Doppler radar coexistence

Abstract

A method for providing reduced interference to at least two co-located FMCW (Frequency Modulated Continuous Wave) Doppler radars, each of said radars being used in a system to detect the distance and speed of objects moving in space, the method may include a propagation determination step in which an expected electromagnetic wave propagation time between the radar pair is determined, a sweep time offset synchronization step in which a different respective sweep time offset is selected for each radar in a first group of radars, and a sweep frequency offset synchronization step in which a second sweep frequency offset is selected for a second group of radars, the second sweep frequency offset being relative to the sweep frequency pattern used for the radars belonging to the previous first group. The invention also relates to a system and a computer software product.

Description

[Background technology]

[0001] The present invention relates to a method for providing reduced interference to several co-located Doppler radars, in particular FMCW (Frequency Modulated Continuous Wave) radars provided for automatically tracking the trajectory of moving objects, for example sporting projectiles like golf balls. The present invention also relates to a system for performing such a method, and to a computer software product arranged to perform such a method.

[0002] It is known to use FMCW Doppler radar to automatically track such objects moving in space relative to the radar. With FMCW Doppler radar it is possible to measure both the distance (sometimes also referred to in the art as "range") of the moving object as well as the (radial) velocity of the moving object relative to the radar. Typically such radars are also able to measure the direction of the object relative to the tracking radar. Summary of the Invention

[0003] In many cases, several FMCW Doppler radars are distributed over an area to monitor a defined volume. To achieve this, such FMCW Doppler radars may need to be in one close location, in other words to cooperate to monitor the combined volume with respect to the tracked object. This is the case, for example, at a golf driving range. In such cases, there is a problem of interference between such co-located FMCW Doppler radars. In other words, electromagnetic waves emitted from one such radar are received, directly or via reflection, by one or several other co-located radars. Such received signals then run the risk of being erroneously interpreted as emanating from the tracked object or of bringing about other interference. For example, interference may reduce sensitivity, such that important weak target signals of interest may not be detected.

[0004] Known methods for solving the radar interference problem include time and frequency multiplexing. However, time multiplexing generally reduces the granularity and accuracy of the measurements, and the available frequency bandwidth is often limited. In particular, for example at a golf driving range, it is often preferable to be able to use a relatively simple set of identical radar devices.

[0005] US 6,856,276 B2, relating to ocean wave monitoring, describes an FMCW radar system using several coexisting radars, where the interference problem is addressed by introducing relative time delays for the chirps (frequency sweeps) emitted by the individual radars.

[0006] The present invention may solve one or more, or all, of the problems set forth above.

[0007] Thus, the present invention relates to a method for providing reduced interference to at least two co-located FMCW (Frequency Modulated Continuous Wave) Doppler radars, each of which is used in a system to detect a respective distance and velocity of an object moving in space relative to a respective FMCW Doppler radar, the method comprising: determining an expected electromagnetic wave propagation time between a pair of FMCW Doppler radars; selecting a different respective sweep time offset for each FMCW Doppler radar in a first group of FMCW Doppler radars; and selecting a second non-zero sweep frequency offset for a second group of FMCW Doppler radars, the second non-zero sweep frequency offset being relative to a sweep frequency pattern used by the FMCW Doppler radars belonging to the first group.

[0008] In some embodiments, the method further comprises the system measuring, using some of the FMCW Doppler radars, respective positions and velocities of one or several moving objects relative to a respective measuring FMCW Doppler radar by each of the FMCW Doppler radars emitting repeated FMCW sweeps with a selected respective sweep time offset and, if applicable, a selected respective sweep frequency offset.

[0009] In some embodiments, the FMCW sweep comprises a respective return ramp waveform, and the return ramp waveform is blanked.

[0010] In some embodiments, the method further comprises repeating determining an expected electromagnetic wave propagation time, selecting a different respective sweep time offset, and selecting a second non-zero sweep frequency offset, the repeating occurring periodically and / or when adding additional FMCW Doppler radars to the system.

[0011] In some embodiments, the method further comprises synchronizing respective clocks of each of the FMCW Doppler radars based, for example, on a common received geolocation signal or a common received time signal from a central control module of the system, the clocks being used in combination with respective sweep time offsets to trigger FMCW sweeps.

[0012] In some embodiments, determining the expected electromagnetic wave propagation times comprises one or more of the FMCW Doppler radars emitting respective electromagnetic test signals that are detected and timed by one or more of the other FMCW Doppler radars.

[0013] In some embodiments, determining the expected electromagnetic wave propagation time comprises each FMCW Doppler radar receiving an electromagnetic test signal and determining a maximum detectable propagation time of the electromagnetic test signal, taking into account any multipath wave trajectories, where the received electromagnetic test signal has at least a predetermined minimum power upon reception.

[0014] In some embodiments, the electromagnetic test signal is an FMCW wave.

[0015] In some embodiments, an electromagnetic test signal is emitted from each emitting FMCW Doppler radar having a carrier frequency that is offset relative to the carrier frequency used for the respective electromagnetic test signal by each of the other FMCW Doppler radars.

[0016] In some embodiments, determining the expected electromagnetic wave propagation time comprises measuring a maximum power of the received electromagnetic test signal at each FMCW Doppler radar receiving the electromagnetic test signal.

[0017] In some embodiments, the respective sweep waveforms used by each of the FMCW Doppler radars are identical across the FMCW Doppler radars, except for the respective sweep time offsets and sweep frequency offsets used by the FMCW Doppler radars.

[0018] In some embodiments, selecting the different respective sweep time offsets comprises ranking the FMCW Doppler radars in a first ranking group of FMCW Doppler radars in a first ranking according to interference severity relative to other FMCW Doppler radars in the first ranking group and based on expected electromagnetic wave propagation times; and selecting, for each of the FMCW Doppler radars in the first ranking group, in order from the most severely interfering FMCW Doppler radars to the less severely interfering FMCW Doppler radars, taking into account both the minimum relative sweep time offset and the propagation time delay of the FMCW Doppler radar determined relative to other FMCW Doppler radars in the first ranking group for which the respective first sweep time offsets have already been selected.

[0019] In some embodiments, selecting a different respective sweep time offset includes identifying a second ranked group of one or more FMCW Doppler radars in the first ranked group for which it has been determined that it is not possible to select a respective sweep time offset that does not result in a predetermined minimum interference level during the object tracking operation; ranking the FMCW Doppler radars in the second ranked group of FMCW Doppler radars in the second ranking, the second ranking being made according to interference severity relative to other FMCW Doppler radars in the second ranked group and based on expected electromagnetic wave propagation times; and for each of the FMCW Doppler radars in the second ranked group, selecting the FMCW Doppler radar with the largest interference severity among the FMCW Doppler radars. The method further comprises selecting a second sweep time offset for each of the FMCW Doppler radars in order from those interfering with severity to those interfering with less severity of the FMCW Doppler radars, taking into account both the minimum relative sweep time offset and the propagation time delay of the FMCW Doppler radar determined relative to other FMCW Doppler radars in the second ranking group for which the respective second sweep time offsets have already been selected; selecting FMCW Doppler radars that belong to the first ranking group but not to the second ranking group as a first group of FMCW Doppler radars; and selecting one or several FMCW Doppler radars that belong to the second ranking group as a second group of FMCW Doppler radars.

[0020] In some embodiments, selecting the different respective sweep time offsets further comprises determining a minimum relative sweep time offset based on a minimum sweep time offset that produces at least a predetermined attenuation of an IF (intermediate frequency) of one of the FMCW Doppler radars experiencing interference from another one of the FMCW Doppler radars.

[0021] In some embodiments, the sweep frequency offset is selected such that interference between an FMCW Doppler radar belonging to a first group and an FMCW Doppler radar belonging to a second group results in a misinterpretation of an object velocity outside a predetermined velocity range, such misinterpretation corresponding, for example, to a false object detection due to interference.

[0022] In some embodiments, the sweep frequency offset is 0.3 of the sweep repetition frequency used by the FMCW Doppler radar. and The value is chosen to be between 0.7 and 1.

[0023] Furthermore, the present invention relates to a system for detecting respective distances and velocities of objects moving in space relative to at least one of at least two co-located FMCW (Frequency Modulated Continuous Wave) Doppler radars, the system comprising the FMCW Doppler radars and one or more hardware processors, the hardware processors being configured to determine expected electromagnetic wave propagation times between a pair of the FMCW Doppler radars, select different respective sweep time offsets for each FMCW Doppler radar in a first group of the FMCW Doppler radars, and select a second non-zero sweep frequency offset for a second group of the FMCW Doppler radars, the second non-zero sweep frequency offset being relative to a sweep frequency pattern used by the FMCW Doppler radars belonging to the first group.

[0024] In some embodiments, one or more of the one or more hardware processors are located within one or more of the FMCW Doppler radars.

[0025] In some embodiments, the system includes a central control module, where one or more of the one or more hardware processors are located within the central control module.

[0026] Furthermore, the present invention relates to a computer software product configured, when executed on the computer hardware of a central control module and / or a set of distributed FMCW Doppler radar devices, to detect respective distances and velocities of objects moving in space relative to at least one of at least two co-located FMCW (Frequency Modulated Continuous Wave) Doppler radars, the computer software product comprising a control function configured to provide reduced interference to each of the FMCW Doppler radars by determining expected electromagnetic wave propagation times between a pair of FMCW Doppler radars, selecting different respective sweep time offsets for each FMCW Doppler radar in a first group of FMCW Doppler radars, and selecting a second non-zero sweep frequency offset for a second group of FMCW Doppler radars, the second non-zero sweep frequency offset being relative to the sweep frequency pattern used by the FMCW Doppler radars belonging to the first group.

[0027] The computer software product may be implemented by a non-transitory computer-readable medium encoding instructions that cause one or more hardware processors located in at least one of the FMCW Doppler radars in the system and / or in a central control module of the system to determine expected electromagnetic wave propagation times, select different respective sweep time offsets, and select a second non-zero sweep frequency offset.

[0028] The present invention will now be described in detail with reference to exemplary embodiments thereof and the accompanying drawings. [Brief description of the drawings]

[0029] [Figure 1a] 1 is a simplified chart illustrating a frequency sweep or chirp. [Figure 1b] 1 is a simplified chart illustrating a frequency sweep or chirp. [Figure 1c] 1 is a simplified chart illustrating a frequency sweep or chirp. [Diagram 2] This is an overview of an FMCW Doppler radar. [Diagram 3] 1 is a chart showing a mixed signal for a target radar. [Figure 4] 1 is a chart showing a signal that has been range compressed via a Fourier transform. [Diagram 5] 1 is a chart showing the response at the output of a victim radar receiver due to an interferometric radar with a matching waveform (top), the corresponding range-sweep matrix (middle), and the corresponding range-velocity matrix (bottom). [Figure 6] This corresponds to Fig. 5, but shows the case where the waveforms between the radars do not match. [Figure 7] 1 is an overview of a system according to one or more embodiments of the present invention, comprising a central control module and multiple FMCW Doppler radars arranged in several groups. [Figure 8] 1 is a simplified diagram of a data processing device. [Figure 9] 1 is a flow chart illustrating a method. [Figure 10] 1 is a simplified chart showing frequency sweeps for two different radars. [Figure 11] FIG. 1 is a top view layout diagram of multiple co-located FMCW Doppler radars. [Figure 12a] 12 is a plot of IF (Intermediate Frequency) versus time for a first identified group of the multiple FMCW Doppler radars shown in FIG. 11 . [Figure 12b] 12 is a plot of IF (Intermediate Frequency) versus time for a second identified group of the multiple FMCW Doppler radars shown in FIG. 11 . [Figure 13a] FIG. 12b is an enlarged detail of the plot shown in FIG. 12a. [Figure 13b] FIG. 12b is an enlarged detail of the plot shown in FIG. [Figure 14] These are combinations of range-sweep maps (top of each) and range-velocity maps (bottom of each) for three different scenarios. [Figure 15] These are combinations of range-sweep maps (top of each) and range-velocity maps (bottom of each) for three different scenarios. [Figure 16] These are combinations of range-sweep maps (top of each) and range-velocity maps (bottom of each) for three different scenarios. [Figure 17] 16 is a plot of IF versus time for the scenario shown in FIG. 15. [Figure 18] 4 is a chart showing the shape of soft blanking and hard blanking, respectively, as a function of time. [Figure 19] The waveforms of the victim radar and the interferometric radar are shown. [Figure 20] 1 is a flow chart illustrating a method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] The present invention broadly relates to interference mitigation within a group of FMCW (Frequency Modulated Continuous Wave) radar devices 120 operating in the same RF (Radio Frequency) band. The FMCW radar devices 120 may be coordinated by a central control module 110 or in a decentralized manner using an appropriate set of rules that specify device specific behavior and possibly also device-to-device interactions regarding operation and information synchronization. To work together in a coordinated manner, these cooperating radar devices 120 need to have proper knowledge of a global time reference, e.g., as derived from the central control module 110, and / or a geolocation signal, e.g., a GPS (Global Positioning System) receiver in each radar device 120.

[0031] More generally, the present invention relates to Doppler radar technology, in other words, the FMCW radar device 120 is a Doppler radar device. Hereinafter, the expression "FMCW Doppler radar" will be used.

[0032] FMCW radar device 120 may be of the "linear" type, or in other words, an FMCW Doppler radar operating using linear FMCW. Examples of such linear FMCW include the "sawtooth," "inverse sawtooth," and "triangle" frequency / time shapes described below, where the frequency varies linearly with time, although it will be understood that non-linear frequency variations may also be used in some embodiments.

[0033] Traditionally, FMCW radar transmits a continuous waveform with linear frequency modulation, even though various kinds of nonlinear frequency modulation are also possible. For simplicity, in the following description it is assumed that the modulation is of the "sawtooth" type, as shown in Figure 1a.

[0034] Thus, in FIG. 1a, a “sawtooth” (or, using a different term, “upward sawtooth”) shaped FMCW waveform 20 is shown. Reference numeral 21 denotes a sweep repetition interval T Rep 1 indicates the frequency sweep time (corresponding to 1 Hz), which is generally constant over time and may be the same for all FMCW Doppler radars 120. Reference numeral 22 indicates the "up chirp" portion (the actual frequency sweep) and 23 indicates (using a dashed line) the "down chirp" portion where the frequency is returned to the starting value for another frequency sweep.

[0035] It will be understood that time-repeating FMCW waveforms other than such "sawtooth" type are also useful in the present context, and the "sawtooth" type is merely an example provided for clarity. Preferably, in FMCW waveforms useful in the present context, the frequency varies linearly with time. Figures 1b and 1c disclose two examples of different FMCW waveforms useful in the present context.

[0036] However, in some embodiments, the FMCW waveform used is one or more of the following: “upward sawtooth”, “downward sawtooth”, or “triangular”, as described herein and shown in FIGS. 1a-1c.

[0037] Thus, Figure 1b shows a "downward sawtooth" variation, and Figure 1c shows a "triangle" variation. In the latter, the up-sweep and down-sweep sweeps are nominally of the same duration, and the chirp rate of the down-sweep is the negative of the chirp rate of the up-sweep. Of note, Figures 1a-1c share the same reference numbers for corresponding parts.

[0038] Range and Doppler processing when using a triangular waveform is slightly different than with a sawtooth waveform (not described in detail here), but note that in this case the down sweep is just as important as the up sweep (the down sweep in a sawtooth does not contribute to range (or Doppler) determination). Thus, in the example of FIG. 1a, one "tooth" (upward ramp) of the waveform is often referred to as a "chirp" or "sweep", and the upward portion is of a particular duration t Up The chirp rate μ is defined as the change in frequency per unit time during an up-chirp. In FMCW radars using linear frequency modulation, the chirp rate is constant (slope). Between chirps, the radar needs to return to the starting frequency, hence the "down-chirp". Ideally, the transition time back to the starting frequency would be zero, but this is generally not possible. However, for a sawtooth waveform, the downward transition time t Down is usually t Up It's a tiny fraction compared to the rest.

[0039] A block diagram of a typical FMCW Doppler radar 200 is shown in FIG.

[0040] The FMCW Doppler radar 200 comprises a transmitting antenna 201, a receiving antenna 202, and an oscillator 203 used to generate the waveform 20. Thus, a waveform signal is provided from (or based on) the oscillator 203 to the transmitting antenna 201 and also to a mixer 204. The receiving antenna 202 provides a delayed version (due to the target echo) of the transmitted waveform signal. The received signal may be amplified by a low noise amplifier 205 and provided to the mixer 204. The mixer 204 demodulates the received signal by multiplying it with a reference waveform signal, resulting in a constant frequency signal having a frequency proportional to the time delay of the target echo. This signal is provided to and low pass filtered by a filter module 206. The filter module 206 provides a mixed filtered signal, possibly after amplification in another amplifier 207, to an A / D converter 208. Downstream of the A / D converter 208 is a digital filter module 206' configured to perform any filtering, decimation, etc., as is known per se for FMCW Doppler radars 200 of the current general type.

[0041] The resulting signal is provided to local control logic 209 for digital processing. It is preferred that the receiver bandwidths of all participating FMCW Doppler radars 120 in the system 100 are the same or at least differ by no more than 50%.

[0042] The FMCW Doppler radar 200 also comprises a time synchronization means 210, which may comprise a receiving antenna for a global time reference signal, e.g., a geolocation signal such as a GPS signal. Alternatively, the time synchronization means 210 may be configured to receive a time synchronization signal broadcast from the central control module 110 (see below). The time synchronization means 210 may form part of the local control logic 209. For example, the local control logic 209 may include a clock that is synchronized to the clocks of each of the peer FMCW Doppler radars 200.

[0043] Clock synchronization across the FMCW Doppler radar 120 may occur periodically, such as at least once every 10 seconds, or at least once every 5 seconds, such as once every second.

[0044] The FMCW Doppler radar 200 also comprises a communication means 211, for example a wireless or wired communication means, configured to enable the FMCW Doppler radar 200 to digitally communicate with peer FMCW Doppler radars of a corresponding type and / or with the central control module 110. Possible communication channels include WiFi or any other wireless communication standard, and wired communication channels. For example, per se conventional Internet communication protocols may be used for said communication.

[0045] An FMCW waveform 20 is transmitted by a transmit antenna 201, reflects off a target object 10 (see below) and returns towards a receive antenna 202. On reception, the (weak) received signal is multiplied with a copy of the transmitted signal. The round trip delay produces a constant frequency signal at the output of a mixer 204, with a frequency proportional to the round trip delay and proportional to the distance (range) to the target object 10. The signal after the mixer 204 is known as the IF (Intermediate Frequency) signal, and is sometimes also referred to as the "beat signal".

[0046] Since the output signal at the mixer 204 is a sine wave with a frequency proportional to the range (or more precisely the relative delay), the range can be extracted by taking a Fourier transform of the received signal. For example, a per se conventional Fast Fourier algorithm can be used. Furthermore, since superposition is applied, a group of target objects 10 can be resolved in terms of distance since, as long as they have different radial distances to the radar 220, they will be associated with different IF frequencies.

[0047] Figure 3 shows the real part of the complex baseband signal after mixing and filtering. The target object in this example is located at a range of 49.8 meters.

[0048] After a Fourier transform of the received signal, see FIG. 4, a focal response is received at the target range.

[0049] Next, some theoretical aspects are explained in relation to the range-Doppler (i.e., distance-velocity of the target object 10) processing performed in the present system for detection of the target object 10. These details may be useful for understanding the relationship between IF frequency and peak position in range-velocity space.

[0050] A sequence of sweeps (chirps) is considered, and the receiver response to a single chirp is referred to as "fast time." The received data may be arranged in a matrix, e.g., row 1 may be the sampled response to sweep 1, row 2 may be the sampled response to sweep 2, and so on. Thus, the "fast time" is along the columns (for a particular row), whereas the row dimension is often referred to as "slow time," which means, for example, that for a column, the time increment from one row to another is equal to the sweep repetition interval T Rep This is because it is equal to

[0051] As mentioned above, the range profile is obtained by applying a Fourier transform along the fast time dimension (along the columns of each row in the matrix above). The Fourier transform of each receive sweep gives the IF frequency

number

number

[0052] The mapping between IF frequency and range is as follows:

number

[0053] In radar processing, the assumption when mapping IF frequencies to equivalent ranges is that the signal undergoes two-way propagation, and the response is due to its own transmitted signal reflecting off a target object 10 (for the same carrier frequency). If instead the response due to an interfering FMCW radar device with a matching waveform (see below) is considered, then at a range of about 50 meters, a peak occurs at half that range, or in other words 25 meters.

[0054] The term "matched waveform" refers to an interfering FMCW radar device having an identical waveform with the same time offset and the same carrier frequency as the victim radar (the radar being interfered with).

[0055] When the Fourier transform is applied across the row dimension of the above matrix, T Rep is the sampling interval in slow time, so the frequency

number

number

[0056] Doppler frequency and radial velocity

number

number

[0057] moreover,(

number

number

number

[0058] In the above formula, the distinct Doppler and velocity intervals are [0,1 / T Rep ]Hz and [0,v PRF ]. Usually, the symmetric interval [-0.5 / T Rep ,0.5 / T Rep ]Hz and [-0.5v PRF ,0.5v PRF ]m / s is taken into account.

[0059] To provide a theoretical background of the present principles, some general characteristics of FMCW interference are now described. In general, the interference characteristics depend on the waveform parameters of the interfering radar and the victim radar.

[0060] First, the case where the waveforms of the interferometric radar and the victim radar are coincident, in other words the frequency sweep has the same shape characteristics for both radars, is analyzed. Furthermore, for simplicity, we assume an ideal situation where the interferometric and victim radars generate perfect chirps that are synchronized with each other. In this case, see Figure 5, the interferometric radar generates a response similar to a point target at the victim radar.

[0061] The top panel of Figure 5 shows the response at the output of the victim radar receiver due to an interferometric radar emitting a waveform matching the victim waveform at a range of 99.2 m from the victim radar in the boresight direction. A strong peak response is shown at 49.6 m, half of this range, with the target object velocity being zero.

[0062] The middle part of Figure 5 shows the corresponding "range-sweep" matrix (the range Fourier transform is performed, but not the velocity Fourier transform). The lower part of Figure 5 shows the corresponding "range-velocity" matrix, showing the compressed point-like response at zero velocity and half range for the interferometric radar (one-way propagation for the interferometric radar).

[0063] Now referring to the case of having mismatched waveforms, in other words the waveform parameters of the interfering radar devices are different, the interference is blurred to various degrees depending on how different the waveforms are. The example shown in FIG. 6 shows the interference that occurs when the victim radar uses a 37.6 MHz waveform and the interfering radar uses a 9.7 MHz waveform. The nominal carrier frequency of the two radars is the same as well as the nominal chirp repetition frequency and (up)chirp length, so the chirp rate is the main parameter that differs. The geometry of the problem (and all other relevant system parameters, output power, antennas, etc.) is the same as in the example shown in FIG. 5. However, in addition to the change to the mismatched waveforms, a clock drift (oscillator drift) of 7 ppm has also been added to the interfering radar (the interfering radar's clock runs 7 microseconds per second faster than the victim radar). The mismatched oscillators and independent drifts result in varying degrees of blurring of the interference response in addition to the blurring caused by the mismatched nominal waveform parameters.

[0064] Thus, the top part of Figure 6 shows the response at the output of the victim radar receiver due to an interferometric radar having a waveform that does not match the victim waveform. As is evident from the top part of this Figure 6, a "mini-chirp" is produced that has a nominal chirp rate equal to the difference between the chirp rates of the victim and interferometric radars. The duration of the mini-chirp depends on the chirp rate of the mini-chirp and the bandwidth of the victim radar's receiver chain.

[0065] The middle part of Figure 6 shows the "range-sweep" matrix, corresponding to the middle part of Figure 5 above (where a range Fourier transform is performed but no velocity Fourier transform is performed). The bottom part of Figure 6 shows the "range-velocity" matrix based on a 20 ms coherent processing time interval (corresponding to the bottom part of Figure 5 above), where it is clear that the response to the interfering FMCW signal is smeared, primarily due to the mismatched waveforms.

[0066] Thus, from the bottom of Figure 6, we can see that there is smeared interference, and that it is located along a ridge in the range dimension. Due to the relative drift of the oscillators, the location of the ridge has moved away from zero velocity. Although the nominal carrier frequency is the same, the drift creates a difference in the effective carrier frequency, the magnitude of which determines the location of the ridge in the velocity dimension (the drift is modeled as constant during the 20 ms coherent processing interval).

[0067] As mentioned above, the present solution is particularly useful when time and / or frequency multiplexing is not possible or desirable. Therefore, it is generally preferred that the present method and system does not utilize such multiplexing, in other words, all FMCW Doppler radars 100 of the present system 100 coexist and transmit simultaneously in the same frequency (sub)band. However, in some embodiments, the present synchronization method can be used in conjunction with a certain multiplexing, where the multiplexing itself is not sufficient to adequately suppress or avoid the occurrence of interference.

[0068] Instead of a multiplexed approach, the FMCW Doppler radars 120 of the system 100 generally use the same nominal waveforms and parameters, except for time offset parameters and slightly offset carrier frequencies, as described below.

[0069] As explained above, non-cooperative FMCW Doppler radars generate varying degrees of smearing of inter-radar interference in the range-velocity domain, depending on how different their waveforms are. In current applications, such as in tracking golf balls, it is desirable to maintain high sensitivity to weak target signals and high requirements for the estimation accuracy of parameters of these signals. Furthermore, in many specific applications, the interference between FMCW Doppler radars 120 can be very strong, with interference-to-noise ratios often reaching as much as 50-60 dB before any coherent integration. Thus, such strong interference smearing is undesirable, as it effectively reduces sensitivity over large areas in the range-velocity domain.

[0070] At the same time, there may be regions within the range-velocity region where requirements are less stringent, for example there may be a maximum velocity above which the target object 10 is expected not to be detected.

[0071] Therefore, using a set of FMCW Doppler radars with "mismatched" waveforms results in a significant degradation of system performance in terms of sensitivity and estimation accuracy, because the interference is usually very strong and widespread, and there is no control over where the interference is located in the range-velocity domain.

[0072] Instead, the approach used here is to tune the FMCW Doppler radar 120 to concentrate interference in favorable regions in the range-velocity space that are relatively less sensitive to interference. Moreover, it is desirable to prevent interference energy in such regions from "leaking" into regions associated with high requirements for sensitivity and parameter estimation accuracy. As explained below, the latter can be achieved by carefully selecting the window used for processing (consisting of a compromise between resolution and lobe level) and possibly also considering the use of Tx blanking / tapering of the transmit chirp to reduce the effect of short portions of the chirp repetition interval corresponding to the down chirp.

[0073] As also described below, one important means for mitigating mutual interference within a set of cooperating FMCW Doppler radars 120 is to schedule the initiation of the individual chirps of the radars 120 in question to have time delays with respect to each other. If the set of delays is properly selected and the nominal waveform parameters are otherwise the same, the intentionally imposed delays will cause the IF frequencies of the other FMCW Doppler radars to be outside the receiver bandwidth of the victim radar (i.e., in the "range" where they will not be searched for the target object 10) at the victim radar. As a result, the interference is significantly attenuated.

[0074] As will be described later, the number of coexisting FMCW Doppler radars 120 is often greater than can be accommodated by simply delaying the sweeps. In such cases, the inventors have developed a method to solve this problem using subgroups, where time offsets are selected such that systems within such subgroups can operate without interfering with each other, but there is interference between subgroups.

[0075] The filter chain (i.e., the combination of the analog filter 206 and the digital filtering step in 206') of each FMCW Doppler radar 120 in the system 100 should have the sharpest possible transition band (above the maximum IF range, i.e., the maximum range for target detection) and the highest possible out-of-band attenuation. This allows a dense set of time offsets to be selected while still meeting the interference rejection requirements, thereby increasing the number of FMCW Doppler radars 120 that can coexist.

[0076] Therefore, by introducing an intentional relative chirp delay between the systems, it is possible to locate interference outside the range interval of interest.

[0077] However, in the general case, N groups may be defined. These groups may be defined during the determination of the time offsets of the sweep delays, for example in the manner described below with a "first" group 121 and a "second" group 122, and possibly further groups. Each group is then given its own set of time offsets that overlap (i.e., of similar span) with the time offsets of the other groups. The time offsets given to the FMCW Doppler radars 120 in a group ensure interference-free operation between the radars 120 in that group. However, as mentioned above, significant inter-group interference may exist.

[0078] To reduce the effects of subgroup interference, the center frequency of the sweep of the FMCW Doppler radars 120 in one subgroup can be changed relative to all other subgroups, so that the interference that this group of radars 120 causes to other groups of FMCW Doppler radars 120 arrives at a rate that is insignificant for target detection and estimation in the other radar groups. Specifically, to reduce the effects of inter-group interference, all subgroups can be given a uniquely tuned center frequency (the same for all FMCW Doppler radars 120 in a subgroup).

[0079] Therefore, a small difference in center frequencies (carrier frequency difference) between the groups 121, 122 can be selected to achieve separation of the groups 121, 122 in the velocity domain and reduce the effects of mutual interference between the groups 121, 122 of the FMCW Doppler radar 120.

[0080] This achieves "subgroup isolation," as described in more detail below.

[0081] As in the case of the receive filter chain 206, 206' (filtering in the IF dimension), velocity filtering (Doppler filtering) may also be applied to reduce the side lobe levels in the velocity domain extending from the localized interference response, which side lobes are dictated by the processing window used. The Doppler filtering then operates continuously along the slow time dimension, allowing the use of very long filters with high stopband attenuation. This filtering may be bandpass filtering.

[0082] As used herein, the “filter chain” 206, 206′ of the receive FMCW Doppler radar 120 refers to the total transfer function from the output terminal of the receive antenna 201 to the final output of the radar, e.g., before a two-dimensional Fourier transform is performed to generate a range-velocity matrix.

[0083] 7 illustrates a system 100 in accordance with one or more embodiments of the present invention that is particularly adapted for detecting the range and velocity of each of target objects 10 moving in space relative to one or more of a plurality of co-located FMCW Doppler radars 120. It is understood that FIG. 7 is provided for the purpose of clarifying the principles used in the present invention, and that the geometric layout of system 100 may differ in practice.

[0084] The system 100 may include a central control module 110 configured to communicate with each of the FMCW Doppler radars 120 to synchronize their behavior with respect to the type of sweep transmissions described above. Alternatively, the system 100 may not include such a central control module 110, in which case each FMCW Doppler radar 120 may include a predetermined set of behavior rules and / or communications capabilities for communicating between the radars 120 to establish peer-to-peer direct coordination aimed at achieving the type of synchronization described above coupled with a common time reference as described above.

[0085] In any case, the system 100 comprises a control function implemented in hardware and / or software that is centralized in a central control module 110 and / or distributed to several of the FMCW Doppler radars 120 and then executed locally in the respective control logic 209 of each participating radar 120. This control function is configured to provide reduced interference to each of the FMCW Doppler radars 120 by performing the synchronization between the FMCW Doppler radars 120.

[0086] The central control module 110 may include a centralized clock function 111, which may include a receiving antenna for a global time reference signal, such as a GPS signal, which the central control module 110 may use to perform the above-mentioned synchronization for time offsets across the FMCW Doppler radars 120.

[0087] The central control module 110 may also include control logic 112, which may be implemented in suitable software. The control logic 112 may be part of or may constitute the control functions described above.

[0088] Furthermore, the central control module 110 may comprise communication means 113 corresponding to the communication means 211 described above, for communicating with the individual FMCW Doppler radars 120 .

[0089] 7 also illustrates various FMCW Doppler radars 120, which are categorized into a first group 121 of such FMCW Doppler radars 120 and a second group 122 of such FMCW Doppler radars 120. It is understood that the groups of such FMCW Doppler radars 120 may number more than two.

[0090] In general, the inventors contemplate that system 100 may include at least three FMCW Doppler radars 120, possibly at least five FMCW Doppler radars 120, or even at least ten FMCW Doppler radars 120. Furthermore, each of the FMCW Doppler radars 120 of system 100 that are synchronized using the methods disclosed herein may be located up to 5,000 meters, e.g., up to 3,000 meters, e.g., up to 1,000 meters, e.g., up to 500 meters, e.g., up to 100 meters, etc., from the nearest peer FMCW Doppler radar 120.

[0091] 7 shows electromagnetic waves 123 (sweeping radar signals) emitted from each of the co-located FMCW Doppler radars 120. Note that in a real scenario, the antennas 201, 202 have a particular directivity, but this fact is ignored for the sake of simplicity in FIG.

[0092] In this context, FMCW Doppler radars 120 are "co-located" means that they affect each other at least pairwise with respect to interference.

[0093] 7, each of the FMCW Doppler radars 120, when configured according to at least some possible configurations, may be subject to interference from at least one other FMCW Doppler radar 120. In particular, there are preferably no completely isolated "islands" of FMCW Doppler radars 120 in the sense that no single FMCW Doppler radar 120 in an "island" experiences interference from the FMCW Doppler radars 120 in any other such "island." If two or more such "islands" exist, they are considered to belong to different systems 100 as that terminology is used herein.

[0094] In this context, "affected by interference" means that the interfering electromagnetic radar waves reaching the victim FMCW Doppler radar 120 have a power that exceeds the minimum sensitivity of the victim FMCW Doppler radar 120 such that, after filtering in the victim FMCW Doppler radar 120, they adversely affect the output information from the victim FMCW Doppler radar 120.

[0095] Each target object 10 to be detected is within the detection range of at least one FMCW Doppler radar 120 .

[0096] 2, the FMCW Doppler radar 200 includes only one receiving antenna 202. However, it is understood that one or some of the FMCW Doppler radars 120 used in the system 100 may include multiple receiving antennas 202. This may, for example, enable the FMCW Doppler radar 200 to detect a relative direction to a tracked target object 10. The principles described herein are equally applicable in this case.

[0097] In an exemplary embodiment of the invention, system 100 is configured to track a target object 10 in the form of a sports object in flight, such as a ball in flight, e.g., a baseball or golf ball in flight. In some embodiments, system 100 is used at a golf driving range, e.g., a driving range having multiple sections for hitting golf balls that are tracked using system 100. In this case, multiple FMCW Doppler radars 120 are provided to jointly cover a desired ground area and / or air volume associated with the range.

[0098] Various types of computers may be used in the system 100. The central control module 110 (particularly the control logic 112) and each of the FMCW Doppler radars 120 (particularly the control logic 209) constitute examples of such computers.

[0099] In general, the essential elements of a computer are a processor for executing instructions and one or more memory devices for storing instructions and data. As used herein, a "computer" may include a server computer, a client computer, a personal computer, an embedded programmable circuit, or a dedicated logic circuit. Such a computer may be connected to one or more other computers via a network, such as the Internet, or via any suitable peer-to-peer connection for digital communication, such as a Bluetooth connection.

[0100] Each computer may include various software modules that may be distributed in the application layer and the operating system. These may include executable and / or interpretable software programs or libraries, including various programs operating as, for example, a target object 10 detection program, a 2D target object 10 tracking program, and / or a 3D target object 10 tracking program. Other examples include FMCW Doppler radar time sweep offset and / or frequency sweep offset synchronization programs. The number of software modules used may vary from implementation to implementation and from such computer to computer. Each of the above programs may be implemented in embedded firmware and / or as software modules distributed in one or more data processing devices connected by one or more computer networks or other suitable communication networks.

[0101] 8 illustrates an example of such a computer, which is a data processing device 300, which may include hardware or firmware devices including one or more hardware processors 312, one or more additional devices 314, a non-transitory computer-readable medium 316, a communication interface 318, and one or more user interface devices 320. The processor 312 may process instructions for execution within the data processing device 300, such as instructions stored on the non-transitory computer-readable medium 316, which may include a storage device, such as one of the additional devices 314. In some implementations, the processor 312 is a single-core or multi-core processor, or two or more central processing units (CPUs). The data processing device 300 communicates with one or more other computers 390, for example via a network 380, using the communication interface 318. Thus, in various implementations, the processes described may be executed in parallel or serially on a single-core or multi-core computing machine, and / or on a computer cluster / cloud, etc.

[0102] The data processing device 300 includes various software modules that may be distributed in the application layer and the operating system. These may include executable and / or interpretable software programs including a program 330 that constitutes the local control logic 209 of the individual FMCW Doppler radar 200 as described herein and that is configured to perform at least a part of the method of the type described herein, such as, for example, one or more of the propagation determination step S2, the clock synchronization step S3, the sweep time offset synchronization step S4, the sweep frequency offset synchronization step S5, and the object detection step S6. The program 330 may also constitute the control logic 112 as described herein, that is configured to perform at least a part of the method of the type described herein, such as, for example, one or more of the propagation determination step S2, the clock synchronization step S3, the sweep time offset synchronization step S4, the sweep frequency offset synchronization step S5, and the object detection step S6. Additionally, the program 330 may be a program for detecting the range, velocity, and / or direction of each of the objects 10 moving in space relative to at least one of the at least two co-located FMCW Doppler radars 120 .

[0103] Examples of user interface devices 320 include a display, a touch screen display, a speaker, a microphone, a haptic feedback device, a keyboard, and a mouse. Furthermore, the user interface device(s) need not be local device(s) 320, but may be remote from the data processing apparatus 300, for example user interface device(s) 390 accessible via one or more communication network(s) 380. The data processing apparatus 300 may store instructions for performing operations as described herein on a non-transitory computer readable medium 316, which may include, for example, one or more additional devices 314, for example one or more of a floppy disk device, a hard disk device, an optical disk device, a tape device, and a solid state memory device (e.g. a RAM drive). Additionally, instructions for performing the operations described herein may be downloaded to the non-transitory computer-readable medium 316 over a network 380 from one or more computers 390 (e.g., from the cloud), and in some implementations the RAM drive is a volatile memory device to which instructions are downloaded each time the computer is powered on.

[0104] It is understood that the computer hardware described may be physical hardware, virtual hardware, or any combination thereof.

[0105] Thus, the system 100 is configured to perform methods according to one or more embodiments of the present invention to provide reduced interference to each of the FMCW Doppler radars 120 described above.

[0106] The present invention further relates to a computer software product configured to perform the above-described method when executed on each or some of the computer hardware of the central control module 110 and / or the set of distributed FMCW Doppler radar devices 120 of the above-described type. The computer software product may thus be deployed as part of the system 100 to provide the functionality necessary to perform the method.

[0107] Thus, both the system 100 and the computer software product are configured to detect the distance and velocity of each of the target objects 10 moving in space relative to one or several of the multiple co-located FMCW (Frequency Modulated Continuous Wave) Doppler radars 120 by carrying out the method steps described in this specification, in particular the steps belonging to the synchronization procedure described below, by comprising or embodying the above-mentioned control functions configured to provide reduced interference to each of the FMCW Doppler radars 120.

[0108] In general, everything that is described in relation to the present method is equally applicable to the present system and the present computer software product, and vice versa.

[0109] Referring now to FIG. 9 , a method according to one or more embodiments of the present invention is shown for providing reduced interference to multiple co-located FMCW Doppler radars 120 of the type described above, each of which is used in a system 100 to detect the range and velocity of a respective target object 10 moving in space relative to the FMCW Doppler radar 120.

[0110] The FMCW Doppler radars 120 are preferably interconnected for inter-radar communication, either directly or through a central control module 110, for synchronizing the radars 120, as described above.

[0111] However, in an alternative embodiment, at least some, e.g., all, of the FMCW Doppler radars 120 are not interconnected for inter-radar communication. Instead, they may be synchronized using a common reliable time reference (e.g., a GPS signal received locally at each of the unconnected FMCW Doppler radars 120) and a set of predefined local synchronization parameter settings applied to each of the unconnected FMCW Doppler radars 120. In the latter case, synchronization may be achieved by determining and implementing values ​​of such synchronization parameters prior to operation of the system 100 to detect and track the target object 10, and such synchronization may then be repeated or updated as necessary. Such synchronization and resynchronization may be performed based on measurement information obtained by measuring inter-radar interference of the type described below. In the case of unconnected FMCW Doppler radars 120, such interference measurements may involve manual measurement steps or may be based on a predefined schedule of transmissions of test signals to each of the participating FMCW Doppler radars relative to a common time reference, allowing each of the unconnected FMCW Doppler radars 120 to reliably measure the time-of-flight and total interference of electromagnetic radar signals received from a particular (or each) identified other FMCW Doppler radar 120.

[0112] Synchronization of the FMCW Doppler radars 120 (with respect to determining an appropriate inter-radar time offset) may generally be based on initial and / or repeated measurements of the time of flight or propagation from each interfering FMCW Doppler radar 120 to each of a set of corresponding victim FMCW Doppler radars 120. Such time of flight or propagation may include several times due to multipath propagation. Due to multipath, several different values ​​of the propagation time delay may be estimated, one for each path. In general, the time of flight or propagation time relevant for purposes of the present invention is the longest time of flight or propagation that reaches the victim FMCW Doppler radar 120 with a signal strength that, after filtering, exceeds the associated signal-to-noise ratio and / or is strong enough at the victim FMCW Doppler radar 120 to cause detrimental interference to the detection or tracking of the target object 10 by the victim FMCW Doppler radar 120.

[0113] As an alternative to such measurements, in some embodiments, the propagation time and / or interference level are calculated based on the known geographical location of each of the FMCW Doppler radars 120 considered. Such geographical location may be obtained by physical measurements and / or by the FMCW Doppler radars 120 reading their respective positions from received geolocation signals, such as GPS. It is understood that this alternative does not automatically take multipath propagation into account, but may provide sufficient information in some cases and may be preferred due to its simpler configuration. It is also feasible to introduce an extra time margin into the procedure to take into account multipaths with the maximum assumed propagation time delay. However, the mutual interference level may be estimated based on radar equations that take into account free space propagation losses and the known positions and attitudes of the radars. In some embodiments, a predefined model of the physical environment that affects the propagation between the FMCW Doppler radars 120 may be used.

[0114] In general, according to one or more embodiments of the method, a respective frequency swept transmission time offset and a respective swept frequency offset are selected for each FMCW Doppler radar 120. In particular, the swept time offset may be adjusted individually for each FMCW Doppler radar 120 (relative to the other radars) to allocate interference within the IF, or target object 10 range region, and the swept frequency offset may be adjusted individually for each FMCW Doppler radar 120 (relative to the other radars) to allocate interference within the target object 10 velocity region, with the aim of minimizing the effect of such interference on the measured output of that FMCW Doppler radar 120. Such individual adjustment may possibly mean that one of the offsets is kept zero or unchanged for one or some FMCW Doppler radars 120, while for other FMCW Doppler radars 120, the offset is changed or set to a non-zero value. The adjustment is made based on the interference level and propagation time between the FMCW Doppler radars 120 determined as described above. As shown below, the swept frequency offset may be performed at a group level such that all FMCW Doppler radars 120 that belong to a particular group 121, 122 are frequency offset by the same amount.

[0115] The method may comprise an iterative evaluation procedure in which mutual interference and propagation time delays of the FMCW Doppler radars 120 are estimated. Based on the information output from that procedure, an optimized set of transmit (chirp) transmission time offsets and carrier frequency offsets is determined by the above (central or distributed) control function, for example the control module 110. The severity of interference from one FMCW Doppler radar 120 to some or all of the remaining FMCW Doppler radars 120 is evaluated, and the measured propagation time delays can then be used to optimize the time slot distribution. For this purpose, each FMCW Doppler radar 120 may comprise a measurement function to allow the derivation of this information. Measurement results may be shared by the FMCW Doppler radars 120 via the network and processed in a distributed manner, and the measurements may be transmitted to the central control module 110 for further information aggregation.

[0116] The method starts in a first step S1.

[0117] In a subsequent propagation determination step S2, the expected electromagnetic wave propagation time between said pair of FMCW Doppler radars 120 is determined / estimated. This propagation determination step S2 may be performed as described above and may in particular comprise measuring the expected interference level between such pair of FMCW Doppler radars 120. The term "expected" refers to the time and level expected during the target object 10 detection step S6 described below.

[0118] The expected electromagnetic wave propagation time and / or expected interference level may be measured for each pair of two FMCW Doppler radars 120 in the system 100.

[0119] Such mutual interference determination may be made without considering the attenuation in the filter chains 206, 206' of the receiving FMCW Doppler radar 200. Instead, the propagation determination step S2 may be designed to measure interference that falls within the passband of all listening (victim) radars, as described below. This means that the mutual interference determination / estimation that may be used to classify the FMCW Doppler radars 120 with respect to the interference they provide, according to the method steps described below, may not directly depend on the IF frequency attenuation of the filter chains 206, 206'.

[0120] The propagation determination step S2 may comprise one FMCW Doppler radar 120 in turn activating its transmitter 201 and all other FMCW Doppler radars 120 simultaneously listening to the transmitting FMCW Doppler radars 120 in order to extract information about the interference levels they experience from the transmitting FMCW Doppler radars 120. Such transmission and listening may be performed using the same antennas 201, 202 as for the detection of the target object 10 in step S6.

[0121] Furthermore, the propagation determination step S2 may comprise one or several of the FMCW Doppler radars 120 emitting respective electromagnetic test signals which may then be detected, for example in terms of interference strength, and timed by one or several of the other FMCW Doppler radars 120.

[0122] In particular, the transmitting FMCW Doppler radar 120 may use a dedicated waveform specially designed for interference assessment, which may be, for example, a pure (monotonous) CW (continuous wave) waveform at a predetermined frequency known to all participating FMCW Doppler radars 120, or a standard FMCW waveform as described below.

[0123] The propagation time may be estimated as the longest respective path (among a set of possible multipath transmission paths) that has a power level above a certain threshold.

[0124] If the test signal is a pure CW electromagnetic signal, the propagation time may not be measured directly, but instead may be calculated as a direct path propagation time using the known 2D or 3D position of the FMCW Doppler radar 120. For example, this information may be conveniently derived from the respective built-in GPS devices within each FMCW Doppler radar 120, as described above.

[0125] However, as another alternative, the electromagnetic test signal is an FMCW wave with a waveform having a non-constant frequency. For example, such an FMCW wave may have a common waveform known and used by some or all of the FMCW Doppler radar 120. The common waveform may be, for example, the same basic waveform used by the normal radar function when performing detection of the target object 10 in step S6 (see below).

[0126] More generally, the propagation determining step S2 may thus comprise each FMCW Doppler radar 120 receiving the electromagnetic test signal from a particular one of the transmitting FMCW Doppler radars 120 and determining a maximum detectable propagation time of the received electromagnetic test signal wave. Preferably, this determination takes into account any multipath wave trajectories by considering only the received electromagnetic test signal having the lowest power at reception by that receiving FMCW Doppler radar 120.

[0127] Additionally, the propagation determining step S2 may also comprise measuring, at each FMCW Doppler radar 120 receiving the electromagnetic test signal, the maximum power of the received electromagnetic test signal, or at least the maximum interference detected based on the received electromagnetic test signal.

[0128] The FMCW electromagnetic test signals emitted from different FMCW Doppler radars 120 may in some embodiments not be identical and may be frequency offset relative to one another. For example, the FMCW electromagnetic test signals having a common waveform emitted from each emitting FMCW Doppler radar 120 may have a carrier frequency that is offset relative to the carrier frequency used by each of the other FMCW Doppler radars 120 for their respective electromagnetic test signals.

[0129] By slightly offsetting the frequency of the transmitting FMCW Doppler radar 120 relative to the peer radar 120, a problem with any clutter filter used in the receiving FMCW Doppler radar 120 that filters out zero velocity (zero Doppler) information is avoided. Such an offset may be small, e.g., less than 10% across any two FMCW Doppler radars 120.

[0130] The FMCW test signal alternative allows for the estimation of the propagation time delay, with the basic accuracy being determined by how accurately each receiving FMCW Doppler radar 120 can lock onto a common time reference. It is expected that deviations from a common clock reference in practical applications can be as little as 0.05 microseconds (corresponding to a bias of about 15 m).

[0131] The propagation determination step S2 may comprise estimating the propagation time and / or the interference magnitude for each FMCW Doppler radar 120 as a receiving radar, or for only one or a subset of the FMCW Doppler radars 120. In either case, the propagation determination step S2 may be relaunched later for the same or different FMCW Doppler radars 120 depending on the current situation. As an example, a new FMCW Doppler radar 120 is added to the system 100. In this case, it is preferable to perform the propagation determination step S2 for all FMCW Doppler radars 120 in order to take into account new operating preconditions resulting from the addition of the FMCW Doppler radar 120 to the system 100. Such an addition may change the operating preconditions throughout the system 100.

[0132] If the test signal is FMCW, the test signal may be adapted so that the received test signal at one or some, e.g., all, of the receiving radars falls within their filter passbands. This may be accomplished in a number of different ways, as described below.

[0133] Using the following methodology, the propagation path delays can be estimated from an interfering FMCW Doppler radar 120 in a multipath environment, along with the power levels associated with each path, at the victim FMCW Doppler radar 120, by designing the test signal to fall within the receiver bandwidth of the victim radar, among other things.

[0134] With reference to FIG. 19 it can be seen why there is a risk of missing important multipath components in this situation.

[0135] Thus, Figure 19 shows the separation of the mixer sweeps of the test signal (dashed line) and the victim radar (solid line) at the input of the mixer of the victim radar. v The "dotted sweep" located at is the receiver bandwidth B v The IF frequencies at the output of the mixer (before any filtering) are (there are two components) IFL and IFH. Tvict -T Interf +τ path As IFK and IFH change, so do IFK and IFH. Naturally, to measure multipath components, T must be adjusted so that the test signal sweep falls within the area enclosed by the dotted line. vict -T Interf must be selected, i.e. IFL v (not satisfied in Figure 19).

[0136] First, it is assumed that the time offset of the chirp sequence for the radar transmitting the test signal is the same as the time offset used by the victim (listening) radar, and further that the A / D converter 208 in the victim radar has a large sampling frequency so that any aliasing can be neglected.

[0137] The test signal is a chirp with a repetition interval of T Rep Since the direct path propagation time delay τ + distance cT Rep (one-way propagation) will nominally produce the same IF frequency at the output of the mixer at the victim radar. This is because when the victim radar receives the direct path component corresponding to transmitted chirp i, previously transmitted chirps i-1, i-2, etc. will also arrive at the victim radar (τ+kT Rep , τ+(k+1)T Rep (In the presence of strong multipath components with propagation time delays such as IF 1, IF 2, IF 3, IF 4, IF 5, IF 6, IF 7, IF 8, IF 9, IF 10, IF 11, IF 12, IF 13, IF 14, IF 15, IF 16, IF 17, IF 18, IF 19, IF 20, IF 21, IF 22, IF 23, IF 24, IF 25, IF 26, IF 27, IF 28, IF 29, IF 30, IF 31, IF 32, IF 33, IF 34, IF 35, IF 36, IF 37, IF 38, IF 39, IF 40, IF 41, IF Max =μT Rep is the upper bound and μ is the (common) chirp rate.

[0138] Note that with increasing delay, the separation between the incoming interfering chirp and the victim radar mixer sweep increases, resulting in negative IFs (negative range that can be filtered out depending on the radar filter design) and positive IFs (which are maintained). Furthermore, the power of the positive frequency components decreases with increasing separation. Approximately 0.5T Rep ​A time separation of 1000 will reduce the power of the positive frequency components by 3 dB.

[0139] The IF frequency is f IF It depends on the delay as = μτ, where τ is the propagation delay time. Finite sampling frequency F s Considering this, the maximum apparent IF frequency that can be observed is f IF =μτ≦0.5F s This is because (in the case of one-way propagation, τ=R / c)

number

[0140] Therefore, if this fact is not taken into account when transmitting the test signals (i.e., using the same or random time offsets for the interference sweep start times and the victim sweep start times), then all significant multipath components may not be fully detected.

[0141] The inventors foresee at least two approaches to solving this problem, as described below, which may be used individually or simultaneously.

[0142] In the first of these two approaches, the transmitting FMCW Doppler radar 120 uses a sequence of test signals.

[0143] Subsequently, if each of such test signals has an individual time offset, the effective apparent time delay in the sacrificial radar varies, similar to the resulting IF frequency at the output of the corresponding mixer 204. If the different time steps (with respect to the transmitter radar time offset) used within the sequence of test signals are appropriately designed, it can be ensured that all multipath components (with respect to a certain time offset) fall within the passband of the sacrificial radar. Thus, the power level of each multipath component and its propagation time delay (taking into account the known delay used by the radar transmitting the sequence of test signals) can be determined.

[0144] As an example, it is considered that all receiver radar bandwidths are the same and are denoted by B v Furthermore, the bandwidth of the chirp is denoted by B Chirp Next,

Number

[0145] The time increment is set as ΔT = T UpChirp / n (5) As an example, when B Chirp = 40 MHz, B v = 2 MHz, and k = 0.9,

Number

[0146] Of course, using μ = B Chirp / T UpChirp we have

number

[0147] In the second of these approaches, the victim radar uses a time offset derived from the position data: in this case, the transmitter radar transmits only one test signal (the test signal is typically a number of chirps) and the time offset is set to zero.

[0148] The listening radar (victim radar) uses a time offset (i.e., the receiver radar delays the reference waveform input to the mixer 204) corresponding to the direct path propagation derived from the position data (own radar and the transmitting radar). Nominally, the victim radar's time offset should be the same as the calculated propagation time, possibly modified to some extent to account for the filter characteristics in the receiver filter chain 206, 206'. However, the time offset should be close to the nominal propagation time so that the direct path response gets a lower IF frequency, i.e., appears at closer range.

[0149] If there are significant multipath components, then a higher IF frequency is obtained (appears at a longer range). Thus, the maximum multipath delay that can be detected relative to the direct path delay is nominally equal to the range coverage of normal radar function. Since the direct path propagation delay is known, the additional multipath propagation time can be calculated from the range difference of the multipath components that appear in the range velocity map relative to the direct path components.

[0150] The propagation determination step S2 can be divided into several sub-steps, which are arranged together to obtain a complete picture of the determined mutual interference and propagation path delays. In such an approach, each such sub-step can cover (repeatedly, not simultaneously) a respective subset of one or several transmitting FMCW Doppler radars 120, followed by a time interval of normal radar target object 10 tracking operation until the next sub-step of the propagation determination step S2 is called to cover the next set of FMCW Doppler radars 120, and so on.

[0151] Finally, if the propagation determination step S2 determines that one or more of the FMCW Doppler radars 120 does not experience interference from or impart interference to any other FMCW Doppler radars 120, then such radars 120 may be eliminated from further consideration. In a sense, such non-interfering FMCW Doppler radars 120 may be considered not to be part of the system 100 for present purposes. Such determinations may be made iteratively or repeatedly over time, so which radars are considered to belong to the same "system" may change over time.

[0152] The optional propagation decision step S2 may be followed by synchronization steps S4, S5 (see below).

[0153] In a clock synchronization step S3, the respective clocks of each of the FMCW Doppler radars 120 may be synchronized. This synchronization may be performed as described above, for example based on a common received geolocation signal or a common received time signal broadcast from the central control module 110. The respective synchronized clocks of each FMCW Doppler radar 120 may then be used in combination with the respective sweep time offsets to trigger FMCW sweeps (see below). The clock synchronization step S3 may be performed continuously, intermittently, or when specifically triggered. It may be performed before or after the propagation determination step S2. In a preferred embodiment, all clocks are synchronized across the FMCW Doppler radars 120 when performing each propagation determination step S2.

[0154] In a sweep time offset synchronization step S4, a different respective sweep time offset is selected for each FMCW Doppler radar 120 within the particular first group 121 of FMCW Doppler radars 120.

[0155] In a sweep frequency offset synchronization step S5, a particular non-zero sweep frequency offset (herein referred to as a second sweep frequency offset) is selected for a particular second group 122 of the FMCW Doppler radars 120. This second sweep frequency offset is relative to the sweep frequency pattern used for the FMCW Doppler radars 120 belonging to the first group 121. It should be noted that in this context, the terms "sweep frequency offset," "carrier frequency offset," and "center frequency offset" have the same meaning and should be construed as synonyms.

[0156] The first group 121 may be disjoint from the second group 122. The first group 121 and the second group 122, when combined, may comprise all of the FMCW Doppler radars 120 in the system 100, although there may be additional disjoint groups of FMCW Doppler radars 120 (a third group, a fourth group, etc.), which collectively may comprise all of the FMCW Doppler radars 120 in the system 100.

[0157] It may be the case that all FMCW Doppler radars 120 belonging to the same group (e.g., the first group 121 or the second group 122) are associated with different sweep time offsets, or all FMCW Doppler radars 120 belonging to the same group are associated with the same sweep frequency offset, with the respective sweep frequency offsets used for different groups always being different.

[0158] As will become apparent below, which FMCW Doppler radars 120 should belong to which group may be dynamically determined, for example, as part of the sweep time offset synchronization step S4. This division of FMCW Doppler radars 120 into groups 121, 122 may thus change over time as a result of successive iterations of the method (see below), for example by adding or removing FMCW Doppler radars 120 in the system 100, or by changing other operating preconditions. Such regrouping is preferably performed in conjunction with an iteration of the propagation determination step S2.

[0159] The sweep time offset synchronization step S4 is performed with respect to at least the Doppler radars 120 belonging to the first group 121, although it will be understood that this step may also be performed with respect to other Doppler radars 120, such as, for example, the Doppler radars 120 belonging to the second group 122 and any subsequent group(s). In particular, as exemplified below, the sweep time offset synchronization step may be performed iteratively, one group 121, 122 at a time, after which each such group 121, 122 undergoes a respective sweep frequency offset synchronization step.

[0160] The swept frequency pattern used by the FMCW Doppler radars 120 belonging to the first group 121 may be associated with a “first” swept frequency offset, which may be a zero offset, while the swept frequency pattern used by the FMCW Doppler radars 120 belonging to the second (and any subsequent) group 122 may be associated with subsequent (“second,” “third,” etc.) swept frequency offsets, each of which is non-zero with respect to each of the other groups.

[0161] In a subsequent step S6, the speed, range and / or direction of the target object 10 may be detected by one or several of the FMCW Doppler radars 120, as measured in relation to each of such FMCW Doppler radars 120. Such detection may be performed in a manner conventional per se, using the sweep time and sweep frequency offset determined for each individual FMCW Doppler radar 120 in the manner described herein. Based on the detected speed, range and / or direction of such target object 10, the track of such target object 10 may be determined and displayed to a user of the system 100, or may be used in any other manner, such as for example calculating and showing various launch metrics such as flight distance and ball spin. Such detection and use is known per se and will not be detailed herein.

[0162] In the following step S7 the method ends.

[0163] The method may be implemented such that the FMCW Doppler radars 120 are assigned their respective individual time offsets one by one in an order determined by the extent to which each FMCW Doppler radar 120 interferes with the remaining FMCW Doppler radars 120. Hereinafter, when the term "isolated" FMCW Doppler radar 120 is used, it means any FMCW Doppler radar 120 that is given its given time offset, at least compared to other FMCW Doppler radars 120 that belong to the same group 121, 122 (e.g., depending on whether all FMCW Doppler radars 120 are grouped). In a sense, such FMCW Doppler radar 120 is "isolated" from the remaining FMCW Doppler radars 120 because it causes substantially no interference to the remaining FMCW Doppler radars 120 or is substantially unaware of interference from the remaining FMCW Doppler radars 120. This will be clarified below.

[0164] Below, in relation to Figures 10-13, an example methodology for performing the sweep time offset synchronization step S4 and the sweep frequency offset synchronization step S5 is provided.

[0165] In many cases, the number of coexisting FMCW Doppler radars 120 is so large that it is not possible to assign a time offset to each such radar 120 so that all radars 120 can operate without mutual interference. This methodology uses carrier frequency offsets between subgroups to solve this problem. These subgroups may be determined as part of a subalgorithm for time offset selection, as described in more detail below. In this case, the methodology generates respective time offsets for all radars 120 such that these time offsets enable interference-free operation between radars 120 belonging to the same subgroup.

[0166] It is pointed out that the above mentioned propagation determination step S2 can be configured to determine the inter-radar interference without taking into account the attenuation in the filter chain 206, 206' of the receiving radar 120 in question. In this case, the determined / estimated mutual interference is independent of the IF frequency.

[0167] As used herein, the term "remaining systems" refers to FMCW Doppler radars 120 that have not yet been assigned to a subgroup (have not yet received their time offsets).

[0168] In what follows, the following additional notation is used: S: All FMCW Doppler Radar 120 sets. S i :i-th subgroup of FMCW Doppler radar 120. S Rem : The remaining set of FMCW Doppler radars 120 (at this stage in the procedure). M: A temporary set of FMCW Doppler radars 120 used within the procedure. n is :(isolated)S i The number of FMCW Doppler radars in the region is 120. δ: the minimum time offset required for a given receiver filter chain 206 , 206 ′ to attenuate the corresponding interfering IF signal entering the receiver filter chain 206 , 206 ′ of the victim FMCW Doppler radar 120 . T k : Time offset (algorithmically determined) of FMCW Doppler radar 120k. T Rep : Chirp repetition interval common to all FMCW Doppler radars 120. τ kl : The propagation time between the FMCW Doppler radar 120k and the FMCW Doppler radar 120l. Note that τ kl = τ lk .

[0169] The remaining set of FMCW Doppler radars 120 is S and all subgroups S determined so far. i is the difference between the set of

number

[0170] A high level flowchart of such a procedure is shown as part of Figure 9 (steps S4A-S4E), while a more detailed flowchart of a particular example of such a procedure is shown in Figure 20. The following description refers to both Figures 9 and 20.

[0171] First, in step S4A (FIG. 9), a suitable nominal minimum time offset δ is determined, for example based on an analysis of the filter chains 206, 206′ of each of the receiving FMCW Doppler radars 120. The offset δ is selected such that the corresponding IF frequency is sufficiently attenuated by the filter chains 206, 206′ of the FMCW Doppler radars 120, for example at least 80 dB, or at least 100 dB. If the receiver filter chains 206, 206′ of different FMCW Doppler radars 120 are not identical, particularly if they are associated with different IF frequency attenuations, the nominal minimum time offset δ may be determined based on the receiver filter chain 206, 206′ of the FMCW Doppler radar 120 with the largest bandwidth. Alternatively, the nominal minimum time offset δ may be treated as a variable and incorporated into the procedure in the sense that it is specifically and individually calculated for at least some, for example all, of the FMCW Doppler radars 120, possibly interfering pairs. In general, the nominal minimum time offset δ may be selected as the shortest time offset that achieves the smallest predetermined attenuation in the context considered.

[0172] A certain additional margin may be added to the nominal minimum time offset δ to take into account the expected clock drift during the time interval between synchronizations to a common reference time source. The size of this margin is determined on a case-by-case basis. Typically, however, it may be chosen to be the relative clock drift times twice the clock synchronization interval. For example, the relative clock drift m of the FMCW Doppler radar 120 may be set to ε m and the clock drift n of the FMCW Doppler radar 120 is expressed as ε n Then, the margin added to δ is, for example, (ε m +ε n )T sync If GPS is used as the time reference, T sync = 1, and ε m =ε n = 0.1 ppm (which may be slightly higher for a synchronized FMCW Doppler radar 120), the margin is 0.2 microseconds.

[0173] In general, a nominal minimum relative sweep time offset δ may be determined based on a minimum sweep time offset that produces a desired attenuation (at least a predetermined attenuation) of one IF of the FMCW Doppler radar 120 experiencing interference from another FMCW Doppler radar 120, using the minimum relative time offset, e.g., such a minimum sweep time offset across all pairs of FMCW Doppler radars 120 in the system 100 that achieves at least such desired attenuation.

[0174] The determination of the minimum relative sweep time offset may be part of the propagation determination step S2 and may therefore be performed each time step S2 is performed.

[0175] Next, in step S4B, all FMCW Doppler radars 120 may be ranked based on the total interference to other radars. In other words, the ranking is an interference ranking. For example, a sum of the interference-to-noise ratio of each of the other FMCW Doppler radars 120 due to each particular interfering FMCW Doppler radar 120 considered may be formed.

[0176] This interference ranking may be performed without considering any filter chain 206, 206′ attenuation if the interference is determined as described above. In other words, in such a case, the interference ranking is performed based only on the interference that falls within the respective passbands of each victim FMCW Doppler radar 120.

[0177] Thus, the interference ranking of all systems S is based on the estimated / determined interference inflicted by FMCW Doppler radar 120 on the remaining FMCW Doppler radars in S. The FMCW Doppler radar 120 with rank 1 is assigned to the first subgroup 121, i.e., S1, and assigned a zero time offset. The subgroup counter i is set to 1, and the set S of the remaining FMCW Doppler radars 120 is Rem is S Rem =S\S1.

[0178] Generally, this ranking is denoted as the "first" ranking, and the FMCW Doppler radars 120 that receive this ranking are denoted as the "first ranked group" of FMCW Doppler radars 120. Note the difference between a "first / second group" and a "first / second ranked group" here.

[0179] In a first iteration, a first ranking group may comprise all FMCW Doppler radars 120 in the system 100.

[0180] Furthermore, the first ranking may be made according to the severity of interference to other FMCW Doppler radars 120 in the first ranking group, particularly based on such data measured or otherwise determined in the propagation determination step S2.

[0181] Next, in step S4C, for each of the FMCW Doppler radars 120 in the first ranked group, in order from the most severely interfering FMCW Doppler radars 120 to the less severely interfering FMCW Doppler radars 120, a respective first sweep time offset is selected for the FMCW Doppler radar 120 taking into account both the determined minimum relative sweep time offset and the propagation time delay of the FMCW Doppler radar 120 relative to the other FMCW Doppler radars 120 in the first ranked group for which respective first sweep time offsets have already been selected. Note that, herein, the "first sweep time offset" refers to the sweep time offset selected for the FMCW Doppler radars 120 belonging to the first ranked group.

[0182] The rank 1 radar (the radar that causes the most interference to all others) may be associated with a zero sweep time offset, and the other first ranking group radars 120 may be associated with sweep time offsets related to the sweep time offset of the rank 1 radar.

[0183] Thereafter, one or more of the other FMCW Doppler radars 120 in the first ranked group may be successively associated with respective sweep time offsets that correspondingly provide interference-free operation for the other FMCW Doppler radars 120 in the first ranked group to which the respective sweep time offsets are assigned. Thus, in other words, the former FMCW Doppler radar 120 is “isolated” from the other FMCW Doppler radars 120 in the above terminology.

[0184] After each such assignment of a sweep time offset to an FMCW Doppler radar 120, another interference ranking may be performed for any not-yet-isolated FMCW Doppler radars 120 in the first ranking group. Isolation by associating individual radars with respective sweep time offsets may continue until no FMCW Doppler radars 120 in the first ranking group can be assigned a sweep time offset that meets the current conditions in terms of interference-free operation.

[0185] The remaining FMCW Doppler radars 120, in other words the non-isolatable FMCW Doppler radars 120 of the first ranking group, are then denoted a “second ranked group” of FMCW Doppler radars 120, and the isolated FMCW Doppler radars 120 become the first group 121. Then, only the second ranked group may be sent to the time offset selection described above for a second run (using the second ranked group as the first ranked group in the procedural description above), and in order to be isolated from each other (within the second ranked group), they obtain a sweep time offset relative to each other.

[0186] The sweep time offsets of the isolated members of the second ranking group are then such that the members of the second ranking group interfere with (and are interfered with by) the members of the first group 121.

[0187] However, the second ranked group can then be separated (from an interference perspective) from the first group by introducing a common (shared) change in carrier frequency of the FMCW Doppler radars 120 in the second ranked group such that interference from the second ranked group to the FMCW Doppler radars 120 of the first group 121 will be located in a low sensitivity target object 10 velocity interval. By symmetry, interference to the second ranked group from the first group 121 will be located in a corresponding negative target object 10 velocity interval.

[0188] More generally, the second ranking group may be identified as a set of radars for which it has been determined that it is not possible to select, among one or several FMCW Doppler radars 120 in the first ranking group, a respective sweep time offset that does not result in a predetermined minimum interference level during the tracking operation of the target object 10. In some cases, this is identified as a set of radars 120 in said first ranking group for which the respective required sweep time offset (to avoid potentially harmful interference) is detected to be longer than a threshold, i.e. a maximum allowable value. This threshold may be selected such that, if violated, sweep-to-sweep interference becomes a risk.

[0189] Thus, after examining the FMCW Doppler radars 120 in the first ranked group, if there is an FMCW Doppler radar 120 that could not be isolated, the same procedure is performed again using only the remaining FMCW Doppler radars 120 in preparation for sweep frequency offset selection (described below).

[0190] In other words, as shown in FIG. 9, in step S4D, the FMCW Doppler radars 120 in the second ranking group are ranked in order by interference, similar to the ranking of the first ranking group described above.

[0191] Thus, in ranking the FMCW Doppler radars 120 of this second ranking group, the ranking is performed according to the interference severity relative to the other FMCW Doppler radars 120 of said second ranking group and based on the data measured in the propagation decision step S2.

[0192] Thereafter, in step S4E, respective sweep time offsets are determined for the FMCW Doppler radars 120 in the second ranked group, also similar to that described above with respect to the first ranked group.

[0193] Thus, for each of the FMCW Doppler radars 120 in the second ranked group, in order from the most severely interfering one of the FMCW Doppler radars 120 to the less severely interfering one of the FMCW Doppler radars 120, a respective sweep time offset may be selected for that FMCW Doppler radar 120 taking into account both the determined minimum relative sweep time offset and the propagation time delay of that FMCW Doppler radar 120 relative to other FMCW Doppler radars 120 in the second ranked group for which a respective sweep time offset has already been selected.

[0194] As described above, the FMCW Doppler radars 120 that belong to the first ranking group but not to the second ranking group may be selected as the first group 121 of the FMCW Doppler radars 120. One or several FMCW Doppler radars 120 that belong to the second ranking group may be selected as the second group 122 of the FMCWs.

[0195] As will be appreciated, this procedure may be implemented as a recursive procedure.

[0196] FIG. 20 shows a practical implementation of such a procedure, according to the following.

[0197] Step 1: At the start, S Rem = S\S1, i=1, where S1 contains the single FMCW Doppler radar 120 in S that has the highest interference rank.

[0198] Step 2: The "middle set" M is the set S of the remaining FMCW Doppler radars 120. Rem It is defined by setting it equal to

[0199] Step 3: Now, subgroup S i All FMCW Doppler radars 120 that are not capable of time offset are excluded from M so that mutual interference with already isolated FMCW Doppler radars 120 in S can be avoided.i The conditions checked for each FMCW Doppler radar 120 in M ​​relative to all already isolated FMCW Doppler radars 120 in are as follows: T 21m ≦T 22m (8) where the subscript m denotes the mth system in set M,

number

[0200] Before returning to the flow chart shown in FIG. 20, and with brief reference to FIG. 10, a time offset constraint that may be used in the procedure will now be described.

[0201] In particular, FIG. 10 illustrates an individual determination of the time offset for a particular subgroup, such as the first group 121 or the second group 122.

[0202] For simplicity, the description herein begins by considering the special case of two FMCW Doppler radars 120, so that the number of previously isolated FMCW Doppler radars in the current subgroup is one, n is = 1. is The general case requirement of >1 is simply a function of the requirement derived below: is This must be maintained for all of the previously isolated FMCW Doppler radars 120. An example of this is provided below.

[0203] Assume that FMCW Doppler radar 120 having number 1 has been assigned a time offset T1 (i.e., isolated). Next, constraints on the time offset T2 of FMCW Doppler radar 120 number 2 and how to select T2 are sought. First, T2 must be selected such that neither FMCW Doppler radar 120 number 1 nor FMCW Doppler radar 120 number 2 experiences interference.

[0204] The four constraints A, B, C, and D may be written as shown in Figure 10. The upper part of Figure 10 shows the relative timing of the waveforms of FMCW Doppler radar 120 number 1 ("System 1") and FMCW Doppler radar 120 number 2 ("System 2") at the location of FMCW Doppler radar 120 number 1 (for any propagation time delay τ and time offset T2 relative to FMCW Doppler radar 120 number 2). Similarly, the lower part of Figure 10 shows the relative timing of the waveforms of FMCW Doppler radar 120 number 1 and FMCW Doppler radar 120 number 2 at the location of FMCW Doppler radar 120 number 2 (for any propagation time delay τ and time offset T2 relative to FMCW Doppler radar 120 number 2).

[0205] The sweep sequences shown with thick dashed lines belong to FMCW Doppler radar 120 number 1, and the sweep sequences shown with thick solid lines belong to FMCW Doppler radar 120 number 2. The thin lines in the same style as the thick sweep lines represent "minimum separation constraints" on the IF frequencies, i.e., sweeps from different FMCW Doppler radars 120 must not be closer than either of the thin lines.

[0206] Here, the minimum IF frequency separation is determined by the filter chain 206, 206′ of the receiver radar 120 being Δf min Δf determined to have a specified attenuation at minThus, identical FMCW Doppler radar 120 with identical filter chains 206, 206' is assumed here, although it would be straightforward to allow for different receiver filter chains 206, 206' with different cutoff frequencies corresponding to the different range intervals over which the radar 120 searches for the target object 10. Also, the waveforms are assumed to be identical (except for time and carrier frequency offsets).

[0207] Nominal minimum time offset δ and minimum IF frequency separation Δf min The relationship between

number

[0208] Constraints A and B described in the upper part of FIG. 10 relate to the requirement that FMCW Doppler radar 120 number 2 does not interfere with the already isolated FMCW Doppler radar 120 number 1. Constraint A is referred to as the "left constraint" and constraint B is referred to as the "right constraint". Constraint A describes that T2 must be selected large enough not to cause interference to FMCW Doppler radar 120 number 1. Constraint B describes that T2 must not be selected too large, because the chirp of FMCW Doppler radar 120 number 2 will reach the next chirp of FMCW Doppler radar 120 number 1 and cause interference to FMCW Doppler radar 120 number 1.

[0209] Constraints C and D, described at the bottom of Figure 10, relate to the requirement that FMCW Doppler radar 120 number 1 not interfere with FMCW Doppler radar 120 number 2. Constraint C is referred to as the "left-side constraint" and constraint D is referred to as the "right-side constraint." Constraints C and D are similar to constraints A and B, but are constraints from the perspective of FMCW Doppler radar 120 number 2 (FMCW Doppler radar 120 number 2 must not experience interference from FMCW Doppler radar 120 number 1).

[0210] The constraints are as follows: A: T2 ≧ T1 + δ-τ B: T2≦T1+T Rep -δ-τ C: T2 ≥ T1 + τ + δ D: T2≦T1+T Rep +τ-δ (11)

[0211] Considering both the "≧" constraints A and C and the "≦" constraints B and D, the "hardest" of each pair is selected. Since τ is positive, constraint C is harder than constraint A. For the same reason, constraint B turns out to be harder than constraint D. Thus, the final constraint that ensures interference-free operation between FMCW Doppler radar 120 number 1 and FMCW Doppler radar 120 number 2 is: T2 ≥ T1 + τ + δ T2≦T1+T Rep -δ-τ (12) It becomes.

[0212] There are already several isolated FMCW Doppler radars 120, n is In the general case of ≥ 1, the constraint defined in (12) applies to n is This is maintained for all of the FMCW Doppler radars 120.

[0213] τ i , i=1,2,...n is is defined as the propagation delay time between the FMCW Doppler radar 120 number 2 (the FMCW Doppler radar 120 for which the offset is to be determined) and the i-th FMCW Doppler radar 120 in the subset of previously isolated FMCW Doppler radars 120 (the propagation time delay is symmetric even in the case of multipath), and the time offset of the i-th isolated FMCW Doppler radar 120 is defined as T i (substituting the T1 parameter into the above equation). The time offset notation T2 for the new FMCW Doppler radar 120 is maintained. Furthermore,

number

[0214] Therefore, T2 ≥ T 21 And T2≦T 22 Note that in most practical applications, the propagation time delay τ i and the minimum time offset δ is T Rep This is much smaller than the start of the procedure (n is =1) at T 21 < <T 22 This indicates that.

[0215] At this point it is possible to explain how to iteratively select the time offsets. Thus, for a given n is For each isolated FMCW Doppler radar 120, condition T 21 ≦T 22 If true, the time offset of the new FMCW Doppler radar 120 is T2=T 21 can be chosen as T to ensure interference-free operation. 21 >T 22 If T2 is the current FMCW Doppler radar 120 under consideration, then the set {1,...n is}. In this case, the FMCW Doppler radar 120 is moved to the pool of FMCW Doppler radars 120 that cannot be isolated with respect to the already isolated FMCW Doppler radars 120 in the current subgroup and is excluded from future iterations for the current subgroup. Such an FMCW Doppler radar 120 will eventually become a member of another subgroup.

[0216] Step 4: In this step, the procedure checks if M is empty. If not, the procedure proceeds to step 5, otherwise it proceeds to step 8.

[0217] Step 5: In this step, the FMCW Doppler radars 120 in M ​​are ranked based on the interference they cause to the remaining FMCW Doppler radars 120 in M. This is thus the type of "interference ranking" described above. For example, the interference-to-noise ratios of other FMCW Doppler radars 120 due to the interfering FMCW Doppler radars 120 can be summed.

[0218] Step 6: As determined by step 4, index

number

number

number

[0219] Step 7: In this step, it is checked whether M is empty. If it is not empty, the procedure goes to step 3 and repeats steps 3 to 6. Eventually, M becomes empty and the procedure reaches step 8.

[0220] Step 8: In this step, S Rem =S Rem \S i By setting S Remis updated, i.e., the subgroup S defined at this point is i The FMCW Doppler radar 120 in Rem are excluded.

[0221] Step 9: In this step, S Rem is empty. If it is empty, all FMCW Doppler radars 120 have been assigned to their respective subgroups and given time offsets such that the FMCW Doppler radars 120 may operate without causing interference to or experiencing interference from any other FMCW Doppler radars 120 in their own subgroups. In this case, the procedure ends (goes to step 11), otherwise it moves to step 10.

[0222] Step 10: Now, the subgroup index i is incremented by 1 (i=i+1). S Rem An interference ranking of the FMCW Doppler radars 120 in i,Si is performed, and the FMCW Doppler radar 120 with rank 1 is placed as the first FMCW Doppler radar 120 (member) in a new subgroup i,Si (which now contains only a single member). This FMCW Doppler radar 120 is assigned a zero time offset. The set S of remaining FMCW Doppler radars 120 is Rem S determined from i A new “middle set” M is defined by subtracting the rank 1 FMCW Doppler radar 120 in Rem \S i Then, the procedure goes to step 3 and repeats steps 3 to 9.

[0223] Step 11: This is the final state, where all FMCW Doppler radars 120 have been assigned their respective subgroups and given time offsets such that the FMCW Doppler radars 120 may operate without causing interference to or experiencing interference from any other FMCW Doppler radars 120 in their subgroups.

[0224] The reason for the iterative interference ranking performed in step 5 is that when one FMCW Doppler radar 120 is removed from consideration (as may occur when it is "isolated"), the rank order among the remaining FMCW Doppler radars 120 typically changes. Thus, by re-performing the interference ranking at each iteration of the isolation procedure, the "worst" FMCW Doppler radar 120 in terms of interference is isolated in an effective manner with a time offset.

[0225] As is clear from the above, the mutual interference between the FMCW Doppler radars 120 in the subgroups has been mitigated at this point by defining appropriate time offsets. However, significant interference may exist between the subgroups of the FMCW Doppler radars 120 (e.g., between the first group 121 and the second group 122). This may be mitigated by introducing a common change in the carrier frequency of each of the FMCW Doppler radars 120 in the subgroup relative to the carrier frequencies of the other subgroups, as described below. Preferably, these carrier frequency offsets are selected so that the interference between the subgroups is located in a speed region where it is not significant.

[0226] However, before describing the selection of carrier frequency offsets in more detail, an illustration of sweep time offset selection is provided based on the layout of an FMCW Doppler radar 120, as shown in Figure 11. In this Figure 11, a designated victim FMCW Doppler radar 120 (numbered 1) is located at (0,0). Nineteen interfering FMCW Doppler radars 120 are distributed across range and angle directions. The diagram in Figure 11 is a top view, with the axes of the antenna coordinate system drawn to the location of each individual FMCW Doppler radar 120.

[0227] After performing the above-mentioned procedure, the number of isolated FMCW Doppler radars 120 was 13 (first group 121). In addition, there were seven FMCW Doppler radars 120 (second group 122) that were assigned sweep time offsets relative to each other but not to the first group 121. In Fig. 12a and Fig. 13a (enlarged version), the resulting IF frequencies are plotted for all pairs of isolated FMCW Doppler radars 120 belonging to the first group 121. From an examination of Fig. 13a, it is clear that the isolated FMCW Doppler radars 120 in the first group 121 do not interfere with each other, as will be explained next.

[0228] In particular, Fig. 12a shows (in a magnified view in Fig. 13a) the IF frequencies for all pairs of the isolated FMCW Doppler radars 120 in the first group 121. The close horizontal dashed lines indicate the receiver bandwidth plus a margin determined so that all interfering IF components are attenuated by at least 115 dB.

[0229] 12b and 13b correspond to FIGS. 12a and 13a, but show the situation for the second group 122. FIG.

[0230] To summarize some of the above, once the individual specific sweep time offsets have been determined for all possible FMCW Doppler radars 120 in the first ranked group, this isolated group of FMCW Doppler radars 120 becomes the "first group" 121 in the terminology used above. At this point, the FMCW Doppler radars 120 in the second ranked group are considered, and if all FMCW Doppler radars 120 in this second ranked group are assigned individual group specific sweep time offsets, they form the "second group" 122 in the terminology used above. This completes the assignment of all FMCW Doppler radars 120 to groups 121, 122. However, if at this point there are still FMCW Doppler radars 120 that have not been assigned to either the first group 121 or the second group 122, the procedure continues to form a third (and possibly further) ranked group. Thus, the remaining FMCW Doppler radars 120 form a third ranked group, and the procedure repeats again in a corresponding manner as described above. Each time it is determined that a particular maximum number of the FMCW Doppler radars 120 in the currently considered ranked group can be assigned a group-specific sweep time offset, the FMCW Doppler radars 120 in that ranked group form the next group in the above terminology, and the procedure continues until all of the FMCW Doppler radars 120 in the system 100 have been assigned to groups.

[0231] Once the first and second groups 121, 122 (and possibly any additional groups) have been formed and all FMCW Doppler radars 120 have been assigned group-specific sweep time offsets, step S4 ends.

[0232] 9, the swept frequency offset synchronization step S5 may take part in any time relationship to step S4 (indeed, this is also the case for steps S3, S4 and S5). Thus, step S5 may take place before step 4, in parallel with step S4 or after step S4 has been completed. Alternatively, a predefined set of frequency offsets may be used for the various groups 121, 122 identified in step S4, and the synchronization of the swept frequency offsets in step S5 may simply involve assigning each of the identified groups 121, 122 to a respective unique swept frequency offset from the predefined set of swept frequency offsets.

[0233] To avoid the risk of strong local interference within the range interval of the target object 10 of interest of the first group 121 of FMCW Doppler radars 120 (which would be located at low speeds if the carrier frequencies were nominally the same), the carrier frequency (sweep center frequency) of the second group 122 of FMCW Doppler radars 120 can be adjusted (decreased or increased) to locate the interference in a speed region that does not affect the ability of the system 100 to accurately detect and track the target object 10. It is understood that such a speed region can vary across different specific embodiments. In the specific case where the target object 10 is a golf ball, the interference can be localized (by selecting an appropriate sweep frequency offset across the groups 121, 122) to detection speeds below 0 m / s, or below -35 m / s, and / or above 80 m / s, or above 100 m / s.

[0234] In the special case where there are only two groups 121, 122, the carrier / center frequency of the second group 122 may be increased (or decreased) by half the chirp repetition frequency with respect to the first group 121. In this way, the interference experienced by the FMCW Doppler radars 120 in the first group 121 is moved to a well-defined minimum (or maximum) velocity that does not cause harmful interference in many embodiments. More specifically, in order to move the interference generated by the second group 122 in the first group 121 to a well-defined minimum velocity component (on the negative side), the common carrier frequency of the FMCW Doppler radars 120 in the second group 122 is increased (or decreased) by Δ HF =0.5 / T Rep (selecting a positive shift). This carrier frequency shift is interpreted as a (positive) Doppler shift in the radar processing and is mapped to a negative velocity (as described above). Note that due to symmetry, the interference from the first group 121 to the second group 122 is located in the corresponding positive velocity interval.

[0235] In other embodiments, carrier frequency selection can be performed to introduce interference at any velocity value by introducing a swept frequency shift of corresponding magnitude.

[0236] In general, offsetting the false velocities across different groups 121, 122 using swept frequency offsets as described above may not only locate object 10 tracking velocity intervals that are problem-free (by selection of the corresponding swept frequency) so that no false detection of object 10 occurs, but such velocity shifting may also increase the sensitivity for tracking real object 10 in velocity intervals that are close to or include velocity-generated interference if not frequency shifted.

[0237] Thus, each sweep frequency offset may be selected, for example, for one or several, e.g., each, of the groups 121, 122, to be sufficiently large so that interference between the FMCW Doppler radar 120 belonging to the first group 121 and the FMCW Doppler radar 120 belonging to the second group 122 results in misinterpretation or reduced detection sensitivity of the velocity of the tracked object 10, which is outside a predetermined velocity range. In particular, the sweep frequency offset may be selected between 0.3 and 0.7 times, e.g., between 0.4 and 0.6 times, e.g., about or exactly 0.5 times, the sweep repetition frequency used by the plurality of FMCW Doppler radars 120.

[0238] It is understood that such a sweep frequency shift may be introduced for all identified groups 121, 122, etc., even if there are more than two groups of FMCW Doppler radars 120 in the system 100 identified according to the methodology described herein. The first group 121 may be assigned a "zero" sweep frequency shift, and the other groups (the second group 122 and any subsequent groups) may be assigned various unique non-zero sweep frequency shifts. The important point is that no two such groups 121, 122 have the same relative sweep frequency shift, and all such groups 121, 122 of FMCW Doppler radars 120 are associated with respective sweep frequency shifts, making inter-group interference appearing at the target object 10 problematic for the application in question.

[0239] After said synchronization steps S3, S4, S5, in a subsequent target object 10 detection step, the system 100 may use some of said plurality of FMCW Doppler radars 120 of the system 100 to measure the respective positions and velocities of one or several moving target objects 10 relative to the respective measuring FMCW radars 120. This measurement may be performed in a manner conventional per se, by some or each of said FMCW Doppler radars 120 repeatedly emitting FMCW sweeps with said selected respective sweep time offsets and said selected respective sweep frequency offsets.

[0240] As mentioned above, the multiple FMCW Doppler radars 120 may comprise one or several FMCW Doppler radars 120 capable of detecting a direction to a detected target object 10, for example, using multiple receiving radar antennas 202. In some embodiments, several different FMCW Doppler radars 120 simultaneously detect or track one and the same target object 10 from different angles.

[0241] Also as described above, the respective sweep waveforms used by each of the FMCW Doppler radars 120 may be the same across the FMCW Doppler radars 120, except for the respective sweep time and sweep frequency offsets used by that FMCW Doppler radar 120. Thus, the waveforms used by all of the FMCW Doppler radars 120 may be the same linear FMCW waveform, which may have the same chirp rate and / or the same duration and / or the same bandwidth.

[0242] 9, the propagation determination step S2, the sweep time offset synchronization step S4, and the sweep frequency offset step S5 may be performed repeatedly, for example, periodically and / or when additional FMCW Doppler radars 120 are added to the system 100, or when any other configuration or topology updates or changes are made to the system 100. The clock synchronization step S3 may also be performed repeatedly / intermittently.

[0243] For example, the FMCW Doppler radars 120 are typically deployed and connected to the network one at a time. Once the FMCW Doppler radars 120 are activated and locked to a common time reference source, they may be configured to wait for a cue message, for example from the central control module 110 or from a user installing via a user interface of the radar 120, to exercise their TX function using some dedicated waveforms (the test signal waveforms described above) or to listen for the test signal transmission of another cooperating FMCW Doppler radar 120. In other words, this cue message then defines the start of the synchronization steps S3, S4, S5 described above, which may then involve some or all of the FMCW Doppler radars 120 already present in the system 100.

[0244] The synchronization steps S3, S4, S5 may additionally or alternatively be performed periodically to assess mutual interference and propagation time delays between the FMCW Doppler radars 120 in the system 100 and to take appropriate measures as exemplified above. The synchronization steps S3, S4, S5 may also be performed on an as-needed basis, either based on automatic detection of changes in any parameter values ​​affecting the operating prerequisites, or manually triggered by a user of the system 100.

[0245] Examples are presented below in connection with Figures 14-17 to show what the range-velocity response may look like for the victim radar 120 without any adjustments to the FMCW Doppler radar 120 of the system 100, and how it may look when adjustments are applied using the procedures described above.

[0246] The scenario shown in Figure 11 is used. As mentioned above, 20 FMCW Doppler radars 120 are included, one of which is designated as the "victim radar" (the radar at the origin). For this radar, the interference experience is shown in the range-velocity domain. The data in this example is derived from a simulation of a simplified system 100 for clarity and simplicity.

[0247] The maximum distance (range) for an interfering FMCW Doppler radar is 756m. For a matched waveform, assuming zero time offset, the IF frequency produces a point response in the signal processing at half this distance, i.e. about 378m. The receiver bandwidth of the victim radar (or all radars for that matter) is such that the corresponding -3dB range is 304m. In view of this, it is noted that if all radars use zero time offset in the chirp sequence, radars at ranges farther than about 610m will be increasingly attenuated by filters in the receiver chain of the victim radar. There are 14 interfering radars (other than the victim radar) with ranges less than 610m. Furthermore, the range corresponding to the IF value at which there is 115dB attenuation (assuming two-way propagation) is 400m, which translates to 800m in the case of one-way propagation (with respect to interference). Thus, there are 14 radars in the "-3dB range", but all radars are in the "-115dB range" interval. The 19-15=4 radars within the "transition band" (610-800m) are attenuated to various degrees depending on the actual IF frequency and the detailed characteristics of the receiver transmitter capabilities. In this description the effects of free space attenuation are ignored so in reality radars at far ranges have additional large relative attenuation. Otherwise the radars would be identical in terms of output power, antenna gain etc and all interference would be directed straight at the victim radar.

[0248] Additionally, in these simulations, thermal noise was added to the victim radar. The interference-to-noise ratio generated by the interferometric radar was accurately modeled with the radar equations, including antenna gains, different losses in transmission and reception, etc.

[0249] First, looking at the unadjusted case, all sweep time offsets were set to zero, which means that the main factor affecting the resulting IF frequency is the propagation time delay.

[0250] Furthermore, for this unaligned case, the FMCW Doppler radars 120 have no common time reference to lock on to. This means that the clocks / oscillators of the FMCW Doppler radars 120 have a large drift with respect to each other. Thus, the waveforms are somewhat different (even if the nominal specifications are the same), resulting in some smearing. Both the IF frequency and the "Doppler" varied with time because the oscillators run at different speeds. The relative clock drift for this simulation was randomized (uniformly distributed) in the interval -25 ppm to 25 ppm (x ppm means that the clock changes x microseconds every second). The drift was assumed to be constant over a coherent integration time of 20 ms.

[0251] The results for this case are shown in Figure 14, where the bottom shows the range-velocity map of the victim FMCW Doppler radar 120 for the unaligned case described above. In this case, about 14 significant interference components are counted, and the labels indicate the ones with the highest power. Note also that there is a relatively large defocus of the interference response even when the nominal waveform parameters are matched, which is due to the large relative clock drift.

[0252] An additional problem of a system 100 with an FMCW Doppler radar 120 associated with an individual drift is that it usually increases the risk of a false detection trajectory of the target object 10. This is because detections due to interference are then regularly generated over time, especially if the time offset and propagation time delay are such that the IF components are within the range interval of interest for the target object 10 detection. The detections change in range and (slightly) velocity, and the tracker may falsely detect information such as the target object 10 trajectory due to the interference.

[0253] Next, the results of applying a synchronization procedure of the type described above on the same system are investigated.

[0254] Here, the FMCW Doppler radars 120 are locked onto a common time reference, and the relative clock drift between the FMCW Doppler radars 120 is very small. In this simulation part, a relative clock drift of 0.05 ppm was used, randomized with uniform distribution within the interval of -0.05 ppm to 0.05 ppm. For reference, the procedure shown in Figure 20 was used to automatically determine the two groups 121, 122 as follows:

[0255] 1st group = [1 13 7 10 8 15 2 17 9 6 18 5 4]. 2nd group = [3 14 19 20 11 16 12].

[0256] The results are shown in FIG. 15, which shows the range-sweep and range-velocity maps of the victim radar for this adjusted case.

[0257] It can therefore be seen that the approach for coexistence works as intended. The interfering radars of the first group 121 (there was a total of 13 FMCW Doppler radars 120 that could be isolated in the first step, including the victim radar) were placed outside the range interval of interest. Furthermore, the interfering radars of the second group 122 of seven radars got a small upward carrier frequency shift, corresponding to 0.45 PRF (instead of the value of 0.5 PRF mentioned above to make this effect more visible in the plot). As a result of this carrier frequency shift, when the clock drift is zero, the carrier frequency shift is -120.9m / sv=-λf D This results in a speed shift of 1 / 2. However, the clock drift is not exactly zero and the speed change is not exact, but it is sufficient for our purposes, i.e. to move the interference far enough away from the small speed.

[0258] From Figure 17, which shows (zoomed in) the relative IF frequencies experienced by the victim radar 120, it can be seen that it is radar number 3 of the second group 122 (as expected) that produces the sharp point response (producing an IF frequency within the passband of the victim receiver) labeled in Figure 15. Radar number 3 is a member of the second group 122. By coincidence, the other radars in the second group 122 displayed total relative delays whose corresponding IF frequencies are within the stopband of the victim receiver. In Figure 17, the dashed lines in the diagram indicate the frequencies at which the victim receiver chain 206, 206' has an attenuation of 115 dB.

[0259] Inspecting Figure 15 (with "hard blanking" rather than "soft blanking", see below) there is some residual interference (ridges) at low velocities. This is due to short transients generated by the down sweep of the interfering radar. These transients are "wideband" and have high spectral content far from the "main IF frequency", some of which leak into the receiver passband of the victim radar. This phenomenon is explained in more detail below, along with some measures to reduce it.

[0260] Solely for validation, the same example was simulated without clock drift in Figure 16, which shows the range-sweep and range-velocity maps. The label at the bottom of Figure 16 indicates that the interference component travels with a velocity of -120.9 m / s, similar to what would be expected for a perfectly synchronized clock.

[0261] As mentioned above, it is proposed to use deliberate delays of the transmit chirps to separate the FMCW Doppler radar 120 in the range dimension. However, the short-term down-chirps generate short transient signals at the output of the mixer in the victim FMCW Doppler radar 120. These transient signals are "wideband" and have high spectral content far from the "main IF frequency". These spectral components leak more or less into the passband of the victim receiver and generate excess noise along ridges in the range domain. Such interference ridges can be identified at low velocities, for example in FIG. 15. If these ridges are located exactly at zero velocity, it is a small problem because clutter filters are usually applied as part of the radar signal processing to filter out zero velocity. However, with some residual oscillator drift, these ridges will "wander around" in some small velocity intervals around zero velocity where good sensitivity is usually desired for target object 10 detection.

[0262] Such interference can be reduced using a combination of blanking of the return ramp (e.g., down-chirp with a normal sawtooth pattern or up-chirp with an inverse sawtooth pattern) and tapering to "soften" the transient signal and reduce the magnitude of the high spectral content of the transient signal. This principle can be applied to both the transmit signal and the mixer signal. However, the application of this principle to the transmit signal is important with respect to mitigating the above-mentioned high spectral content of the transient signal.

[0263] As used herein, the term "blanking" means reducing or completely switching off the transmit power during the return ramp of the frequency pattern. "Hard blanking" means a tapering function with a value of 1 during the up-chirp and a value of 0 (transmit power switched off completely) during the down-chirp. "Soft blanking" means a modification of hard blanking to have a smoother shape. Soft blanking affects the range resolution of normal radar functions since the transmit bandwidth is reduced and reduces the target response to some extent, the reduction in peak response is referred to as tapering loss. This further typically reduces the transmit energy, resulting in further SNR (signal to noise ratio) degradation.

[0264] In general, the FMCW sweeps described above may comprise respective return ramp waveforms which are then hard blanked or smoothed.

[0265] Examples of hard and soft blanking functions are shown in FIG. 18, which shows an example of "hard" blanking and an example of "soft" blanking. The down-chirp (return ramp) starts at t=51.1 μs and ends at the chirp repetition interval t=53.3 μs. One interesting point is how "soft" or "smooth" to make the soft blanking. On the one hand, the softer the blanking, the less interference experienced. On the other hand, the range resolution is reduced. If the radar is equipped with digital synthesis of the TX waveform and a high-performance flexible linear variable amplifier, almost any tapering function can be designed. However, the design of FIG. 18 shows a relatively simple solution with limited flexibility in terms of design parameters. The design of the soft tapering at TX for this radar involved careful selection of values ​​of the RC constants of the electronics. Thus, a perfectly symmetrical tapering function could not be designed, and this soft blanking was a compromise based on an investigation of the spectral characteristics (symmetry, etc.) of the soft blanking.

[0266] No blanking, hard blanking and soft blanking were compared for a simple two-radar scenario using the hard and soft blanking functions shown in Figure 18. As the interferometric radar time offset was stepped from zero to the chirp repetition interval relative to the victim sweep time offset, maximum interference levels for each example were observed (as before within the target range interval of interest of approximately 300m). The results were as follows:

[0267] No blanking: -45.3dB Hard Blanking: -71.9dB Soft Blanking: -77.4dB

[0268] Hard blanking reduced the interference by 26.6 dB, and soft blanking provided an additional 5.5 dB improvement over hard blanking. Thus, soft blanking reduced the interference by 32.1 dB. The bandwidth reduction by using soft blanking was 1.71%, which represents a 1.74% increase in the width of the point target response compared to hard blanking.

[0269] Hard blanking has generally been used throughout the examples shown in the various figures of this application.

[0270] Although preferred embodiments have been described above, it will be apparent to those skilled in the art that numerous modifications can be made to the disclosed embodiments without departing from the fundamental concepts of the invention.

[0271] For example, system 100 may include numerous additional components for detecting and tracking target objects 10, such as, for example, a greater number of radar instruments and various computer systems configured to process object 10 tracking data, for example to determine object tracking feasibility and 3D trajectory in real time.

[0272] Thus, the invention is not limited to the described embodiments, but may be modified within the scope of the appended claims.

Claims

1. A method for providing reduced interference to at least two co-located frequency modulated continuous wave (FMCW) Doppler radars, each of said FMCW Doppler radars being used in a system to detect the respective range and velocity of an object moving in space relative to each of said FMCW Doppler radars, said method comprising: determining an expected electromagnetic wave propagation time between the pair of FMCW Doppler radars, wherein determining the expected electromagnetic wave propagation time comprises each FMCW Doppler radar receiving an electromagnetic test signal and determining a maximum detectable propagation time of the electromagnetic test signal, taking into account any multipath wave trajectories, wherein the received electromagnetic test signal has at least a predetermined minimum power upon reception; selecting a different respective sweep time offset for each FMCW Doppler radar in the first group of FMCW Doppler radars; selecting a second non-zero sweep frequency offset for a second group of the FMCW Doppler radars, the second non-zero sweep frequency offset being relative to a sweep frequency pattern used by the FMCW Doppler radars belonging to the first group, the first group and the second group being disjoint groups of FMCW Doppler radars; A method comprising:

2. The system uses some of the FMCW Doppler radars to measure respective positions and velocities of one or several moving objects relative to a respective measuring FMCW Doppler radar by: i) a first radar of a first set of some of the FMCW Doppler radars, the first set being of the first group, the first radar emitting FMCW sweeps repeatedly with a selected respective sweep time offset; and / or ii) a second radar of a second set of some of the FMCW Doppler radars, the second set being of the second group, the second radar emitting FMCW sweeps repeatedly with a selected second non-zero sweep frequency offset. The method of claim 1 further comprising:

3. The method of claim 2 , wherein the FMCW sweep comprises a respective return ramp waveform, and the return ramp waveform is blanked.

4. The method comprises: It is something that is done repeatedly, determining the expected electromagnetic wave propagation time; selecting different respective sweep time offsets; selecting a second non-zero sweep frequency offset; and further comprising repeatedly performing The method of claim 1 , wherein the repeating occurs periodically and / or when additional FMCW Doppler radars are added to the system.

5. Selecting a different respective sweep time offset for each FMCW Doppler radar in the second group of FMCW Doppler radars. The method of claim 1 , comprising:

6. Synchronizing a respective clock of each of said FMCW Doppler radars, said clock then being used in combination with said respective sweep time offset to trigger an FMCW sweep. The method of claim 5 further comprising:

7. 6. The method of claim 5, wherein the electromagnetic test signal is emitted by one or more of the FMCW Doppler radars, and the electromagnetic test signal is detected and timed by each of the other FMCW Doppler radars.

8. The method of claim 7 , wherein the electromagnetic test signal is an FMCW wave.

9. 8. The method of claim 7, wherein the electromagnetic test signal is emitted from each emitting FMCW Doppler radar having a carrier frequency that is offset relative to the carrier frequency used for the respective electromagnetic test signal by each of the other FMCW Doppler radars.

10. 10. The method of claim 9, wherein determining the expected electromagnetic wave propagation time comprises measuring a maximum power of the received electromagnetic test signal at each FMCW Doppler radar receiving the electromagnetic test signal.

11. 6. The method of claim 5, wherein the respective sweep waveforms used by each of the FMCW Doppler radars are identical across the FMCW Doppler radars except for the respective sweep time offsets and sweep frequency offsets used by the FMCW Doppler radars.

12. Selecting the different respective sweep time offsets comprises: In a first ranking, ranking the FMCW Doppler radars within a first ranking group of FMCW Doppler radars, the first ranking being performed according to interference severity relative to other FMCW Doppler radars in the first ranking group and further based on the expected electromagnetic wave propagation time; selecting, for each of the FMCW Doppler radars in the first ranked group, a first sweep time offset for each of the FMCW Doppler radars, taking into account both a minimum relative sweep time offset and a propagation time delay for the FMCW Doppler radar determined relative to other FMCW Doppler radars in the first ranked group for which a respective first sweep time offset has already been selected, in order from the most severely interfering FMCW Doppler radar to the least severely interfering FMCW Doppler radar; The method of claim 5 , comprising:

13. Selecting the different respective sweep time offsets comprises: Identifying a second ranked group of one or more FMCW Doppler radars in the first ranked group for which it has been determined that it is not possible to select a respective sweep time offset that does not result in a predetermined minimum interference level during object tracking operations; and In a second ranking, ranking the FMCW Doppler radars within the second ranking group of FMCW Doppler radars, the second ranking being performed according to interference severity relative to other FMCW Doppler radars in the second ranking group, and further based on the expected electromagnetic wave propagation time; selecting, for each of the FMCW Doppler radars in the second ranked group, a second sweep time offset for each of the FMCW Doppler radars, taking into account both a minimum relative sweep time offset and a propagation time delay for the FMCW Doppler radar determined relative to other FMCW Doppler radars in the second ranked group for which a respective second sweep time offset has already been selected, in order from the most severely interfering FMCW Doppler radar to the least severely interfering FMCW Doppler radar; selecting the FMCW Doppler radars that belong to the first ranking group but not to the second ranking group as a first group of FMCW Doppler radars; Selecting one or several FMCW Doppler radars belonging to the second ranked group as the second group of FMCW Doppler radars; The method of claim 12 further comprising:

14. 14. The method of claim 13, wherein selecting the different respective sweep time offsets further comprises determining a minimum relative sweep time offset based on a minimum sweep time offset that produces at least a predetermined attenuation of an intermediate frequency (IF) of one of the FMCW Doppler radars experiencing interference from another one of the FMCW Doppler radars.

15. 6. The method of claim 5, wherein the sweep frequency offset is selected so that interference between the FMCW Doppler radar belonging to the first group and the FMCW Doppler radar belonging to the second group causes a misinterpretation of an object velocity outside a predetermined velocity range, and such misinterpretation corresponds to a false object detection due to the interference.

16. 16. The method of claim 15, wherein the sweep frequency offset is selected as between 0.3 and 0.7 times the sweep repetition frequency used by the FMCW Doppler radar.

17. A system comprising: at least two co-located frequency modulated continuous wave (FMCW) Doppler radars; one or more hardware processors, the one or more hardware processors comprising: determining an expected electromagnetic wave propagation time between the pair of FMCW Doppler radars, wherein determining the expected electromagnetic wave propagation time comprises each FMCW Doppler radar receiving an electromagnetic test signal and determining a maximum detectable propagation time of the electromagnetic test signal, taking into account any multipath wave trajectories, wherein the received electromagnetic test signal has at least a predetermined minimum power upon reception; selecting a different respective sweep time offset for each FMCW Doppler radar in the first group of FMCW Doppler radars; selecting a second non-zero sweep frequency offset for a second group of the FMCW Doppler radars, the second non-zero sweep frequency offset being relative to a sweep frequency pattern used by the FMCW Doppler radars belonging to the first group, the first group and the second group being disjoint groups of FMCW Doppler radars; and a hardware processor configured to perform the steps of: A system comprising:

18. 20. The system of claim 17, wherein one or more of the one or more hardware processors are located within one or more of the FMCW Doppler radars.

19. 20. The system of claim 17, wherein the system comprises a central control module, and wherein one or more of the one or more hardware processors are located within the central control module.

20. The system of claim 17, wherein the one or more hardware processors are configured to measure, using some of the FMCW Doppler radars, the respective positions and velocities of one or more moving objects relative to a respective measuring FMCW Doppler radar by: i) a first radar of a first set of some of the FMCW Doppler radars, the first set being of the first group, the first radar emitting FMCW sweeps repeatedly with a selected respective sweep time offset; and / or ii) a second radar of a second set of some of the FMCW Doppler radars, the second set being of the second group, the second radar emitting FMCW sweeps repeatedly with a selected second non-zero sweep frequency offset.

21. The system described in claim 17, wherein the one or more hardware processors are configured to select a different respective sweep time offset for each FMCW Doppler radar in the second group of FMCW Doppler radars.

22. The system described in claim 21, wherein the electromagnetic test signal is emitted by one or more of the FMCW Doppler radars, and the electromagnetic test signal is detected and timed by one or more of the other FMCW Doppler radars.

23. The system described in claim 22, wherein the electromagnetic test signal is emitted from each emitting FMCW Doppler radar having a carrier frequency that is offset relative to the carrier frequency used for the respective electromagnetic test signal by each of the other FMCW Doppler radars.

24. The system described in claim 23, wherein determining the expected electromagnetic wave propagation time comprises measuring the maximum power of the received electromagnetic test signal at each FMCW Doppler radar receiving the electromagnetic test signal.

25. The system described in claim 21, wherein the respective sweep waveforms used by each of the FMCW Doppler radars are identical across the FMCW Doppler radars except for the respective sweep time offsets and sweep frequency offsets used by the FMCW Doppler radars.

26. The system described in claim 17, wherein the one or more hardware processors are configured to repeat i) determining the expected electromagnetic wave propagation time, ii) selecting different respective sweep time offsets, and iii) selecting a second non-zero sweep frequency offset periodically and / or when additional FMCW Doppler radars are added to the system.

27. ​​The system described in claim 17, wherein the one or more hardware processors are configured to synchronize respective clocks of each of the FMCW Doppler radars, which clocks are then used in combination with the respective sweep time offsets to trigger FMCW sweeps.

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