Communications: directive radio wave systems and devices (e.g. – Return signal controls external device – Radar mounted on and controls land vehicle
Reexamination Certificate
2002-09-12
2003-11-11
Tarcza, Thomas H. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Return signal controls external device
Radar mounted on and controls land vehicle
C342S071000, C342S072000, C342S107000, C342S109000, C342S113000, C342S114000, C342S115000, C342S196000
Reexamination Certificate
active
06646589
ABSTRACT:
BACKGROUND OF THE INVENTION
1 Technical Field of the Invention
The present invention relates generally to a radar such as an FMCW (Frequency Modulated Continuous Wave) radar which is designed to transmit a frequency-modulated radar wave and receive a return thereof from an object through a plurality of antennas to determine the distance to, relative speed, and azimuth or angular direction of the object.
2 Background Art
Recently, radars are tried to be used in an anti-collision device of automotive vehicles. As such as radars, FMCW radars designed to measure both the distance to and relative speed of a target are proposed for ease of miniaturization and reduction in manufacturing cost thereof.
Typical FMCW radars transmit a radar signal Ss, as indicated by a solid line in FIG.
9
(
a
), which is frequency-modulated with a triangular wave to have a frequency increasing and decreasing, i.e., sweeping upward and downward cyclically in a linear fashion and receive a radar return of the transmitted radar signal Ss from a target. The received signal Sr, as indicated by a broken line, usually undergoes a delay of time Tr the radar signal Ss takes to travel from the radar to the target and back, that is, a time lag depending upon the distance to the target and is doppler-shifted in frequency by Fd as a function of the relative speed of the target.
The received signal Sr and the transmitted signal Ss are mixed together by a mixer to produce a beat signal B, as shown in FIG.
9
(
b
), whose frequency is equal to a difference in frequency between the received signal Sr and the transmitted signal Ss. If the frequency of the beat signal B when the frequency of the transmitted signal Ss is increasing or sweeping upward, which will be referred to below as a beat frequency in a modulated frequency-rising range, is defined as fb
1
, the frequency of the beat signal B when the frequency of the transmitted signal Ss is sweeping downward, which will be referred to below as a beat frequency in a modulated frequency-falling range, is defined as fb
2
, then the frequency fr due to the time delay Tr and the doppler-shifted frequency fd may be expressed as:
fr
=
fb1
+
fb2
2
(
1
)
fd
=
fb1
-
fb2
2
(
2
)
Using the frequencies fr and fd, the distance R to and relative speed V of the target may be expressed as:
R
=
c
·
fr
4
·
fm
·
Δ
F
(
3
)
V
=
c
·
fd
2
·
F
o
(
4
)
where c is the propagation speed of a radio wave, fm is a modulation frequency of the transmitted signal Ss, &Dgr;F is a variation in frequency (i.e., amplitude) of the transmitted signal Ss, and Fo is a central frequency of the transmitted signal Ss.
The determination of the beat frequencies fb
1
and fb
2
is made usually using a signal processor. Specifically, the beat signal B is sampled in sequence and subjected to fast Fourier transform (FFT) in each of the modulated frequency-rising and -falling ranges to find a frequency spectrum of the beat signal B. Frequency components showing peaks in signal strength within the modulated frequency-rising and -falling ranges are determined as the beat frequencies fb
1
and fb
2
, respectively.
The sampling frequency fs of the beat signal B, as is well known in the art, needs to be twice an upper frequency limit of the beat signal B. Specifically, the frequency variation A F and a modulation cycle 1/fm of a radar wave are so set that frequency components of the beat signal B due to returns of the radar wave from targets present within a preset target detecting range may fall within a band preset below the upper frequency limit of the beat signal B.
Usually, returns of the radar wave from stationary objects such as footbridges or buildings near a road much bigger in size than ordinary automotive vehicles are strong in level even if they are out of the target detecting range (such objects will be referred to as long range targets below). Therefore, when the radar receives a radar return from the long range target, it will cause the beat signal B to contain, as shown in FIG.
10
(
a
), a frequency component exceeding the upper frequency limit. FIG.
10
(
a
) illustrates a frequency spectrum of the beat signal B. In this case, when the beat signal B is sampled and subjected to the FFT, it will cause the frequency component due to the long range target exceeding the upper frequency limit of the beat signal B to be shifted, as indicated by a broken line, to a location that is symmetric with respect to half the sampling frequency fs, so that it appears as a frequency peak within the preset band. This causes the radar to identify the long range target as lying within the target detecting range in error.
Even in the absence of the long range targets, the FFT of samples of the beat signal B may cause any noise components, as shown in FIG.
10
(
b
), to move from outside the upper frequency limit of the beat signal B to inside the preset band, thereby resulting in rise of a noise floor within the preset band, which leads to a drop in SN ratio, thus resulting in lowering of the radar performance. In order to avoid this problem, an anti-aliasing filter may be coupled to an output of the mixer to remove, as shown in FIG.
10
(
c
), noise components lying outside the preset band, especially frequency components over half the sampling frequency fs from the beat signal B produced by the mixer.
Electronically-scanned radar systems are also known as being designed for spreading the target detecting range or improving the accuracy of determining the angular direction of a target. Such a type of radar system works to receive a return of a radar wave from a target through a plurality of antennas and determine the angular direction of the target based on differences in phase and level of the signals received by the antennas. For instance, U.S. Pat. No. 6,292,129 to Matsugatani et al. (corresponding to Japanese Patent First Publication No. 2000-284047), assigned to the same assignee as that of this application, teaches the electronically-scanned radar system which uses a single mixer for decreasing manufacturing costs. The mixer is designed to process signals received by a plurality of antennas in time division to produce a beat signal. In the following discussion, combinations of transmit antennas and receive antennas will be referred to as channels, respectively.
The use of an anti-aliasing filter in the above system in which the mixer processes inputs from the antennas in time division gives rise to a difficulty in deriving information about targets accurately. Specifically, if a cycle in which the channels are switched from one to another is defined as 1/fx, a time division-multiplexed signal inputted to the mixer contains a harmonic that is an integral multiple of a frequency fx. An output of the mixer, that is, the beat signal B, thus, contains a frequency component arising from that harmonic, resulting in an increase in frequency band of the beat signal B. This causes the anti-aliasing filter to eliminate information as well which is required to demultiplex the time division-multiplexed signal into discrete components for the respective channels. This results in overlapping of the discrete components, thus leading to a difficulty in sampling the levels of the signals received by the antennas accurately.
Radar systems using a plurality of transmit antennas work to multiplex a radar wave in time division. Therefore, even if mixers are provided one for each receive antenna, they receive returns of time division-multiplexed components of the radar wave, respectively, thus giving rise to the same problem as described above.
The determination of an angular direction of a target as functions of differences in phase and intensity of return signals received by a plurality of channels requires time consistency or synchronization between the return signals. To this end, all data items required for the FFT in each channel need to be acquired within a sweep time T (=1/2fm) required for each of upward and downward sweeps of the frequ
Alsomiri Isam
Denso Corporation
Posz & Bethards, PLC
Tarcza Thomas H.
LandOfFree
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