Method for LFM radar accuracy improvement without increasing FFT length

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First Claim
1. A method of determining a frequency of an echo signal obtained by a radar system, comprising:
 receiving the echo signal at a receiver in response to a source signal of the radar system;
multiplying the echo signal by a harmonic component signal to obtain a combined signal; and
performing, on a processor, a Fast Fourier Transform (FFT) on the combined signal to obtain a peak in a frequency space, wherein a central frequency of a frequency bin in the frequency space is shifted with respect to the frequency of the echo signal by the harmonic component signal; and
determining the frequency of the echo signal from the shifted central frequency.
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Abstract
A method and apparatus for determining a frequency of an echo signal obtained by a radar system is disclosed. The echo signal in response to a source signal of the radar system is received at a receiver. A harmonic oscillator generates a harmonic component signal, and a multiplier multiplies the echo signal with the harmonic component signal to obtain a combined signal. A Fast Fourier Transform (FFT) is performed on the combined signal to obtain a peak in a frequency space, wherein a central frequency of a frequency bin in the frequency space is shifted with respect to the frequency of the echo signal by the harmonic component signal. The frequency of the echo signal is determined from the shifted central frequency.
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RADAR DETECTION AND LOCATION OF RADIO FREQUENCY (RF) DEVICES  
Patent #
US 20120256783A1
Filed 06/29/2007

Current Assignee
The Boeing Co.

Sponsoring Entity
The Boeing Co.

16 Claims
 1. A method of determining a frequency of an echo signal obtained by a radar system, comprising:
receiving the echo signal at a receiver in response to a source signal of the radar system; multiplying the echo signal by a harmonic component signal to obtain a combined signal; and performing, on a processor, a Fast Fourier Transform (FFT) on the combined signal to obtain a peak in a frequency space, wherein a central frequency of a frequency bin in the frequency space is shifted with respect to the frequency of the echo signal by the harmonic component signal; and determining the frequency of the echo signal from the shifted central frequency.  View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
 9. An apparatus for determining a frequency of an echo signal obtained using radar, comprising:
a receiver configured to receive the echo signal in response to reflection of a source signal generated by the radar from a target; a harmonic oscillator configured to generate a harmonic component signal; a multiplier for multiplying the echo signal with the harmonic component signal to obtain a combined signal; and a processor configured to; perform a Fast Fourier Transform (FFT) on the combined signal to obtain a peak in a frequency space, wherein a central frequency of a frequency bin in the frequency space is shifted with respect to the frequency of the echo signal, and determine the frequency of the echo signal from the shifted central peak.  View Dependent Claims (10, 11, 12, 13, 14, 15, 16)
1 Specification
The subject invention relates to collision avoidance systems for vehicles and, in particular, to a method of increasing an accuracy of frequency measurements in radar systems used in collision avoidance systems.
Recent automobiles and vehicles have been built with onboard safety systems which include radar technologies for detecting a location of an object or target with respect to the vehicle so that a driver or collisionavoidance device can react accordingly. A radar system includes a transmitter for sending out a source signal and a receiver for receiving an echo or reflection of the source signal from the target. The received signal is sampled at a selected sampling frequency and the sampled data points of the received signal are entered into a Fast Fourier Transform (FFT) in order to determine a frequency of the returning signal. A range or relative velocity of the target with respect to the vehicle can be determined from this frequency.
The frequency resolution in such radar systems is limited due to the discrete nature of the FFT. Such frequency resolution is a function of a sampling frequency and a number of samples of the received signal. One way to increase accuracy is to take more samples. This however increases the length of the FFT, which increase the number of computations need to perform the FFT, often requiring a prohibitively large number of computations. Additionally, the FFT often produces frequency sidelobes which are considered aberrations. Increasing the length of the FFT without increasing the sample frequency creates higher sidelobes, leading to higher noise levels. Thus, any improvement in accuracy that results from increasing FFT length comes with a corresponding reduction in signal quality. Accordingly, it is desirable to provide a method for improving radar accuracy without increasing a length of FFTs.
In an exemplary embodiment of the invention, a method of determining a frequency of an echo signal obtained by a radar system includes: receiving the echo signal at a receiver in response to a source signal of the radar system; multiplying the echo signal by a harmonic component signal to obtain a combined signal; performing, on a processor, a Fast Fourier Transform (FFT) on the combined signal to obtain a peak in a frequency space, wherein a central frequency of a frequency bin in the frequency space is shifted with respect to the frequency of the echo signal by the harmonic component signal; and determining the frequency of the echo signal from the shifted central frequency.
In another exemplary embodiment of the invention, an apparatus for determining a frequency of an echo signal obtained using radar includes: a receiver configured to receive the echo signal in response to reflection of a source signal generated by the radar from a target; a harmonic oscillator configured to generate a harmonic component signal; a multiplier for multiplying the echo signal with the harmonic component signal to obtain a combined signal; and a processor configured to: perform a Fast Fourier Transform (FFT) on the combined signal to obtain a peak in a frequency space, wherein a central frequency of a frequency bin in the frequency space is shifted with respect to the frequency of the echo signal, and determine the frequency of the echo signal from the shifted central peak.
The above features and advantages and other features and advantages of the invention are readily apparent from the following detailed description of the invention when taken in connection with the accompanying drawings.
Other features, advantages and details appear, by way of example only, in the following detailed description of embodiments, the detailed description referring to the drawings in which:
The following description is merely exemplary in nature and is not intended to limit the present disclosure, its application or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.
In accordance with an exemplary embodiment of the invention,
While the radar system 102 is discussed herein as being onboard vehicle 100, the radar system 102 may also be part of an immobile or stationary object in alternate embodiments. Similarly, the target 104 can be a vehicle or moving object or can be an immobile or stationary object.
The FFT performed on the digital signal 215 produces a discrete frequency space which includes multiple frequency bins for representing a frequency component of the digital signal 215. The length of the FFT is related to the number of samples. Therefore, N samples of the echo signal at the ADC 212 indicates an FFT of length N. The resultant frequency space has N frequency bins, each characterized by a central frequency. A bin resolution ΔF of the frequency space provided by the FFT is therefore given by ΔF=f_{s}/N, where f_{s }is the sampling rate.
The ability of the collision avoidance system 112 or of the driver to avoid the target 104 is related to an accuracy with which one knows the location and/or velocity of the target 104 relative the radar system 102 or vehicle 100. This distance and/or velocity resolution is related to frequency resolution of the echo signal 122. The frequency resolution has previously been limited by sampling rate and sample size or FFT length of the radar system. The method described herein improves the frequency resolution without changing the length of the FFT.
The second frequency space 304 shows, via squares, how sampled signal frequencies are more likely to be located with respect to the frequency bins {ΔF(n−2), ΔF(n−1), ΔF(n), ΔF(n+1), ΔF(n+2)}. Rather than producing peaks at a central frequency of the frequency bins, the frequency peaks of the actual echo signal are located at squares. The difference between the location of the frequency peak of the actual echo signal and the central frequency of the frequency bins is represented by frequency Ω. Due to the nature of the discrete frequency space, the FFT returns a frequency ΔF(n) for the echo signal which is different than the actual frequency G(n) of the echo signal, where G(n)=ΔF(n)+Ω.
A harmonic signal generator 402 provides a harmonic signal component H to the DSP 214. The harmonic signal component H is multiplied with the signal s(nΔt) at multiplier 404 to obtain combined signal Z and the combined signal Z is provided to FFT 406. The harmonic signal component is a complex harmonic signal that has a selected frequency that is related resolution of the FFT 406. In an embodiment, the harmonic signal component H is a sinusoidal signal H=exp(j2πΩt), where Ω is a frequency component given by Ω=ΔF/k, wherein ΔF is the frequency resolution of the FFT and k is an arbitrary resolution factor. Multiplying the signal s(nΔt) by the harmonic signal component H and performing the FFT allows one to adaptively shift the resulting frequency bins to coincide with the peaks of the digital signal s(nΔt) within a selected resolution criterion. A frequency tuner 408 can adaptively adjust K at the harmonic signal generator 402 based on the results of the FFT 406 in order to provide a frequency Ω which aligns a central frequency of a frequency bin with the actual frequency of the echo signal to within the selected criterion. The frequency tuner 408 can select a frequency k (and thereby Ω) in order to shift the central frequency toward the actual frequency. The resolution factor k can be selected to improve the resolution of the echo signal by factors of 2, 4, etc., or to within any other resolution threshold. By doing so, the frequency resolution is increased without increasing the length of the FFT (i.e., without increasing a number of samples). The results of multiplying s(nΔt) by the harmonic component H are demonstrated herein with respect to
Frequency space 504 shows the results of multiplying the echo signal s(nΔt) by the harmonic signal component H=exp(j2πΩt), in which Ω=ΔF/k and k=2 to obtain combined signal Z and performing the FFT on the combined signal Z. As a result, the central frequencies of the bins ΔF(n) and ΔF(n+1) have been shifted by an amount Ω to G(n)=ΔF(n)+Ω and G(n+1)=ΔF(n+1)+Ω. The central frequencies of the new frequency bins G(n) and G(n+1) better coincide with the actual echo frequency 510 of echo signal s(nΔt) and thus provide a more accurate reading of the echo signal.
It is to be noted that the method disclosed herein provides an improvement in frequency resolution and thus an improvement in distance and/or velocity measurements. This improvement is obtained without increasing the length of the FFT and thus without an increase in computational complexity or computation time. By increasing accuracy while not increasing computation time, the distance or velocity of the target 104 can be provided to the driver or collision avoidance system in less time than when using methods which require increasing the length of the FFTs. The driver or collision avoidance system then has more time to react to avoid the target, thus increasing a safety of the driver and vehicle.
While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the application.