Radar signal frame for communication between two motor vehicles

Abstract

The invention relates to a radar signal frame (Radar frame) transmitted by a first radar of a first motor vehicle, the frame comprising a control sub-frame (Control frame) preceding a sensing sub-frame (Sensing frame), wherein the control sub-frame (Control frame) is transmitted in a second frequency range, Pfpart, of a frequency band (Bf) and is capable of communicating frequency information regarding a frequency range (Pf) used by the first radar to transmit the sensing sub-frame (Sensing frame), and wherein the control sub-frame (Control frame) is capable of being received by a second narrow-band radar of a second vehicle tuned to the second frequency range (Pfpart).

Description

DESCRIPTION Title of the invention: Radar signal frame for communication between two motor vehicles

[0001] Technical field

[0002] The present invention relates to the definition of a radar signal frame intended to be transmitted by a radar of a motor vehicle to another motor vehicle. The invention also relates to a method for managing a frequency range for communication between said motor vehicles.

[0003] Radar systems are known to be used to detect distances, speeds, and angles between targets. With technological advances, radar systems are now being applied to automotive safety. For example, 77 GHz FMCW (Frequency Modulation Continuous Wave) radars or FCM (Fast Chirp Modulation) radars can be used with MIMO (Multiple-Input, Multiple-Output) networks as sensors in ADAS (Advanced Driver Assistance System) systems and autonomous driving.

[0004] A radar used as an automotive sensor is designed to provide information about a vehicle's surroundings. To do this, the radar emits an original radio frequency signal. This original radio frequency signal is reflected by various objects in the vehicle's immediate vicinity to form reflected radio frequency signals. These reflected signals are picked up by the radar. They have a lower intensity than the original signal. Therefore, noise near the radar can interfere with the reflected signals, thereby reducing the radar's effectiveness.

[0005] Such noise is generated, for example, by mutual interference between different radars present on vehicles within line of sight. This mutual interference phenomenon is a growing problem. Indeed, studies illustrating the penetration of the radar market show that by 2023, more than 50% of vehicles in circulation will be equipped with radars. This represents approximately 700 million vehicles equipped with 3.5 billion radars. However, there is currently no regulation or standardization for the use of an available frequency range or a waveform to be used.

[0006] The lack of standardization combined with this future exponential growth of advanced driver assistance systems and autonomous driving systems will increase the risks of radar interference, which can endanger the lives of users.

[0007] One way to limit these risks is to promote the attenuation (or "mitigation" in English) of radar interference.

[0008] Today, most interference mitigation techniques are based on repair at the signal processing level. These techniques attempt to repair an interfered signal, or to eliminate disturbances coming from another radar after receiving the jammed signal.

[0009] Other methods rely on random changes of radar parameters several times in a row to reduce the risk of interference.

[0010] Publications propose management algorithms such as RadarMAC. This method assumes that radars / cars are connected to a base station that will estimate the interference probabilities of each radar and distribute resources accordingly. However, this method is based on an estimation of lines of sight between radars.

[0011] In another method, a RadChat management algorithm proposes to modify the hardware of the radars to allow them to communicate with each other (for example via Wi-Fi communication). Such a method makes it possible to identify radars in line of sight because the communication antenna is identical to the radar antenna, while reserving resources.

[0012] There is therefore a need to propose a method to improve the "mitigation" of interference between radars by limiting modifications to the "hardware" in these radars.

[0013] Summary of the invention An object of the invention relates to a radar signal frame emitted by a first motor vehicle capable of using a first narrowband type radar for sending said radar signal frame in a first frequency range of a band of frequency, said first radar having a partial perception of the frequency band. Said radar signal frame comprises: - a detection subframe, said detection subframe comprising: - a period of emission of a detection signal at a determined frequency included in the first frequency range, said first radar being able to listen to a frequency zone around said determined frequency to pick up at least one echo, said echo coming from a reflection of all or part of the signal by an object placed at a certain distance from the first motor vehicle; - a processing period following said transmission period, said first radar being capable of processing said echo in said processing period so as to detect said object; said radar signal frame being characterized in that it comprises: - a control subframe preceding said detection subframe, said control subframe being transmitted in a second frequency range of the frequency band and capable of communicating frequency information concerning the first frequency range used by said first radar for sending said detection subframe, said control subframe being capable of being received by a second narrowband type radar of a second vehicle listening to said second frequency range, said second radar having partial perception of the frequency band.

[0014] The invention thus allows a narrowband radar to communicate to other radars in line of sight information on the frequency range used to transmit communication frames. Indeed, the other radars are also narrowband radars, they therefore have a partial perception of the frequency band. The use of the frequency information present in the control subframe allows these other radars to precisely determine the frequency range used by the transmitting radar. They can then, if necessary, knowingly modify their own frequency range for sending communication frames.

[0015] In a preferred embodiment, the emitted signal is of the FMCW type, such that said emitted signal comprises a grouping of chirps, each chirp corresponding to a continuous sinusoidal signal modulated in frequency so that the frequency of said chirp varies linearly over time.

[0016] FMCW radars are particularly suitable for use in the automotive sector. These radars emit a continuous sinusoidal signal that is frequency modulated so that its frequency varies linearly over time, creating a "chirp".

[0017] In a preferred embodiment, the control subframe extends over a frequency range less than or equal to 40 MHz.

[0018] In a preferred embodiment, the control subframe is formed by a matrix of blocks, each block of said matrix of blocks being capable of containing a signal to be transmitted for the transmission of data to the second vehicle.

[0019] The block matrix comprises at least n rows and at least m columns, the n rows of said block matrix extending successively in frequency and the m columns of said block matrix extending successively in time, each block of the same row of the matrix being associated with a vehicle identifier, each block of the same column of the matrix being associated with the same type of data to be transmitted.

[0020] Thus, the control subframe is divided into blocks. Vehicle identifiers are positioned in frequency and various predefined communicable data are positioned in time. Since the blocks only consume a small part of the frequency range, the emitted signal will be detectable by all radars in line of sight.

[0021] In a preferred embodiment, the block matrix comprises at least three dimensions, said dimensions making it possible to extend the block matrix successively in polarization or in coding of the signal such as in time or in frequency. In a preferred embodiment, each block of said block matrix has a height at least equal to 10 MHz.

[0022] The blocks thus have a certain extension in height so as to avoid radar emission in a wrong vehicle identifier. In a preferred embodiment, each block of said block matrix has a width at least equal to 4 ^s.

[0023] The blocks thus have a certain extension in width so as to prevent the echo of a signal from ending up in a following block.

[0024] In a preferred embodiment, for at least one block of the plurality of blocks, the signal to be transmitted contained in said block is distinguishable from another signal to be transmitted contained in another block via a Fourier transformation.

[0025] It is thus possible to increase the number of vehicle identifiers despite the frequency limitation and the imprecision of quartz.

[0026] In a preferred embodiment, the signal to be transmitted contained in said block comprises a first part with a constant frequency and a second part with an ascending frequency or a descending frequency.

[0027] In this way, even if two signals are emitted in the same frequency part, it is possible to distinguish them, which makes it possible to double the number of possible vehicle identifiers.

[0028] In a preferred embodiment, the block matrix comprises on a first column synchronization blocks intended to identify time offsets between a time signal associated with the first radar and a time signal associated with the second radar.

[0029] The synchronization block is used to identify time offsets from other radars. This synchronization block is located at the beginning of the control subframe and all radars must transmit within it. Another subject of the invention relates to a method for managing a frequency band for communication between a first motor vehicle and a second motor vehicle, said first motor vehicle being able to use a first narrowband type radar for sending a radar signal frame according to the invention, in a first frequency range of said frequency band, said second motor vehicle being able to use a second narrowband type radar for receiving said radar signal frame, said radar signal frame comprising a detection subframe, said detection subframe comprising: - a period of emission of a detection signal at a determined frequency included in the first frequency range, said first radar being able to listen to a frequency zone around said determined frequency to capture at least an echo, said echo resulting from a reflection of all or part of the signal by an object placed at a certain distance from the first motor vehicle; - a processing period following said transmission period, said first radar being capable of processing said echo in said processing period so as to detect said object; said radar signal frame comprising: - a control subframe preceding said detection subframe, said control subframe being transmitted in a second frequency range of the frequency band and capable of communicating frequency information concerning the first frequency range used by the first radar for sending said detection subframe, said control subframe being capable of being received by said second radar listening to said second frequency range to obtain the first frequency range used by said first radar for sending said radar signal frame, said management method comprising in the second motor vehicle: - a step of processing the frequency information to determine the first frequency range used by the first radar for sending the radar signal frame; - a comparison of the frequency range used by the first radar with a frequency range used by the second radar to send a radar signal frame.

[0030] In a preferred embodiment, if the frequency range used by the first radar at least partially overlaps the frequency range used by the second radar, the second vehicle is able to modify its radar signal frame sending frequency range.

[0031] In a preferred embodiment, the management method comprises a synchronization step for sending control subframes respectively by the first radar and by the second radar, at the same time and on the same second frequency range.

[0032] This synchronization can be carried out by a V2X system (for “Vehicle-of-Everything”), by GNSS (for “Geolocation and Navigation by a Satellite System”) or by an external communication channel.

[0033] Description of figures

[0034] Other characteristics and advantages of the invention will appear during the reading of the detailed description which follows for the understanding of which reference will be made to the appended drawings in which:

[0035] Figure 1 illustrates the principle of radar interference between two vehicles;

[0036] Figure 2 illustrates the principle of an FMCW radar chirp according to the prior art;

[0037] Figure 3 illustrates the organization of a radar frame according to the prior art;

[0038] Figure 4 illustrates a radar signal frame according to the invention integrating the organization of a radar frame according to Figure 2;

[0039] Figure 5 illustrates a control subframe belonging to the radar signal frame according to the invention;

[0040] Figure 6 illustrates an example of distinguishable signals according to the invention;

[0041] Figure 7 illustrates a radar signal frame comprising synchronization blocks according to the invention;

[0042] Figure 8 illustrates the steps of a method for managing a frequency band using a radar signal frame according to Figures 4 to 7.

[0043] Figure 1 is a simplified block diagram illustrating a direct interference scenario between a first motor vehicle 1 and a second motor vehicle 2.

[0044] The two motor vehicles 1, 2 are approaching from opposite sides of a road 100. Thus, the first motor vehicle 1 is moving from left to right in Figure 1 and the second vehicle 2 is moving from right to left. Each vehicle 1, 2 has a narrowband radar 11, 21 facing the front of the respective vehicle. The first radar 11 and the second radar 21 are directly facing each other and in line of sight of each other. In such a scenario, there is a risk of direct interference between these radars 11, 21.

[0045] This direct interference is problematic because the power received by a radar that is a "victim" of the "jamming" radar follows the equation:

[0047] In which P reçu is the interference power received by the “victim” radar, Ptransmis esi the power of the signal transmitted by the “jamming” radar, G is the antenna gain, A (i)is the effective antenna aperture of the “victim” radar and R is the distance between the two radars.

[0048] In this scenario, the power of the interfering signal decreases with the square of the distance ^ / r2 separating the two radars. This creates significant interference with the “victim” radar.

[0049] Figure 2 illustrates the principle of an FMCW radar. Such a radar is adapted to emit a series of chirps.

[0050] A chirp is a continuous sinusoidal signal that is frequency modulated so that its frequency varies linearly over time. Such a signal is referred to as S e in Figure 2. The frequency of this signal S e varies here between f c and f c +Z, where Z corresponds to the frequency difference between the start of emission of signal S e and the end of emission of this signal S e .

[0051] When emitting a chirp, the radar listens in a frequency zone Zf around the signal frequency S e in order to capture an echo signal S eC ho resulting from the reflection of the signal S e on an object in the immediate environment of the vehicle. The frequency zone Zf is represented here by dotted lines around this signal S e

[0052] In the example of Figure 2, a first chirp is separated from a second chirp by a transition part T. This transition part T allows the signal S e to go back down from the frequency f c +Z to frequency f c in view of the emission of the second chirp.

[0053] Chirps can be repeated hundreds of times (usually 512 or 1024 times) and then followed by a pause time for processing. The combination of a period T_chirp*N_chirp for sending a group of chirps with a period Processing time for processing the echo signals Secho forms a Sensing frame, as shown in Figure 3.

[0054] Figure 4 illustrates a radar signal frame according to the invention. This radar signal frame comprises the detection subframe Sensing frame and a control subframe Control frame.

[0055] As already indicated, the detection subframe comprises the sending of a succession of chirps. This sending can be done according to a chirp sending cycle. Alternatively, as illustrated in Figure 4, this sending can be done during several chirp sending cycles. The detection subframe here extends in frequency over a frequency range Pt included in the frequency band Bf.

[0056] The radar signal frame also includes the Control frame control subframe. This Control frame control subframe precedes the detection subframe in time. This Control frame control subframe extends over a frequency range Pfpart which is common and known to all the radars so that they can communicate with each other during this phase. This frequency range is defined beforehand. Alternatively, this frequency range is communicated via a V2X (Vehicle-of-Everything) system. The choice of the frequency range of the Control frame control subframe has no link with the frequency range on which the radar then transmits. Preferably, the frequency range of the Control frame control subframe is less than or equal to 40 MHz.

[0057] The control subframe Control frame is thus able to communicate frequency information to the second radar 21 concerning the frequency range Pt used by said first radar 11 for sending said radar signal frame Radar frame.

[0058] Following the Control frame, each radar resumes normal operation, i.e. operation on its frequency band (which it communicated during the Control frame) which cannot include the range of the control subframe.

[0059] Figure 5 illustrates this control subframe more specifically. It is formed by a matrix of several blocks bÿ, with i ranging from 1 to m, m representing the maximum number of columns in the matrix, and with j ranging from 1 to n, n representing the maximum number of rows in the matrix. The columns of the matrix extend successively along the time line and the rows of the matrix extend successively along the frequency line. Each block bij is capable of containing a signal to be transmitted St.

[0060] Each block bij in the same row of the matrix is ​​associated with a vehicle identifier v ldj. Similarly, each block bij in the same column of the matrix is associated with the same type of frequency information datai to be transmitted via the signal to be transmitted St.

[0061] In order for a radar to communicate a predefined frequency datai, it needs an identifier v_ldj. Such an identifier is randomly generated by the vehicle carrying the radar. A signal to be transmitted St is then emitted in the block bij corresponding to the frequency associated with the identifier and the time range corresponds to that of datai.

[0062] Since all blocks are located in a small frequency band, the signal to be transmitted St will be detectable by all other radars in line of sight.

[0063] Using a v_ldj identifier allows to detect if another radar is communicating the same datai information. As the signal is sent, it can bounce off objects in the environment and return to the original radar. It is then difficult to differentiate between the own echoes of the sent signal and the transmitted signal of another radar. By v_ldj identifiers, the frequencies are different, allowing to have a transmitted signal that bounces and that of the other radars even though the transmitted datai information is the same. In the particular case where the number of rows of the block matrix is ​​equal to 4, there is then a probability of 1 in 4 of having the same v ldj identifier, which would prevent the correct communication of this same datai information.

[0064] Alternatively, motor vehicles are equipped with the V2X system. They can then communicate their v_ldj identifier to each other. In this way, it is possible to identify which motor vehicles the radars that are in line of sight belong to. This can be useful for additional interference mitigation strategies.

[0065] Regarding the design of the bij blocks, since the control subframe is limited in frequency (and not in time), this frequency is used for the v_ldj identifier and the time for the datai information. In the same way, since radar quartz is quite imprecise, there is a certain imprecision on the transmitted frequency. For this reason, the bij blocks must be quite thick to prevent a radar from transmitting in a bad v_ldj identifier. These blocks thus have a height at least equal to 10 MHz.

[0066] In time, the blocks must also be wide enough to prevent the echo of one signal from ending up in the next block. Since the signal travels at about 300 m / s, the blocks will need to be at least a few microseconds long. The wider the blocks, the longer the transmitted signals can be, allowing for better detection by other radars.

[0067] It should be noted that even if the blocks have durations similar to the duration of a chirp (20 ps for a long-range FMCW), with 50 different communicable information, the duration of the control subframe would be approximately 2.5% to 5% of the complete radar subframe, which would have little impact on radar performance.

[0068] Note that bij blocks are also called frequency blocks.

[0069] One way to increase the number of possible v_ldj identifiers, despite the frequency limitation and the imprecision of quartz, is to use signals that are distinguishable after Fourier transforms. For example, in Figure 6, the signal present in block bi2 comprises a first part Su with a constant frequency and a second part St2 with a falling frequency. In the same way, the signal present in block bin comprises a first part S'u with a constant frequency and a second part S't2 with a rising frequency. By a Fourier transform, it is possible to distinguish them. Indeed, the Fourier transform of the signal Sti St2 forms a right-shifted lobe, while the Fourier transform of the signal S'u S't2 forms a left-shifted lobe. This makes it possible to double the number of possible v_l dj identifiers.

[0070] Figure 7 illustrates an alternative embodiment in which the radar signal frame comprises synchronization blocks b sy nc on the first column of the block matrix. These synchronization blocks will allow the detection of A time shifts of other radars. These synchronization blocks are thus located at the beginning of the control subframe and all radars must transmit in it. It is important that these synchronization blocks are at the start. In this way, the very first signal detected for a certain identifier v_l dj will always be considered as the synchronization signal, allowing the detection of A shifts greater than the width of the blocks.

[0071] The frequency information datai can be used in a method for managing a frequency band Bf. Such a method is illustrated in particular in Figure 8.

[0072] In a first step E1, the first motor vehicle 1 sends a radar signal frame Radar frame. This frame includes the frequency information datai. This radar signal frame also includes the identifier v_ldj associated with the first vehicle.

[0073] In a second step E2, the frequency information datai is processed by the radar of the second motor vehicle 2 in order to determine the frequency range Pf used by the first radar 11 for sending the radar signal frame.

[0074] In a third step E3, the second radar 21 compares the frequency range Pf used by the first radar with a frequency range P'f used by the second radar 21 to send a radar signal frame.

[0075] In the case where there is no overlap (Pf n P'f = 0) between the frequency range Pf used by the first radar 11 and the frequency range P'f used by the second radar 21, there is no interference between these two radars.

[0076] In the case where there is an overlap (Pf n P'f t 0) between the frequency range Pf used by the first radar 11 and the frequency range P'f used by the second radar 21, there is a risk of interference between the two radars. In this case, the second vehicle 2 can modify its frequency range P'f for sending radar signal frames in a fourth step E4.

[0077] In a particular embodiment, the second vehicle 2 performs this modification action if an additional criterion is satisfied. This criterion is, for example, linked to the identifier of the motor vehicles. Since the identifiers of the motor vehicles can be classified according to their position in the frequency scale, the radar associated with the vehicle having the highest identifier changes frequency range. The radar associated with the vehicle having the lowest identifier then does not change resource.

[0078] Note that if the resource change is also predictable (for example, choosing the nearest unused resource), then all radars can anticipate the resources used even if there are changes in the meantime.

Claims

CLAIMS 1. Radar signal frame emitted by a first motor vehicle (1) capable of using a first radar (11) of the narrow band type for sending said radar signal frame (Radar frame) in a first frequency range (Pf) of a frequency band (Bf), said first radar (11) having a partial perception of the frequency band (Bf), characterized in that said radar signal frame (Radar frame) comprises: - a detection subframe (Sensing frame), said detection subframe (Sensing frame) comprising: - a period of emission of a detection signal at at least one determined frequency (f c , fc+Z) included in the first frequency range (Pf), said first radar (11) being able to listen to a frequency zone (Zf) around said at least one determined frequency (f c , f c+Z) to capture at least one echo (Secho), said echo (Secho) resulting from a reflection of all or part of the signal by an object placed at a certain distance from the first motor vehicle (1); - a processing period (Processing time) following said transmission period, said first radar (11) being capable of processing said echo in said processing period (Processing time) so as to detect said object; said radar signal frame (Radar frame) comprising: - a control subframe (Control frame) preceding said detection subframe (Sensing frame), said control subframe (Control frame) being transmitted in a second frequency range (Pfpart) of the frequency band (Bf) and capable of communicating frequency information (datai) concerning the first frequency range (Pt) used by said first radar (11) for sending said detection subframe (Sensing frame), said control subframe (Control frame) being capable of being received by a second radar (21) of the narrowband type of a second vehicle (2) listening to said second frequency range (Pfpart), said second radar (21) having a partial perception of the frequency band (Bf).

2. Radar signal frame according to claim 1, wherein the transmitted signal is of the FMCW type, such that said transmitted signal comprises a grouping of chirps, each chirp corresponding to a continuous sinusoidal signal (S e) frequency modulated so that the frequency of said chirp (f c , f c +Z) varies linearly over time.

3. Radar signal frame according to any one of claims 1 or 2, in which the control sub-frame (Control frame) extends over a frequency range less than or equal to 40 MHz.

4. Radar signal frame according to any one of claims 1 to 3, wherein said control sub-frame (Control frame) is formed by a matrix of blocks, each block (bij) of said matrix of blocks being capable of containing a signal to be transmitted (St) for the transmission of data to the second vehicle (2).

5. Radar signal frame according to claim 4, wherein said block matrix comprises at least n rows and at least m columns, the n rows of said block matrix extending successively in frequency and the m columns of said block matrix extending successively in time, each block (b) of the same row of the matrix being associated with a vehicle identifier (v_ldj), each block (by) of the same column of the matrix being associated with the same type of data (datai) to be transmitted.

6. Radar signal frame according to claim 4, wherein the block matrix comprises at least three dimensions, said dimensions allowing the block matrix to be extended in polarization or in coding of the signal such as in time or in frequency.

7. Radar signal frame according to any one of claims 4 to 6, wherein each block (by) of said matrix of blocks has a height at least equal to 10 MHz.

8. Radar signal frame according to any one of claims 4 to 7, wherein each block (by) of said matrix of blocks has a width at least equal to 4 [1S.

9. Radar signal frame according to any one of claims 4 to 8, wherein for at least one block (by) of the plurality of blocks, the signal to be transmitted (St) contained in said block is distinguishable from another signal to be transmitted contained in another block via a Fourier transform.

10. Radar signal frame according to claim 9, wherein the signal to be transmitted contained in said block (bij) comprises a first part (Sti) with a constant frequency and a second part (St2) with an ascending frequency or a descending frequency.

11. Radar signal frame according to any one of claims 4 to 10, in which the block matrix comprises on a first column synchronization blocks (b sync) intended to identify time shifts between a time signal (St) associated with the first radar (11) and a time signal (S't) associated with the second radar (21).

12. Method for managing a frequency band (Bf) for communication between a first motor vehicle (1) and a second motor vehicle (2), said first motor vehicle (1) being able to use a first radar (11) of narrowband type for sending (E1) a radar signal frame (Radar frame) according to any one of claims 1 to 11, in a first frequency range (Pf) of said frequency band (Bf), said second motor vehicle (2) being able to use a second radar (21) of narrowband type for receiving said radar signal frame (Radar frame), said radar signal frame (Radar frame) comprising a detection sub-frame (Sensing frame), said detection sub-frame (Sensing frame) comprising: - a period of emission of a detection signal at at least one determined frequency (f c , f c +Z) included in the first frequency range (Pf), said first radar (11) being able to listen to a frequency zone (Zf) around said at least one determined frequency (f c , f c +Z) to capture at least one echo (S ec ho), said echo (Secho) being the result of a reflection of all or part of the signal by an object placed at a certain distance from the first motor vehicle (1); - a processing period (Processing time) following said transmission period, said first radar (11) being capable of processing said echo in said processing period (Processing time) so as to detect said object; said radar signal frame (Radar frame) comprising: - a control subframe (Control frame) preceding said detection subframe (Sensing frame), said control subframe (Control frame) being transmitted in a second frequency range (Pfpart) of the frequency band (Bf) and capable of communicating frequency information (datai) concerning the first frequency range (Pf) used by the first radar (11) for sending said detection subframe (Sensing frame), said control subframe (Control frame) being capable of being received by said second radar (21) listening to said second frequency range (Pfpart) to obtain the first frequency range (Pf) used by said first radar (11) for sending said radar signal frame (Radar frame), said management method comprising in the second motor vehicle (2): - a step (E2) of processing the frequency information (datai) to determine the first frequency range (Pf) used by the first radar (11) for sending the radar signal frame (Radar frame); - a comparison (E3) of the frequency range (Pf) used by the first radar (11) with a frequency range (P'f) used by the second radar (21) to send a radar signal frame (Radar frame), said first radar (11) and said second radar (21) having a partial perception of the frequency band (Bf).

13. Management method according to claim 12, wherein if the frequency range (Pf) used by the first radar (11) at least partially overlaps the frequency range (P'f) used by the second radar (21), the second vehicle (2) is able to modify (E4) its frequency range (P'f) for sending radar signal frames.

14. Management method according to any one of claims 12 or 13, in which the management method comprises a synchronization step for sending control subframes respectively by the first radar (11) and by the second radar (21), at the same time and on the same second frequency range (Pfpart).

Images