Phase Noise Compensation in Digital Beamforming Radar Systems

Abstract

A radar system (600) includes a transmitter (610) that transmits a pulse sequence (112); a receiver (620) that receives reflections (114) of the transmitted pulse sequence (112); and a speed detection circuit system (115) that determines the speed of an object (101) in the path of the transmitted pulse sequence (112) based at least in part on the transmitted pulse sequence (112) and the reflected pulses (114). The transmitter (610) includes a plurality of digital-to-analog converters (DACs) (614) that generate the pulse sequence (112) in response to a clock signal (TX / RX CLK). The receiver (620) includes a plurality of analog-to-digital converters (ADCs) (624) that sample the reflected pulses (114) in response to the clock signal (TX / RX CLK). Thus, the ADCs (614) and DACs (624) are in phase lock.

Description

Technical Field

[0001] Aspects of the present disclosure relate generally to radar systems and, more particularly, to phase noise compensation in digital beamforming radar systems. Background Art

[0002] A radar system can be used to measure the distance and / or speed of objects in its vicinity. A radar system operates by transmitting one or more modulated pulses. Any object in the path of the pulses will reflect at least a portion of the transmitted pulses back to the radar. The radar system can determine the distance and / or speed of (multiple) objects based on the time it takes the radar to receive the reflected pulses. For example, a radar pulse may take longer to reach (and return) an object that is farther away from the radar system than an object that is closer to the radar system. Therefore, the radar system can determine the distance of the object based on the round-trip time (RTT) of the pulses reflected from the object.

[0003] Doppler radar systems are often used to measure the speed of moving objects. For example, Doppler radars operate by transmitting a series (or sequence) of pulses at a target frequency. The movement of an object in the path of the radar pulses affects the frequency of the reflected pulses. For example, radar pulses reflected by an object moving in the direction of (e.g., toward) the radar system will exhibit a frequency higher than the target frequency (e.g., a shorter duration between consecutive pulses). On the other hand, radar pulses reflected by an object moving in the opposite direction of (e.g., away from) the radar system will exhibit a frequency lower than the target frequency (e.g., a longer duration between consecutive pulses). The change in frequency (e.g., target frequency) between the reflected pulse and the transmitted pulse is often referred to as a "Doppler shift." Therefore, a Doppler radar system can determine the speed of an object based on the amount of Doppler shift exhibited by the pulses reflected from the object. Summary of the invention

[0004] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to limit the scope of the claimed subject matter.

[0005] Aspects of the present disclosure relate to radar systems and methods of operating the same. An example radar system includes a transmitter that transmits a pulse train; a receiver that receives reflections of the transmitted pulses; and a speed detection circuit system that determines a speed of a first object in a path of the transmitted pulses based at least in part on the transmitted pulses and the reflected pulses. The transmitter includes a plurality of digital-to-analog converters (DACs) that generate the pulse train in response to a clock signal. The receiver includes a plurality of analog-to-digital converters (ADCs) that sample the reflected pulses in response to the clock signal.

[0006] In some implementations, the ADC is in phase lock with the DAC. For example, at least one of the DACs can be configured to generate a first pulse in response to a first transition of a clock signal, and at least one of the ADCs can be configured to sample a reflection of the first pulse in response to the first transition of the clock signal.

[0007] The speed detection circuit system can also be configured to detect a Doppler shift between the frequency of the transmitted pulse and the frequency of the reflected pulse, and determine the speed of the first object based at least in part on the Doppler shift. Phase noise in the clock signal may cause changes in the frequency of the transmitted pulse and corresponding changes in the frequency of the reflected pulse. Therefore, in some aspects, when sampling the reflected pulse, the receiver matches the frequency change of the reflected pulse with the frequency change of the transmitted pulse.

[0008] In some implementations, the reflected pulses may include reflections from a second object in the path of the transmitted pulses. Further, the phase noise may cause the frequency of at least some of the reflected pulses from the second object to overlap with the frequency of the reflected pulses from the first object. In some aspects, the first object may be a moving object and the second object may be stationary. Therefore, the speed detection circuit system may also be configured to filter the reflected pulses from the second object when a Doppler shift is detected.

[0009] In some implementations, the radar system may also include a clock generator circuit for generating a clock signal; and a delay element for delaying the clock signal provided to the ADC. For example, phase noise may be generated by one or more components of the clock generator circuit. In some aspects, the amount of delay implemented by the delay element may be based at least in part on the round-trip time between the transmission of a pulse and the reception of a reflection of the pulse.

[0010] The example method disclosed herein can be used to operate a radar system. The method can include: generating a pulse sequence via a plurality of digital-to-analog converters (DACs) in response to a clock signal; transmitting the pulse sequence; sampling reflections of the transmitted pulses via a plurality of analog-to-digital converters (ADCs) in response to the clock signal; determining a speed of a first object in a path of the transmitted pulses based at least in part on the transmitted pulses and the reflected pulses.

[0011] In some implementations, the ADC is in phase lock with the DAC. For example, at least one of the DACs can be configured to generate a first pulse in response to a first transition of a clock signal, and at least one of the ADCs can be configured to sample a reflection of the first pulse in response to the first transition of the clock signal.

[0012] In some aspects, the step of determining the velocity of the first object may further include the steps of: detecting a Doppler shift between the frequency of the transmitted pulses and the frequency of the reflected pulses; and determining the velocity of the first object based at least in part on the Doppler shift. Phase noise in the clock signal may cause changes in the frequency of the transmitted pulses and corresponding changes in the frequency of the reflected pulses. Therefore, in some aspects, sampling the reflections may further include the steps of: matching changes in the frequency of the reflected pulses with changes in the frequency of the transmitted pulses.

[0013] In some implementations, the reflected pulses may include reflections from a second object in the path of the transmitted pulses. Further, phase noise may cause the frequency of at least some of the reflected pulses from the second object to overlap with the frequency of the reflected pulses from the first object. In some aspects, the first object may be a moving object and the second object may be stationary. Therefore, the step of determining the speed of the first object may also include the step of sampling the reflected pulses from the second object when the Doppler shift is detected.

[0014] In some implementations, the method may further include the steps of generating a clock signal via a clock generator circuit and delaying the clock signal provided to the ADC. For example, phase noise may be generated by one or more components of the clock generator circuit. In some aspects, the amount of delay implemented by the delay element may be based at least in part on the round-trip time between the transmission of the pulse and the reception of the reflection of the pulse. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The example embodiments are illustrated by way of example and are not intended to be limited by the figures of the accompanying drawings. Throughout the drawings and description, like numerals refer to like elements.

[0016] Figure 1A and Figure 1BAn example radar sensing environment is shown in which the present embodiments may be implemented.

[0017] Figure 2 is a block diagram of an example analog front end (AFE) for a radar system.

[0018] Figure 3 is a block diagram of an example clock generator.

[0019] Figure 4 is a timing diagram depicting an example radar pulse sequence that may be transmitted and received by a Doppler radar system.

[0020] Figure 5 is a frequency diagram depicting an example frequency domain representation of a reflected pulse train that may be received by a Doppler radar system.

[0021] Figure 6 is a block diagram of a phase-matched Doppler radar AFE, according to some embodiments.

[0022] Figure 7 is a frequency diagram depicting an example frequency domain representation of a reflected pulse train that may be received by a phase-matched Doppler radar AFE.

[0023] Figure 8 is a block diagram of another phase-matched Doppler radar AFE, according to some embodiments.

[0024] Fig. 9 is an illustrative flow chart depicting example operations of a phase-matched Doppler radar system in accordance with some embodiments. DETAILED DESCRIPTION

[0025] In the following description, many specific details (such as examples of specific components, circuits and processes) are elaborated to provide a thorough understanding of the present disclosure. As used herein, the term "coupling" means directly coupled to or coupled by one or more intermediate components or circuits. In addition, in the following description and for the purpose of explanation, specific nomenclature and / or details are used to provide a thorough understanding of the exemplary embodiments. However, it is obvious to those skilled in the art that these specific details may not be necessary for practicing the exemplary embodiments. In other examples, well-known circuits and devices are shown in block diagram form to avoid making the present disclosure obscure. Any signal in the signal provided by the various buses described herein can be time-multiplexed with other signals and provided by one or more common buses. Additionally, the interconnection between circuit elements or software blocks can be shown as a bus or a single signal line. Each bus in the bus can alternatively be a single signal line, and each single signal line in the single signal line can alternatively be a bus, and a single line or bus can represent any one or more of the countless physical or logical mechanisms for communication between components. The example embodiments should not be construed as limited to the particular examples described herein, but rather all embodiments defined by the appended claims are to be included within their scope.

[0026] Figure 1A and Figure 1B An example radar sensing environment in which the present embodiment may be implemented is shown. Figure 1A , environment 100A includes a first object 101, a second object 102, and a radar system 110. Radar system 110 may be configured to measure distances and / or velocities of other objects (eg, first object 101 and second object 102) in environment 100A.

[0027] In some embodiments, radar system 110 may transmit a sequence of pulses 112 (which may also be referred to herein as a “radar pulse” or collectively as a “radar beam”) in the direction of one or more objects in environment 100A. For example, radar pulses 112 may include a radio frequency (RF) carrier signal having a predetermined pulse width (τ) and pulse repetition frequency (PRF) corresponding to the number of pulses transmitted in a given duration. Figure 1B As shown, at least a portion of transmitted pulse 112 may be reflected by objects 101 and 102 and returned to radar system 110 as reflected pulses 114 and 116, respectively. Since electromagnetic signals propagate at the speed of light (c), radar system 110 may determine a distance or range (R) to each of objects 101 and 102 based on a round trip time (RTT) between the transmission of a radar pulse and the receipt of a corresponding reflection of the radar pulse:

[0028]

[0029] It should be noted that according to the above equation, as the distance between radar system 110 and an object changes, the RTT between successive radar pulses reflected by the object also changes. Therefore, in some embodiments, radar system 110 may include speed detection circuitry 115 to measure the speed of a moving object using Doppler sensing techniques. For example, radar system 110 may detect a moving object at a predetermined “target” frequency (f T ) transmits a pulse train 112. Radar pulses reflected by an object moving in the direction of (e.g., toward) radar system 110 will exhibit a frequency higher than the target frequency (e.g., a shorter duration between consecutive pulses). On the other hand, radar pulses reflected by an object moving in the opposite direction of (e.g., away from) radar system 110 will exhibit a frequency lower than the target frequency (e.g., a longer duration between consecutive pulses). The reflected pulses (f R ) and the transmitted pulse (f T The change in frequency between the two frequencies is usually called the "Doppler shift" (f D ). Thus, velocity detection circuitry 115 may detect a pulse reflected from an object based on the amount of Doppler shift (e.g., f D =f R -f T ) to determine the velocity (v) of a moving object:

[0030]

[0031] exist Figure 1B In the example of FIG. 1 , first object 101 is a moving object (e.g., an airplane) traveling at a speed (V) toward radar system 110, while second object 102 is a stationary object (e.g., a tree). Therefore, the frequency of reflected pulses 114 from first object 101 (e.g., corresponding to a positive Doppler shift) may be higher than the frequency of transmitted pulses 112 (e.g., f T ). However, because the second object 102 is not moving, the frequency of the reflected pulse 116 from the second object 102 (e.g., without Doppler shift) may be the same as the frequency of the transmitted pulse 112. It should be noted that the power (P) or energy of the reflected pulses 114 and 116 may depend on the distance, size, and / or shape of the reflecting object. For example, Figure 1B As shown, first object 101 is farther from radar system 110 than second object 102. Thus, the power of reflected pulse 114 from first object 101 may be significantly less than the power of reflected pulse 116 from second object 102. More generally, received power (P RX ) can be expressed as the transmission power (P TX), the antenna gain (G) of the radar system 110, the wavelength (λ) of the transmitted pulse 112, the radar cross section (σ) of the reflecting object t ) and distance (R), as well as any external or internal losses (L s ) function:

[0032]

[0033] It should be noted that according to the above equation, the power loss in the received (reflected) radar pulse varies exponentially with respect to the distance between the radar system 110 and the target object. Therefore, in order to maximize the power in the reflected pulses, radar system 110 may use beamforming techniques to focus transmitted pulses 112 in a particular direction (such as the direction of a target object). For example, radar system 110 may include multiple antennas (e.g., an antenna array), each antenna configured to transmit the same radar pulse with a different phase offset. As a result, pulses transmitted by the various antennas can be combined along an intended path (e.g., to increase power) and can cancel each other out of the intended path. In some embodiments, radar system 110 may be configured to transmit radar pulses 112 using analog beamforming techniques (e.g., applying phase shifts to individual pulses in the analog domain). In some other embodiments, radar system 110 may be configured to transmit radar pulses 112 using digital beamforming techniques (e.g., applying phase shifts to individual pulses in the digital domain).

[0034] Figure 2 1 is a block diagram of an example analog front end (AFE) 200 for a radar system. AFE 200 can be implemented in a Doppler radar system such as radar system 110 of FIG. 1 to transmit and receive radar pulses that can be used to detect the velocity or speed of an object in a surrounding environment. More specifically, AFE 200 includes a transmitter 210 that transmits radar pulses and a receiver 220 that receives reflections of the radar pulses. In some embodiments, AFE 200 is configured to transmit and receive radar pulses using digital beamforming techniques.

[0035] The transmitter 210 receives digital representations of outgoing (TX) pulses and transmits the TX pulses via a plurality of TX antennas 201(1) to 201(n) in a target direction (e.g., the direction of a target object). In some implementations, the transmitter 210 may include a plurality of first-in-first-out (FIFO) buffers 212(1) to 212(n), a plurality of digital-to-analog converters (DACs) 214(1) to 214(n), a plurality of TX filters 216(1) to 216(n), and a plurality of power amplifiers (PAs) 218(1) to 218(n), each coupled to a respective one of the TX antennas 201(1) to 201(n). Each of the FIFOs 212(1) to 212(n) may apply a respective phase shift (e.g., a digital delay) to the TX pulses to steer or focus the analog pulses transmitted by the antennas 201(1) to 201(n) in the target direction. DACs 214(1) to 214(n) can convert the digital outputs of FIFOs 212(1) to 212(n), respectively, into a plurality of phase-shifted analog pulses, which are filtered by TX filters 216(1) to 216(n). The phase-shifted analog pulses are amplified by PAs 218(1) to 218(n) and then transmitted via TX antennas 201(1) to 201(n).

[0036] Receiver 220 may receive reflections of transmitted pulses (eg, transmitted by transmitter 210) via a plurality of RX antennas 202(1) to 202(n) and recover digital representations of the reflected (RX) pulses. Figure 2In the example of FIG. 1 , the RX antennas 202(1)-202(n) are shown as being separate from the TX antennas 201(1)-201(n). However, in some embodiments, the transmitter 210 and the receiver 220 may share one or more antennas (e.g., via antenna switching circuitry (not shown for simplicity)). In some implementations, the transmitter 210 may include a plurality of FIFO buffers 222(1)-222(n), a plurality of analog-to-digital converters (ADCs) 224(1)-224(n), a plurality of RX filters 226(1)-226(n), and a plurality of low noise amplifiers (LNAs) 228(1)-228(n), each coupled to a respective one of the RX antennas 202(1)-202(n). The LNAs 228(1)-228(n) may amplify analog pulses received via the RX antennas 202(1)-202(n), respectively. The amplified pulses may be filtered by RX filters 226(1) to 226(n) and converted to digital format via ADCs 224(1) to 224(n). Each of FIFOs 222(1) to 222(n) may apply a corresponding phase shift (e.g., digital delay) to the output of ADCs 224(1) to 224(n), respectively, to align the phases of the digital pulses (e.g., to counter the effects of beamforming). The phase-aligned digital pulses may then be combined or added together (e.g., via summing circuit 221) to recover the corresponding RX pulses.

[0037] When AFE 200 is implemented in a Doppler radar system, transmitter 210 may receive a digital bit stream corresponding to a series of TX pulses to be transmitted continuously at a predetermined (e.g., target) frequency. Thus, receiver 220 may receive a series of corresponding reflected pulses (e.g., to be converted into RX pulses) from one or more objects in the path of the transmitted pulses. As described above, a processing system (such as a processor) coupled to AFE 200 may be configured to receive a series of corresponding reflected pulses (e.g., to be converted into RX pulses). Figure 1A and Figure 1B The speed detection circuit system 115 of the present invention can determine the speed of the object based at least in part on the frequency of the RX pulses. More specifically, the speed of the object can be determined by the amount of Doppler shift detected in the frequency of the RX pulses reflected by the object (e.g., relative to the frequency of the TX pulses). As described in more detail below, in order to accurately detect such a Doppler shift, the RX pulses should be correlated with the TX pulses (e.g., phase matched).

[0038] It should be pointed out that in Figure 2In the example of , DACs 214(1) to 214(n) sample or generate analog pulses in response to a TX clock signal (CLK). More specifically, each of the DACs 214(1) to 214(n) can update the amplitude of its electronic output (e.g., corresponding to an RX pulse) with each transition of the TX CLK (e.g., from low to high or from high to low or both). On the other hand, ADCs 224(1) to 224(n) sample reflections of the analog pulses in response to the RX CLK. More specifically, each of the ADCs 224(1) to 224(n) can update the amplitude of its electronic output (e.g., corresponding to an RX pulse) with each transition of the RX CLK (e.g., from low to high or from high to low or both). In a conventional radar system, the TX CLK is provided by a local clock generator (not shown) residing on the transmitter 210, while the RX CLK is provided by a different clock generator (not shown) residing on the receiver 220.

[0039] Figure 3 is a block diagram of an example clock generator 300. For example, the clock generator 300 may be used to generate Figure 2 The clock generator 300 includes a crystal oscillator (XO) 310 and a phase-locked loop (PLL) circuit 320. The crystal oscillator 310 typically includes a piezoelectric material (such as a crystal) that resonates at a precise frequency (e.g., the resonant frequency of the crystal) when a voltage is applied across the material. The mechanical oscillation of the crystal produces a very precise sinusoidal waveform that can be used as a reference (REF) signal for the PLL 320. The PLL 320 receives the REF signal from the crystal oscillator 310 and generates a clock (CLK) signal based at least in part on the REF signal. More specifically, the PLL 320 can generate a CLK signal that is in phase with the REF signal. However, the oscillation frequency of the CLK signal may be different from the oscillation frequency of the REF signal. For example, in some aspects, the oscillation frequency of the CLK signal may be higher than the frequency of the REF signal.

[0040] The PLL circuit 320 includes a phase and frequency detector (PFD) 322, a charge pump 324, a loop filter 326, a voltage controlled oscillator (VCO) 328, and a frequency divider 329. The PFD 322 compares the phase of the REF signal and the feedback (FB) signal to generate an "up" (UP) and a "down" (DN) control signal. The charge pump 324 converts the UP control signal and the DN control signal into a charge (Q) proportional to the phase difference between the REF signal and the FB signal. The charge Q is filtered (e.g., integrated) by the loop filter 326 and provided to the VCO 328 as a control voltage (V). The VCO 328 outputs a CLK signal at an oscillation frequency based on the control voltage V. The CLK signal is also provided to the frequency divider 329 (e.g., the frequency of the CLK signal is scaled to the frequency of the REF signal) and fed back to the PFD 322 as the FB signal.

[0041] The CLK signal can be generated by Figure 2 AFEs such as AFE 200 of the embodiment of the present invention are used to time the transmission of radar pulses. For example, in some aspects, a CLK signal can be provided to DACs 214 (1) to 214 (n) (e.g., as TX CLK) to control the sampling time of the TX pulses. Therefore, the timing (e.g., phase) of the analog pulses output by the DACs 214 (1) to 214 (n) can depend at least in part on the frequency of the CLK signal (e.g., the rising edge transition or the falling edge transition of the CLK signal). However, it should be noted that one or more components of the clock generator 300 may introduce jitter noise or phase noise (e.g., random phase fluctuations) into the CLK signal. For example, although the crystal oscillator 310 can produce a fairly stable REF signal, even the slightest fluctuation in the phase of the REF signal may contribute to "close" phase noise (e.g., relatively small frequency changes) in the transmitted pulses. Temperature changes and other changes in the operating conditions of the VCO 328 and / or PLL 320 may cause the frequency of the CLK signal to further vary.

[0042] As described above, in conventional radar systems, the transmitter and receiver of the AFE typically receive their CLK signals from different clock generators. For example, if the clock generator 300 is to reside locally on the transmitter 210 of the AFE 200 (e.g., to provide the TX CLK), a different clock generator typically resides on the receiver 220 (e.g., to provide the RX CLK). However, when the TX CLK and the RX CLK are generated from different sources, phase noise in either of the clock signals may affect the accuracy with which the RX pulses are mapped to the TX pulses. Specifically, excessive phase noise in the TX CLK or the RX CLK may cause the TX pulses and the RX pulses to become uncorrelated. This in turn may affect the radar system's ability to detect or distinguish Doppler shifts in RX pulses reflected by moving objects (e.g., compared to RX pulses reflected by stationary objects).

[0043] Figure 4 is a timing diagram depicting an example radar pulse sequence that may be transmitted and received by a Doppler radar system. Figure 2 , radar pulse 401 may correspond to an analog pulse transmitted via antennas 201(1)-201(n), and reflected pulse 402 may correspond to a reflection of such an analog pulse incident on antennas 202(1)-202(n). More specifically, each of reflected pulses 402 may be a reflection of a corresponding one of radar pulses 401. For example, reflected pulse 402 received immediately after time t0 may be a reflection of radar pulse 401 transmitted at time t0, and each successive reflected pulse 402 received thereafter may be a reflection of a corresponding radar pulse 401 transmitted just before. It should be noted that the amplitude (e.g., power) of reflected pulse 402 is significantly less than the amplitude of corresponding radar pulse 401.

[0044] Radar pulse 401 is typically at target frequency f from time t0 to t3 T The reflected pulse 402 is also typically transmitted at the target frequency f from time t0 to t3. T 4. Since the frequency of reflected pulse 402 is substantially equal to the frequency of radar pulse 401 (e.g., without Doppler shift), reflected pulse 402 received between time t0 and t3 may be reflected by a stationary object (e.g., second object 102 of FIG. 1). Figure 4In the example of FIG. 4 , phase noise in the TX CLK causes a change in the timing of successive radar pulses 401 transmitted between times t1 and t2. This causes a corresponding change or variation (Δf) in the frequency of radar pulses 401 between times t1 and t2. More specifically, the phase noise in the TX CLK between times t1 and t2 causes radar pulses 401 to be transmitted from times t1 and t2 at a frequency (Δf) that is higher than the target frequency. T As a result, the reflected pulse 402 from the stationary object is also transmitted at this higher frequency (f T +Δf) is reflected back.

[0045] It should be noted that changes in the frequency of reflected pulses 402 (and radar pulses 401) may hinder the radar system's ability to measure and / or detect the Doppler shift in the frequency of reflected pulses from moving objects. For example, in some instances, the frequency of reflected pulses 402 from stationary objects may overlap or coincide with the frequency (e.g., Doppler frequency) of reflected pulses from moving objects. Moreover, when the radar system is closer to stationary objects relative to non-moving objects, the amplitude of reflected pulses 402 from stationary objects is significantly greater than the amplitude of reflected pulses from moving objects. As a result, the energy in reflected pulses 402 from stationary objects may "swallow" (e.g., overwhelm) a receiver.

[0046] Figure 5 is a frequency diagram 500 depicting an example frequency domain representation of a reflected pulse train that may be received by a Doppler radar system. Figure 1A and Figure 1B , the frequency graph 500 may depict the frequencies of the reflected pulses 114 and 116 from the first object 101 and the second object 102 , respectively.

[0047] exist Figure 5 In the example of FIG. 5 , the frequencies of the reflected pulses are generally distributed into two frequency intervals 510 and 520. The first frequency interval 510 is at the target frequency f T (eg, the frequency at which radar system 110 transmits radar pulses 112), and second frequency interval 520 is centered slightly above target frequency f T The Doppler frequency f D As described above, the first object 101 may be a moving object, and the second object 102 may be stationary. Therefore, the first frequency interval 510 may include the reflected pulse 116 from the second object 102, and the second frequency interval 520 may include the reflected pulse 114 from the first object 101.

[0048] Phase noise in the TX CLK may cause the frequency of the transmitted pulse 112 to vary, thereby causing the frequency of the reflected pulses 114 and 116 to vary. For example, the frequency variation of the reflected pulse 116 from the stationary object 102 is depicted by the skirt 501 around the first frequency interval 510. Further, the frequency variation of the reflected pulse 114 from the moving object 101 is depicted by the skirt 502 around the second frequency interval 520. Figure 1A and Figure 1B As shown, the distance between radar system 110 and second object 102 is significantly shorter than the distance between radar system 110 and first object 101. Therefore, the amplitude of the reflected pulse in first frequency interval 510 is significantly greater than the amplitude of the reflected pulse in second frequency interval 520.

[0049] like Figure 5 As shown, due to the phase noise in TX CLK, at the Doppler frequency f D In this case, the energy in the reflected pulse 116 from the stationary object 102 is greater than the energy in the reflected pulse 114 from the moving object 101 (e.g., the energy in the second frequency interval 520 is swallowed up by the energy from the phase noise (e.g., in the skirt 501)). As a result, the conventional radar system may not be able to detect the Doppler frequency f of the reflected pulse 114 from the moving object 101. D Or distinguishing the Doppler frequency f from the phase noise in the reflected pulse 116 from the stationary object 102 D Conventional techniques for mitigating phase noise in radar pulses have focused on suppressing phase noise in the TX CLK (e.g., by implementing a more stable crystal oscillator 310 and / or adjusting Figure 3 320 within the clock generator 300. However, as described above, even the slightest variation in the reference clock (REF) signal may contribute to the close phase noise in the reflected pulse 116, thereby causing the Doppler shift to tend to be relative to the radar pulse f T The frequency is very small (for example, |f T -f D |< <f T ), so it may engulf the Doppler frequency f of the reflected pulse 114 D .

[0050] Aspects of the present disclosure will recognize that by correlating (e.g., matching) the phase noise in the TX CLK with the phase noise in the RX CLK, the close phase noise in the reflected pulses 114 and 116 can be mitigated (e.g., rather than suppressed) in the receiver of the radar system 110. For example, returning to Figure 4, the phase noise in TX CLK causes the frequency of radar pulse 401 transmitted between time t1 and t2 to change (Δf). This frequency change may be due to a sudden increase in the frequency of TX CLK during the same time period. For example, assume that TX CLK normally increases (e.g., between time t0 and t1 and between time t2 and t3) by 2f T The phase noise between time t1 and t2 may cause TX CLK to be equal to 2f T +Δf, so that the DAC in the transmitter (for example, Figure 2 The DACs 214(1) to 214(n)) sample the TX pulses at a faster rate.

[0051] For example, refer to Figure 2 By synchronizing the phase noise of the RX CLK with the phase noise of the TX CLK, the reflected pulse 402 will also be transmitted at a higher frequency (e.g., 2f) between time t1 and t2. T +Δf) is sampled. As a result, the phase of the radar pulse 401 generated by the DAC will be different from that of the ADC (e.g., Figure 2 The phases of the reflected pulses 402 sampled by the ADCs 224(1) to 224(n)) are correlated (e.g., matched) such that the radar system effectively detects that the frequency of the reflected pulses 402 received between times t1 and t2 has not changed. In other words, to the radar system, the reflected pulses 402 appear to be at the target frequency f throughout the duration from time t0 to t3 (e.g., even between times t1 and t2). T take over.

[0052] Figure 6 1 is a block diagram of a phase-matched Doppler radar AFE 600 according to some embodiments. AFE 600 may be implemented in a Doppler radar system such as radar system 110 of FIG. 1 to transmit and receive radar pulses that may be used to detect the velocity or speed of objects in the surrounding environment. More specifically, AFE 600 may be Figure 2 6. An example embodiment of the AFE 200 of FIG. 600 includes a transmitter 610 that transmits radar pulses and a receiver 620 that receives reflections of the radar pulses. In some embodiments, the AFE 600 is configured to transmit and receive radar pulses using digital beamforming techniques.

[0053] The transmitter 610 receives a digital representation of a series of TX pulses and transmits the TX pulses via a plurality of TX antennas 601(1) to 601(n) in a target direction (e.g., the direction of a target object). In some implementations, the transmitter 610 may include a plurality of FIFOs 612(1) to 612(n), a DAC 614(1) to 614(n), a plurality of TX filters 616(1) to 616(n), and a plurality of PAs 618(1) to 618(n), each coupled to a corresponding TX antenna in the TX antennas 601(1) to 601(n). Each of the FIFOs 612(1) to 612(n) may apply a corresponding phase shift (e.g., a digital delay) to the TX pulses to steer or focus the analog pulses transmitted by the antennas 601(1) to 601(n) in the target direction. DACs 614(1) to 614(n) can convert the digital outputs of FIFOs 612(1) to 612(n), respectively, into a plurality of phase-shifted analog pulses, which are filtered by TX filters 616(1) to 616(n). The phase-shifted analog pulses are amplified by PAs 618(1) to 618(n) and then transmitted via TX antennas 601(1) to 601(n).

[0054] Receiver 620 may receive reflections of transmitted pulses (eg, transmitted by transmitter 610) via multiple RX antennas 602(1) to 602(n) and recover digital representations of the reflected (RX) pulses. Figure 6In the example of FIG. 6 , the RX antennas 602(1)-602(n) are shown as being separate from the TX antennas 601(1)-601(n). However, in some embodiments, the transmitter 610 and the receiver 620 can share one or more antennas (e.g., via antenna switching circuitry (not shown for simplicity)). In some implementations, the transmitter 610 can include a plurality of FIFO buffers 622(1)-622(n), a plurality of ADCs 624(1)-624(n), a plurality of RX filters 626(1)-626(n), and a plurality of LNAs 628(1)-628(n), each coupled to a respective one of the RX antennas 602(1)-602(n). The LNAs 628(1)-628(n) can amplify analog pulses received via the RX antennas 602(1)-602(n), respectively. The amplified pulses may be filtered by RX filters 626(1) to 626(n) and converted to digital format via ADCs 624(1) to 624(n). Each of FIFOs 622(1) to 622(n) may apply a corresponding phase shift (e.g., digital delay) to the output of ADCs 624(1) to 624(n), respectively, to align the phases of the digital pulses (e.g., to counter the effects of beamforming). The phase-aligned digital pulses may then be combined or added together (e.g., via summing circuit 621) to recover the corresponding RX pulses.

[0055] When AFE 600 is implemented in a Doppler radar system, transmitter 610 may receive a digital bit stream corresponding to a series of TX pulses to be transmitted continuously at a predetermined (e.g., target) frequency. Thus, receiver 620 may receive a series of corresponding reflected pulses (e.g., to be converted into RX pulses) from one or more objects in the path of the transmitted pulses. As described above, a processing system (such as a processor) coupled to AFE 600 may be configured to receive a series of TX pulses to be transmitted continuously at a predetermined (e.g., target) frequency. Figure 1A and Figure 1B The speed detection circuit system 115 can determine the speed of the object based at least in part on the frequency of the RX pulses. More specifically, the speed of the object can be determined by the amount of Doppler shift detected in the frequency of the RX pulses reflected by the object (e.g., relative to the frequency of the TX pulses).

[0056] It should be pointed out that in Figure 6 In the example of FIG. 6 , DACs 614(1) to 614(n) and ADCs 624(1) to 624(n) are coupled to receive the same TX / RX CLK. More specifically, the TX / RX CLK is provided by a clock generator 630 that is shared between the transmitter 610 and the receiver 620. In some implementations, the clock generator 630 may be Figure 3An example embodiment of a clock generator circuit 300 is provided. By distributing the TX / RX CLK generated by the same clock generator 630 to the transmitter 610 and the receiver 620, the sampling time (including phase noise) of the DACs 614(1) to 614(n) is matched to the sampling time of the ADCs 624(1) to 624(n). As a result, the phase noise in the ADCs 624(1) to 624(n) will effectively cancel the phase noise in the DACs 614(1) to 614(n), thereby mitigating any changes in the frequency of the transmitted pulses and / or reflected pulses.

[0057] Figure 7 FIG. 7 is a frequency diagram 700 depicting an example frequency domain representation of a reflected pulse train that may be received by a phase-matched Doppler radar AFE. Figure 1A and Figure 1B , the frequency graph 700 may depict the frequencies of the reflected pulses 114 and 116 from the first object 101 and the second object 102, respectively. More specifically, the frequency graph 700 may depict the frequencies of the reflected pulses 114 and 116 from the first object 101 and the second object 102, respectively. Figure 6 A phase-matched Doppler radar AFE such as AFE 600 senses the frequencies of the reflected pulses 114 and 116 .

[0058] exist Figure 7 In the example of FIG. 7 , the frequencies of the reflected pulses are generally distributed into two frequency intervals 710 and 720. The first frequency interval 710 is at the target frequency f T (eg, the frequency at which radar system 110 transmits radar pulses 112), and second frequency interval 720 is centered at a frequency slightly higher than target frequency f T The Doppler frequency f D As described above, the first object 101 may be a moving object, and the second object 102 may be a stationary object. Therefore, the first frequency interval 710 may include the reflected pulse 116 from the second object 102, and the second frequency interval 720 may include the reflected pulse 114 from the first object 101.

[0059] Phase noise in the TX CLK may cause the frequency of the transmitted pulse 112 to vary, thereby causing the frequencies of the reflected pulses 114 and 116 to vary. Figure 5, the frequency variation of the reflected pulse 116 from the stationary object 102 is depicted by the skirt 501 around the first frequency interval 510, and the frequency variation of the reflected pulse 114 from the moving object 101 is depicted by the skirt 502 around the second frequency interval 520. However, because the phase of the RX CLK (e.g., the TX / RX CLK in the receiver 620) is correlated or matched to the phase of the TX CLK (e.g., the TX / RX CLK in the transmitter 610), the AFE 600 does not perceive the frequency variation in the reflected pulses 114 and 116 due to the phase noise in the TX CLK. Therefore, the frequency diagram 700 does not include Figure 5 The skirt 501 or 502 (eg, phase noise) is depicted in the frequency graph 500 .

[0060] exist Figure 7 In the example of FIG. 1 , the energy in the reflected pulse 116 from the stationary object 102 (e.g., in the first frequency interval 701) is separate and different from the energy in the reflected pulse 114 from the moving object 101 (e.g., in the second frequency interval 702). Therefore, the phase-matched Doppler radar system can clearly identify the Doppler frequency f in the reflected pulse 114 received from the moving object 101. D and / or Doppler frequency shift (e.g., |f T -f D |). Moreover, the phase-matched Doppler radar system can be based at least in part on the Doppler frequency f D To more accurately determine the speed of the moving object 101.

[0061] Figure 8 8 is a block diagram of another phase-matched Doppler radar AFE 800 according to some embodiments. AFE 800 may be implemented in a Doppler radar system such as radar system 110 of FIG. 1 to transmit and receive radar pulses that may be used to detect the velocity or speed of an object in a surrounding environment. More specifically, AFE 800 may be Figure 2 800 includes a transmitter 810 that transmits radar pulses and a receiver 820 that receives reflections of the radar pulses. In some embodiments, the AFE 800 is configured to transmit and receive radar pulses using digital beamforming techniques.

[0062] The transmitter 810 receives a digital representation of a series of TX pulses and transmits TX pulses via a plurality of TX antennas 801(1) to 801(n) in a target direction (e.g., the direction of a target object). In some implementations, the transmitter 810 may include FIFOs 812(1) to 812(n), DACs 814(1) to 814(n), a plurality of TX filters 816(1) to 816(n), and a plurality of PAs 818(1) to 818(n), each coupled to a corresponding TX antenna in the TX antennas 801(1) to 801(n). Each of the FIFOs 812(1) to 812(n) may apply a corresponding phase shift (e.g., a digital delay) to the TX pulses to steer or focus the analog pulses transmitted by the antennas 801(1) to 801(n) in the target direction. DACs 814(1) to 814(n) can convert the digital outputs of FIFOs 812(1) to 812(n), respectively, into a plurality of phase-shifted analog pulses, which are filtered by TX filters 816(1) to 816(n). The phase-shifted analog pulses are amplified by PAs 818(1) to 818(n) and then transmitted via TX antennas 801(1) to 801(n).

[0063] Receiver 820 may receive reflections of transmitted pulses (e.g., transmitted by transmitter 810) via multiple RX antennas 802(1) to 802(n) and recover digital representations of the reflected (RX) pulses. Figure 8In the example of FIG. 8 , the RX antennas 802(1)-802(n) are shown as being separate from the TX antennas 801(1)-801(n). However, in some embodiments, the transmitter 810 and the receiver 820 can share one or more antennas (e.g., via antenna switching circuitry (not shown for simplicity)). In some implementations, the transmitter 810 can include a plurality of FIFO buffers 822(1)-822(n), a plurality of ADCs 824(1)-824(n), a plurality of RX filters 826(1)-826(n), and a plurality of LNAs 828(1)-828(n), each coupled to a respective one of the RX antennas 802(1)-802(n). The LNAs 828(1)-828(n) can amplify analog pulses received via the RX antennas 802(1)-802(n), respectively. The amplified pulses may be filtered by RX filters 826(1) to 826(n) and converted to digital format via ADCs 824(1) to 824(n). Each of FIFOs 822(1) to 822(n) may apply a corresponding phase shift (e.g., digital delay) to the output of ADCs 824(1) to 824(n), respectively, to align the phases of the digital pulses (e.g., to counter the effects of beamforming). The phase-aligned digital pulses may then be combined or added together (e.g., via summing circuit 821) to recover the corresponding RX pulses.

[0064] When AFE 800 is implemented in a Doppler radar system, transmitter 810 may receive a digital bit stream corresponding to a series of TX pulses to be transmitted continuously at a predetermined (e.g., target) frequency. Thus, receiver 820 may receive a series of corresponding reflected pulses (e.g., to be converted into RX pulses) from one or more objects in the path of the transmitted pulses. As described above, a processing system (such as a processor) coupled to AFE 800 may be configured to receive a series of corresponding reflected pulses (e.g., to be converted into RX pulses). Figure 1A and Figure 1B The speed detection circuit system 115 can determine the speed of the object based at least in part on the frequency of the RX pulses. More specifically, the speed of the object can be determined by the amount of Doppler shift detected in the frequency of the RX pulses reflected by the object (e.g., relative to the frequency of the TX pulses).

[0065] It should be pointed out that in Figure 8 In the example of FIG. 8 , DACs 814(1) to 814(n) and ADCs 824(1) to 824(n) are coupled to receive the same TX / RX CLK. More specifically, the TX / RX CLK is provided by a clock generator 830 that is shared between the transmitter 810 and the receiver 820. In some implementations, the clock generator 830 may be Figure 3824(1) to 824(n) is matched to the sampling time of the ADCs 824(1) to 824(n). As a result, the phase noise in the ADCs 824(1) to 824(n) will effectively cancel the phase noise in the DACs 814(1) to 814(n), thereby mitigating any changes in the frequency of the transmitted pulses and / or reflected pulses.

[0066] In some embodiments, receiver 820 may receive a delayed version of the TX / RX CLK provided to transmitter 810. For example, delay element 840 may be coupled between the output of clock generator 830 and the clock inputs of ADCs 824(1) to 824(n). The amount of delay applied by delay element 840 may be based at least in part on the round trip time (RTT) of the transmitted pulse to be reflected and received by AFE 800. Figure 4 As noted, there is typically some propagation delay between the time a radar pulse 401 is transmitted via TX antennas 801(1)-801(n) and the time a reflection of the radar pulse 402 arrives at RX antennas 802(1)-802(n). The RTT of a radar pulse typically depends on the distance between the radar system and the reflecting object. For example, a pulse reflected from an object farther from the radar system may have a longer RTT than a pulse reflected from an object closer to the radar system.

[0067] Because radar pulses effectively propagate at the speed of light, the RTT of radar pulses reflected by objects relatively close to the radar system may be negligible. Thus, in some embodiments, when AFE 800 (or a corresponding radar system) is configured to detect the velocity of an object relatively close to the radar system, delay element 840 does not apply a delay to the TX / RX CLK provided to receiver 820. However, when AFE 800 is configured to detect the velocity of an object far from the radar system, delay element 840 may be configured to delay the TX / RX CLK provided to receiver 820 by an amount commensurate with the range of one or more target objects so that the sampling time (including phase noise) of radar pulse 401 will more accurately align with the sampling time of reflected pulse 402. This may allow the radar system to more accurately mitigate frequency variations in radar pulse 401 and / or phase noise in TX CLK.

[0068] Fig. 9 is an illustrative flow chart depicting example operations 900 of a phase-matched Doppler radar system according to some embodiments. Figure 6 and Figure 8The example operations 900 may be performed by a radar system coupled to any of the phase-matched Doppler radar AFEs 600 and / or 800 described herein. More specifically, the example operations 900 may allow the radar system to mitigate phase noise in the TX CLK by correlating or synchronizing the TX CLK and the RX CLK.

[0069] The radar system can transmit a pulse train, where the pulse train is generated by a plurality of DACs in response to transitions of a clock signal (910). Figure 6 , the transmitter 610 may receive a digital bit stream corresponding to a series of TX pulses to be transmitted continuously at a predetermined (e.g., target) frequency. More specifically, each of the FIFOs 612(1) to 612(n) may apply a corresponding phase shift (e.g., digital delay) to the TX pulses to steer or focus the analog pulses transmitted by the antennas 601(1) to 601(n) in a target direction. The DACs 614(1) to 614(n) may convert the digital outputs of the FIFOs 612(1) to 612(n) into a plurality of phase-shifted analog pulses, respectively. In some embodiments, the DACs 614(1) to 614(n) sample or generate analog pulses in response to the TX / RX CLK generated by the clock generator 630. For example, each of the DACs 614(1) to 614(n) may update the amplitude of its analog output (e.g., corresponding to the TX pulse) with each transition (e.g., from low to high or from high to low or both) of the TX / RX CLK.

[0070] The radar system may also receive reflections of the transmitted pulses, wherein the reflected pulses are sampled by a plurality of ADCs in response to transitions of the clock signal (920). Figure 6, the receiver 620 can receive a series of corresponding reflected pulses (e.g., to be converted into RX pulses) from one or more objects in the path of the transmitted pulses. More specifically, the received pulses can be converted into a digital format via ADCs 624(1) to 624(n). Each of the FIFOs 622(1) to 622(n) can apply a corresponding phase shift (e.g., a digital delay) to the output of the ADCs 624(1) to 624(n), respectively, to align the phases of the digital pulses (e.g., to counter the effects of beamforming). The phase-aligned digital pulses can then be combined or added together (e.g., via summing circuit 621) to recover the corresponding RX pulses. In some embodiments, the ADCs 224(1) to 224(n) can sample the received pulses in response to the TX / RX CLK generated by the same clock generator 630. For example, each of ADCs 624(1)-624(n) may update the amplitude of its digital output (eg, corresponding to an RX pulse) with each transition of the TX / RX CLK (eg, from low to high or from high to low or both).

[0071] The radar system can then determine the speed of the object in the path of the transmitted pulse based at least in part on the transmitted pulse and the reflected pulse (930). Figure 1A and Figure 1B As described, a Doppler radar system can determine the velocity of an object based at least in part on the frequency of the RX pulses. More specifically, the velocity of an object can be determined by the amount of Doppler shift detected in the frequency of the RX pulses reflected by the object (e.g., relative to the frequency of the TX pulses). Therefore, in order to accurately detect such Doppler shift, the RX pulses should be correlated with (e.g., phase matched to) the TX pulses. By distributing the TX / RX CLK generated by the same clock generator 630 to the transmitter 610 and the receiver 620, the sampling time (including phase noise) of the DACs 614(1) to 614(n) is matched to the sampling time of the ADCs 624(1) to 624(n). As a result, the phase noise in the ADCs 624(1)-624(n) will effectively cancel the phase noise in the DACs 614(1) to 614(n), thereby mitigating any changes in the frequency of the transmitted pulses and / or the reflected pulses.

[0072] Those skilled in the art will appreciate that any of a variety of different technologies and techniques may be used to represent information and signals. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced in the above description may be represented by voltage, current, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof.

[0073] Further, it will be appreciated by those skilled in the art that the various illustrative logic blocks, modules, circuits and algorithm steps described in conjunction with the various aspects disclosed herein can be implemented as electronic hardware, computer software or a combination of the two. In order to clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits and steps have been generally described above with respect to their functionality. Whether this functionality is implemented as hardware or software depends on the specific application and the design constraints imposed on the entire system. The technician can implement the described functions in different ways for each specific application, but such implementation determination should not be interpreted as departing from the scope of the present disclosure.

[0074] The methods, sequences or algorithms described in conjunction with the various aspects disclosed herein may be embodied directly in hardware, in software modules executed by a processor, or in a combination of the two. The software modules may reside in RAM latches, flash latches, ROM latches, EPROM latches, EEPROM latches, registers, hard disks, removable disks, CD-ROMs or any other form of storage medium known in the art. An example storage medium is coupled to a processor so that the processor can read information from the storage medium and write information to the storage medium. As an alternative, the storage medium may be integrated with the processor.

[0075] In the foregoing description, the exemplary embodiments have been described with reference to specific exemplary embodiments of the exemplary embodiments. However, it will be apparent that various modifications and changes may be made thereto without departing from the broader scope of the present disclosure as set forth in the appended claims. Accordingly, the description and drawings are to be regarded as illustrative rather than restrictive.

Claims

1. A radar system, comprising: A transmitter configured to transmit a pulse sequence, the transmitter comprising a plurality of digital-to-analog converters DAC, the digital-to-analog converters being used to generate the pulse sequence in response to a clock signal; a receiver configured to receive reflections of the transmitted pulse train, the receiver comprising a plurality of analog-to-digital converters ADC that sample the reflected pulses in response to the clock signal, wherein the reflected pulses include reflections from a first object and a second object in a path of the transmitted pulse train, and phase noise in the clock signal causes a frequency of at least some of the reflected pulses from the second object to overlap a frequency of the reflected pulses from the first object; as well as The speed detection circuit system is configured to: detecting a Doppler shift between the frequency of the transmitted pulse train and the frequency of the reflected pulses; filtering the reflected pulses from the second object when the Doppler shift is detected; as well as A velocity of the first object is determined based at least in part on the Doppler shift. 2 . The radar system of claim 1 , wherein the ADC is in phase locked with the DAC.

3. The radar system of claim 1 or claim 2, wherein at least one of the DACs is configured to generate a first pulse in the pulse train in response to a first transition of the clock signal, and wherein at least one of the ADCs is configured to sample a reflection of the first pulse in the pulse train in response to the first transition of the clock signal.

4. The radar system of claim 1 , wherein phase noise in the clock signal causes a change in the frequency of the transmitted pulse train and a corresponding change in the frequency of the reflected pulses, and wherein the receiver is configured to match the change in the frequency of the reflected pulses to the change in the frequency of the transmitted pulse train when sampling the reflected pulses. 5 . The radar system of claim 1 , wherein the first object is a moving object and the second object is stationary.

6. The radar system according to claim 1, further comprising: A clock generator circuit is configured to generate the clock signal, which is provided to the plurality of DACs and the plurality of ADCs, wherein the phase noise corresponds to a close phase noise generated by one or more components of the clock generator circuit.

7. The radar system of claim 1, further comprising: A delay element is configured to delay the clock signal provided to the ADC by a delay amount based at least in part on a round trip time between the transmission of a corresponding pulse of the pulse train and the receipt of a reflection of the corresponding pulse.

8. The radar system of claim 1, wherein the transmitter and the receiver are configured to use digital beamforming techniques to transmit the pulse sequence and receive the reflections of the transmitted pulses, respectively.

9. A method of operating a radar system, comprising: In response to a clock signal, a pulse train is generated via a plurality of digital-to-analog converters DAC; transmitting the pulse sequence; sampling, in response to the clock signal, reflections of the transmitted pulse train via a plurality of analog-to-digital converters (ADCs), wherein the reflected pulses include reflections from a first object and a second object in a path of the transmitted pulse train, and phase noise in the clock signal causes a frequency of at least some of the reflected pulses from the second object to overlap a frequency of the reflected pulses from the first object; as well as detecting a Doppler shift between the frequency of the transmitted pulse train and the frequency of the reflected pulses; filtering the reflected pulses from the second object when the Doppler shift is detected; as well as A velocity of the first object is determined based at least in part on the Doppler shift.

10. The method of claim 9, wherein the ADC is phase-locked with the DAC, at least one of the DACs is configured to generate a first pulse in the pulse train in response to a first transition of the clock signal, and at least one of the ADCs is configured to sample a reflection of the first pulse in the pulse train in response to the first transition of the clock signal.

11. The method of claim 9, wherein phase noise in the clock signal causes a change in the frequency of the transmitted pulse train and a corresponding change in the frequency of the reflected pulses, and wherein the sampling further comprises: The change in the frequency of the reflected pulses is matched to the change in the frequency of the transmitted pulse train.

12. The method according to claim 9, further comprising: generating the clock signal via a clock generator circuit, wherein the phase noise corresponds to close phase noise generated by one or more components of the clock generator circuit; or The clock signal provided to the ADC is delayed by an amount of delay based at least in part on a round trip time between the transmission of a corresponding pulse of the pulse train and the receipt of a reflection of the corresponding pulse.

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