Full duplex monostatic radar device

Abstract

There is provided a full duplex monostatic radar device. The full duplex monostatic radar device comprises a transmitter. The transmitter is configured to transmit a modulated signal and to operate first dual-polarized antennas with a first rotating linear polarization when transmitting the modulated signal. The full duplex monostatic radar device comprises a receiver. The receiver is configured to receive the modulated signal and to operate second dual-polarized antennas with a second rotating linear polarization when receiving the modulated signal. The second rotating linear polarization is 90 degrees offset with respect to the first rotating linear polarization.

Description

[0001] FULL DUPLEX MONOSTATIC RADAR DEVICE

[0002] TECHNICAL FIELD

[0003] Embodiments presented herein relate to a method, a full duplex monostatic radar device, a computer program, and a computer program product for operating the full duplex monostatic radar device.

[0004] BACKGROUND

[0005] In general terms, radar is a system that uses radio waves to determine the distance (ranging), direction (azimuth and elevation angles), and radial velocity of objects relative to the radar device. A radar device configured to perform both transmission of radar signals and reception of echoes of the radar signals from objects is referred to as a monostatic radar device. This type of radar device is popular as it is not dependent on the presence of other radar devices, but can on its own perform a complete radar measurement. However, having to transmit and receive from the same radar device puts stringent requirements on the hardware of the radar device. Two schemes that can be used to realize this will be disclosed next.

[0006] In a first scheme, the radar device is in transmit mode and transmits a short pulse, then switches to receive mode, and waits for the signal echoes to return so that the signal echoes can be received and measured by the receiver. The radar device then switches back to the transmission mode and transmits new pulse, and so on. Thus, when switching from transmit mode to receive mode, and vice versa, an antenna switch is made whereby the antennas are selectively connected to the transmitter in the transmit mode and to the receiver in the receive mode. To be able to detect nearby targets, however, the antenna switch has to be very fast. The length of the pulse also has to be very short for the radar device to be able to detect nearby targets. This limits the signal energy. A radar device operating according to the first scheme can be referred to as a pulsed monostatic radar device.

[0007] In a second scheme, the receiver and transmitter operate simultaneously. Typically, different antennas are then used for the transmit and receive operations. However, a significant distance may be needed between the antennas of the transmitter at the one side and the antennas of the receiver at the other side to achieve a sufficient isolation between them. This is to limit the risk of strong signals from the transmitter desensitizing the receiver. A radar device operating according to the second scheme can be referred to as a full duplex monostatic radar device.

[0008] US 10,983,192 B2 relates to millimeter-wave transmitters and receivers and, more particularly, to polarimetric phased array millimeter-wave transceivers. The polarimetric transceiver front-end includes two receive paths configured to receive signals from an antenna, each including a respective variable phase shifter. A first transmit path is connected to the variable phase shifter of one of the two receive paths and is configured to send signals to the antenna. A transmit / receive switch is configured to select between the first transmit path and the two receive paths for signals. The transmit / receive switch has an element that adds a high impedance to the transmit path when the transmit / receive switch is in a receiving state.

[0009] For the pulsed monostatic radar device, the antenna switch needs to be very fast for the pulsed monostatic radar device to be able to detect nearby objects. The pulse length must also be very short for the pulsed monostatic radar device to be able to detect nearby objects. If the same pulsed monostatic radar device is to be used also for detection of far-away targets, the time spent in receive mode needs to be much longer than the time spent in transmit mode. In other words, the duty-cycle of the transmit mode becomes small, impacting the average power and hence the long- range performance of the pulsed monostatic radar device.

[0010] On the other hand, full duplex monostatic radar device may, depending on its operational frequency, become rather large to ensure sufficient antenna isolation for high output power to be possible. There will then be a tradeoff of size versus radar performance in terms of resolving small targets at large distances.

[0011] Another general issue for both pulsed monostatic radar devices and full duplex monostatic radar devices is that nearby objects will provide much stronger echoes than objects further away from the radar device. If the objects have the same size (i.e., same radar cross-section), the power of their echoes are inversely proportional to their distance to the power of four. This means that if one target is at a 5 m distance and another target is a at 50 m distance, the received power of the target at 5 m will be 10000 times stronger, i.e. 40 dB. This makes the dynamic range requirements high in the analog part of the receiver, and also creates difficulties in the signal design and signal processing for detecting and characterizing small echoes resulting from targets far away in presence of large echoes resulting from targets near the radar device.

[0012] Hence, there is still a need for improved monostatic radar devices.

[0013] SUMMARY

[0014] An object of embodiments herein is to provide a full duplex monostatic radar device where the above issues have been resolved, or at least mitigated or reduced.

[0015] A particular object is to provide a full duplex monostatic radar device with improved antenna isolation and capable of resolving targets at different distances.

[0016] According to a first aspect there is presented a full duplex monostatic radar device. The full duplex monostatic radar device comprises a transmitter. The transmitter is configured to transmit a modulated signal and to operate first dual-polarized antennas with a first rotating linear polarization when transmitting the modulated signal. The full duplex monostatic radar device comprises a receiver. The receiver is configured to receive the modulated signal and to operate second dual-polarized antennas with a second rotating linear polarization when receiving the modulated signal. The second rotating linear polarization is 90 degrees offset with respect to the first rotating linear polarization.

[0017] According to a second aspect there is presented a method for operating a full duplex monostatic radar device according to the first aspect. The method comprises transmitting a modulated signal and operating first dual-polarized antennas with a first rotating linear polarization when transmitting the modulated signal. The method comprises receiving the modulated signal and operating second dual-polarized antennas with a second rotating linear polarization when receiving the modulated signal. The second rotating linear polarization is 90 degrees offset with respect to the first rotating linear polarization.

[0018] According to a third aspect there is presented a computer program for operating a full duplex monostatic radar device. The computer program comprises computer code which, when run on processing circuitry of the full duplex monostatic radar device, causes the full duplex monostatic radar device to perform actions. One action comprises the full duplex monostatic radar device to transmit a modulated signal and to operate first dual-polarized antennas with a first rotating linear polarization when transmitting the modulated signal. One action comprises the full duplex monostatic radar device to receive the modulated signal and to operate second dual-polarized antennas with a second rotating linear polarization when receiving the modulated signal. The second rotating linear polarization is 90 degrees offset with respect to the first rotating linear polarization.

[0019] According to a fourth aspect there is presented a computer program product comprising a computer program according to the third aspect and a computer readable storage medium on which the computer program is stored. The computer readable storage medium could be a non-transitory computer readable storage medium.

[0020] Advantageously, this full duplex monostatic radar device and the related operation of the full duplex monostatic radar device enable the transmitter-to-receiver leakage to be cancelled, and thus improves isolation.

[0021] Advantageously, this full duplex monostatic radar device and the related operation of the full duplex monostatic radar device enable the full duplex monostatic radar device to have reduced dynamic range requirements for detecting targets at different distances.

[0022] Advantageously, by having a changeable rotation frequency this full duplex monostatic radar device and the related operation of the full duplex monostatic radar device enable the full duplex monostatic radar device to have an attenuation profile that is adjustable for different target distances.

[0023] Advantageously, by being full duplex, the average transmit power of the full duplex monostatic radar device is not limited by the pulse duration, and the full duplex monostatic radar device does not need any fast antenna switch.

[0024] Advantageously, by implementing polarization signal combination in the current domain, pre-cancellation signal voltage gain can be avoided.

[0025] Advantageously, this full duplex monostatic radar device and the related operation of the full duplex monostatic radar device enable targets with low radar cross-sections for one polarization to still be detected, and also targets with cross-polarization dominating over co-polarization can be detected.

[0026] Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

[0027] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a / an / the element, apparatus, component, means, module, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, module, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

[0028] BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The inventive concept is now described, by way of example, with reference to the accompanying drawings, in which:

[0030] Fig. 1 is a schematic diagram illustrating a system comprising a full duplex monostatic radar device according to embodiments;

[0031] Fig. 2 is a block diagram of a full duplex monostatic radar device according to an embodiment;

[0032] Fig. 3 schematically illustrates rotating polarizations of the transmitter and the receiver according to embodiments;

[0033] Fig. 4 is a flowchart of methods according to embodiments;

[0034] Figs. 5, 6, 7, and 8 show simulation results according to embodiments;

[0035] Fig. 9 is a schematic diagram showing structural units of a full duplex monostatic radar device according to an embodiment; and

[0036] Fig. 10 shows one example of a computer program product comprising computer readable storage medium according to an embodiment. DETAILED DESCRIPTION

[0037] The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the inventive concept are shown. This inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description. Any step or feature illustrated by dashed lines should be regarded as optional.

[0038] Fig. 1 is a schematic diagram illustrating a system 100 comprising a full duplex monostatic radar device no according to embodiments. The full duplex monostatic radar device 110 is configured to transmit a modulated signal (as indicated by the notation “Tx”), and to receive the modulated signal (as indicated by the notation “Rx”) after reflection at some physical object, representing a target. In the example of Fig. 1 is schematically illustrated a first target 120a located at a distance di from the full duplex monostatic radar device 110 and a second target 120b located at a distance d2 from the full duplex monostatic radar device no.

[0039] As noted above, there is still a need for improved monostatic radar devices.

[0040] At least some of the herein disclosed embodiments are based on a full duplex monostatic radar device comprising a transmitter, attached to, or comprising, a dualpolarized antenna or antenna system, being configured to transmit a modulated signal which, by the dual-polarized antenna or antenna system, is rotated in polarization over time. At least some of the herein disclosed embodiments are based on a full duplex monostatic radar device comprising a receiver, attached to, or comprising, a dual-polarized antenna or antenna system, which is configured to receive the modulated signal with a polarization that, by the dual -polarized antenna or antenna system, is rotated in the same way as the transmitted signal but leading or lagging it by 90 degrees. As will be disclosed in further detail below, the difference does not need to be exactly 90 degrees. The rotation frequency of the polarizations can be adjusted depending on at what object distance the minimum attenuation is desired. Fig. 2 is a block diagram of a full duplex monostatic radar device 200 according to an embodiment.

[0041] The full duplex monostatic radar device 200 comprises a transmitter 210. The transmitter 210 is configured to transmit a modulated signal. The transmitter 210 is configured to operate first dual-polarized antennas 230 with a first rotating linear polarization when transmitting the modulated signal. The transmitter may have, or be attached to, a dual polarized antenna or antenna array 230, where each polarization is connected to a transmit chain, similar to a communication signal transmitter. The signal modulation in this case is the same in both channels, but the amplitudes (including the sign) of the two polarizations are altered according to sine and cosine functions with time, rotating the linear polarization of the transmitted signal.

[0042] The full duplex monostatic radar device 200 further comprises a receiver 220. The receiver 220 is configured to receive the modulated signal. The receiver 220 is configured to operate second dual-polarized antennas 240 with a second rotating linear polarization when receiving the modulated signal. The second rotating linear polarization is 90 degrees offset with respect to the first rotating linear polarization. As noted above and as will be disclosed in further detail below, the difference does not need to be exactly 90 degrees. In comparison to the transmitter 210, the receiver 220 is thus also configured to have a rotating polarization, with the same frequency and direction as the transmitter 210, but with a 90-degree offset, or at least near 90 degrees. In this respect, the offset can be fine-tuned so that when the echo of the transmitted signal arrives at the receiver antenna 240, the receiver 220 will have an orthogonal polarization and not receive any strong direct transmit signal leakage. The receiver 210 will then cancel the direct transmit leakage. However, while the signal propagates back and forth to a target 120a, 120b, the receiver 220 will rotate its polarization and some signal energy will be received.

[0043] In Fig. 2 is illustrated a transmitter 210 and a receiver 220 with antenna systems 230, 240 having one single antenna each for each polarization, indicated as H and V, as in horizontal and vertical polarization, respectively, in Fig. 2. However, the herein disclosed embodiments are applicable to both single and multi-antenna full duplex monostatic radar devices 200. In the transmitter 210, the weighting with — sin(wt) and cos(wt) can be performed at different places in the signal chain, at digital baseband, at intermediate frequency, or at radio frequency. The same holds for the receiver 220, but performing the operation early in the signal chain, i.e. at radio frequency, is favorable. The signals can then be combined to suppress direct transmitter-to-receiver leakage, relaxing linearity requirements in succeeding parts of the receiver. Rf is the transmitted radar signal and w is the angular frequency of the polarization rotation. If the signals can be represented in the current domain before being combined, the maximum transmitter leakage signal voltage in the receiver can be minimized, and the required transmitter-to-receiver antenna isolation can be reduced.

[0044] The modulated signal is transmitted using a carrier frequency, denoted fc. The first rotating linear polarization has a rotation frequency, denoted frot, which is lower than the carrier frequency fc. As a non-limiting example, the rotation frequency can be in the order of 1 MHz, such as between 0.5 MHz and 5 MHz. Hereinafter a fixed rotation frequency will be assumed. However, it is also possible to use a time-varying rotation frequency, for instance a chirp or a frequency modulation. Having a time-varying rotation frequency would spread out the minima and maxima in the polarization loss versus distance. However, as will be disclosed below with reference to Table 1, the maximum is already rather flat, and the minimum at two times the distance of the maximum is far away, so in practical implementations a fixed rotation frequency is a good choice with low complexity.

[0045] If the target 120b is at a distance d2 = 37.5 m from the full duplex monostatic radar device, for the example of 1 MHz polarization rotation frequency, the receiver polarization will have rotated by 90 degrees when the signal comes back, and the signal is received without any attenuation due to polarization difference. If the target 120a is at half that distance (i.e., di = 18.75 m), however, the signal will have rotated only by 45 degrees and the attenuation is 3dB. At 25% of the distance (i.e., at a distance d2 / 4) the attenuation becomes 8.3dB, and at 10% of the distance (i.e., at a distance d2 / 10) the attenuation becomes i6dB, and so on. This attenuation due to polarization will thus not only cancel the transmit to receive direct leakage, but it will also significantly reduce the aforementioned dynamic range problem caused by targets 120a, 120b at different distances having very different path loss. In some aspects, the rotation frequency frotcan be selected depending on the maximum distance to targets 120b to detect, or at what distance minimum attenuation due to polarization difference is desired. In particular, in some embodiments, rotation frequency, frot, is by the full duplex monostatic radar device 200 adjustable to a distance di, d2 from the full duplex monostatic radar device 200 at which detection of a target 120a, 120b is either to be avoided or to occur. If the rotation frequency is frot= 1 MHz as in the example, it will take 0.25 ps for the transmitter output signal polarization to rotate 90 degrees, and in that time the signal propagates 75 m, that is 37.5m forward and back to a target. Targets at that distance will then have minimum attenuation due to the difference in polarization between the reflected signal and the receiver. If instead frot= 2 MHz, targets at 18.75 m will instead have minimum attenuation due to polarization difference.

[0046] More generally, and with reference to Fig. 1, assume that the second target 120b is to be detected, and that the first target 120a is to cause as little impact on the detection of the second target 120b as possible. With this in mind, the rotation frequency frotcan be determined as function of a first distance di from the full duplex monostatic radar device 200 at which detection of a first target 120a is to be avoided and / or as function of a second distance d2 from the full duplex monostatic radar device 200 at which detection of a second target 120b is to occur. In some embodiments, as in Fig. 1, the null is closer than the target to be detected. In other words, the first distance di is shorter than the second distance d2.

[0047] As disclosed above, the polarization of the received signal is rotated in the same way as the transmitted signal but leading or lagging by 90 degrees. Reference is here made to Fig. 3 which schematically illustrates rotating polarizations of the transmitter 210 and the receiver 220 according to embodiments. In the examples of Fig. 3, the polarization is illustrated as rotating counterclockwise, but it could equally well be rotated clockwise. At reference numeral 300a is illustrated an example where the polarization at the receiver 220 is leading by 90 degrees. This represents an embodiment where the second rotating linear polarization is leading the first rotating linear polarization by 90 degrees. At reference numeral 300b is illustrated an example where the polarization at the receiver 220 is lagging by 90 degrees. This represents an embodiment where the second rotating linear polarization is lagging the first rotating linear polarization by 90 degrees. If the second rotating linear polarization is leading the first rotating linear polarization, it should be with slightly less than 90 degrees to compensate for the time the wave travels between antennas, and if lagging it should be with slightly more than 90 degrees. In the example of frot= 1 MHz rotation and with a distance of 0.1m between the antennas, the adjustment from 90 degrees is just 0.12 degrees, and can be omitted in practice. However, if a higher rotation frequency is used, the adjustment will be larger. In case the distance between the first dual-polarized antennas 230 and the second dualpolarized antennas 240 is significant compared to the distance d2 to the target 120b where minimum attenuation is desired, the angle can be finetuned around 90 degrees. In this case when the first rotating linear polarization is leading the second rotating linear polarization, it should be with slightly more than 90 degrees. With typical numbers it will, however, be offset by less than one degree.

[0048] When the signal that is reflected by a target 120a, 120b comes back to the receiver 220, some time has passed since the signal was transmitted. That time is equal to the round-trip time trtt= 2d / c, where c is the speed of light and d is the (one-way) distance between the full duplex monostatic radar device and the target. During that time the receiver has rotated its polarization by 0 = 2n ■ trtt■ frotradians. Due to the difference between the reflected signal polarization and the receiver polarization, the received voltage of the reflected signal (i.e., of the echo) will then be multiplied by a gain factor G determined as:

[0049] Evaluating this equation for different distances d for a constant frot= 1 MHz rotation yields the values in Table 1.

[0050] Table 1: Example of gain versus distance for a 1 MHz rotation frequency

[0051] As can be seen from Table 1, the attenuation at 0.1 m is rather high (48 dB), so finetuning of the receiver offset from 90 degrees seems unnecessary for a distance d=o.i m for frot= 1 MHz.

[0052] It can also be observed that the gain is close to linear with distances up to about 10 m, and there is a rather flat maximum around 37.5 m with no attenuation at the center. There is also a minimum at twice that distance; at 75 meters in the present example. If the rotation frequency frotis increased, both the minimum and the maximum will move closer in distance, which could be desired if targets at certain distances should be detected or suppressed.

[0053] Assume that a second target 120b at a distance d2 = 37.5 meters with a radar crosssection (RCS) of -20 dBsm is to be detected, and at the same time a first target 120a with an RCS of -5 dBsm is located at di = 5 meters. The difference in echo strength without any polarization rotation technique would be 15 dB due to the difference in RCS and 35 dB due to the distance difference, adding up to a total of 50 dB difference in signal power. With herein disclosed full duplex monostatic radar device 200, the attenuation at 5 m compared to 37.5 m would be 14 dB, reducing the power difference from 50 dB to 36 dB, which is significant.

[0054] It is noted that the targets 120a, 120b can also have a cross polarized reflection, but the RCS for that should be about lodB below the co-polarized RCS. Nearby targets, the first target 120a, may then show up with the cross-polarized RCS rather than the attenuated co-polarized RCS, as the cross-polarized response is then slightly larger after the co-polarized has been more attenuated. The improvement of 14 dB of the herein disclosed full duplex monostatic radar device 200 may then be reduced to about 10 dB, but that is still a very significant and valuable reduction of the dynamic range.

[0055] The RCS of the targets can be estimated by combining information about the transmit power, the received power, and the estimated distance. When using the full duplex monostatic radar device 200, the gain due to the polarization rotation, with an example for 1 MHz rotation frequency in Table 1, should be inverted when estimating the radar cross-section of the reflecting object. For instance if the object is 10 m away and 1 MHz rotation frequency is used, then the received power should be multiplied with 1 / 0.41=2.44 when estimating the radar cross-section.

[0056] As disclosed above, the offset can be fine-tuned so that when the echo of the transmitted signal arrives at the receiver antenna 240, the receiver 220 will have an orthogonal polarization and not receive any strong direct transmit signal leakage. Therefore, in some embodiments, the receiver 220, 924 is configured to add a finetuning value, denoted a, to the 90 degrees offset of the second rotating linear polarization when operating the dual-polarized antennas. The angle can be adjusted to control the distance at which the first null appears. As will be disclosed next, the angle can be adjusted by adding (or subtracting) a small offset a to the 90 degrees offset. As disclosed above, if the receiver 220 is leading the transmitter 210, it should be with slightly less than 90 degrees to compensate for the time the wave travels between antennas. Hence, in some embodiments, — 5 < a < 0. As disclosed above, if the receiver 220 is lagging it should be with slightly more than 90 degrees. Hence, in some embodiments, 0 < a < 5. The sign of a thus depends on if the receiver is leading or lagging the transmitter polarization.

[0057] The 90 degrees offset can thereby be adjusted to control the distance at which the first null appears. Therefore, in some embodiments, the fine-tuning value a is a function of a nulling distance, denoted dnull, and the rotation frequency, frot, of the first rotating linear polarization. In particular, in some examples, the fine-tuning value a is: In the receiver 220, when the antennas are not located at far enough distance to have high isolation, transconductance amplifiers with programmable gain and sign can be used for the two polarizations, which are then combined in the current domain. Hence, in some embodiments, the receiver 220 comprises transconductance amplifiers 260 with variable gain and sign for operating the second dual-polarized antennas 240 with the second rotating linear polarization. This avoids voltage gain before the cancellation of the strong transmit signal has occurred. The orientation of the received polarization is controlled by the gain and sign of the transconductance amplifiers. In some examples, as in Fig. 2, the transmitter 210 comprises transconductance amplifiers 250 with variable gain and sign for operating the first dual-polarized antennas 230 with the first rotating linear polarization. Alternatively, the signal multiplication in the transmitter may be performed earlier in the signal chain.

[0058] In some aspects, the signal combination of the signals received in the two polarizations takes place in the current domain, after the variable or programmable gain transconductance amplifiers. Therefore, in some embodiments, the receiver 220, comprises a signal combiner 270 configured to combine the modulated signal as received with a first polarization of the second dual-polarized antennas 240 and the modulated signal as received with a second polarization of the second dual-polarized antennas 240.

[0059] Fig. 4 is a flowchart illustrating embodiments of methods for operating the full duplex monostatic radar device 200. The methods are performed by the full duplex monostatic radar device 200. The methods are advantageously provided as computer programs.

[0060] S102: The full duplex monostatic radar device 200 transmits a modulated signal and operates first dual-polarized antennas 230 with a first rotating linear polarization when transmitting the modulated signal.

[0061] S104: The full duplex monostatic radar device 200 receives the modulated signal and operates a second dual-polarized antennas 240 with a second rotating linear polarization when receiving the modulated signal. As above, the second rotating linear polarization is 90 degrees offset with respect to the first rotating linear polarization. Embodiments relating to further details of operating the full duplex monostatic radar device 200 will now be disclosed with continued reference to Fig. 4.

[0062] As disclosed above, the modulated signal as received with a first polarization of the second dual-polarized antennas 240 is combined with the modulated signal as received with a second polarization of the second dual-polarized antennas 240. Hence, in some embodiments, the method further comprises (optional) step S106.

[0063] S106: The full duplex monostatic radar device 200 combines the modulated signal as received with the first polarization of the second dual-polarized antennas 240 with the modulated signal as received with the second polarization of the second dualpolarized antennas 240.

[0064] Some implementational aspects will be disclosed next.

[0065] Assume that the full duplex monostatic radar device 200 has 4 dual-polarized antennas with an antenna element gain of 5 dB available for the transmitter 210 and 4 dual-polarized antennas available for the receiver 220, and with a transmit power per antenna of 15 dBm. The antenna gain then becomes 5 dB + 101og(4) dB = 11 dB for both the transmitter and the receiver. The total radiated power (TRP) becomes i5dBm + 6dB = 2idBm. The effective isotropic radiated power (EIRP) is the combination of TRP and antenna gain; 2idBm + ndB = 32dBm. Assuming a 6dB noise figure (NF) and a 1 ms integration time the received effective radar signal noise Pnoise becomes:

[0066] Pnoise= —174dBm + NF — lOlog

[0067] The received radar signal PreCeived after reflection at a target can be expressed as:

[0068] The received radar signal, when having a carrier frequency of 30 GHz, for a target at a distance 50 m and with a radar cross-section of 0.01m2then becomes:

[0069] Preceived = 32dBm + lldB — 161dB = — 118dBm The signal-to-noise-ratio (SNR) can then be calculated as:

[0070] SNR = ^received—^noise= —118 dBm — (— 138dBm) = 20 dB

[0071] However, there will also be a loss due to polarization mismatch between the echo signal and the receiver. The polarization mismatch depends on target distance and polarization rotation frequency. If the rotation frequency is 1 MHz, as an example, the loss can be found using Table i, and for a target with a distance 50 m the loss will be i.25dB (since the gain is -i.25dB at 50m). The received signal strength will then be reduced by i.25dB, to -nq.isdB, and the SNR will drop by i.25dB, to i8.85dB. To eliminate the loss at 50m, the polarization rotation frequency can be reduced from 1 MHz to 0.75 MHz.

[0072] As disclosed above, Table 1 shows how the gain (loss) due to polarization mismatch between the echo signal and the receiver depends on the distance d to the target for a 1 MHz rotation frequency, from 0.1 m to 75 m target distance. However, the gain will be repetitive with the distance and will thereby create spatial filtering for certain distances. This can be seen as if a “null” can be placed at a certain distance with very strong suppression of radar reflections with that time delay. There are numerous scenarios where such a feature can be used, for instance when a rather small target is placed in front of large building, but with some distance to the building. Using the example from Table 1, it could be considered that the small target is located at 50 m distance and a large building with perhaps 10 000 times (40 dB) larger cross-section is located at 75 m distance. The larger distance to the building will only suppress the reflection by (75 / 5o)-4, i.e., 7 dB, so the reflection from the building is expected to be 33 dB higher, which however, ideally will be completely suppressed when using a 1MHz frequency of the polarization rotation. In practice, however, the building will have some cross-polarized reflections, so full suppression may not be possible. But the dynamic range can be improved substantially. Had the distance been different, then another rotation frequency could be selected. Assume now that detection is to be performed of a target that is further away than multiple small objects that are at a certain distance, but not very close to the radar device. In such a scenario another best choice of rotation frequency would be to place a null at the approximate distance of the multiple small objects, but still enabling a low gain reduction at the desired distance, i.e. the distance of the desired target. In Figs. 5(a), 5(b), 5(c), and 5(d) is illustrated simulation results for four scenarios with increasing rotation frequency. Curves with dashed lines show the prior art, i.e., no rotating polarization, whilst the curves with solid lines show the herein disclosed embodiments with increasing polarization rotation frequency. In the upper plots in each figure the difference between solid line and dashed line curves shows the reduction of the maximum signal power that needs to be handled by the receiver. The lower plots in each of these figures show the corresponding SNR. The curves with solid lines show a lower SNR, but only for levels where the SNR is very high or for distances further away than c / (8 x rotation frequency), i.e. 37.5 m from Table 1.

[0073] Further, in Fig. 6 is shows how the attenuation varies when adjusting the nulling distance from lqm (all positive) for a constant rotation frequency of 1 MHz.

[0074] Further simulation results will be disclosed next with reference to Figs. 7 and 8.

[0075] Both the transmitter and the receiver use dual-polarized antennas with horizontal (H) and vertical (V) linear polarization. All signals transmitted and received in horizontal and vertical domains are shown, and so are the received signals after weighting and after combination. It can be seen that the modulated signal after combination in the receiver is identical in shape to the modulated transmitted signal, unaffected by the polarization rotation, but with an additional attenuation that is distance dependent.

[0076] To illustrate the functionality of the transmitter, on-off keying (OOK) modulation with a pseudo-random signal pattern is created and used as the transmitted signal. Any modulation can be used in this radar system, since the modulated signal will at the receiver be recovered exactly, and no additional modulation of the received signal after combination will occur due to the rotation of polarization.

[0077] Fig. 7(a) shows the amplitude of the OOK signal, being 1 during ON periods, and o during OFF periods. The angle of the linear polarized transmit signal is rotated counterclockwise, starting at zero degrees, which is defined as positive polarity and horizontal polarization. Fig. 7(b) shows the desired polarization angle versus time. The frequency of rotation in the example is 1 MHz, and thus the polarization angle increases linearly by 360 degrees per microsecond. Fig. 7(c) shows the weighting functions to project the OOK signal to the horizontal and vertical domains. In this case a cosine function is used for the horizontal weighting and a sine function for the vertical weighting. The frequency of the sine and cosine functions is 1MHz, the same as the rotation frequency. Fig. 7(d) and Fig. 7(e) show the horizontal and vertical domain transmit signals that have been generated by multiplying the modulated signal with the two different weighting functions. By transmitting these signals using horizontally and vertically polarized antennas, respectively, the modulated transmit signal is transmitted with a linear polarization that is rotating by 1MHz counterclockwise, as desired.

[0078] In the radar device the polarization of the received signal is rotated in the same direction, counterclockwise, and with the same frequency, in this simulation 1MHz. However, a 90-degree phase offset is introduced between the transmitter and receiver linear polarization angle, to reduce the effect of short propagation delay leakage between the transmitter and the receiver. In the present example the receiver polarization is lagging the transmitter by 90 degrees, meaning that at t=o when the transmitter linear polarization angle is o degrees (positive horizontal polarization), the receiver linear polarization angle is -90 degrees (negative vertical polarization). To further aid the understanding, the example excludes effects of path loss (and cross polarization) of the reflection, so that the reflected signals reaching the receiver are normalized to unit amplitude.

[0079] According to the results in Fig. 8, the reflecting object is placed at 18.75m distance from the transmitter. The receiving antennas will then receive time delayed versions of the two corresponding transmitted polarization signals, with respect to the horizontal and vertical signals of Fig. 7(d) and Fig. 7(e). The horizontal and vertical waveforms are the same in Fig. 8(a) as in Fig. 7(d) and the same in Fig. 8(b) as in Fig. 7(e), but with different time axis. The same applies for the polarization angle versus time in Fig. 8(c) and the weighting functions in Fig. 8(d) to project the OOK signal to the horizontal and vertical domains.

[0080] The weighting functions in the receiver is a sine function for the horizontal domain and a negative cosine function for the vertical domain. In the receiver the weighting functions are used in a first step to scale the two received polarization signals. The signals after this step can be seen in Fig. 8(e) and Fig. 8(f), for the horizontal and vertical domains, respectively. After the scaling of the two polarizations (multiplying with sine and minus cosine), they are combined by addition of the scaled signals, with the result shown in Fig. 8(g). Since a normalized unity amplitude is used for the received horizontal and vertical domain signals and a reflecting object distance with a propagation time equal to a 90-degree polarization rotation, the same amplitude as that of the transmitted signal is retrieved in the receiver. For this distance to the reflecting object there is thus no additional signal attenuation due to the polarization rotation. It is also clear that the modulated signal is retrieved without any distortions or additional modulations due to the polarization rotation.

[0081] Similar results can be obtained for reflections at shorter distances, with more and more attenuation from the polarization rotation as the distance is reduced, and with no distortion caused to the modulation in the received combined signal.

[0082] Fig. 9 schematically illustrates, in terms of a number of structural units, the components of a full duplex monostatic radar device 900 according to an embodiment. Processing circuitry 210 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), etc., capable of executing software instructions stored in a computer program product 1010 (as in Fig. 10), e.g. in the form of a storage medium 930. The processing circuitry 210 may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA).

[0083] Particularly, the processing circuitry 210 is configured to cause the full duplex monostatic radar device 900 to perform a set of operations, or steps, as disclosed above. For example, the storage medium 930 may store the set of operations, and the processing circuitry 210 maybe configured to retrieve the set of operations from the storage medium 930 to cause the full duplex monostatic radar device 900 to perform the set of operations. The set of operations maybe provided as a set of executable instructions.

[0084] Thus, the processing circuitry 210 is thereby arranged to execute methods as herein disclosed. The storage medium 930 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory. The full duplex monostatic radar device 900 may further comprise a communications (comm.) interface 920 at least configured for communications with other entities, functions, nodes, and devices. As such the communications interface 920 may comprise one or more transmitters and receivers, comprising analogue and digital components. For example, the communications interface 920 may comprise a transmitter 922 being configured to transmit a modulated signal and to operate first dual-polarized antennas with a first rotating linear polarization when transmitting the modulated signal and a receiver 924 being configured to receive the modulated signal and to operate second dual-polarized antennas with a second rotating linear polarization, the second rotating linear polarization being 90 degrees offset with respect to the first rotating linear polarization, when receiving the modulated signal, as disclosed above. In some examples, the communications interface 920 further comprises the first dual-polarized antennas and the second dual-polarized antennas.

[0085] The processing circuitry 210 controls the general operation of the full duplex monostatic radar device 900 e.g. by sending data and control signals to the communications interface 920 and the storage medium 930, by receiving data and reports from the communications interface 920, and by retrieving data and instructions from the storage medium 930. Other components, as well as the related functionality, of the full duplex monostatic radar device 900 are omitted in order not to obscure the concepts presented herein.

[0086] The full duplex monostatic radar device 900 maybe provided as a standalone device or as a part of at least one further device. For example, the full duplex monostatic radar device 900 may be provided in a user equipment, a network equipped vehicle, a gaming controller, etc.

[0087] Fig. 10 shows one example of a computer program product 1010 comprising computer readable storage medium 1030. On this computer readable storage medium 1030, a computer program 1020 can be stored, which computer program 1020 can cause the processing circuitry 210 and thereto operatively coupled entities and devices, such as the communications interface 920 and the storage medium 930, to execute methods according to embodiments described herein. The computer program 1020 and / or computer program product 1010 may thus provide means for performing any steps as herein disclosed. In the example of Fig. 10, the computer program product 1010 is illustrated as an optical disc, such as a CD (compact disc) or a DVD (digital versatile disc) or a Blu-Ray disc. The computer program product 1010 could also be embodied as a memory, such as a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM), or an electrically erasable programmable read-only memory (EEPROM) and more particularly as a non-volatile storage medium of a device in an external memory such as a USB (Universal Serial Bus) memory or a Flash memory, such as a compact Flash memory. Thus, while the computer program 1020 is here schematically shown as a track on the depicted optical disk, the computer program 1020 can be stored in any way which is suitable for the computer program product 1010.

[0088] The inventive concept has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended patent claims.

Claims

CLAIMS1. A full duplex monostatic radar device (no, 200, 900), comprising: a transmitter (210, 922), the transmitter (210, 922) being configured to transmit a modulated signal and to operate first dual-polarized antennas (230) with a first rotating linear polarization when transmitting the modulated signal; and a receiver (220, 924), the receiver (220, 924) being configured to receive the modulated signal and to operate second dual-polarized antennas (240) with a second rotating linear polarization, the second rotating linear polarization being 90 degrees offset with respect to the first rotating linear polarization, when receiving the modulated signal.

2. The full duplex monostatic radar device (no, 200, 900) according to claim 1, wherein the modulated signal is transmitted using a carrier frequency, fc, and wherein the first rotating linear polarization has a rotation frequency, frot, being lower than the carrier frequency fc.

3. The full duplex monostatic radar device (no, 200, 900) according to any preceding claim, wherein the first rotating linear polarization has a rotation frequency, frot, that by the full duplex monostatic radar device (110, 200, 900) is adjustable to a distance (di, d2) from the full duplex monostatic radar device (110, 200, 900) at which detection of a target (120a, 120b) is either to be avoided or to occur.

4. The full duplex monostatic radar device (110, 200, 900) according to any preceding claim, wherein the first rotating linear polarization has a rotation frequency, frot, determined as function of a first distance (di) from the full duplex monostatic radar device (no, 200, 900) at which detection of a first target (120a) is to be avoided and / or as function of a second distance (d2) from the full duplex monostatic radar device (110, 200, 900) at which detection of a second target (120b) is to occur.

5. The full duplex monostatic radar device (no, 200, 900) according to claim 4, wherein the first distance (di) is shorter than the second distance (d2).

6. The full duplex monostatic radar device (no, 200, 900) according to any preceding claim, wherein the second rotating linear polarization is leading the first rotating linear polarization by 90 degrees.

7. The full duplex monostatic radar device (110, 200, 900) according to any preceding claim, wherein the second rotating linear polarization is lagging the first rotating linear polarization by 90 degrees.

8. The full duplex monostatic radar device (no, 200, 900) according to any preceding claim, wherein the receiver (220, 924) is configured to add a fine-tuning value a to the 90 degrees offset of the second rotating linear polarization when operating the dual-polarized antennas.

9. The full duplex monostatic radar device (no, 200, 900) according to claims 6 and 8, wherein — 5 < a < 0.

10. The full duplex monostatic radar device (110, 200, 900) according to claims 7 and 8, wherein 0 < a < 5.

11. The full duplex monostatic radar device (110, 200, 900) according to any of claims 8 to 10, wherein the fine-tuning value a is a function of a nulling distance, dnull, and a rotation frequency, frot, of the first rotating linear polarization.

12. The full duplex monostatic radar device (no, 200, 900) according to claim 11, wherein the fine-tuning value a is:where c is speed of light.

13. The full duplex monostatic radar device (no, 200, 900) according to any preceding claim, wherein the receiver (220, 924) comprises transconductance amplifiers (260) with variable gain and sign for operating the second dual-polarized antennas (240) with the second rotating linear polarization.14- The full duplex monostatic radar device (no, 200, 900) according to any preceding claim, wherein the receiver (220, 924) comprises a signal combiner (270)configured to combine the modulated signal as received with a first polarization of the second dual-polarized antennas (240) and the modulated signal as received with a second polarization of the second dual-polarized antennas (240).

15. A method for operating a full duplex monostatic radar device (no, 200, 900) according to any preceding claim, wherein the method comprises: transmitting (S102) a modulated signal and operating first dual-polarized antennas (230) with a first rotating linear polarization when transmitting the modulated signal; and receiving (S104) the modulated signal and operating second dual-polarized antennas (240) with a second rotating linear polarization, the second rotating linear polarization being 90 degrees offset with respect to the first rotating linear polarization, when receiving the modulated signal.

16. A computer program (1020) for operating a full duplex monostatic radar device (no, 200, 900) according to any of claims 1 to 14, the computer program comprising computer code which, when run on processing circuitry (910) of the full duplex monostatic radar device (110, 200, 900), causes the full duplex monostatic radar device (110, 200, 900) to: transmit (S102) a modulated signal and to operate first dual-polarized antennas (230) with a first rotating linear polarization when transmitting the modulated signal; and receive (S104) the modulated signal and to operate second dual-polarized antennas (240) with a second rotating linear polarization, the second rotating linear polarization being 90 degrees offset with respect to the first rotating linear polarization, when receiving the modulated signal.

17. A computer program product (1010) comprising a computer program (1020) according to claim 16, and a computer readable storage medium (1030) on which the computer program is stored.

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