Communications: directive radio wave systems and devices (e.g. – With particular circuit – Digital processing
Reexamination Certificate
2000-03-30
2002-01-15
Lobo, Ian J. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
With particular circuit
Digital processing
Reexamination Certificate
active
06339395
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates generally to a radar apparatus designed to transmit a continuous wave for detecting a target object, and more particularly, to a radar apparatus designed to process beat signals to form digital beams using the complex Fast Fourier Transform (FFT) for frequency analysis of the beat signals.
2. Background Art
Automotive radar systems are known which are designed to measure the distance to, the azimuth, and the relative speed of an object present ahead of an automotive vehicle for cruise control and/or anti-collision control. Radar systems of this type usually use as a radar wave a continuous wave (CW) or a frequency-modulated continuous wave (FM-CW). The radar systems receive a return of the radar wave from an object, mix it with a local signal having the same frequency as that of the transmit signal (i.e., the transmitted radar wave) to produce a beat signal whose frequency is equivalent to a difference in frequency between the received signal and the local signal, and analyze the frequency of the beat signal to derive information on the target object.
Specifically, the return of the radar wave undergoes the Doppler shift as a function of the speed of the radar system relative to the target object. The travel of the radar wave to and from the target object causes the received signal to be changed in frequency and phase from the transmit signal. This change appears at the beat signal. The distance to and relative speed of the target object may, thus, be determined by analyzing the beat signal.
The detectable range of the radar system is defined geometrically by a beam emitted from or received by an antenna. It is, thus, advisable that multiple beams be provided in order to increase the detectable range without decreasing the detectable distance to a target or determine the azimuth of the target in the limited detectable range. The provision of the multiple beams is typically achieved using a plurality of antennas oriented in different directions or a known phased array, but in recent years when it has become possible to process digital signals at high speeds, attention is being paid to digital beam forming (DBF) which forms a plurality of beams through digital signal processing.
FIG. 8
shows one example of conventional radar systems adapted to produce beams using the DBF technique.
A transmitting antenna AS installed in a transmitter
104
radiates a radar wave. A return of the radar wave from an object is received by a plurality of receiving antennas AR
1
to ARN simultaneously. A receiver
106
mixes a signal outputted from each receiving antenna ARi (i=1, 2, . . . , N) with a local signal whose frequency is equal to that of the radiated radar wave to produce a beat signal. The beat signals thus produced are inputted to a signal processor
110
through an A/D converter circuit
108
. The signal processor
110
performs a phase/weighting operation on the beat signals in a digital form for producing beams. Specifically, the signal processor
110
realizes functions of analog phase shifters installed in each radiating element in a conventional phased array system and of combining outputs of the analog phase shifters in an analog form.
Usually, the DBF requires expression of the beat signals B
1
to BN produced by signals received by the receiving antennas AR
1
to ARN in baseband complex signal each made of a real signal I and an imaginary signal Q. The receiver
106
and the A/D converter circuit
108
, thus, require not only dual channels each made of a mixer and an A/D converter for each receiving antenna, but also increasing of power of the local signals L supplied to the mixers, which will result in an increase in circuit size.
The reason why the DBF requires the baseband complex signals is because it is difficult to specify the phase of a baseband scalar signal derived only by a received signal in one channel at any time, thus resulting in a difficulty in determining the direction of incoming of a return of the radar wave based on results of operations performed in the DBF.
FIG.
6
(
a
) illustrates signals received by antennas each expressed in a baseband complex signal made up of a real signal I and an imaginary signal Q and corresponding baseband scalar signals each expressed in vector in a first case where a radar return enters a plane of an array of antennas at an angle of a° to a line perpendicular to the plane from a right direction, as viewed in the drawing. FIG.
6
(
b
) illustrates for a second case where a radar return enters the antenna-arrayed plane at an angle of −a° to the line perpendicular to the plane from a left direction, as viewed in the drawing. As clearly shown in the drawings, each baseband complex signal in the first case has a sign reverse to that of a corresponding one of the baseband complex signals in the second case, thereby enabling the radar returns to be discriminated between the first and second cases. It is, however, impossible to use the baseband scalar signals each made up of only the real signal I to discriminate the radar returns between the first and second cases.
In other words, the vector of each baseband scalar signal becomes, as shown in
FIG. 7
, equivalent to that of a corresponding one of the baseband complex signal in a case where radar returns having the same level enter the antenna-arrayed plane from two directions of ±a°. Therefore, if the complex Fourier transform is performed on the baseband scalar signals in the direction of a spatial axis to form the beams, it may cause peaks to appear on resultant frequency components in the both directions of ±a°, which leads to a difficulty in determining whether the baseband scalar signals are produced by the radar return traveling from the direction of +a° or −a° or to an erroneous decision that two signals traveling from the both directions of ±a° have entered the antennas simultaneously.
There has been also proposed a technique for digitizing received signals using high frequency A/D converters before the beat signals undergo a frequency conversion and realizing a two-channel mixer function in a computer through digital signal processing. For example, Japanese Patent First Publication No. 10-63645 teaches such a technique. This, however, requires a large number of expensive A/D converters, thus resulting in an increase in total production cost of the system.
Further, precise measurement of the azimuth or angular direction of a target object within a limited angular range generally requires use of a large number of antennas (i.e., received signals). This also requires many receivers, thus resulting in an increased size of the system.
SUMMARY OF THE INVENTION
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide a radar system having a simple structure which is smaller in load on operations to form digital beams and analyze the frequency of each digital beam.
It is a further object of the invention to provide a radar system which has a compact structure capable of measuring the azimuth of a target with high accuracy.
According to one aspect of the invention, there is provided a radar apparatus which comprises: (a) a transmitter providing a transmit signal having a preselected frequency to produce an output signal to be transmitted as a radar wave to a radar detectable zone; (b) an array of receiving antennas; (c) a plurality of receivers each of which mixes an input signal that is a return of the radar wave from a target object received by one of the receiving antennas with a local signal having the same frequency as that of the transmit signal to produce a single beat signal including a frequency component corresponding to a difference in frequency between the output signal and the input signal; and (d) a signal processing circuit forming beams made of components of the beat signals corresponding to angular directions predetermined in t
Hazumi Hiroshi
Natsume Kazuma
Denso Corporation
Law Office of David G. Posz
Lobo Ian J.
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