EP2425273A1

EP2425273A1 - Method and apparatus for digitally processing ofdm signals for radar applications

Info

Publication number: EP2425273A1
Application number: EP10730345A
Authority: EP
Inventors: Christian Sturm; Werner Wiesbeck; Thomas Zwick
Original assignee: Karlsruher Institut fuer Technologie KIT
Current assignee: Karlsruher Institut fuer Technologie KIT
Priority date: 2009-04-27
Filing date: 2010-04-22
Publication date: 2012-03-07
Also published as: DE102009019905A1; WO2010124673A1; US20120076190A1

Abstract

The present invention relates to a method and to an apparatus for digitally processing OFDM signals, which are emitted by a transmitter using modulation symbols as information carriers, at least partially reflected by one or more objects, and received by a receiver. Without prior channel equalization, the modulation symbols are extracted from the OFDM signals that are received, and the extracted modulation symbols are standardized for the particular emitted modulation symbol by way of complex division. The radar evaluation for determining the distance and/or for determining the speed of the objects is then carried out on the basis of the standardized modulation symbols. The method and the apparatus can be used to determine both the distance and the speed of the objects independently from each other. In addition, the method operates very reliably because it is not influenced by the transmitted information.

Description

Method and device for digital processing of QFDM signals for radar applications

Technical application

The present invention relates to a method and a device for the digital processing of OFDM signals, which are transmitted by a transmitting device with modulation symbols as information carrier, at least partially reflected at one or more objects and received by a receiving device, wherein the modulation symbols from the received OFDM Signals are extracted.

For radar sensors, in addition to military applications, there are also numerous civil applications, for example in the field of intelligent driver assistance systems

(Radar cruise control, collision prevention) or in the monitoring of production processes. Radar technology offers the advantage over other sensor technologies of being able to determine both distances and speeds quickly and accurately, and radar sensors are resistant to external influences such as steam, rain or fog.

State of the art

In the field of radar technology, there are different classical methods and hardware concepts in which the signal processing takes place largely analogously in electronic circuits. Examples include chirp radar, pulse radar or FMCW (Frequency Modulated Continuous Wave) radar. These methods are largely exhausted and optimized. The now available performance in the field of digital signal processing opens up completely new possibilities both with regard to the formation of novel transmission signals and with regard to the application of complex processing algorithms in the receiver. The performance of future radar sensors will therefore be determined primarily by the signal forms used and digital processing techniques.

The present invention is concerned with the digital processing and use of OFDM signals (Orthogonal Frequency Division Multiplexing) for an application in radar sensor technology. The generation of OFDM signals can only be done on a digital level. So far, the OFDM technique has been used primarily for the transmission of information or data, since the OFDM signal is composed of modulation symbols and thus specifically serves the purpose of transmitting information. A use of the OFDM technology for radar applications has already been discussed. For example, A. Garmatyuk et al. , Proc. 37 ^th European Microwave Conference, pages 1473 to 1476, October 2007, an OFDM radar system that can be used simultaneously for information transmission In this implementation of OFDM radar, cross-correlation of the received signal with the transmitted signal is performed for the processing of the OFDM radar signals in the receiver transmitted and y (t) the received baseband signal, then this process can be mathematically described with the following equation:

By this processing, however, the achievable dynamics are dependent on the autocorrelation properties of the transmission signal. In order to achieve high dynamics, special codes such as M-sequences must be sent. Their properties then determine the dynamics. However, if the system is to be used for simultaneous information transmission, then the achievable dynamic is unpredictable, since it depends on the autocorrelation properties of the transmitted information. Reliable operation is therefore not possible. Furthermore, with the processing based on the cross correlation, although the distance of objects, but not their speed can be determined.

The object of the present invention is to specify a method and a device for processing OFDM signals, which, with the simultaneous use of these signals for radar and for information transmission, enables reliable operation and, if required, also the determination of the speed of objects.

Presentation of the invention

The object is achieved with the method and the device according to claims 1 and 8. Advantageous embodiments of the method and the device are the subject of the dependent claims or can be found in the following description and the exemplary embodiment.

In the proposed method of digitally processing OFDM signals, the transmitted modulation symbols, which may also be referred to as data symbols, are first extracted from the received OFDM signals without prior channel equalization. These extracted modulation symbols or at least some of these modulation symbols are then normalized by a complex division to the respectively originally transmitted modulation symbol. Radar evaluation for distance determination and / or

Velocity determination of the objects on which the OFDM signals were reflected then takes place on the basis of the normalized modulation symbols. The radar evaluation for distance determination and / or velocity determination of the objects comprises in particular the creation of a radar image from which the distance of the objects or the distance and speed of the objects can be derived.

With the proposed method and the device mentioned below for performing the method, the radar processing is completely independent of the transmitted information or the transmitted data. This is achieved by normalizing the modulation symbols extracted from the signal to the originally transmitted modulation symbols. No special codes are needed so that the OFDM radar signal modulates with arbitrary payload can be transmitted via the common OFDM signal. Due to the independence of the transmitted information, a very high dynamic can be achieved, which is limited only by the secondary maxima of the required Fourier transform and noise. A particular advantage of the proposed method and the associated device is that the distance or distance of the objects can be determined independently of their speed. Distance and speed are not linked here. During speed determination, the integration time and thus the Doppler resolution or speed resolution can be adapted as required during operation. The computational effort required for the method is comparatively low, since no cross-correlations have to be calculated, as has hitherto been the case in the prior art.

The processing of the radar signals is carried out in the proposed method not with the aid of the baseband signals, but with the aid of the transmitted and received modulation symbols. These are picked up in the receiver before the optional equalization because they still contain the complete distortions that occur during the transmission, which ultimately contain the information about reflective objects. Each received and selected modulation symbol is normalized by means of a complex division by the transmitted modulation symbol in amplitude and phase. This normalization makes the method completely independent of the transmitted modulation symbols. The calculation of the radar image for distance determination is then carried out via an inverse Fourier transformation.

The evaluation of the Doppler information for determining the relative velocity of reflecting objects preferably takes place with the aid of a Fourier transformation via time-sequential OFDM symbols. The duration of the OFDM symbols is suitably parameterized for this purpose. This processing is also based on the modulation symbols and not on the baseband signals.

For the discrete Fourier transforms or inverse Fourier transformations carried out to determine the distance and / or velocity, in the present method, in each case, all transmitted modulation symbols of an OFDM symbol or a sequence of OFDM symbols or only a few of these modulation symbols can be used. In the evaluation itself, both distance and speed of the objects at which the OFDM signals were reflected are preferably determined. Of course, however, the method and the associated device can also be operated so that only the distance or only the speed is determined from the received OFDM signals.

The proposed device for carrying out the method comprises, in a known manner, a receiving antenna with which OFDM signals can be received, a mixing device for mixing down the received signals, an analog-to-digital converter for the digitization of the signals and a processing unit. processing device which extracts the modulation symbols from these signals, normalizes them to the transmitted modulation symbols and carries out a radar evaluation on the basis of these normalized modulation symbols for determining the distance and / or speed of the objects at which the signals were reflected. The device can be designed like a conventional receiver for OFDM signals, wherein only the processing unit is designed for the implementation of the proposed method, ie, extracts the modulation symbols without prior channel equalization and performs the radar evaluation for distance determination and / or velocity determination on the basis of these modulation symbols, in particular the corresponding radar or Doppler images calculated.

BRIEF DESCRIPTION OF THE DRAWINGS The proposed method and the associated apparatus will be explained in more detail below on the basis of an exemplary embodiment in conjunction with the drawings. Hereby show:

Fig. 1 is a schematic representation of the

Structure of an OFDM transmission signal; Fig. 2 is a comparison of a radar image of classical processing with a radar image obtained according to the proposed method;

3 is a schematic representation for determining the speed; 4 shows a schematic illustration for determining the distance;

5 shows an example of a processed radar image; Fig. 6 is a schematic representation of a

OFDM transmitter; and

Fig. 7 is a schematic representation of an OFDM receiver.

Ways to carry out the invention

The entire processing in the implementation of the proposed method will be described in detail below with reference to an exemplary embodiment. In this case, example results are shown, which were calculated using a computer simulation.

Em OFDM signal is described in the time domain as follows:

* "= Σ Σ i <j M + n) ψ _n (t-μT) (2)

/ 1 = 0 n = 0

In this case, I describes the modulation symbols to be transmitted, which already consist of the binary information to be transmitted by means of discrete

Phase modulation (eg PSK: phase shift keying) were generated. The index n indicates the total number of N OFDM subtransagers, μ indicates the time-sequential OFDM symbols and ψ _n represents the orthogonal OFDM subtransagers with:

where for the distance of the subtrager to maintain the orthogonality .DELTA.f = l / T must apply. Where T denotes the OFDM symbol duration.

The structure of the OFDM transmission signal from individual modulation symbols can also be illustrated by means of a matrix, as shown in FIG. Each cell of the matrix contains a modulation symbol, each column of the matrix represents an OFDM symbol.

For a simpler discussion of the procedure for the normalization of the modulation symbols and the determination of the distance or of a radar image on the basis of the modulation symbols, only the first OFDM symbol with μ = 0 is considered. The time signal of this OFDM symbol can be printed as:

N- x (0 = Σ I (n) tπp (j2πf _n t), O≤t≤T (4)

In the receiver, the OFDM signal is decoded, the individual received modulation symbols I _r are directly after the discrete Fourier transform in

OFDM receiver and used for processing before the channel equalization. The standardization is done by a complex division:

(5) The radar image in the distance direction is obtained by a mverse discrete Fourier transform of the normalized modulation symbols

h (k) = IDFT ({I _d M}) ^ ~ ^ I _ώv (n) cxp ^ jψnky k = 0,, N- (6)

Where k is the discrete time variable. The uniqueness is limited to the distance d _max = Tco / 2, the dynamics only by the secondary maxima of the Fourier transform.

With the aid of a simulation model, the functionality of the modulation-symbol-based processing was verified and its performance compared with the classical approach of cross-correlation according to equation (1). The images shown in FIG. 2 show a radar simulation for a point target at a distance of 30 m. The left picture shows the result of classical processing, the right picture shows the result of the modulation symbol based processing according to equations (5) and (6). It can be clearly seen from the figure that in conventional processing substantially higher secondary maxima occur than in the processing according to the proposed method.

These sub-maxima result from the autocorrelation properties of the random data with which the signal was modulated and can not be reduced by suitable techniques such as windowing. The dynamic range is therefore very limited, which makes object detection m scenarios with many objects virtually impossible. In the modulation symbol based on the proposed method according to the proposed method, the secondary maxima can be brought to a constantly very low level by means of a fenestration. For the result in the right image of Figure 2, a Hamming window was used. The

Secondary maxima are here exclusively caused by the Fourier transformation. Secondary maxima due to poor autocorrelation properties can not occur due to the principle of modulation-symbol-based processing.

The realization of the Doppler processing or the determination of the speed is likewise carried out on the basis of the modulation symbols standardized according to equation (5). In this case, however, temporally successive modulation symbols are considered. The evaluation takes place in each case via a defined frame of M OFDM symbols. For any OFDM subtransager n, the Doppler spectrum is calculated using a discrete Fourier transform

where v is the discrete frequency variable.

Preferably, the two above method variants for determining the distance and for determining the speed are combined into a two-dimensional method which enables independent processing of distance and speed. The entire processing takes place at this Processing method based on the received signal xn three steps:

1st step: normalization by complex division The received modulation symbols are normalized before the channel equalization by means of the transmitted modulation symbols according to equation (5) by a complex division.

2nd step: Fourier transformation in time direction

To determine the velocity, a discrete Fourier transformation in time direction is calculated in the normalized modulation symbol matrix for each OFDM subtrager within a frame of length M. The result is a matrix in which the

Time axis is replaced by a Doppler axis, which represents the Doppler shift. This is illustrated in FIG.

3rd step: inverse Fourier transformation m frequency direction

To determine the distance, an inverse discrete Fourier transform in the frequency direction is calculated in the result matrix of step 2 for each OFDM symbol within a frame of the length M. The result is a matrix containing a two-dimensional radar image with the dimensions of distance and doubling displacement. The third process is illustrated in FIG. 4.

Steps 2 and 3 can be reversed in order. Alternatively, both steps can be summarized and represented by a two-dimensional discrete Fourier transform or a two-dimensional discrete inverse Fourier transform with subsequent mirroring of the matrix to be replaced.

FIG. 5 shows an example result from FIG

Simulation for the processing of three point targets of the same reflectivity with an OFDM signal of N = 1024 sub-carriers with a spacing of Δf = 90.909 kHz over a frame duration of M = 128. The simulated objects have the following distances and speeds:

In the processed radar image, all three objects are clearly mapped in distance and speed. A coupling between distance and speed does not occur. Objects at the same distance and objects at the same speed are separable, as can be seen in FIG.

Finally, FIG. 6 shows an example of a transmission device for transmitting the OFDM radar signals. The input bits 1, which represent the information to be transmitted, are first converted in a digital modulator 2, in the present example by means of PSK, into complex modulation symbols, ie into modulation symbols having a complex value (I, Q). With the aid of a serial-to-parallel converter 3, a serial-parallel conversion of the data stream is performed. In this case, the serial data stream is divided into m N parallel parallel data sequences which are assigned to N sub-carriers. In a Fourier transformation unit 4, a digital time-discrete OFDM signal is formed with the aid of an inverse discrete Fast Founer Transform (IFFT) for the respective OFDM symbol-forming, currently parallel applied modulation symbols, which is subsequently converted into a parallel-serial converter 5 is serialized. After adding a cyclic prefix (cyclic prefix) of the thus-obtained signal (unit 6) for preventing intersymbol interference, the digital signal stream is converted into a digital-to-analog converter 7 in an analog transmission signal a low-pass filter 8 and a mixing unit 9 is emitted on a high-frequency carrier as a radar signal via the transmitting antenna 10. Such a transmitting device is known to those skilled in the field of OFDM radar technology or OFDM information transmission technology.

An example of a receiving device, which may also be arranged in the same device as the transmitting device, is shown schematically in FIG. In this receiving device, the radar signal reflected by the objects is received via the receiving antenna 11, mixed down again in a mixing unit 12 ms baseband and converted after passing through a low-pass filter 13 in an analog-to-digital converter 14 into a digital signal. In the case of an application for OFDM radar is typically the same high-frequency oscillator for controlling the mixing unit 9 in the transmitter and the Mixing unit 12 used in the receiver. Subsequently, in the receiver, the cyclic prefix (unit 15) is removed and the digital signal is parallelized in a serial-to-parallel converter 16 according to the number of subtransagers to subject the individual channels to a discrete Fast Fourier Transform (FFT unit 17) and to convert again into a serial signal in a parallel-to-serial converter 18. In a conventional receiving device, the serial signal is fed to a channel equalization unit 20 in which the samples of the individual channels are equalized. After equalization, an extraction of the modulation symbols 21 and a demodulator 22, a conversion of the modulation symbols in the transmitted data bits, which are then available as output bits 23 in an extraction unit 21.

The receiving device of the device proposed here comprises, alternatively or in addition to the channel equalization unit 20, the extraction unit 21 and the demodulator 22, a processing unit 19 which extracts the modulation symbols without prior channel equalization, normalizes the transmitted modulation symbols and calculates the speeds and / or distances of the objects or the corresponding radar images on the basis of the normalized modulation symbols durchfuhrt.

For the realization of such a receiving device, as hardware components in addition to the I / Q mixing unit 12, an analog-to-digital converter 14 and FFT or IFFT processors are required, which are accessed by the processing unit 19. The method can be carried out, for example, in the ISM band at 24 GHz, since a signal bandwidth of approximately 100 MHz can be used license-free worldwide. Basically, when choosing the carrier frequency, the wavelength should be smaller than the reflective structures. At the same time, the carrier frequency must not be too high, otherwise the propagation steam is too high and thus only an application over small distances is possible.

Reference sign list

I input bits 2 modulator

3 serial-parallel converters

4 Fourier transform unit

5 parallel-to-serial converter

6 Unit for adding a prefix 7 Digital-to-analog converter

8 low-pass filters

9 mixing unit

10 transmitting antenna

II receiving antenna 12 mixing unit

13 low pass filters

14 analog-to-digital converter

15 unit for removing the prefix

16 Serial-to-Parallel Converters 17 FFT unit

18 parallel-to-serial converter

19 processing unit

20 channel equalization unit

21 Extraction Unit 22 Demodulator

23 output bits

Claims

claims

1. A method for the digital processing of OFDM signals, which are transmitted by a transmitting device with modulation symbols as information carrier, at one or more objects at least partially reflected and received by a receiving device, wherein

the modulation symbols without previous channel equalization are extracted from the received OFDM signals,

- some or all of the extracted modulation symbols are normalized by a complex division to the respective transmitted modulation symbol, and

- A radar evaluation for distance determination and / or speed determination of the objects based on the normalized modulation symbols is carried out.

2. The method of claim 1, wherein the distance determination via a reverse Fourier transform of the normalized modulation symbols of at least one OFDM symbol of the received OFDM signals.

3. The method of claim 1 or 2, wherein the velocity determination by evaluation of a phase shift temporally successive modulation symbols of at least one sub-carrier of the OFDM signals.

4. The method of claim 3, wherein the evaluation of the phase shift by means of a Fourier transform of the normalized modulation symbols.

5. The method of claim 1, wherein the distance determination and the speed determination are carried out simultaneously via a two-dimensional inverse or normal Fourier transformation of the normalized modulation symbols.

6. The method of claim 1, wherein for the distance determination, an inverse Fourier transform of the normalized modulation symbols of a plurality of subtrager temporally successive OFDM symbols is performed and the data obtained therefrom are then subjected to a Fourier transform for the velocity determination.

7. Method according to claim 1, in which a Fourier transformation of the normalized modulation symbols of a plurality of sub-carriers of temporally successive OFDM symbols is carried out for the velocity determination and the data obtained therefrom are subsequently subjected to averse Fourier transformation for the distance determination.

8. An apparatus for the digital processing of OFDM signals, by a transmitting device with modulation symbols as information carrier be sent, and at least partially reflected on one or more objects, with

a receiving antenna (11) for receiving the OFDM signals, a mixing device (12) for mixing down the received signals,

- An analog-to-digital converter (14) for the digitization of the signals and

- A processing device (19) which extracts the modulation symbols without prior channel equalization and based on these modulation symbols radar evaluation for distance determination and / or speed determination durchfuhrt, in particular radar or Doppler images calculated.