There will be below explained a transmission signal generating unit according to an embodiment of the present invention in detail with reference to several figures.
The radar device comprises a transmission signal generating unit 10, a D/A converter 11, a local oscillator 12, a transmitting side mixer 13, a transmission signal amplifier 14, a circulator 15, an antenna 16, a received signal amplifier 17, a receiving side mixer 18, an A/D converter 19, a pulse compressor 20, a frequency analyzer 21, and a target detector 22.
The transmission signal generating unit 10 generates a digital signal (pulse signal) as a transmission signal and transmits it to the D/A converter 11. The D/A converter 11 converts the transmission signal transmitted by the transmission signal generating unit 10 to an analog signal and transmits it to the transmitting side mixer 13. The local oscillator 12 generates a local signal having a local frequency and transmits it to the transmitting side mixer 13 and the receiving side mixer 18. The transmitting side mixer 13 mixes the transmission signal transmitted by the D/A converter 11 and the local signal transmitted by the local oscillator 12 to obtain a radio frequency signal and transmits it to the transmission signal amplifier 14.
The transmission signal amplifier 14 amplifies the radio frequency signal transmitted by the transmitting side mixer 13 to a predetermined signal level and transmits it to the circulator 15. The circulator 15 switches between the first operation that outputs the radio frequency signal transmitted by the transmission signal amplifier 14 to the antenna 16 and the second operation that outputs a received signal received by the antenna 16 to the received signal amplifier 17.
The antenna 16, such as an array antenna, transmits the radio frequency signal, transmitted by the transmission signal amplifier 14 through the circulator 15, toward a target. Also, the antenna 16 receives a reflected wave from the target and then transmits it to the circulator 15 as a received signal.
The received signal amplifier 17 amplifies the received signal, received from the antenna 16 through the circulator 15, with a low noise and transmits it to the receiving side mixer 18. The receiving side mixer 18 converts the received signal received from the received signal amplifier 17 to an intermediate frequency signal (IF signal) by mixing the received signal and the local signal received from the local oscillator 12 and transmits it to the A/D converter 19. The A/D converter 19 converts the IF signal transmitted by the receiving side mixer 18 to a digital signal and transmits it to the pulse compressor 20.
The frequency analyzer 21 performs Fourier transformation on a signal compressed by the pulse compressor 20 to transform data from time-domain to frequency-domain. Then, the received signal is decomposed to detect the relative speed of the target. The target detector 22 extracts Doppler components from the decomposed components, which represent the speed components of the target, to detect the target.
Next, there is explained the detail of the transmission signal generating unit 10 according to the embodiment of the present invention.
The window function calculator 31 generates a window function H that makes all frequencies without a center frequency of an input signal (phase-modulated rectangular pulse) and its adjacent frequencies zero and that makes the SNR of the center frequency maximum, and transmits the generated window function H to the transmission signal generator 32. The detail of the window function calculator 31 will be explained later.
The transmission signal generator 32 generates a transmission signal by modulating the amplitude of the input signal using the window function H transmitted by the window function calculator 31.
However, there is explained a transmission signal generating method, in particular, how to calculate the window function H in the window function calculator 31.
There is shown how to calculate a window function H that theoretically makes a filter with loss minimum under a constraint condition to make a transmission signal spurious free, below called a “spurious free condition”.
Let W be a weight vector corresponding to the sampled data of a transmission pulse,
W=[w1 w2 . . . wN
where the subscript “Nf” denotes all sampling numbers of the transmission pulse in an aperture time.
Further, let y be a spectrum pattern vector expressing the frequency spectrum of these data,
y=[y1 y2 . . . yN
Then, we can describe a relationship between the weight vector W and the spectrum pattern vector y as
where “Q” represents a fast Fourier transform matrix (FFT matrix) and n,k=1,2, . . . ,Nf. It is noted here that the subscript “Nf” defined above also represents the number of FFT points, and the superscript “T” represents transpose.
The inverse matrix (IFFT matrix) of the FFT matrix (4) is calculated as
where “*” denotes complex conjugate.
It is noted that the convolution of the spectrum pattern calculated by (3) and that of an input pulse is a spectrum pattern to be observed.
Now, let us suppose the width of the transmission pulse satisfying a predetermined basic performance, such as range resolution ability, as effective data, and suppose that the effective data is in the central area of the weight vector W as shown in
Then, a weight vector Wm is expressed as
that makes outputs from a spurious-frequency area zero and the SNR of the center frequency maximum. Here “S” is a steering vector showing the center frequency.
Thus, a window function H except for a constant term is represented by
H=u*{(uTu*)−1}uT, (8)
u={circumflex over (Q)}Qm{circumflex over (Q)}Qs (9)
that is the weight of an aperture excluding the steering vector S.
For reference, the calculation processes from (7) to (9) is described in detail below.
When the data outside the effective data are made zero as shown in
y
m=└0 . . . 0 yK−N, . . . yK . . . yK+N
Then, we can describe a relationship between the weight vector Ws and the frequency vector ym as
As described above, since the frequency vector ym shows a spectrum pattern where all side-lobes without the main lobe and its neighborhood are made zero. When the weight vector W=Wm that satisfies
ymT=QWT (14)
is the weighted vector to be intended. Here (14) is obtained by substituting ym into (3). When (12) is set as a constraint condition for (14), there is obtained
ymT=QWmT=QmWsT. (15)
WmT={circumflex over (Q)}QmWsT. (16)
“SNR” is defined as the ratio (unit: dB) of noise to an output signal, and the SNR of the present case is represented by
where “S” is a vector that shows the series of sample values of the input signal corresponding to the center frequency of the frequency filter, and is written as
Under the above described constraint condition (side-lobe free condition), the following identity is introduced to obtain the weight vector Wm which makes the SNR represented by (17) maximum,
QWsT=QsWT (20)
where
WsT={circumflex over (Q)}QsWT. (22)
Using (22), (16) can be rewritten as
where
u≡{circumflex over (Q)}Qm{circumflex over (Q)}Qs. (24)
where
z=u
T
u* (26)
where
v≡S u. (28)
and Schwarts's inequality for arbitrary vectors F and G
(FGT*)(FGT*)T*≦(FFT*)(GGT*), (30)
(17) can be rewritten as
When the equality is established in (31), the SNR takes a maximum value. Then, the condition for the equality is given as
G=αF (32)
where α is a constant.
Substituting (29) into (32), we get
This window function H has a filter band width corresponding to the number of effective data set initially and makes the SNR maximum under the side-lobe free condition. It is clear that above calculations do not use any convergence method.
By using the window function H obtained above it is possible to generate a transmission signal where the SNR of the center frequency of the input signal is made maximum and the spurious components of the input signal are reduced. That is, according to the transmission signal generating unit 10, since the signal loss of the center frequency is made minimum, the signal level can be ensured and the frequency band can be narrowed.
In the above example, the waveform of the input signal formed with the data number Nf including a predetermined center frequency is defined as an original waveform and further the original waveform is defined as the steering vector S. Then the window function H is applied to generate the weighted vector W corresponding to the transmission signal. It is however possible to store the window function H that is pre-calculated in the above steps in a memory unit (not shown).
It is also possible to use a signal with a predetermined frequency as the original waveform and also to use a frequency-modulated waveform, such as a chirp signal, as the original waveform. In addition, it is also possible to transmit continuously or intermittently a plurality of phase-modulated pulses with a waveform whose amplitude is modulated using the above window function.
The transmission signal generating unit 10 according to the present embodiment comprises: the window function calculator 31 that calculates a window function that makes all frequencies without a center frequency of an input signal and its adjacent frequencies zero and makes the SNR of the center frequency maximum; and the transmission signal generator 32 that generates a transmission signal whose amplitude is modulated in a shape of an envelope curve.
This enables to generate the transmission signal where the spurious components are reduced and the signal level of the center frequency is made maximum.
When the direct generation of the transmission signal is difficult, it is also possible to make a required center frequency by frequency-converting the transmission signal from the transmission signal generating unit 10 to a signal with a higher frequency.
A radar transmission device 40 comprises an intermediate frequency signal (IF signal) generating unit 10a as the transmission signal generating unit 10 in
The radar transmission device 40 is applied with the transmission signal generating unit 10a according to the present embodiment. Thus, it generates a transmission signal by modulating the amplitude of an input signal based on a window function that makes, for the input signal, all of the outer frequencies excluding a center frequency and its adjacent frequencies zero and at the same time makes the SNR of the center frequency maximum. It is therefore possible to suppress spurious components and to make the signal level of the center frequency maximum. Such radar transmission device is applicable to transmission units of radar systems, and so on.
This application is based upon the Japanese Patent Applications No. 2006-145733, filed on May 25, 2006, the entire content of which is incorporated by reference herein.
Number | Date | Country | Kind |
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P2006-145733 | May 2006 | JP | national |