DE102019217804B4 - Method for operating a MIMO radar system and a radar system designed for it - Google Patents

Abstract

Method for operating a MIMO radar system with several transmission and reception channels, in which each reception channel has a mixer with which a radar reception signal can be mixed down to a baseband, with several in the mixer between a signal input for the radar reception signal and a signal output series-connected mixers (4, 5) are used and each of the mixers (4, 5) is supplied with a LO signal at a frequency that is lower than a transmission frequency of the radar system, and the frequencies of the supplied LO signals in total the Resulting transmission frequency, characterized in that two mixers (4, 5) connected in series are used in the mixing device and a LO signal with half the transmission frequency is fed to each of the mixers (4, 5).

Description

Technical field of application

The present invention relates to a method for operating a MIMO radar system (MIMO: Multiple Input Multiple Output) with several transmission and reception channels, in which a mixer is used in each reception channel with which a radar reception signal can be mixed down to a baseband. The invention also relates to a MIMO radar system which is designed to carry out the method.

The basic principle of a MIMO radar system is the use of several transmit and receive channels. Only one transmission channel is active at the same time, while all reception channels receive at the same time. A larger, virtual antenna array can then be calculated from the individual received signals by means of signal processing. It is crucial that every received signal is based on exactly one transmitted signal. While a transmission channel is active, no signal in the relevant frequency range may be transmitted elsewhere at the same time.

1 shows a schematic structure of an exemplary, monostatic transmit and receive channel for FMCW operation in a MIMO radar system according to the prior art. The transmission signal (TX signal) is sent via a directional coupler 1 the antenna 2 fed. The transmit signal is from the receive mixer 3 isolated. That with the same antenna 2 received received signal is this receiving mixer 3 via the directional coupler 1 where it is mixed down to baseband with the help of the LO signal (local oscillator signal). The LO signal corresponds to the TX signal, so it has the same frequency f _LO = f _TX . In order to only use the channel concerned as a receiver for MIMO operation, the transmit signal is switched off during this time, so that only the LO signal for the receive mixer 3 remains active and the channel thus works as a receiver.

The main problem in this configuration, however, is an unavoidable crosstalk from the LO input of the receiving mixer to the receiving input (RF input). The path of this parasitic signal is indicated by the dashed arrows in 1 indicated. The strength of this crosstalk depends on the LO-RF isolation of the mixer, which is the attenuation of the LO signal at the reception input (RF input) of the mixer 3 indicates. Although this LO-RF isolation can be influenced by various measures and mixer architectures, it cannot be completely avoided. Due to the finite LO-RF isolation, a parasitic LO signal that corresponds to the TX signal, that is to say is in the useful frequency range of the radar system, is emitted even when the channel is in reception mode when the transmit signal is not present. Thus, in MIMO mode with n channels, in addition to the active transmit signal n-1, there are further parasitically radiating attenuated channels. These parasitically radiated signals lead to undesired artifacts in MIMO signal processing or can also make correct signal evaluation impossible because the individual transmitters can no longer be clearly separated from one another or, in the case of a received signal, no longer between a weak reflection of the intended transmission signal and a strong reflection of a parasitic transmission signal can be distinguished.

To avoid this problem, it must be ensured that the parasitic signal, which is caused by the LO signal and can reach the antenna via the receiving input of the mixer, is suppressed on the way to the antenna. In the case of MIMO radar systems of the prior art, this is achieved by using a low noise amplifier (LNA) in front of the reception input of the reception mixer. Since LNAs naturally have a very high level of reverse isolation from the output to the input, they are a suitable method for blocking the parasitic LO signal and preventing it from being radiated. Since in most cases an LNA is provided to amplify the received signal anyway, this solution is already implemented inherently for many systems.

However, with this solution, problems arise with the development of ever higher frequencies by modern radar sensors. Modern SiGe technologies enable operating frequencies of over 300 GHz, even for complex, multi-channel MIMO systems. In these frequency ranges, signals can be generated using various circuit concepts, but at these high frequencies at the limit of the available technologies, no significant gain factors are achieved. Amplification is therefore no longer possible and only a more or less pronounced attenuation is achieved by means of an LNA. This contradicts the use of LNAs in the receiving path, as they still act as an isolator for the LO-RF crosstalk, but have negative effects on the signal-to-noise ratio and the receiver's noise figure due to the additional attenuation. Without restrictions with regard to the signal-to-noise ratio and noise figure, there is currently no solution available to prevent the parasitic radiation of the receiver mixer in the useful frequency range when using high frequencies close to the technological limits.

From the US 2005/0227660 A1 is a method for operating a MIMO radar system according to the preamble of the patent claim 1 known.

the DE 10 2017 208 904 A1 describes a method and an arrangement for generating IQ signals in which a reference signal is first doubled in frequency with a frequency doubler and then fed to a master-slave flip-flop operated as a divider to generate an I component and a Q component of the To obtain reference signal.

The object of the present invention is to provide a method for operating a MIMO radar system and a radar system designed for this, which enable the undesired emission of a parasitic LO signal in the useful frequency range even when the radar system is operated at very high frequencies> 200 GHz without cause negative effects on the signal-to-noise ratio and the noise figure of the receiver.

Presentation of the invention

The object is achieved with the method and the radar system according to claims 1 and 4. Advantageous configurations of the method and of the radar system are the subject matter of the dependent claims or can be found in the following description and in the exemplary embodiment.

In the proposed method for operating a MIMO radar system with a plurality of transmission and reception channels, a mixer is used in a known manner in each reception channel, with which a radar reception signal can be mixed down to a baseband. The method is characterized in that a mixer is used which has two mixers connected in series between the signal input for the radar received signal and a signal output for the downmixed signal, each of which is fed with a LO signal for mixing, which is half Having the transmission frequency of the MIMO radar system.

Through this use of a mixing device, which has several successive receiving mixers that are subharmonic with respect to the transmission frequency, crosstalk of the LO signals used from the LO input to the receiving input of the mixing device leads to the emission of signals whose frequency deviates from the transmission frequency. This means that there is no longer any radiation in the usable frequency range of the transmission frequency, so that MIMO operation is no longer impaired. In contrast to the known solution, in which the parasitic radiation is only attenuated by a finite value through the backward isolation of an LNA, the proposed method systematically suppresses parasitic radiation in the useful frequency range. Since mixers can be controlled comparatively well even at high frequencies close to the technological limits, the proposed method is superior to an LNA without amplification or with attenuation as an isolator with regard to the signal-to-noise ratio and noise figure.

In the proposed method, two mixers connected in series are used in the mixing device, each of which is fed with a LO signal at half the transmission frequency f _LO = ½ f _TX . If a conversion gain is hardly achieved in the first mixer stage or the first mixer on the input side due to the high frequencies, this can be compensated in the second mixer stage, ie the mixer on the output side, which is only operated at half the frequency, so that compared to a simple , harmonic mixer structure usually no disadvantages are to be expected.

In an advantageous embodiment of the proposed method, the antennas for receiving and possibly transmitting the radar signals are designed so that they exhibit a band-limited radiation behavior in which radiation at the frequencies of the LO signals used is avoided. Due to the use of LO signals at half the transmission frequency, the antenna is then designed in such a way that it does not allow any radiation _{at half the design frequency f ANT / 2} = f _{TX / 2.}

The proposed MIMO radar system has several transmission and reception channels, each of which has at least one antenna for receiving and emitting radar signals, a signal generator with an oscillator, in particular a VCO, for generating radar transmission signals with the desired transmission frequency, which may be can also have one or more frequency doublers, as well as a suitable division structure of frequency and / or power dividers for generating the LO signals from the signals inside or outside the signal generator. In an alternative embodiment, the dividing structure generates signals from the signals inside or outside the signal generator, from which the LO signals are then generated by separate frequency multiplication, for example again by means of one or more frequency doublers. In a known manner, the radar system also has a signal processing unit for signal processing of the received signals mixed down into the baseband. The receiving channels show accordingly the method a mixing device consisting of two mixers connected in series, which are fed with LO signals of half the transmission frequency. In a known manner, the radar system can also include amplifiers, bandpass filters and analog-digital converters in front of the signal processing unit.

In the proposed method, the MIMO radar system can be designed both as a monostatic and as a bistatic radar system. When configured as a monostatic radar system, a directional coupler is arranged on the signal path between signal generator and antenna or between antenna and mixer, which ensures correct signal routing from the signal generator to the antenna for the transmitted signals and from the antenna to the mixer for the received signals.

The subharmonic, multi-stage receiving mixer structure used in the proposed method and the associated radar system effectively prevents the parasitic radiation in the useful frequency band, caused by a limited LO-RF isolation of the mixing device. This avoids disturbances that make signal processing in MIMO mode difficult or impossible. The method and the associated radar system can be used particularly advantageously in the high-frequency range, i.e. with high-frequency MIMO radar systems with transmission frequencies> 200 GHz, in particular in connection with MMICs based on SiGe or III-V semiconductor technologies or systems in waveguide technology.

Figure list

• 1 eine schematische Darstellung des Aufbaus eines beispielhaften monostatischen Sende- und Empfangskanals eines MIMO-Radarsystems der Standes der Technik; und
• 2 eine schematische Darstellung eines Ausführungsbeispiels der Empfangsmischerstruktur in einem Sende- und Empfangskanal gemäß der vorliegenden Erfindung.

The proposed method and the associated radar system are explained again below using an exemplary embodiment in conjunction with the drawings. Here show:

• 1 a schematic representation of the structure of an exemplary monostatic transmission and reception channel of a MIMO radar system of the prior art; and
• 2 a schematic representation of an embodiment of the receiving mixer structure in a transmitting and receiving channel according to the present invention.

Ways of Carrying Out the Invention

The problem of crosstalk of the LO signal in the usual construction of a transmission and reception channel of a MIMO radar system of the prior art was already discussed in connection with the introduction to the description 1 explained. In the proposed method and the associated radar system, this problem was solved by a receiving mixer structure or mixing device, as exemplified in FIG 2 is shown. In this way, the receiving channels can be designed without LNAs or isolators and operated in the useful frequency range even at high frequencies> 200 GHz without parasitic emissions. 2 shows the schematic structure of an exemplary, monostatic transmission and reception channel of a MIMO radar system for FMCW operation according to the present invention. The transmission signal (TX signal) with the frequency f _TX is as in the 1 via a directional coupler 1 the antenna 2 fed and broadcast through the antenna. That from the antenna 2 received signal is received via the directional coupler 1 fed to the mixing device, which consists of two successive receiving mixers, the first mixer 4th and second mixer 5 , is formed. A signal of half the frequency f _LO = ½ f _{TX of} the transmission signal (TX signal) is used as the LO signal. Owing to the signal generation by frequency doublers, which is often encountered when operating such radar systems at high frequencies, these signals are available in the radar system anyway in many systems with half the transmission frequency. This LO signal is generated by a power divider 5 , in the present example a differential Wilkinson divider, divided into two LO signals, each of which has a mixer 4th , 5 to drive. The received signal is thus initially passed through the first mixer _{at twice the frequency f TX} 4th _mixed down to an intermediate frequency f IF1 and then through the second mixer 5 , which in this case is operated harmoniously _{to mix} down _{this intermediate frequency} signal f IF1 into the baseband f IF2. On the part of the received signal, the same effect is achieved as in the conventional construction of the mixing device according to FIG 1 . The main difference, however, is that the crosstalk from the LO input of the mixer to the RF input of the first mixer 4th takes place at half the useful frequency f _Lo = ½ f _TX and thus no radiation takes place in the useful frequency range around f _TX , that is, it has no influence on MIMO operation.

The signal path of this parasitic LO signal is in 2 again indicated by the dashed arrows.

List of reference symbols

1

Directional coupler

2

antenna

3rd

Receiving mixer

4th

first mixer

5

second mixer

6th

Power divider

Claims (6)

Method for operating a MIMO radar system with several transmission and reception channels, in which each reception channel has a mixer with which a radar reception signal can be mixed down to a baseband, with several in the mixer between a signal input for the radar reception signal and a signal output series-connected mixers (4, 5) are used and each of the mixers (4, 5) is supplied with a LO signal at a frequency that is lower than a transmission frequency of the radar system, and the frequencies of the supplied LO signals in total the Resulting transmission frequency, characterized in that two mixers (4, 5) connected in series are used in the mixing device and a LO signal with half the transmission frequency is fed to each of the mixers (4, 5). Procedure according to Claim 1 , characterized in that the LO signals for the two mixers (4, 5) are obtained from a common LO signal by power division. Procedure according to Claim 1 or 2 , characterized in that antennas (2) used for receiving the radar received signals are designed for a transmission and reception bandwidth within which the transmission frequency lies, but not the frequency of the LO signals. MIMO radar system with - a signal generator for generating radar transmission signals at a transmission frequency, - Several transmission and reception channels, each having at least one antenna (2) for the emission and / or reception of radar signals, each reception channel having a mixer with which a radar reception signal can be mixed down to a baseband, the mixer between a Signal input for the radar received signal and a signal output has two mixers (4, 5) connected in series and each of the mixers (4, 5) is supplied with a LO signal at half the transmission frequency, - One or more frequency and / or power dividers for branching off the LO signals or signals from which the LO signals are generated by frequency multiplication, from the signal generator, and - A signal processing device for processing the downmixed radar received signals. MIMO radar system Claim 4 , characterized in that the antennas (2) are designed for a transmission and reception bandwidth within which the transmission frequency, but not the frequency of the LO signals, lies. MIMO radar system Claim 4 or 5 , characterized in that the signal generator has one or more frequency doublers for generating the radar transmission signals at the transmission frequency, the LO signals or the signals from which the LO signals are generated by frequency multiplication by a power divider in front of the or one of the Frequency doublers are branched off.

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