on the hardware-agnostic radar data  502  provided by the hardware-abstraction module  220  or operate on other data that is provided by one of the modules. 
     The Doppler-windowing module  820  filters the hardware-agnostic radar data  502  along the Doppler dimension according to a Doppler interval  826 , which specifies a set of Doppler bins  618 , such as the high-Doppler bins  712 . The amplitude-thresholding module  822  identifies cells  622  with amplitudes that are greater than or equal to a far-range detection threshold  828 . The far-range detection threshold  828  can be a constant-false-alarm-rate threshold and set based on a target false-alarm rate. In some cases, the far-range detection threshold  828  and the near-range detection threshold  812  are the same. In other cases, the far-range detection threshold  828  is less than the near-range detection threshold  812 . The spatial-coverage module  824  compares a percentage of cells  622  with amplitudes that are greater than or equal to the far-range detection threshold  828  to a far-range spatial threshold  830 . Responsive to the percentage being less than or equal to the far-range spatial threshold  830 , the spatial-coverage module  824  generates the far-range detection data  506 , which indicates the presence of the far-range object  710 . The far-range spatial threshold  830  can be smaller than the near-range spatial threshold  814 . As an example, the far-range spatial threshold  830  can be set such that a detection is declared responsive to the amplitude-selected cells  834  including a few cells (e.g., one cell, two cells, or less than four cells). 
     Consider an example operation of the far-range detection module  224  given the range-Doppler map  620  of  FIG.  7   . During operation, the Doppler-windowing module  820  filters the far-range portion  704  of the range-Doppler map  620  to generate a windowed far-range portion  832 , which includes the range bins ( 706 - 1 )+1 to  706 -A and the high-Doppler bins  712 . Although the windowed far-range portion  832  is shown to include the positive high-Doppler bins  712  in  FIG.  8 - 3   , it is to be understood that the negative high-Doppler bins  712  can also be included within the windowed far-range portion  832 . By performing this operation, the Doppler-windowing module  804  can filter any interference or clutter associated with the low-Doppler bins  714 . 
     The amplitude-thresholding module  822  identifies cells  622  with amplitudes that are greater than the far-range detection threshold  828 . In this case, the amplitude-threshold module  822  identifies a portion of the cells  622  associated with the far-range object  710 , which are shown as amplitude-selected cells  834 . The amplitude-threshold module  822  provides the amplitude-selected cells  834  to the spatial-coverage module  824 . 
     The spatial-coverage module  824  compares a percentage of the windowed far-range portion  832  that is associated with the amplitude-selected cells  834  to the far-range spatial threshold  830 . If the percentage is less than or equal to the far-range spatial threshold  830 , the spatial-coverage module  808  generates the far-range detection data  506  based on the amplitude-selected cells  834 . In this way, the far-range detection module  224  detects the far-range object  710  and the far-range detection data  506  includes information associated with the far-range object  710 . The far-range detection data  506  can include the range bin  616  and Doppler bin  618  associated with the highest amplitude within the amplitude-selected cells  834 . 
     Although described with respect to the range-Doppler map  620 , the near-range Detection module  222  and the far-range detection module  224  can also perform similar operations on other types of data. In some implementations, for instance, a digital beamformer is connected to an output of the hardware-abstraction module  220 . In this case, the near-range detection module  222  and the far-range detection module  224  can operate on maps with angular dimensions, which are generated by the digital beamformer. An example map can include a range-Doppler-azimuth-elevation map. In this case, the spatial-coverage modules  808  and  824  can be tailored to evaluate other spatial thresholds across an angular dimension. 
     Example Method 
       FIG.  9    depicts an example method  900  for performing operations of a smart-device-based radar system capable of near-range detection. Method  900  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, reorganized, or linked to provide a wide array of additional and/or alternate methods. In portions of the following discussion, reference may be made to the environment  100 - 1  to  100 - 6  of  FIG.  1   , and entities detailed in  FIG.  2 - 1  or  5   , reference to which is made for example only. The techniques are not limited to performance by one entity or multiple entities operating on one device. 
     At  902 , radar transmit signals are transmitted using an antenna array of a radar system. For example, the antenna array  212  of the radar system  102  transmits radar transmit signals  306 , as shown in  FIG.  4   . 
     At  904 , radar receive signals are received using the antenna array. The radar receive signals comprise respective reflected versions of the radar transmit signals. The radar transmit signals are reflected by a user. For example, the antenna array  212  receives the radar receive signals  308 , as shown in  FIG.  4   . The radar receive signals  308  include respective reflected versions of the radar transmit signals  306 . The radar transmit signals  306  are reflected by the user  302 . 
     At  906 , range-Doppler maps are generated based on the radar receive signals. For example, the hardware-abstraction module  220  generates range-Doppler maps  620  based on the digital beat signals  428  associated with the radar receive signals  308 . Each range-Doppler map  620  is associated with a particular pulse-mode feature frame  318  or burst-mode feature frame  320  within one of the radar receive signals  308 . 
     At  908 , far-range portions of the range-Doppler maps are processed using a far-range detection module of the radar system. For example, the far-range detection module  224  processes far-range portions  704  of the range-Doppler maps  620 , as shown in  FIG.  8 - 1   . 
     At  910 , the user is detected approaching the smart device within the far-range portions of a set of the range-Doppler maps. For example, the far-range detection module  224  detects, within the far-range portions  704  of a set of the range-Doppler maps  620 , the user  302  approaching the smart device  104 . In  FIGS.  7  and  8 - 3   , the far-range object  710  can represent a portion of the user  302  within the far-range interval. 
     At  912 , near-range portions of the range-Doppler maps are processed using a near-range detection module of the radar system. The near-range portions of the range-Doppler maps include at least one interference artifact. The processing is effective to filter the at least one interference artifact from the near-range portions of the range-Doppler maps. For example, the near-range detection module  222  processes the near-range portions  702  of the range-Doppler maps  620 , as shown in  FIG.  8 - 1   . The near-range portions  702  of the range-Doppler maps  620  include at least one interference artifact  716 , such as the interference artifacts  716 - 1 ,  716 - 2 , and/or  716 - 3  of  FIG.  7   . The processing is effective to filter the at least one interference artifact  716  from the near-range portions  702  of the range-Doppler maps  620 , as shown in  FIG.  8 - 2   . 
     At  914 , the user is detected interacting with the smart device within the near-range portions of another set of the range-Doppler maps. For example, the near-range detection module  222  detects the user  302  interacting with the smart device  104  within the near-range portions  702  of another set of the range-Doppler maps  620 . The user  302  can be watching a movie on the smart device  104 , performing a gesture to control a feature of the smart device  104 , or physically interacting with the smart device  104 . In  FIGS.  7  and  8 - 2   , the near-range object  708  can represent a portion of the user  302  within the near-range interval. 
     Example Computing System 
       FIG.  10    illustrates various components of an example computing system  1000  that can be implemented as any type of client, server, and/or computing device as described with reference to the previous  FIG.  2 - 1    to implement near-range detection. 
     The computing system  1000  includes communication devices  1002  that enable wired and/or wireless communication of device data  1004  (e.g., received data, data that is being received, data scheduled for broadcast, or data packets of the data). Although not shown, the communication devices  1002  or the computing system  1000  can include one or more radar systems  102 . The device data  1004  or other device content can include configuration settings of the device, media content stored on the device, and/or information associated with a user  302  of the device. Media content stored on the computing system  1000  can include any type of audio, video, and/or image data. The computing system  1000  includes one or more data inputs  1006  via which any type of data, media content, and/or inputs can be received, such as human utterances, the radar-based application  206 , user-selectable inputs (explicit or implicit), messages, music, television media content, recorded video content, and any other type of audio, video, and/or image data received from any content and/or data source. 
     The computing system  1000  also includes communication interfaces  1008 , which can be implemented as any one or more of a serial and/or parallel interface, a wireless interface, any type of network interface, a modem, and as any other type of communication interface. The communication interfaces  1008  provide a connection and/or communication links between the computing system  1000  and a communication network by which other electronic, computing, and communication devices communicate data with the computing system  1000 . 
     The computing system  1000  includes one or more processors  1010  (e.g., any of microprocessors, controllers, and the like), which process various computer-executable instructions to control the operation of the computing system  1000  and to enable techniques for, or in which can be embodied, gesture recognition in the presence of saturation. Alternatively or in addition, the computing system  1000  can be implemented with any one or combination of hardware, firmware, or fixed logic circuitry that is implemented in connection with processing and control circuits which are generally identified at  1012 . Although not shown, the computing system  1000  can include a system bus or data transfer system that couples the various components within the device. A system bus can include any one or combination of different bus structures, such as a memory bus or memory controller, a peripheral bus, a universal serial bus, and/or a processor or local bus that utilizes any of a variety of bus architectures. 
     The computing system  1000  also includes a computer-readable media  1014 , such as one or more memory devices that enable persistent and/or non-transitory data storage (i.e., in contrast to mere signal transmission), examples of which include random access memory (RAM), non-volatile memory (e.g., any one or more of a read-only memory (ROM), flash memory, EPROM, EEPROM, etc.), and a disk storage device. The disk storage device may be implemented as any type of magnetic or optical storage device, such as a hard disk drive, a recordable and/or rewriteable compact disc (CD), any type of a digital versatile disc (DVD), and the like. The computing system  1000  can also include a mass storage media device (storage media)  1016 . 
     The computer-readable media  1014  provides data storage mechanisms to store the device data  1004 , as well as various device applications  1018  and any other types of information and/or data related to operational aspects of the computing system  1000 . For example, an operating system  1020  can be maintained as a computer application with the computer-readable media  1014  and executed on the processors  1010 . The device applications  1018  may include a device manager, such as any form of a control application, software application, signal-processing and control module, code that is native to a particular device, a hardware abstraction layer for a particular device, and so on. 
     The device applications  1018  also include any system components, engines, or managers to implement near-range detection. In this example, the device applications  1018  includes the radar-based application  206  and the near-range detection module  222  of  FIG.  2 - 1   . 
     Conclusion 
     Although techniques using, and apparatuses including, a smart-device-based radar system performing near-range detection have been described in language specific to features and/or methods, it is to be understood that the subject of the appended claims is not necessarily limited to the specific features or methods described. Rather, the specific features and methods are disclosed as example implementations of a smart-device-based radar system performing near-range detection. 
     Some examples are described below. 
     Example 1: A method performed by a radar system implemented within a smart device, the method comprising: 
     transmitting radar transmit signals using an antenna array of the radar system; 
     receiving radar receive signals using the antenna array, the radar receive signals comprising respective reflected versions of the radar transmit signals, the radar transmit signals reflected by a user; 
     generating range-Doppler maps based on the radar receive signals; 
     processing far-range portions of the range-Doppler maps using a far-range detection module of the radar system; 
     detecting, within the far-range portions of a set of the range-Doppler maps, the user approaching the smart device; 
     processing near-range portions of the range-Doppler maps using a near-range detection module of the radar system, the near-range portions of the range-Doppler maps including at least one interference artifact, the processing effective to filter the at least one interference artifact from the near-range portions of the range-Doppler maps; and 
     detecting, within the near-range portions of another set of the range-Doppler maps, the user interacting with the smart device. 
     Example 2: The method of example 1, wherein: 
     the near-range portions of the range-Doppler maps represent distances that are less than or equal to a threshold; and 
     the far-range portions of the range-Doppler maps include other distances that are greater than the threshold. 
     Example 3: The method of example 2, wherein the threshold corresponds to a distance that is less than or equal to twenty centimeters. 
     Example 4: The method of example 2 or 3, wherein the distances are greater than or equal to another threshold, the other threshold being less than the threshold. 
     Example 5: The method of example 4, wherein: 
     the threshold is greater than or equal to a distance associated with the at least one interference artifact; and 
     the other threshold is greater than or equal to another distance between the radar system and an exterior of the smart device. 
     Example 6: The method of any one of examples 2 to 5, wherein the near-range portions include a smaller quantity range bins than the far-range portions. 
     Example 7: The method of any preceding example, wherein: 
     the range-Doppler maps include a set of low-Doppler bins and a set of high-Doppler bins; 
     the at least one interference artifact is associated with at least one Doppler bin within the set of high-Doppler bins; 
     the processing of the far-range portions of the range-Doppler maps comprises analyzing the far-range portions of the range-Doppler maps that are associated with the set of high-Doppler bins; and 
     the processing of the near-range portions of the range-Doppler maps comprises analyzing the near-range portions of the range-Doppler maps that are associated with the set of low-Doppler bins to filter the at least one interference artifact. 
     Example 8: The method of any preceding example, wherein: 
     the processing of the far-range portions of the range-Doppler maps comprises:
         comparing amplitudes of cells within the far-range portions of the range-Doppler maps to a far-range detection threshold;   detecting the user responsive to one or more of the cells having amplitudes that are greater than or equal to the far-range detection threshold; and/or the processing of the near-range portions of the range-Doppler maps comprises:   comparing amplitudes of other cells within the near-range portion of the range-Doppler maps to a near-range detection threshold, the near-range detection threshold being greater than the far-range detection threshold; and   detecting the user interacting with the smart device responsive to one or more of the other cells having amplitudes that are greater than or equal to the near-range detection threshold.       

     Example 9: The method of any of examples 1 to 7, wherein: 
     the processing of the far-range portions of the range-Doppler maps comprises:
         comparing amplitudes of cells within the far-range portions of the range-Doppler maps to a far-range detection threshold;   determining a percentage of the cells that have amplitudes that are less than or equal to the far-range detection threshold; and   detecting the user responsive to the percentage of the cells being less than or equal to a far-range spatial threshold; and/or       

     the processing of the near-range portions of the range-Doppler maps comprises:
         comparing amplitudes of other cells within the near-range portions of the range-Doppler map to a near-range detection threshold;   determining a percentage of the other cells that have amplitudes that are greater than or equal to the near-range detection threshold; and   detecting the user interacting with the smart device responsive to the percentage of the other cells being greater than or equal to a near-range spatial threshold.       

     Example 10: The method of example 9, wherein: 
     the near-range detection threshold is greater than the far-range detection threshold; or 
     the near-range detection threshold is equal to the far-range detection threshold. 
     Example 11: The method of any preceding example, wherein the interference artifact represents at least one of the following: 
     an audible sound; or 
     a wireless communication signal; 
     noise on a power line of the smart device that is connected to the radar system; or 
     cross-coupling within the antenna array of the radar system. 
     Example 12: The method of any preceding example, further comprising: 
     determining, prior to the processing of the near-range portions of the range-Doppler maps, whether the user is within a proximity of the smart device; and
         responsive to the user being within the proximity, enabling the processing of the near-range portions of at least a portion of the range-Doppler maps; or   responsive to the user being outside the proximity, disabling the processing of the near-range portion of the range-Doppler maps.       

     Example 13: The method of example 0, wherein the determining that the user is within the proximity comprises: 
     determining that the user is approaching the smart device based on the processing of the far-range portions of the range-Doppler maps; or 
     detecting the user using a proximity sensor of the smart device. 
     Example 14: An apparatus comprising: 
     a radar system comprising:
         an antenna array;   a transceiver; and   a processor and computer-readable storage media configured to perform any of the methods of examples 1 to 13.       

     Example 15: The apparatus of example 14, wherein the apparatus comprises a smart device, the smart device comprising one of the following: 
     a smartphone; 
     a smart watch; 
     a smart speaker; 
     a smart thermostat; 
     a security camera; 
     a vehicle; or 
     a household appliance.