Patent Publication Number: US-2021173065-A1

Title: Multistatic radar measurement

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application claims priority to U.S. Provisional Patent Application No. 62/944,900 entitled “MULTISTATIC RADAR MEASUREMENT” and filed on Dec. 6, 2019, which is assigned to the assignee hereof. The disclosure of the prior application is considered part of and are incorporated by reference in this patent application. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to wireless communication, and more specifically, to multistatic radar measurements in wireless communication systems. 
     DESCRIPTION OF THE RELATED TECHNOLOGY 
     Radar is a ranging technique that can be used to determine the distances of objects relative to a given location. A radar system operates by transmitting and receiving electromagnetic pulses. Some of the pulses reflect off objects or surfaces along the transmission path, producing “echoes.” The radar system may determine the distances of the objects or surfaces based on a round trip time between the transmission of a pulse to the reception of an echo of that pulse. In a monostatic radar system, the antennas used to transmit the pulses (“transmit antennas”) are collocated with the antennas used to receive the echoes (“receive antennas”). For example, the transmit antennas and receive antennas are often disposed on the same device. This allows for simple synchronization between the timing of the transmitted pulses and the timing of the received echoes since the same device (or system) clock may be used for both. 
     In a multistatic radar system, the transmit antennas are located a substantial distance away from the receive antennas. The spatial diversity afforded by multistatic systems provides a high accuracy of target location and allows different aspects of a target to be viewed simultaneously. However, to achieve the necessary amount of separation between the transmit antennas and receive antennas, many multistatic radar systems are implemented using multiple devices with no wired communication between them. For example, the transmit antennas may be disposed on a transmit device and the receive antennas may be disposed on one or more spatially diverse receive devices (with a shared or partially shared coverage area). The transmit device may transmit pulses in various directions and the receive devices may detect the resulting echoes. The transmit and receive devices use separate clocks for the timing of pulse transmissions and echo detection. Accordingly, a mechanism is needed to synchronize the transmission of pulses by the transmit device with the reception of echoes by receive devices and to exchange radar measurement information between the transmit and receive devices. 
     SUMMARY 
     The systems, methods and devices of this disclosure each have several innovative aspects, no single one of which is solely responsible for the desirable attributes disclosed herein. 
     One innovative aspect of the subject matter described in this disclosure can be implemented as a method of wireless communication. The method may be performed by a wireless communication device, and may include receiving a radar alert frame carrying timing information indicating a timing relationship between a codeword sequence and one or more pulses; detecting a codeword of the codeword sequence; determining a first time at which the codeword is detected; detecting an echo of a pulse of the one or more pulses; determining a second time at which the echo is detected; and determining a distance of an object to the wireless communication device based at least in part on the first time, the second time, and the timing information. 
     In some implementations, the radar alert frame may be a clear-to-send (CTS)-to-self frame having a control trailer that carries the timing information. In some implementations, each codeword of the codeword sequence may include a respective Golay sequence. In some implementations, the detecting of the codeword may include tuning a plurality of antennas of the wireless communication device in a direction of a transmitting device. In some implementations, the one or more pulses may include a sequence of pulses and the timing information may indicate a duration of each of the one or more pulses and a delay between the detected codeword and the beginning of the sequence of pulses. In some implementations, the detecting of the echo may include positioning a plurality of antennas of the wireless communication device in a plurality of directions. 
     In some implementations, the method may further include calculating a time at which the pulse is transmitted based on the first time, the second time, and the timing information; and estimating a range of distances associated with the object based at least in part on the second time and the calculated time. In some implementations, the determining of the distance of the object may include determining an angle of arrival of the echo at the wireless communication device; and calculating the distance of the object based at least in part on the angle of arrival and the estimated range of distances. In some implementations, the method may further include transmitting feedback to a transmitting device responsive to detecting the echo, where the feedback indicates the estimated range of distances. In some other implementations, the method may further include transmitting feedback to a transmitting device responsive to detecting the echo, where the feedback indicates the second time. 
     Another innovative aspect of the subject matter described in this disclosure can be implemented in a wireless communication device. In some implementations, the wireless communication device may include at least one modem, at least one processor communicatively coupled with the at least one modem, and at least one memory communicatively coupled with the at least one processor and storing processor-readable code. In some implementations, execution of the processor-readable code by the at least one processor causes the wireless communication device to perform operations including receiving a radar alert frame carrying timing information indicating a timing relationship between a codeword sequence and one or more pulses; detecting a codeword of the codeword sequence; determining a first time at which the codeword is detected; detecting an echo of a pulse of the one or more pulses; determining a second time at which the echo is detected; and determining a distance of an object to the wireless communication device based at least in part on the first time, the second time, and the timing information. 
     Another innovative aspect of the subject matter described in this disclosure can be implemented as a method of wireless communication. The method may be performed by a wireless communication device, and may include transmitting, to a first receiving device, a first radar alert frame carrying timing information indicating a timing relationship between a codeword sequence and one or more pulses; transmitting a first codeword of the codeword sequence, using beamforming, in a direction of the first receiving device; transmitting the one or more pulses, using beamforming, in a plurality of directions; receiving feedback from the first receiving device responsive to the transmission of the one or more pulses; and determining a distance of an object to the wireless communication device based at least in part on the feedback received from the first receiving device. 
     In some implementations, each codeword of the codeword sequence may include a respective Golay sequence. In some implementations, the first radar alert frame may be a CTS-to-self frame having a control trailer that carries the timing information. In some implementations, the CTS-to-self frame may be transmitted, using beamforming, in a direction of the first receiving device. In some implementations, the codeword sequence and the one or more pulses may be transmitted in a PPDU including a PHY preamble and header. 
     In some implementations, the determining of the distance of the object may include determining, based on the feedback, a time at which an echo of a first pulse of the one or more pulses is detected by the first receiving device; determining an angle of departure of the first pulse at the wireless communication device; and calculating the distance of the object based at least in part on the angle of departure, the time at which the echo is detected by the first receiving device, and a time at which the first pulse is transmitted by the wireless communication device. In some implementations, the one or more pulses may include a sequence of pulses and the timing information may indicate a duration of each of the one or more pulses and a delay between the transmission of the codeword sequence and the beginning of the sequence of pulses. 
     In some implementations, the method may further include transmitting a second radar alert frame carrying the timing information to a second receiving device; transmitting a second codeword of the codeword sequence, using beamforming, in a direction of the second receiving device; and receiving feedback from the second receiving device responsive to the transmission of the one or more pulses. In some implementations, the feedback received from the first receiving device may indicate a first range of distances associated with the object and the feedback received from the second receiving device may indicate a second range of distances associated with the object. 
     In some implementations, the determining of the distance of the object may include determining, based on the feedback received from the first receiving device, a time at which an echo of a first pulse of the one or more pulses is detected by the first receiving device; and determining, based on the feedback received from the second receiving device, a time at which an echo of a second pulse of the one or more pulses is detected by the second receiving device, where the distance of the object is determined based on the time at which the echo of the first pulse is detected by the first receiving device and the time at which the echo of the second pulse is detected by the second receiving device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Details of one or more implementations of the subject matter described in this disclosure are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages will become apparent from the description, the drawings and the claims. Note that the relative dimensions of the following figures may not be drawn to scale. 
         FIG. 1  shows a pictorial diagram of an example wireless communication network. 
         FIG. 2A  shows an example protocol data unit (PDU) usable for communications between an access point (AP) and a number of stations (STAs) 
         FIG. 2B  shows an example field in the PDU of  FIG. 2A . 
         FIG. 3  shows another example PDU usable for communications between an AP and each of a number of STAs. 
         FIG. 4  shows an example physical layer convergence protocol (PLCP) protocol data unit (PPDU) usable for communications between an AP and a number of STAs. 
         FIG. 5  shows a block diagram of an example wireless communication device. 
         FIG. 6A  shows a block diagram of an example access point (AP). 
         FIG. 6B  shows a block diagram of an example station (STA). 
         FIG. 7  shows an example bistatic radar system according to some implementations. 
         FIG. 8  shows an example multistatic radar system according to some implementations. 
         FIG. 9  shows a timing diagram illustrating example communications between a radar transmitter and multiple radar receivers according to some implementations. 
         FIG. 10A  shows a flowchart illustrating an example process for wireless communication that supports multistatic radar measurements according to some implementations. 
         FIG. 10B  shows a flowchart illustrating an example process for wireless communication that supports multistatic radar measurements according to some implementations. 
         FIG. 10C  shows a flowchart illustrating an example process for wireless communication that supports multistatic radar measurements according to some implementations. 
         FIG. 11A  shows a flowchart illustrating an example process for wireless communication that supports multistatic radar measurements according to some implementations. 
         FIG. 11B  shows a flowchart illustrating an example process for wireless communication that supports multistatic radar measurements according to some implementations. 
         FIG. 11C  shows a flowchart illustrating an example process for wireless communication that supports multistatic radar measurements according to some implementations. 
         FIG. 11D  shows a flowchart illustrating an example process for wireless communication that supports multistatic radar measurements according to some implementations. 
         FIG. 12  shows a block diagram of an example radar receiver according to some implementations. 
         FIG. 13  shows a block diagram of an example radar transmitter according to some implementations. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     The following description is directed to certain implementations for the purposes of describing innovative aspects of this disclosure. However, a person having ordinary skill in the art will readily recognize that the teachings herein can be applied in a multitude of different ways. The described implementations can be implemented in any device, system or network that is capable of transmitting and receiving radio frequency (RF) signals according to one or more of the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards, the IEEE 802.15 standards, the Bluetooth® standards as defined by the Bluetooth Special Interest Group (SIG), or the Long Term Evolution (LTE), 3G, 4G or 5G (New Radio (NR)) standards promulgated by the 3rd Generation Partnership Project (3GPP), among others. The described implementations can be implemented in any device, system or network that is capable of transmitting and receiving RF signals according to one or more of the following technologies or techniques: code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), orthogonal FDMA (OFDMA), single-carrier FDMA (SC-FDMA), single-user (SU) multiple-input multiple-output (MIMO) and multi-user (MU) MIMO. The described implementations also can be implemented using other wireless communication protocols or RF signals suitable for use in one or more of a wireless personal area network (WPAN), a wireless local area network (WLAN), a wireless wide area network (WWAN), or an internet of things (IOT) network. 
     Various implementations relate generally to synchronizing spatially diverse transmit and receive devices in multistatic radar systems. Some implementations more specifically relate to using signaling techniques and packet formats conforming to the IEEE 802.11 family of standards for performing multistatic radar measurements by one or more wireless communication devices. A WLAN may be formed by one or more access points (APs) that provide a shared wireless communication medium for use by a number of client devices also referred to as stations (STAs). The basic building block of a WLAN conforming to the IEEE 802.11 family of standards is a Basic Service Set (BSS), which is managed by an AP. Each BSS is identified by a Basic Service Set Identifier (BSSID) that is advertised by the AP. Wireless communication devices (such as APs and STAs) communicate by transmitting and receiving electromagnetic signals in the RF spectrum. Electromagnetic signals transmitted by a transmitting device may reflect off objects and surfaces along the transmission path before reaching a receiving device located a distance away. Accordingly, signaling techniques used in wireless communications (such as beamforming) may be well-suited for multistatic radar. 
     In some implementations, a transmitting device (also referred to as a “radar transmitter”) may transmit, to a receiving device (also referred to as a “radar receiver”), a radar alert frame followed by a radar frame. The radar frame may include a codeword sequence (also referred to as a “synchronization sequence”) and one or more radar pulses. As used herein, the term “radar pulse” may refer to any wireless signal that can be used to detect objects along its path of transmission based on radar techniques. The radar pulses may be transmitted, using beamforming, in a number of directions to detect objects in a vicinity of the radar system. Beamforming focuses the energy of each radar pulse in a narrow direction to compensate for path loss and achieve greater range. The radar alert frame may alert the receiving device of the upcoming radar frame. In some implementations, the radar alert frame may carry timing information that can be used to synchronize a receive (RX) clock of the receiving device with a transmit (TX) clock of the transmitting device. For example, the timing information may indicate a timing offset or delay between one or more codewords of the codeword sequence and the beginning of the radar pulses. The receiving device may detect one or more codewords of the codeword sequence and an echo of at least one of the radar pulses, and determine the time at which the corresponding pulse was transmitted by the transmitting device. The receiving device may compare the timing of the echo with the timing of the transmitted pulse to determine a relative distance of a target object that produced the echo. 
     Particular implementations of the subject matter described in this disclosure can be implemented to realize one or more of the following potential advantages. By leveraging wireless communication techniques defined by the IEEE 802.11 family of standards, aspects of the present disclosure may enable multistatic radar to be implemented by existing wireless communication systems and networks. For example, an AP (or STA) may perform the function of a radar transmitter and one or more STAs (or APs) may perform the functions of each radar receiver. In particular, the radar transmitter may use beamforming techniques to transmit highly directional radar pulses which may produce respective echoes when interacting with objects in the environment. The radar transmitter may use packet formats conforming to the IEEE 802.11 family of standards to communicate timing and synchronization information regarding the radar pulses to each radar receiver. Each radar receiver also may use such packet formats to provide feedback regarding the echoes to the radar transmitter. Further, multistatic radar implementations of the present disclosure obey the link access rules defined by existing IEEE 802.11 standards, thereby enabling radar functionality in frequency bands commonly used for wireless communications. Accordingly, aspects of the present disclosure may enable multistatic radar functionality to be implemented anywhere a WLAN is, or can be, deployed. 
       FIG. 1  shows a block diagram of an example wireless communication network  100 . According to some aspects, the wireless communication network  100  can be an example of a wireless local area network (WLAN) such as a Wi-Fi network (and will hereinafter be referred to as WLAN  100 ). For example, the WLAN  100  can be a network implementing at least one of the IEEE 802.11 family of standards (such as that defined by the IEEE 802.11-2016 specification or amendments thereof including, but not limited to, 802.11ah, 802.11ad, 802.11ay, 802.11ax, 802.11az, 802.11ba and 802.11be). The WLAN  100  may include numerous wireless communication devices such as an access point (AP)  102  and multiple stations (STAs)  104 . While only one AP  102  is shown, the WLAN network  100  also can include multiple APs  102 . 
     Each of the STAs  104  also may be referred to as a mobile station (MS), a mobile device, a mobile handset, a wireless handset, an access terminal (AT), a user equipment (UE), a subscriber station (SS), or a subscriber unit, among other possibilities. The STAs  104  may represent various devices such as mobile phones, personal digital assistant (PDAs), other handheld devices, netbooks, notebook computers, tablet computers, laptops, display devices (for example, TVs, computer monitors, navigation systems, among others), music or other audio or stereo devices, remote control devices (“remotes”), printers, kitchen or other household appliances, key fobs (for example, for passive keyless entry and start (PKES) systems), among other possibilities. 
     A single AP  102  and an associated set of STAs  104  may be referred to as a basic service set (BSS), which is managed by the respective AP  102 .  FIG. 1  additionally shows an example coverage area  106  of the AP  102 , which may represent a basic service area (BSA) of the WLAN  100 . The BSS may be identified to users by a service set identifier (SSID), as well as to other devices by a basic service set identifier (BSSID), which may be a medium access control (MAC) address of the AP  102 . The AP  102  periodically broadcasts beacon frames (“beacons”) including the BSSID to enable any STAs  104  within wireless range of the AP  102  to “associate” or re-associate with the AP  102  to establish a respective communication link  108  (hereinafter also referred to as a “Wi-Fi link”), or to maintain a communication link  108 , with the AP  102 . For example, the beacons can include an identification of a primary channel used by the respective AP  102  as well as a timing synchronization function for establishing or maintaining timing synchronization with the AP  102 . The AP  102  may provide access to external networks to various STAs  104  in the WLAN via respective communication links  108 . 
     To establish a communication link  108  with an AP  102 , each of the STAs  104  is configured to perform passive or active scanning operations (“scans”) on frequency channels in one or more frequency bands (for example, the 2.4 GHz, 5 GHz, 6 GHz or 60 GHz bands). To perform passive scanning, a STA  104  listens for beacons, which are transmitted by respective APs  102  at a periodic time interval referred to as the target beacon transmission time (TBTT) (measured in time units (TUs) where one TU may be equal to 1024 microseconds (μs)). To perform active scanning, a STA  104  generates and sequentially transmits probe requests on each channel to be scanned and listens for probe responses from APs  102 . Each STA  104  may be configured to identify or select an AP  102  with which to associate based on the scanning information obtained through the passive or active scans, and to perform authentication and association operations to establish a communication link  108  with the selected AP  102 . The AP  102  assigns an association identifier (AID) to the STA  104  at the culmination of the association operations, which the AP  102  uses to track the STA  104 . 
     As a result of the increasing ubiquity of wireless networks, a STA  104  may have the opportunity to select one of many B 14  within range of the STA or to select among multiple APs  102  that together form an extended service set (ESS) including multiple connected B 14 . An extended network station associated with the WLAN  100  may be connected to a wired or wireless distribution system that may allow multiple APs  102  to be connected in such an ESS. As such, a STA  104  can be covered by more than one AP  102  and can associate with different APs  102  at different times for different transmissions. Additionally, after association with an AP  102 , a STA  104  also may be configured to periodically scan its surroundings to find a more suitable AP  102  with which to associate. For example, a STA  104  that is moving relative to its associated AP  102  may perform a “roaming” scan to find another AP  102  having more desirable network characteristics such as a greater received signal strength indicator (RSSI) or a reduced traffic load. 
     In some cases, STAs  104  may form networks without APs  102  or other equipment other than the STAs  104  themselves. One example of such a network is an ad hoc network (or wireless ad hoc network). Ad hoc networks may alternatively be referred to as mesh networks or peer-to-peer (P2P) networks. In some cases, ad hoc networks may be implemented within a larger wireless network such as the WLAN  100 . In such implementations, while the STAs  104  may be capable of communicating with each other through the AP  102  using communication links  108 , STAs  104  also can communicate directly with each other via direct wireless links  110 . Additionally, two STAs  104  may communicate via a direct communication link  110  regardless of whether both STAs  104  are associated with and served by the same AP  102 . In such an ad hoc system, one or more of the STAs  104  may assume the role filled by the AP  102  in a BSS. Such a STA  104  may be referred to as a group owner (GO) and may coordinate transmissions within the ad hoc network. Examples of direct wireless links  110  include Wi-Fi Direct connections, connections established by using a Wi-Fi Tunneled Direct Link Setup (TDLS) link, and other P2P group connections. 
     The APs  102  and STAs  104  may function and communicate (via the respective communication links  108 ) according to the IEEE 802.11 family of standards (such as that defined by the IEEE 802.11-2016 specification or amendments thereof including, but not limited to, 802.11ah, 802.11ad, 802.11ay, 802.11ax, 802.11az, 802.11ba and 802.11be). These standards define the WLAN radio and baseband protocols for the PHY and medium access control (MAC) layers. The APs  102  and STAs  104  transmit and receive wireless communications (hereinafter also referred to as “Wi-Fi communications”) to and from one another in the form of physical layer convergence protocol (PLCP) protocol data units (PPDUs). The APs  102  and STAs  104  in the WLAN  100  may transmit PPDUs over an unlicensed spectrum, which may be a portion of spectrum that includes frequency bands traditionally used by Wi-Fi technology, such as the 2.4 GHz band, the 5 GHz band, the 60 GHz band, the 3.6 GHz band, and the 900 MHz band. Some implementations of the APs  102  and STAs  104  described herein also may communicate in other frequency bands, such as the 6 GHz band, which may support both licensed and unlicensed communications. The APs  102  and STAs  104  also can be configured to communicate over other frequency bands such as shared licensed frequency bands, where multiple operators may have a license to operate in the same or overlapping frequency band or bands. 
     Each of the frequency bands may include multiple sub-bands or frequency channels. For example, PPDUs conforming to the IEEE 802.11n, 802.11ac and 802.11ax standard amendments may be transmitted over the 2.4 and 5 GHz bands, each of which is divided into multiple 20 MHz channels. As such, these PPDUs are transmitted over a physical channel having a minimum bandwidth of 20 MHz, but larger channels can be formed through channel bonding. For example, PPDUs may be transmitted over physical channels having bandwidths of 40 MHz, 80 MHz, 160 or 320 MHz by bonding together multiple 20 MHz channels. 
     Each PPDU is a composite structure that includes a PHY preamble and a payload in the form of a PLCP service data unit (PSDU). The information provided in the preamble may be used by a receiving device to decode the subsequent data in the PSDU. In instances in which PPDUs are transmitted over a bonded channel, the preamble fields may be duplicated and transmitted in each of the multiple component channels. The PHY preamble may include both a legacy portion (or “legacy preamble”) and a non-legacy portion (or “non-legacy preamble”). The legacy preamble may be used for packet detection, automatic gain control and channel estimation, among other uses. The legacy preamble also may generally be used to maintain compatibility with legacy devices. The format of, coding of, and information provided in the non-legacy portion of the preamble is based on the particular IEEE 802.11 protocol to be used to transmit the payload. 
       FIG. 2A  shows an example protocol data unit (PDU)  200  usable for communications between an AP and a number of STAs. For example, the PDU  200  can be configured as a PPDU. As shown, the PDU  200  includes a PHY preamble  202  and a PHY payload  204 . For example, the PHY preamble  202  may include a legacy portion that itself includes a legacy short training field (L-STF)  206 , a legacy long training field (L-LTF)  208 , and a legacy signaling field (L-SIG)  210 . The PHY preamble  202  also may include a non-legacy portion (not shown). The L-STF  206  generally enables a receiving device to perform automatic gain control (AGC) and coarse timing and frequency estimation. The L-LTF  208  generally enables a receiving device to perform fine timing and frequency estimation and also to estimate the wireless channel. The L-SIG  210  generally enables a receiving device to determine a duration of the PDU and use the determined duration to avoid transmitting on top of the PDU. For example, the L-STF  206 , the L-LTF  208  and the L-SIG  210  may be modulated according to a binary phase shift keying (BPSK) modulation scheme. The payload  204  may be modulated according to a BPSK modulation scheme, a quadrature BPSK (Q-BPSK) modulation scheme, a quadrature amplitude modulation (QAM) modulation scheme, or another appropriate modulation scheme. The payload  204  may generally carry higher layer data, for example, in the form of medium access control (MAC) protocol data units (MPDUs) or aggregated MPDUs (A-MPDUs). 
       FIG. 2B  shows an example L-SIG field  210  in the PDU of  FIG. 2A . The L-SIG  210  includes a data rate field  212 , a reserved bit  214 , a length field  216 , a parity bit  218 , and a tail field  220 . The data rate field  212  indicates a data rate (note that the data rate indicated in the data rate field  212  may not be the actual data rate of the data carried in the payload  204 ). The length field  216  indicates a length of the packet in units of, for example, bytes. The parity bit  218  is used to detect bit errors. The tail field  220  includes tail bits that are used by the receiving device to terminate operation of a decoder (for example, a Viterbi decoder). The receiving device utilizes the data rate and the length indicated in the data rate field  212  and the length field  216  to determine a duration of the packet in units of, for example, microseconds (μs). 
       FIG. 3  shows another example PDU  300  usable for communications between an AP and a number of STAs. For example, the PDU  300  can be configured as a PPDU. As shown, the PDU  300  includes a PHY preamble  301 , a PHY header  306 , a data field  308 , and a beamforming training field (TRN)  310 . The PHY preamble  301  may include a short training field (STF)  302  and a channel estimation field (CE)  304 . The STF  302  generally enables a receiving device to detect the PDU  300  as well as perform automatic gain control (AGC) and coarse timing and frequency estimation. The CE field  304  may be used by the receiving device to perform channel estimation. The PHY header  306  includes information associated with the data field  308  including, for example, a modulation and coding scheme (MCS), length, or checksum. The data field  308  may include payload data and padding (if any). 
     In high frequency (such as 60 GHz or millimeter wave (mmWave)) wireless communication systems (such as conforming to the IEEE 802.11ad or 802.11ay amendments of the IEEE 802.11 standard), communications may be beamformed using phased array antennas at the transmitter and the receiver. Beamforming generally refers to a wireless communication technique by which the transmitting device and the receiving device adjust transmit or receive antenna settings to achieve a desired link budget for subsequent communications. The procedure to adapt the transmit and receive antennas, referred to as beamforming training, may be performed initially to establish a link between the transmitting and receiving devices and also may be performed periodically to maintain a quality link using optimized transmit and receive beams. 
     The TRN field  310  may be used for beamforming training. More specifically, the TRN field  310  may be used by the receiving device to tune or configure its antennas for directional beamforming. For example, the receiving device may determine, based on the TRN field  310 , which antenna sectors (of a phased array antenna) should be used for transmitting and receiving corresponding beams. The TRN field  310  may include an AGC subfield  312  and one or more TRN subfields  314 . The AGC subfield  312  enables the receiving device to readjust its AGC settings to receive the TRN subfields  314 . The TRN subfields  314  may include Golay sequences that can be used to determine an antenna weight vector (AWV) to be applied to the transmit antennas or receive antennas used for beamforming. For example, the AWV may describe the amplitude or phase to be applied to each transmit or receive antenna. The number of TRN subfields  314  may vary depending on whether the TRN field  310  is used for transmit AWV training or receive AWV training. 
       FIG. 4  shows an example PPDU  400  usable for communications between an AP  102  and a number of STAs  104 . As described above, each PPDU  400  includes a PHY preamble  402  and a PSDU  404 . Each PSDU  404  may carry one or more MAC protocol data units (MPDUs). For example, each PSDU  404  may carry an aggregated MPDU (A-MPDU)  408  that includes an aggregation of multiple A-MPDU subframes  406 . Each A-MPDU subframe  406  may include a MAC delimiter  410  and a MAC header  412  prior to the accompanying MPDU  414 , which comprises the data portion (“payload” or “frame body”) of the A-MPDU subframe  406 . The MPDU  414  may carry one or more MAC service data unit (MSDU) subframes  416 . For example, the MPDU  414  may carry an aggregated MSDU (A-MSDU)  418  including multiple MSDU subframes  416 . Each MSDU subframe  416  contains a corresponding MSDU  420  preceded by a subframe header  422 . 
     Referring back to the A-MPDU subframe  406 , the MAC header  412  may include a number of fields containing information that defines or indicates characteristics or attributes of data encapsulated within the frame body  414 . The MAC header  412  also includes a number of fields indicating addresses for the data encapsulated within the frame body  414 . For example, the MAC header  412  may include a combination of a source address, a transmitter address, a receiver address or a destination address. The MAC header  412  may include a frame control field containing control information. The frame control field specifies the frame type, for example, a data frame, a control frame, or a management frame. The MAC header  412  may further including a duration field indicating a duration extending from the end of the PPDU until the end of an acknowledgment (ACK) of the last PPDU to be transmitted by the wireless communication device (for example, a block ACK (BA) in the case of an A-MPDU). The use of the duration field serves to reserve the wireless medium for the indicated duration, thus establishing the NAV. Each A-MPDU subframe  406  also may include a frame check sequence (FCS) field  424  for error detection. For example, the FCS field  416  may include a cyclic redundancy check (CRC). 
     As described above, APs  102  and STAs  104  can support multi-user (MU) communications; that is, concurrent transmissions from one device to each of multiple devices (for example, multiple simultaneous downlink (DL) communications from an AP  102  to corresponding STAs  104 ), or concurrent transmissions from multiple devices to a single device (for example, multiple simultaneous uplink (UL) transmissions from corresponding STAs  104  to an AP  102 ). To support the MU transmissions, the APs  102  and STAs  104  may utilize multi-user multiple-input, multiple-output (MU-MIMO) and multi-user orthogonal frequency division multiple access (MU-OFDMA) techniques. 
     In MU-OFDMA schemes, the available frequency spectrum of the wireless channel may be divided into multiple resource units (RUs) each including a number of different frequency subcarriers (“tones”). Different RUs may be allocated or assigned by an AP  102  to different STAs  104  at particular times. The sizes and distributions of the RUs may be referred to as an RU allocation. In some implementations, RUs may be allocated in 2 MHz intervals, and as such, the smallest RU may include 26 tones consisting of 24 data tones and 2 pilot tones. Consequently, in a 20 MHz channel, up to 9 RUs (such as 2 MHz, 26-tone RUs) may be allocated (because some tones are reserved for other purposes). Similarly, in a 160 MHz channel, up to 74 RUs may be allocated. Larger 52 tone, 106 tone, 242 tone, 484 tone and 996 tone RUs also may be allocated. Adjacent RUs may be separated by a null subcarrier (such as a DC subcarrier), for example, to reduce interference between adjacent RUs, to reduce receiver DC offset, and to avoid transmit center frequency leakage. 
     For UL MU transmissions, an AP  102  can transmit a trigger frame to initiate and synchronize an UL MU-OFDMA or UL MU-MIMO transmission from multiple STAs  104  to the AP  102 . Such trigger frames may thus enable multiple STAs  104  to send UL traffic to the AP  102  concurrently in time. A trigger frame may address one or more STAs  104  through respective association identifiers (AIDs), and may assign each AID (and thus each STA  104 ) one or more RUs that can be used to send UL traffic to the AP  102 . The AP also may designate one or more random access (RA) RUs that unscheduled STAs  104  may contend for. 
       FIG. 5  shows a block diagram of an example wireless communication device  500 . In some implementations, the wireless communication device  500  can be an example of a device for use in a STA such as one of the STAs  104  described above with reference to  FIG. 1 . In some implementations, the wireless communication device  500  can be an example of a device for use in an AP such as the AP  102  described above with reference to  FIG. 1 . The wireless communication device  500  is capable of transmitting (or outputting for transmission) and receiving wireless communications (for example, in the form of wireless packets). For example, the wireless communication device can be configured to transmit and receive packets in the form of physical layer convergence protocol (PLCP) protocol data units (PPDUs) and medium access control (MAC) protocol data units (MPDUs) conforming to an IEEE 802.11 standard, such as that defined by the IEEE 802.11-2016 specification or amendments thereof including, but not limited to, 802.11ah, 802.11ad, 802.11ay, 802.11ax, 802.11az, 802.11ba and 802.11be. 
     The wireless communication device  500  can be, or can include, a chip, system on chip (SoC), chipset, package or device that includes one or more modems  502 , for example, a Wi-Fi (IEEE 802.11 compliant) modem. In some implementations, the one or more modems  502  (collectively “the modem  502 ”) additionally include a WWAN modem (for example, a 3GPP 4G LTE or 5G compliant modem). In some implementations, the wireless communication device  500  also includes one or more radios  504  (collectively “the radio  504 ”). In some implementations, the wireless communication device  506  further includes one or more processors, processing blocks or processing elements  506  (collectively “the processor  506 ”) and one or more memory blocks or elements  508  (collectively “the memory  508 ”). 
     The modem  502  can include an intelligent hardware block or device such as, for example, an application-specific integrated circuit (ASIC) among other possibilities. The modem  502  is generally configured to implement a PHY layer. For example, the modem  502  is configured to modulate packets and to output the modulated packets to the radio  504  for transmission over the wireless medium. The modem  502  is similarly configured to obtain modulated packets received by the radio  504  and to demodulate the packets to provide demodulated packets. In addition to a modulator and a demodulator, the modem  502  may further include digital signal processing (DSP) circuitry, automatic gain control (AGC), a coder, a decoder, a multiplexer and a demultiplexer. For example, while in a transmission mode, data obtained from the processor  506  is provided to a coder, which encodes the data to provide encoded bits. The encoded bits are then mapped to points in a modulation constellation (using a selected MCS) to provide modulated symbols. The modulated symbols may then be mapped to a number N SS  of spatial streams or a number N STS  of space-time streams. The modulated symbols in the respective spatial or space-time streams may then be multiplexed, transformed via an inverse fast Fourier transform (IFFT) block, and subsequently provided to the DSP circuitry for Tx windowing and filtering. The digital signals may then be provided to a digital-to-analog converter (DAC). The resultant analog signals may then be provided to a frequency upconverter, and ultimately, the radio  504 . In implementations involving beamforming, the modulated symbols in the respective spatial streams are precoded via a steering matrix prior to their provision to the IFFT block. 
     While in a reception mode, digital signals received from the radio  504  are provided to the DSP circuitry, which is configured to acquire a received signal, for example, by detecting the presence of the signal and estimating the initial timing and frequency offsets. The DSP circuitry is further configured to digitally condition the digital signals, for example, using channel (narrowband) filtering, analog impairment conditioning (such as correcting for I/Q imbalance), and applying digital gain to ultimately obtain a narrowband signal. The output of the DSP circuitry may then be fed to the AGC, which is configured to use information extracted from the digital signals, for example, in one or more received training fields, to determine an appropriate gain. The output of the DSP circuitry also is coupled with the demodulator, which is configured to extract modulated symbols from the signal and, for example, compute the logarithm likelihood ratios (LLRs) for each bit position of each subcarrier in each spatial stream. The demodulator is coupled with the decoder, which may be configured to process the LLRs to provide decoded bits. The decoded bits from all of the spatial streams are then fed to the demultiplexer for demultiplexing. The demultiplexed bits may then be descrambled and provided to the MAC layer (the processor  506 ) for processing, evaluation or interpretation. 
     The radio  504  generally includes at least one radio frequency (RF) transmitter (or “transmitter chain”) and at least one RF receiver (or “receiver chain”), which may be combined into one or more transceivers. For example, the RF transmitters and receivers may include various DSP circuitry including at least one power amplifier (PA) and at least one low-noise amplifier (LNA), respectively. The RF transmitters and receivers may in turn be coupled to one or more antennas. For example, in some implementations, the wireless communication device  500  can include, or be coupled with, multiple transmit antennas (each with a corresponding transmit chain) and multiple receive antennas (each with a corresponding receive chain). The symbols output from the modem  502  are provided to the radio  504 , which then transmits the symbols via the coupled antennas. Similarly, symbols received via the antennas are obtained by the radio  504 , which then provides the symbols to the modem  502 . 
     The processor  506  can include an intelligent hardware block or device such as, for example, a processing core, a processing block, a central processing unit (CPU), a microprocessor, a microcontroller, a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a programmable logic device (PLD) such as a field programmable gate array (FPGA), discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. The processor  506  processes information received through the radio  504  and the modem  502 , and processes information to be output through the modem  502  and the radio  504  for transmission through the wireless medium. For example, the processor  506  may implement a control plane and MAC layer configured to perform various operations related to the generation and transmission of MPDUs, frames or packets. The MAC layer is configured to perform or facilitate the coding and decoding of frames, spatial multiplexing, space-time block coding (STBC), beamforming, and OFDMA resource allocation, among other operations or techniques. In some implementations, the processor  506  may generally control the modem  502  to cause the modem to perform various operations described above. 
     The memory  504  can include tangible storage media such as random-access memory (RAM) or read-only memory (ROM), or combinations thereof. The memory  504  also can store non-transitory processor- or computer-executable software (SW) code containing instructions that, when executed by the processor  506 , cause the processor to perform various operations described herein for wireless communication, including the generation, transmission, reception and interpretation of MPDUs, frames or packets. For example, various functions of components disclosed herein, or various blocks or steps of a method, operation, process or algorithm disclosed herein, can be implemented as one or more modules of one or more computer programs. 
       FIG. 6A  shows a block diagram of an example AP  602 . For example, the AP  602  can be an example implementation of the AP  102  described with reference to  FIG. 1 . The AP  602  includes a wireless communication device (WCD)  610 . For example, the wireless communication device  610  may be an example implementation of the wireless communication device  500  described with reference to  FIG. 5 . The AP  602  also includes multiple antennas  620  coupled with the wireless communication device  610  to transmit and receive wireless communications. In some implementations, the AP  602  additionally includes an application processor  630  coupled with the wireless communication device  610 , and a memory  640  coupled with the application processor  630 . The AP  602  further includes at least one external network interface  650  that enables the AP  602  to communicate with a core network or backhaul network to gain access to external networks including the Internet. For example, the external network interface  650  may include one or both of a wired (for example, Ethernet) network interface and a wireless network interface (such as a WWAN interface). Ones of the aforementioned components can communicate with other ones of the components directly or indirectly, over at least one bus. The AP  602  further includes a housing that encompasses the wireless communication device  610 , the application processor  630 , the memory  640 , and at least portions of the antennas  620  and external network interface  650 . 
       FIG. 6B  shows a block diagram of an example STA  604 . For example, the STA  604  can be an example implementation of the STA  104  described with reference to  FIG. 1 . The STA  604  includes a wireless communication device  615 . For example, the wireless communication device  615  may be an example implementation of the wireless communication device  500  described with reference to  FIG. 5 . The STA  604  also includes one or more antennas  625  coupled with the wireless communication device  615  to transmit and receive wireless communications. The STA  604  additionally includes an application processor  635  coupled with the wireless communication device  615 , and a memory  645  coupled with the application processor  635 . In some implementations, the STA  604  further includes a user interface (UI)  655  (such as a touchscreen or keypad) and a display  665 , which may be integrated with the UI  655  to form a touchscreen display. In some implementations, the STA  604  may further include one or more sensors  675  such as, for example, one or more inertial sensors, accelerometers, temperature sensors, pressure sensors, or altitude sensors. Ones of the aforementioned components can communicate with other ones of the components directly or indirectly, over at least one bus. The STA  604  further includes a housing that encompasses the wireless communication device  615 , the application processor  635 , the memory  645 , and at least portions of the antennas  625 , UI  655 , and display  665 . 
     As described above, signaling techniques used in wireless communications (such as beamforming) may be well-suited for multistatic radar. In a multistatic radar system, the transmit antennas are located a substantial distance away from the receive antennas. The spatial diversity afforded by multistatic systems provides a high accuracy of target location and allows different aspects of a target to be viewed simultaneously. However, to achieve the necessary amount of separation between the transmit antennas and receive antennas, many multistatic radar systems are implemented using multiple devices with no wired communication between them. For example, the transmit antennas may be disposed on a transmit device and the receive antennas may be disposed on one or more spatially diverse receive devices (with a shared or partially shared coverage area). The transmit device may transmit pulses in various directions and the receive devices may detect the resulting echoes. The transmit and receive devices use separate clocks for the timing of pulse transmissions and echo detection. 
     In some implementations, a transmitting device may transmit, to one or more receiving devices, a radar alert frame followed by a radar frame. The radar frame may include a codeword sequence and one or more radar pulses. As used herein, the term “radar pulse” may refer to any wireless signal that can be used to detect objects along its path of transmission based on radar techniques. The radar alert frame may alert each receiving device of the upcoming radar frame. The radar pulses may be transmitted, using beamforming, in a number of directions to detect target objects in a vicinity of the radar system. Beamforming focuses the energy of each radar pulse in a narrow direction to compensate for path loss and achieve greater range. In some implementations, the radar alert frame may carry timing information that can be used to synchronize a receive (RX) clock of each receiving device with a transmit (TX) clock of the transmitting device. For example, the timing information may indicate a timing offset or delay between one or more codewords of the codeword sequence and the beginning of the radar pulses. Each receiving device may detect one or more codewords of the codeword sequence and an echo of at least one of the radar pulses, and determine the time at which the corresponding pulse was transmitted by the transmitting device. Each receiving device may compare the timing of the echo with the timing of the transmitted pulse to determine a relative distance of a target object that produced the echo. 
       FIG. 7  shows an example bistatic radar system  700  according to some implementations. The bistatic radar system  700  includes a radar transmitter (RTX)  710  and a radar receiver (RRX)  720 . The radar transmitter  710  and the radar receiver  720  are spatially separated by a baseline (L). In some implementations, the radar transmitter  710  may be one example of the AP  102  of  FIG. 1  or the AP  602  of  FIG. 6A  and the radar receiver  720  may be one example of the STA  104  of  FIG. 1  or the STA  604  of  FIG. 6B . 
     The radar transmitter  710  is configured to transmit radar pulses  712  in a number of directions. Each of the pulses  712  may be a beamformed RF signal having a particular width and directionality. Objects or surfaces along the trajectory of any of the pulses  712  may cause the pulses  712  to reflect or scatter. Reflected pulses may be referred to as “echoes” of the pulses from which they originate. In the example of  FIG. 7 , a target object  701  is located along the path of one of the radar pulses  712 . The radar pulse  712 ( i ) incident on the target object  701  is reflected as an echo  722 . As shown in  FIG. 7 , the echo  722  is reflected in the direction of the radar receiver  720 . In some implementations, the radar receiver  720  may determine a distance (R R ) of the target object  701  relative to the radar receiver  720  based, at least in part, on the baseline distance L between the radar transmitter  710  and the radar receiver  720 , an angle of arrival (θ R ) of the echo  722 , and a time of flight (τ) from the transmission of the incident pulse  712 ( i ) by the radar transmitter  710  to the reception of the resulting echo  722  by the radar receiver  720 . More specifically, the distance R R  can be calculated according to Equation 1. 
         R   R =( R   T   +R   R ) 2   −L   2 /2( R   T   +R   R   −L  cos θ R )  (1)
 
     where R T +R R  represents the combined distances from the target object  701  to each of the radar transmitter  710  and the radar receiver  720 . As shown in  FIG. 7 , R T +R R  defines a range of distances  702  around the radar transmitter  710  and the radar receiver  720  (in the shape of an ellipse) in which the target object  701  may be located. More specifically, R T +R R  can be calculated, according to Equation 2, as a function of the baseline (L), the time of flight of the reflected pulse (τ), and the propagation speed of the radar pulses (c p ). 
         R   T   +R   R   =c   p   τ+L   (2)
 
     With reference to Equations 1 and 2, the baseline L and propagation speed c p  represent fixed or preconfigured values inherent to the radar system  700 . The angle of arrival θ R  may be determined based on a time difference of arrival (TDOA) of the echo  722  between different receive antennas of the radar receiver  720  or based on the antenna sector (corresponding to a preset beam of a phased array antenna) used by the radar receiver  720  to receive the echo  722 . However, to calculate the time of flight τ, the radar receiver  720  must have knowledge of the time at which the incident pulse  712 ( i ) was transmitted at the position of the receiver. More specifically, the time of flight τ can be calculated, according to Equation 3, as a function of the time of transmission of the incident pulse (T pulse ) and the time of reception of the echo (T echo ). 
       τ= T   echo   −T   pulse   (3)
 
     Because the radar transmitter  710  and the radar receiver  720  are implemented in (or correspond to) separate wireless communication devices, the radar transmitter  710  may need to communicate the timing of the transmission of the incident pulse T pulse  to the radar receiver  720 . However, because the radar transmitter  710  transmits pulses  712  in a number of directions, the radar transmitter  710  may be unaware as to which of the pulses  712  is incident on the target object  701 . Accordingly, the radar transmitter  710  may need to communicate the timing of each of the pulses  712  to the radar receiver  720 , and the radar receiver  720  may need to determine which of the pulses  712  resulted in the echo  722 . Aspects of the present disclosure recognize that packet formats in accordance with IEEE 802.11 standards may be well-suited for communicating such timing information (T pulse ) from the radar transmitter  710  to the radar receiver  720 . 
     In some implementations, the radar transmitter  710  may transmit a radar alert frame followed by a codeword sequence to the radar receiver  720  prior to transmitting the radar pulses  712 . The radar alert frame may alert the radar receiver  720  of the upcoming radar pulses  712 . In some aspects, the radar alert frame may include timing information that can be used to synchronize a receiver (RX) clock of the radar receiver  720  with a transmit (TX) clock of the radar transmitter  710 . For example, the timing information may indicate a timing offset or delay between one or more codewords of the codeword sequence and the beginning of the transmission of the radar pulses  712 . Thus, upon detecting one or more codewords of the codeword sequence and the subsequent echo  722 , the radar receiver  720  may determine the time at which the incident pulse  712 ( i ) was transmitted by the radar transmitter  710 . The radar receiver  720  may compare the timing of the echo T echo  with the timing of the transmitted pulse T pulse  to determine the distance R R  of the target object  701  (such as described with respect to Equations 1-3). 
     In some implementations, the radar transmitter  710  also may determine its relative distance R T  to the target object  701 . For example, in some aspects, the radar receiver  720  may feedback timing information to the radar transmitter  710 . The timing information may include the timing of the echo T echo , the timing of the transmitted pulse T pulse , or the time of flight τ. The radar transmitter  710  may then calculate the distance R T  based, at least in part, on the angle of departure θ T  of the incident pulse  712 ( i ). For example, the radar transmitter  710  may calculate the distance R T  by substituting the angle of departure θ T  for the angle of arrival θ R  in Equation 1. The radar transmitter  710  may determine the angle of departure θ T  based on the antenna sector (corresponding to a preset beam of a phased array antenna) used by the radar transmitter  710  to transmit the incident pulse  712 ( i ). 
       FIG. 8  shows an example multistatic radar system  800  according to some implementations. The multistatic radar system  800  includes a radar transmitter RTX and multiple spatially diverse radar receivers RRX 1  and RRX 2 . Although two radar receivers RRX 1  and RRX 2  are shown in the example of  FIG. 8 , the multistatic radar system  800  may include any number of radar receivers in actual implementations. The radar transmitter RTX is spatially separated from each of the radar receivers RRX 1  and RRX 2  by baseline distances L 1  and L 2 , respectively. In some implementations, the radar transmitter RTX may be one example of the AP  102  of  FIG. 1  or the AP  602  of  FIG. 6A  and each of the radar receivers RRX 1  and RRX 2  may be one example of the STA  104  of  FIG. 1  or the STA  604  of  FIG. 6B . 
     In some implementations, the radar transmitter RTX may transmit radar alert (RA) frames  812  and  814  to the radar receivers RRX 1  and RRX 2 , respectively. For example, the first radar alert frame  812  may be transmitted, using beamforming, in a direction of the first radar receiver RRX 1  and the second radar alert frame  814  may be transmitted, using beamforming, in a direction of the second radar receiver RRX 2 . In some implementations, each of the radar alert frames  812  and  814  may be a clear-to-send (CTS)-to-self frame as defined by, for example, the IEEE 802.11ay amendment to the 802.11 standard. The radar alert frames  812  and  814  may alert the respective radar receivers RRX 1  and RRX 2  of an upcoming radar frame  810 . In some aspects, the radar alert frames  812  and  814  may carry timing information that can be used to synchronize RX clocks in the radar receivers RRX 1  and RRX 2 , respectively, with a TX clock of the radar transmitter RTX. In some other aspects, the radar alert frames  812  and  814  may indicate a pulse interval or duration between successive pulses P 1 -P 4 . 
     In some implementations, the radar frame  810  may include a codeword sequence (not shown for simplicity) followed by a sequence of radar pulses P 1 -P 4 . As described in greater detail with reference to  FIG. 9 , the codeword sequence may be used by the radar receivers RRX 1  and RRX 2  to calculate a relative start time of the sequence of pulses P 1 -P 4 . For example, the timing information included in the first radar alert frame  812  may indicate a timing offset or delay between one or more codewords of the codeword sequence and the start of the pulse sequence. After detecting one or more codewords of the codeword sequence, the first radar receiver RRX 1  may determine the start time of the pulse sequence relative to its own internal clock based on the timing information provided in the first radar alert frame  812 . Similarly, the timing information included in the second radar alert frame  814  may indicate a timing offset or delay between one or more portions of the codeword sequence and the start of the pulse sequence. After detecting one or more codewords of the codeword sequence, the second radar receiver RRX 2  may determine the start time of the pulse sequence relative to its own internal clock based on the timing information provided in the second radar alert frame  814 . 
     Each of the pulses P 1 -P 4  may be a beamformed RF signal having a particular width and directionality. Although four radar pulses P 1 -P 4  are shown in the example of  FIG. 8 , the radar frame  810  may include any number of radar pulses in actual implementations. Objects or surfaces along the trajectory of any of the pulses P 1 -P 4  may cause the pulses to reflect or scatter, producing echoes. In the example of  FIG. 8 , a target object  801  is located along the path of two radar pulses P 2  and P 3 . The pulses P 2  and P 3  incident on the target object  801  are reflected as echoes P 2   E  and P 3   E , respectively. As shown in  FIG. 8 , the first echo P 2   E  is reflected in the direction of the first radar receiver RRX 1  and the second echo P 3   E  is reflected in the direction of the second radar receiver RRX 2 . In some implementations, the radar receivers RRX 1  and RRX 2  may detect the echoes P 2   E  and P 3   E , respectively, based on changes in a channel impulse response (CIR). For example, each of the radar receivers RRX 1  and RRX 2  may detect an incident echo when the CIR measured by the radar receiver peaks or exceeds a threshold energy level. 
     In some implementations, each of the radar receivers RRX 1  and RRX 2  may calculate a respective distance (R R ) of the target object  801  according to Equations 1-3. For example, each of the radar receivers RRX 1  and RRX 2  may calculate a time-of-flight (τ) of a respective echo (P 2   E  or P 3   E ) based, at least in part, on the timing information included in the radar alert frames  812  and  814  and the codeword sequence of the radar frame  810 . As described above, each of the radar receivers RRX 1  and RRX 2  may use the timing information and codeword sequence to determine a relative start time of the pulse sequence. In some aspects, the radar receivers RRX 1  and RRX 2  may determine, based on the start time of the pulse sequence and the pulse duration, which of the pulses P 1 -P 4  resulted in the detected echoes P 2   E  and P 3   E . For example, the first radar receiver RRX 1  may determine that the first echo P 2   E  is a reflection of the second pulse P 2  of the pulse sequence due to the temporal proximity of P 2  to P 2   E . Similarly, the second radar receiver RRX 2  may determine that the second echo P 3   E  is a reflection of the third pulse P 3  in the pulse sequence due to the temporal proximity of P 3  to P 3   E . In some other aspects, the pulses P 1 -P 4  may be encoded with information (such as a unique Golay sequence) that can be used by the receiving devices RRX 1  and RRX 2  for identification purposes. 
     After identifying the incident pulses P 2  and P 3  that resulted in the echoes P 2   E  and P 3   E , the radar receivers RRX 1  and RRX 2  may determine the times at which the incident pulses P 2  and P 3 , respectively, were transmitted by the radar transmitter RTX. For example, communications from the radar transmitter RTX to the radar receivers RRX 1  and RRX 2  undergo a propagation delay based on the propagation speed (c p ) of the wireless signals and the distances L 1  and L 2 , respectively, between the devices. Since the propagation speed and distances are known or fixed quantities, each of the radar receivers RRX 1  and RRX 2  also may know its respective propagation delay. Accordingly, each of the radar receivers RRX 1  and RRX 2  may calculate the timing of a transmitted pulse at the radar transmitter RTX (T pulse,Tx ), according to Equation 4, as a function of the timing of the transmitted pulse relative to the radar receiver (T pulse,RX ) and the propagation delay. 
         T   pulse,TX   =T   pulse,RX −Propagation_Delay  (4)
 
     Substituting T pulse,TX  (Equation 4) for T pulse  in Equation 3 yields: 
       τ= T   echo   −T   pulse,RX +Propagation_Delay  (5)
 
     With reference to Equation 5, each of the radar receivers RRX 1  and RRX 2  may calculate a time of flight T based on the time at which an echo is detected T echo , the time at which the corresponding pulse is transmitted T pulse,RX  relative to the radar receiver (as determined based on the timing information included in the radar alert frames  812  or  814  and the codeword sequence in the radar frame  810 ), and the propagation delay between the radar receiver and the radar transmitter RTX. In some implementations, each of the radar receivers RRX 1  and RRX 2  may use its calculated time of flight T to determine a relative distance (R R ) to the target object  801  (according to Equations 1 and 2). 
     In some implementations, the radar transmitter RTX may determine its relative distance (R T ) to the target object  801 . For example, in some aspects, one or more of the radar receivers RRX 1  or RRX 2  may feedback timing information to the radar transmitter RTX. The timing information may include the timing of an echo T echo , the timing of a transmitted pulse T pulse,RX  or T pulse,TX , or a time of flight τ. In some aspects, the radar transmitter RTX may calculate the distance R T  based on the angle of departure θ T  with respect to one or more of the incident pulses (such as described with respect to  FIG. 7 ). 
     In some other aspects, the radar transmitter RTX may calculate a distance of the target object  801  based on the intersection of timing information received from the radar receivers RRX 1  and RRX 2 . For example, as shown in  FIG. 8 , time-of-flight information received from the first radar receiver RRX 1  (τ 1 ) indicates a range of distances  802  around the radar transmitter RTX and the first radar receiver RRX 1  in which the target object  801  may be located. Similarly, time-of-flight information received from the second radar receiver RRX 2  (τ 2 ) indicates a range of distances  804  around the radar transmitter RTX and the second radar receiver RRX 2  in which the target object  801  may be located. Accordingly, the radar transmitter RTX may determine a more precise location or distance of the target object  801  based on the intersection of the range of distances  802  and the range of distances  804  (such as the intersection of the ellipses  802  and  804 ). 
       FIG. 9  shows a timing diagram  900  illustrating example communications between a radar transmitter RTX and multiple radar receivers RRX 1  and RRX 2  according to some implementations. In some implementations, the radar transmitter RTX and radar receivers RRX 1  and RRX 2  may belong to a multistatic radar system, such as the multistatic radar system  800  of  FIG. 8 . Thus, the radar transmitter RTX and the radar receivers RRX 1  and RRX 2  may be examples of the radar transmitter RTX and the radar receivers RRX 1  and RRX 2 , respectively, of  FIG. 8 . 
     At time to, the radar transmitter RTX transmits a radar alert frame  910  to the first radar receiver RRX 1 . In some implementations, the radar alert frame  910  may be a CTS-to-self frame in accordance with the IEEE 802.11ay amendment of the IEEE 802.11 standard. For example, the receiver address (RA) field of the CTS-to-self frame may include the address of the radar transmitter RTX and may be used to indicate, to the first radar receiver RRX 1 , the address to which feedback is to be reported. The radar alert frame  910  may be transmitted, using beamforming, in a direction of the first radar receiver RRX 1  and may alert the first radar receiver RRX 1  of an upcoming radar frame  930 . In some aspects, the CTS-to-self frame may include a control trailer carrying association identifier (AID) values associated with one or more radar receivers (including the first radar receiver RRX 1 ). 
     In some implementations, the control trailer of the radar alert frame  910  may further carry timing information associated with the radar frame  930 . In some aspects, the timing information may identify one or more unique codewords CW 1  or CW 2  in the radar frame  930 . In some other aspects, the timing information may further indicate a timing offset ΔT 1  or ΔT 2  between the codewords CW 1  or CW 2 , respectively, and a start of the pulse sequence (at time t 8 ). Still further, in some aspects, the timing information may indicate a pulse interval between successive radar pulses P 1 -Pn of the radar frame  930 . 
     The first radar receiver RRX 1  receives the radar alert frame  910 , at time t 1 , after a propagation delay  901 . Upon receiving the radar alert frame  910 , the first radar receiver RRX 1  may store the timing offset information and the pulse interval information included therein. The first radar receiver RRX 1  may proceed to listen for the upcoming radar frame  930  to be transmitted by the radar transmitter RTX. For example, the first radar receiver RRX 1  may tune a plurality of receive antennas in a direction of the radar transmitter RTX. More specifically, the first radar receiver RRX 1  may configure its receive antennas to receive beamformed signals from the radar transmitter RTX. In some implementations, the first radar receiver RRX 1  may continue listening for the radar frame  930  until it detects one or more of the codewords CW 1  or CW 2  of the radar frame  930 . 
     At time t 2 , the radar transmitter RTX transmits a radar alert frame  920  to the second radar receiver RRX 2 . In some implementations, the radar alert frame  920  may be a CTS-to-self frame. For example, the RA field of the CTS-to-self frame may include the address of the radar transmitter RTX and may be used to indicate, to the second radar receiver RRX 2 , the address to which feedback is to be reported. The radar alert frame  920  may be transmitted, using beamforming, in a direction of the second radar receiver RRX 2  and may alert the second radar receiver RRX 2  of the upcoming radar frame  930 . In some aspects, the CTS-to-self frame may include a control trailer carrying AID values associated with one or more radar receivers (including the second radar receiver RRX 2 ). 
     In some implementations, the control trailer of the radar alert frame  920  may further carry timing information associated with the radar frame  930 . In some aspects, the timing information may identify one or more of the codewords CW 1  or CW 2  in the radar frame  930 . In some other aspects, the timing information may further indicate a timing offset or delay ΔT 1  or ΔT 2  between the codewords CW 1  or CW 2 , respectively, and the start of the pulse sequence (at time t 8 ). Still further, in some aspects, the timing information may indicate a pulse interval between successive radar pulses P 1 -Pn of the radar frame  930 . 
     The second radar receiver RRX 2  receives the radar alert frame  920 , at time t 3 , after a propagation delay  902 . Upon receiving the radar alert frame  920 , the second radar receiver RRX 2  may store the timing offset information and the pulse interval information included therein. The second radar receiver RRX 2  may further listen for the upcoming radar frame  930  to be transmitted by the radar transmitter RTX. For example, the second radar receiver RRX 2  may tune a plurality of receive antennas in a direction of the radar transmitter RTX. More specifically, the second radar receiver RRX 2  may configure its receive antennas to receive beamformed signals from the radar transmitter RTX. In some implementations, the second radar receiver RRX 2  may continue listening for the radar frame  930  until it detects one or more of the codewords CW 1  or CW 2  of the radar frame  930 . 
     The radar transmitter RTX transmits the radar frame  930  from times t 4  to t 11 . In some implementations, the radar frame  930  may be a new type of PPDU based, at least in part, on a packet structure defined by existing IEEE 802.11 standards (such as the PDU  300  of  FIG. 3 ). For example, the radar frame  930  may include a preamble, a header, a codeword sequence, and one or more radar pulses P 1 -Pn. With reference for example to  FIG. 3 , the preamble and header of the radar frame  930  may be one example of the PHY preamble  301  and PHY header  306 , respectively, of the PDU  300 . However, the TRN field  310  (or TRN subfields  314 ) of the PDU  300  may be repurposed as the codeword sequence and radar pulse sequence of the radar frame  930 . In some aspects, the radar frame  930  may include payload data (such as provided in the data field  308  of  FIG. 3 ). In some other aspects, the radar frame  930  may not include any payload data. The codeword sequence may include one or more unique codewords CW 1  and CW 2 . Although two codewords CW 1  and CW 2  are shown in the example of  FIG. 9 , the radar frame  930  may include any number of codewords in actual implementations. 
     In some implementations, each of the codewords may represent a respective Golay sequence. Due to their auto-correlation characteristics, complementary Golay sequences are often used for channel estimation. For example, existing versions of the IEEE 802.11 standard provide techniques for generating Golay sequences that may be included in a beamforming training field of a PPDU (such as the TRN field  310  of  FIG. 3 ). In some aspects, Golay sequences may be repurposed for the codeword sequence of the radar frame  930 . For example, different Golay sequences may be assigned or otherwise associated with different “colors” which may be known to the radar receivers RRX 1  and RRX 2 . Accordingly, each of the codewords CW 1  and CW 2  may represent a different colored Golay sequence. 
     In some implementations, the radar transmitter RTX may transmit various portions of the radar frame  930 , using beamforming, in different directions. In the example of  FIG. 9 , neither the first radar receiver RRX 1  nor the second radar receiver RRX 2  receives the preamble or header of the radar frame  930 . However, beginning at time t 5 , at least one of the codewords CW 1  or CW 2  in the codeword sequence may be transmitted (using beamforming) in a direction of each of the receivers RRX 1  and RRX 2 . More specifically, in the example of  FIG. 9 , the first codeword CW 1  is transmitted in the direction of the first radar receiver RRX 1  and the second codeword CW 2  is transmitted in the direction of the second radar receiver RRX 2 . In some implementations, the pulses P 1 -Pn may be one example of one of the pulses P 1 -P 4  of  FIG. 8 . Thus, each of the pulses P 1 -Pn may be transmitted, using beamforming, in a different direction (such as shown in  FIG. 8 ). 
     The first radar receiver RRX 1  receives the first codeword CW 1  at time t 6 . Upon receiving the first codeword CW 1 , the first radar receiver RRX 1  may determine a relative start of the pulse sequence (t 6 +ΔT 1 ) based on the timing offset ΔT 1  associated with the first codeword CW 1 . Further, the first radar receiver RRX 1  may begin tuning (or positioning) its receive antennas in various directions to detect echoes of the radar pulses P 1 -Pn. For example, the first radar receiver RRX 1  may reconfigure its receive antennas to detect echoes from objects that may be located anywhere in the vicinity of the first radar receiver RRX 1 . In the example of  FIG. 9 , the first radar receiver RRX 1  detects an echo at time t 9 . The first radar receiver RRX 1  may further determine, based on the relative start of the pulse sequence (t 6 +ΔT 1 ) and the pulse interval, that the received echo is a reflection of the second pulse P 2 . The first radar receiver RRX 1  may then determine a time of flight τ 1  of the second pulse P 2  (such as by using Equation 5). 
     In some implementations, the first radar receiver RRX 1  may calculate a relative distance of a target object based, at least in part, on the time of flight τ 1  of the second pulse P 2  (such as described with respect to  FIGS. 7 and 8 ). In some other implementations, the first radar receiver RRX 1  may transmit feedback to the radar transmitter RTX based on the detected echo. For example, the feedback may indicate the timing of the echo, the timing of the second pulse P 2  (in relation to the first radar receiver RRX 1  or the radar transmitter RTX), or the time of flight τ 1  associated with the second pulse. In such implementations, the radar transmitter RTX may use the feedback from the first radar receiver RRX 1  to calculate its relative distance to the target object (such as described with respect to  FIGS. 7 and 8 ). 
     The second radar receiver RRX 2  receives the first codeword CW 2  at time t 7 . Upon receiving the second codeword CW 2 , the second radar receiver RRX 2  may determine a relative start of the pulse sequence (t 7 +ΔT 2 ) based on the timing offset ΔT 2  associated with the second codeword CW 2 . Similar to the first radar receiver RRX 1 , the second radar receiver RRX 2  may begin tuning (or positioning) its receive antennas in various directions to detect echoes of the radar pulses P 1 -Pn. In the example of  FIG. 9 , the second radar receiver RRX 2  detects an echo at time t 10 . The second radar receiver RRX 2  may further determine, based on the relative start of the pulse sequence (t 7 +ΔT 2 ) and the pulse interval, that the received echo is a reflection of the third pulse P 3 . The second radar receiver RRX 2  may then determine a time of flight τ 2  of the third pulse P 3  (such as by using Equation 5). 
     In some implementations, the second radar receiver RRX 2  may further calculate a relative distance of a target object based, at least in part, on the time of flight τ 2  of the third pulse P 3  (such as described with respect to  FIGS. 7 and 8 ). In some other implementations, the second radar receiver RRX 2  may transmit feedback to the radar transmitter RTX based on the detected echo. For example, the feedback may indicate the timing of the echo, the timing of the third pulse P 3  (in relation to the second radar receiver RRX 2  or the radar transmitter RTX), or the time of flight τ 2  associated with the third pulse. In such implementations, the radar transmitter RTX may use the feedback from the second radar receiver RRX 2  to calculate its distance to the target object (such as described with respect to  FIGS. 7 and 8 ). 
       FIG. 10A  shows a flowchart illustrating an example process  1000  for wireless communication that supports multistatic radar measurements according to some implementations. In some implementations, the process  1000  may be performed by a wireless communication device operating as or within a radar receiver such as the STA  104 , STA  604 , radar receiver  720 , or any of the radar receivers RRX 1  or RRX 2  of  FIGS. 1, 6A, 7, and 8 , respectively. 
     In some implementations, the process  1000  begins in block  1001  with receiving a radar alert frame carrying timing information indicating a timing relationship between a codeword sequence and one or more pulses. The radar alert frame may be received from a transmitting device. In some implementations, the one or more pulses may include a sequence of pulses and the timing information may indicate a duration of each of the one or more pulses and a delay between the detected codeword and the beginning of the sequence of pulses. In some implementations, the radar alert frame may be a CTS-to-self frame having a control trailer that carries the timing information. 
     In block  1002 , the process  1000  proceeds with detecting a codeword of the codeword sequence. In some implementations, the detecting of the codeword may include tuning a plurality of antennas of the wireless communication device in a direction of a transmitting device. In some implementations, each codeword of the codeword sequence may include a respective Golay sequence. In block  1003 , the process  1000  proceeds with determining a first time at which the codeword is detected. 
     In block  1004 , the process  1000  proceeds with detecting an echo of a pulse of the one or more pulses. In some implementations, the detecting of the echo may include positioning a plurality of antennas of the wireless communication device in a plurality of directions. In block  1005 , the process  1000  proceeds with determining a second time at which the echo is detected. 
     In block  1006 , the process  1000  proceeds with determining a distance of an object to the wireless communication device based at least in part on the first time, the second time, and the timing information. In some implementations, the process  1000  may further proceed by transmitting feedback to a transmitting device responsive to detecting the echo, where the feedback indicates the second time. 
       FIG. 10B  shows a flowchart illustrating an example process  1010  for wireless communication that supports multistatic radar measurements according to some implementations. In some implementations, the process  1010  may be performed by a wireless communication device operating as or within a radar receiver such as the STA  104 , STA  604 , radar receiver  720 , or any of the radar receivers RRX 1  or RRX 2  of  FIGS. 1, 6A, 7, and 8 , respectively. 
     With reference for example to  FIG. 10A , the process  1010  may begin, in block  1011 , after the determination of the second time in block  1005 . In block  1011 , the process  1010  begins by calculating a time at which the pulse is transmitted based on the first time, the second time, and the timing information. In block  1012 , the process  1010  proceeds with estimating a range of distances associated with the object based at least in part on the second time and the calculated time. In some implementations, the process  1010  may further proceed, in block  1013 , with transmitting feedback to a transmitting device responsive to detecting the echo, where the feedback indicates the estimated range of distances. 
       FIG. 10C  shows a flowchart illustrating an example process  1020  for wireless communication that supports multistatic radar measurements according to some implementations. In some implementations, the process  1020  may be performed by a wireless communication device operating as or within a radar receiver such as the STA  104 , STA  604 , radar receiver  720 , or any of the radar receivers RRX 1  or RRX 2  of  FIGS. 1, 6A, 7, and 8 , respectively. 
     With reference for example to  FIG. 10A , the process  1020  may be a more detailed implementation of the distance determination operation described in block  1006  of the process  1000 . In some implementations, the process  1020  begins after the process  1010  of  FIG. 10A . For example, the process  1020  may begin, in block  1021 , after the estimation of the range of distances in block  1012  of the process  1010 . In block  1021 , the process  1020  begins by determining an angle of arrival of the echo at the wireless communication device. In block  1022 , the process  1020  proceeds with calculating the distance of the object based at least in part on the angle of arrive and the estimated range of distances. 
       FIG. 11A  shows a flowchart illustrating an example process  1100  for wireless communication that supports multistatic radar measurements according to some implementations. In some implementations, the process  1100  may be performed by a wireless communication device operating as or within a radar transmitter such as the AP  102 , AP  104 , radar transmitter  710 , or radar transmitter RTX of  FIGS. 1, 6A, 7 , and  8 , respectively. 
     In some implementations, the process  1100  begins in block  1101  with transmitting, to a first receiving device, a first radar alert frame carrying timing information indicating a timing relationship between a codeword sequence and one or more pulses. In some implementations, the one or more pulses may include a sequence of pulses and the timing information may indicate a duration of each of the one or more pulses and a delay between transmission of the codeword sequence and the beginning of the sequence of pulses. In some implementations, the first radar alert frame may be a CTS-to-self frame having a control trailer that carries the timing information. In some implementations, the CTS-to-self frame may be transmitting, using beamforming, in a direction of the first receiving device. 
     In block  1102 , the process  1100  proceeds with transmitting a first codeword of the codeword sequence, using beamforming, in a direction of the first receiving device. In block  1103 , the process  1100  proceeds with transmitting the one or more pulses, using beamforming, in a plurality of directions. In some implementations, the codeword sequence and the one or more pulses may be transmitted in a PPDU including a PHY preamble and header. In block  1104 , the process  1100  proceeds with receiving feedback from the first receiving device responsive to the transmission of the one or more pulses. In block  1105 , the process  1100  proceeds with determining a distance of an object to the wireless communication device based at least in part on the feedback received from the first receiving device. 
       FIG. 11B  shows a flowchart illustrating an example process  1110  for wireless communication that supports multistatic radar measurements according to some implementations. In some implementations, the process  1110  may be performed by a wireless communication device operating as or within a radar transmitter such as the AP  102 , AP  104 , radar transmitter  710 , or radar transmitter RTX of  FIGS. 1, 6A, 7 , and  8 , respectively. 
     With reference for example to  FIG. 11A , the process  1110  may be a more detailed implementation of the distance determination operation described in block  1105  of the process  1100 . For example, the process  1110  may begin, in block  1111 , after the reception of the feedback from the first receiving device in block  1104 . In block  1111 , the process  1110  begins by determining, based on the feedback, a time at which an echo of a first pulse of the one or more pulses is detected by the first receiving device. In block  1112 , the process  1110  proceeds with determining an angle of departure of the first pulse at the wireless communication device. In block  1113 , the process  1110  proceeds with calculating the distance of the object based at least in part on the angle of departure, the time at which the echo is detected by the first receiving device, and a time at which the first pulse is transmitted by the wireless communication device. 
       FIG. 11C  shows a flowchart illustrating an example process  1120  for wireless communication that supports multistatic radar measurements according to some implementations. In some implementations, the process  1120  may be performed by a wireless communication device operating as or within a radar transmitter such as the AP  102 , AP  104 , radar transmitter  710 , or radar transmitter RTX of  FIGS. 1, 6A, 7 , and  8 , respectively. 
     In some implementations, the process  1120  begins after the process  1100  of  FIG. 11A . For example, the process  1120  may begin, in block  1121 , after the determination of the distance of the object in block  1105 . In block  1121 , the process  1120  begins by transmitting a second radar frame carrying the timing information to a second receiving device. In block  1122 , the process  1120  proceeds with transmitting a second codeword of the codeword sequence, using beamforming, in a direction of the second receiving device. In block  1123 , the process  1120  proceeds with receiving feedback from the second receiving responsive to the transmission of the one or more pulses. 
       FIG. 11D  shows a flowchart illustrating an example process  1130  for wireless communication that supports multistatic radar measurements according to some implementations. In some implementations, the process  1130  may be performed by a wireless communication device operating as or within a radar transmitter such as the AP  102 , AP  104 , radar transmitter  710 , or radar transmitter RTX of  FIGS. 1, 6A, 7 , and  8 , respectively. 
     With reference for example to  FIG. 11A , the process  1130  may be a more detailed implementation of the distance determination operation described in block  1105  of the process  1100 . In some implementations, the process  1130  begins after the process  1120  of  FIG. 11C . For example, the process  1130  may begin, in block  1131 , after the reception of feedback from the second receiving device in block  1123  of the process  1120 . In block  1131 , the process  1130  begins by determining, based on the feedback received from the first receiving device, a time at which an echo of a first pulse of the one or more pulses is detected by the first receiving device. In block  1132 , the process  1130  proceeds with determining, based on the feedback received from the second receiving device, a time at which an echo of a second pulse of the one or more pulses is detected by the second receiving device, where the distance of the object is determined based on the time at which the echo of the first pulse is detected by the first receiving device and the time at which the echo of the second pulse is detected by the second receiving device. 
       FIG. 12  shows a block diagram of an example radar receiver  1200  according to some implementations. In some implementations, the radar receiver  1200  may be configured to perform any of the processes  1000 - 1020  described above with reference to  FIGS. 10A-10C , respectively. The radar receiver  1200  can be an example implementation of the wireless communication device  500 . For example, the radar receiver  1200  can be a chip, SoC, chipset, package or device that includes at least one processor and at least one modem (for example, a Wi-Fi (IEEE 802.11) modem or a cellular modem). 
     The radar receiver  1200  includes a reception component  1210 , a communication manager  1220 , and a transmission component  1230 . The communication manager  1220  further includes a codeword detection component  1222 , an echo detection component  1224 , and a distance determination component  1226 . Portions of one or more of the components  1222 - 1226  may be implemented at least in part in hardware or firmware. In some implementations, at least some of the components  1222 ,  1224 , or  1226  are implemented at least in part as software stored in a memory (such as the memory  508 ). For example, portions of one or more of the components  1222 ,  1224 , and  1226  can be implemented as non-transitory instructions (or “code”) executable by a processor (such as the processor  506 ) to perform the functions or operations of the respective component. 
     The reception component  1210  is configured to receive RX signals from a radar transmitter. In some implementations, the reception component  1210  may receive timing information indicating a timing relationship between a codeword sequence and one or more pulses. For example, the reception component  1210  may receive the timing information from the radar transmitter. The communication manager  1220  is configured to manage radar communications with the radar transmitter. In some implementations, the codeword detection component  1222  may detect a codeword of the codeword sequence and determine a first time at which the codeword is detected; the echo detection component  1224  may detect an echo of a pulse of the one or more pulses and determine a second time at which the echo is detected; and the distance determination component  1226  may determine a distance of an object to the wireless communication device based at least in part on the first time, the second time, and the timing information. The transmission component  1230  is configured to transmit TX signals to the radar transmitter. In some implementations, the transmission component  1230  may transmit feedback to the radar transmitter based on the detected echo. 
       FIG. 13  shows a block diagram of an example radar transmitter  1300  according to some implementations. In some implementations, the radar transmitter  1300  may be configured to perform any of the processes  1100 - 1130  described above with reference to  FIGS. 11A-11D , respectively. The radar transmitter  1300  can be an example implementation of the wireless communication device  500  of  FIG. 5 . For example, the radar transmitter  1300  can be a chip, SoC, chipset, package or device that includes at least one processor and at least one modem (for example, a Wi-Fi (IEEE 802.11) modem or a cellular modem). 
     The radar transmitter  1300  includes a transmission component  1310 , a communication manager  1320 , and a reception component  1330 . The communication manager  1320  further includes a distance determination component  1322 . Portions of the distance determination component  1322  may be implemented at least in part in hardware or firmware. In some implementations, the distance determination component  1322  is implemented at least in part as software stored in a memory (such as the memory  508 ). For example, portions of the distance determination component  1322  can be implemented as non-transitory instructions (or “code”) executable by a processor (such as the processor  506 ) to perform the functions or operations of the respective component. 
     The transmission component  1310  is configured to transmit TX signals to one or more radar receivers. In some implementations, the transmission component  1310  may transmit, to a first radar receiver, a radar alert frame carrying timing information indicating a timing relationship between a codeword sequence and one or more pulses. The transmission component  1310  may further transmit a first codeword of the codeword sequence, using beamforming, in a direction of the first radar receiver. The transmission component  1310  may further transmit the one or more pulses, using beamforming, in a plurality of directions. The reception component  1330  is configured to receive RX signals from the one or more radar receivers. In some implementations, the reception component  1330  may receive feedback from the first radar receiver responsive to the transmission of the one or more pulses. The communication manager  1320  is configured to manage radar communications with the receiving device. In some implementations, the distance determination component  1322  may determine a distance of an object to the radar transmitter  1300  based at least in part on the feedback received from the first radar receiver. 
     As used herein, a phrase referring to “at least one of” or “one or more of” a list of items refers to any combination of those items, including single members. For example, “at least one of: a, b, or c” is intended to cover the possibilities of: a only, b only, c only, a combination of a and b, a combination of a and c, a combination of b and c, and a combination of a and b and c. 
     The various illustrative components, logic, logical blocks, modules, circuits, operations and algorithm processes described in connection with the implementations disclosed herein may be implemented as electronic hardware, firmware, software, or combinations of hardware, firmware or software, including the structures disclosed in this specification and the structural equivalents thereof. The interchangeability of hardware, firmware and software has been described generally, in terms of functionality, and illustrated in the various illustrative components, blocks, modules, circuits and processes described above. Whether such functionality is implemented in hardware, firmware or software depends upon the particular application and design constraints imposed on the overall system. 
     Various modifications to the implementations described in this disclosure may be readily apparent to persons having ordinary skill in the art, and the generic principles defined herein may be applied to other implementations without departing from the spirit or scope of this disclosure. Thus, the claims are not intended to be limited to the implementations shown herein, but are to be accorded the widest scope consistent with this disclosure, the principles and the novel features disclosed herein. 
     Additionally, various features that are described in this specification in the context of separate implementations also can be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation also can be implemented in multiple implementations separately or in any suitable subcombination. As such, although features may be described above as acting in particular combinations, and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Further, the drawings may schematically depict one more example processes in the form of a flowchart or flow diagram. However, other operations that are not depicted can be incorporated in the example processes that are schematically illustrated. For example, one or more additional operations can be performed before, after, simultaneously, or between any of the illustrated operations. In some circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. 
     Implementation examples are described in the following numbered clauses:
         1. A method for wireless communication performed by a wireless communication device, including:   receiving a radar alert frame carrying timing information indicating a timing relationship between a codeword sequence and one or more pulses;   detecting a codeword of the codeword sequence;   determining a first time at which the codeword is detected;   detecting an echo of a pulse of the one or more pulses;   determining a second time at which the echo is detected; and   determining a distance of an object to the wireless communication device based at least in part on the first time, the second time, and the timing information.   2. The method of clause 1, where the detecting of the codeword includes:   tuning a plurality of antennas of the wireless communication device in a direction of a transmitting device.   3. The method of any of clauses 1 or 2, where the detecting of the echo includes:   positioning a plurality of antennas of the wireless communication device in a plurality of directions.   4. The method of any of clauses 1-3, where the one or more pulses includes a sequence of pulses and the timing information indicates a duration of each of the one or more pulses and a delay between the detected codeword and the beginning of the sequence of pulses.   5. The method of any of clauses 1-4, further including:   transmitting feedback to a transmitting device responsive to detecting the echo, the feedback indicating the second time.   6. The method of any of clauses 1-5, further including:   calculating a time at which the pulse is transmitted based on the first time, the second time, and the timing information; and estimating a range of distances associated with the object based at least in part on the second time and the calculated time.   7. The method of any of clauses 1-6, further including:   transmitting feedback to a transmitting device responsive to detecting the echo, the feedback indicating the estimated range of distances.   8. The method of any of clauses 1-7, where the determining of the distance of the object further includes:   determining an angle of arrival of the echo at the wireless communication device; and   calculating the distance of the object based at least in part on the angle of arrival and the estimated range of distances.   9. The method of any of clauses 1-8, where each codeword of the codeword sequence includes a respective Golay sequence.   10. The method of any of clauses 1-9, where the radar alert frame is a clear-to-send (CTS)-to-self frame having a control trailer that carries the timing information.   11. A wireless communication device including:   at least one modem;   at least one processor communicatively coupled with the at least one modem; and   at least one memory communicatively coupled with the at least one processor and storing processor-readable code that, when executed by the at least one processor in conjunction with the at least one modem, is configured to perform the method of any one or more of clauses 1-10.   12. A method for wireless communication performed by a wireless communication device, including:   transmitting, to a first receiving device, a first radar alert frame carrying timing information indicating a timing relationship between a codeword sequence and one or more pulses;   transmitting a first codeword of the codeword sequence, using beamforming, in a direction of the first receiving device;   transmitting the one or more pulses, using beamforming, in a plurality of directions;   receiving feedback from the first receiving device responsive to the transmission of the one or more pulses; and   determining a distance of an object to the wireless communication device based at least in part on the feedback received from the first receiving device.   13. The method of clause 12, where the determining of the distance of the object includes:   determining, based on the feedback, a time at which an echo of a first pulse of the one or more pulses is detected by the first receiving device;   determining an angle of departure of the first pulse at the wireless communication device; and   calculating the distance of the object based at least in part on the angle of departure, the time at which the echo is detected by the first receiving device, and a time at which the first pulse is transmitted by the wireless communication device.   14. The method of any of clauses 12 or 13, where the one or more pulses includes a sequence of pulses and the timing information indicates a duration of each of the one or more pulses and a delay between the transmission of the codeword sequence and the beginning of the sequence of pulses.   15. The method of any of clauses 12 or 14, further including:   transmitting a second radar alert frame carrying the timing information to a second receiving device;   transmitting a second codeword of the codeword sequence, using beamforming, in a direction of the second receiving device; and   receiving feedback from the second receiving device responsive to the transmission of the one or more pulses.   16. The method of any of clauses 12, 14 or 15, where the determining of the distance of the object includes:   determining, based on the feedback received from the first receiving device, a time at which an echo of a first pulse of the one or more pulses is detected by the first receiving device; and   determining, based on the feedback received from the second receiving device, a time at which an echo of a second pulse of the one or more pulses is detected by the second receiving device, the distance of the object being determined based on the time at which the echo of the first pulse is detected by the first receiving device and the time at which the echo of the second pulse is detected by the second receiving device.   17. The method of any of clauses 12 or 14-16, where the feedback received from the first receiving device indicates a first range of distances associated with the object and the feedback received from the second receiving device indicates a second range of distances associated with the object.   18. The method of any of clauses 12-17, where each codeword of the codeword sequence includes a respective Golay sequence.   19. The method of any of clauses 12-18, where the first radar alert frame is a clear-to-send (CTS)-to-self frame having a control trailer that carries the timing information.   20. The method of any of clauses 12-19, where the CTS-to-self frame is transmitted, using beamforming, in a direction of the first receiving device.   21. The method of any of clauses 12-20, where the codeword sequence and the one or more pulses are transmitted in a physical layer (PHY) convergence protocol (PLCP) protocol data unit (PPDU) including a PHY preamble and header.   22. A wireless communication device including:   at least one modem;   at least one processor communicatively coupled with the at least one modem; and   at least one memory communicatively coupled with the at least one processor and storing processor-readable code that, when executed by the at least one processor in conjunction with the at least one modem, is configured to perform the method of any one or more of clauses 12-21.