JP2987718B2 - Spread spectrum signal demodulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a demodulator for a spread spectrum signal, and more particularly, to a case where a carrier frequency of a received signal fluctuates due to a Doppler effect, a frequency fluctuation of a transmitting oscillator, or a local oscillation frequency of a receiving side. The present invention relates to a spread-spectrum signal demodulator capable of accurately obtaining information data to be demodulated even when it fluctuates.

[0002]

2. Description of the Related Art In a spread spectrum communication system, an information signal to be transmitted is spread and transmitted in a relatively wide frequency band, so that transmission power per unit frequency is small, and there is almost no interference with other communication. In addition, there are many features such as being hardly affected by external noise and being excellent in secrecy. As a method of performing spread spectrum communication via a wireless communication channel, a method shown in FIG. 4 is generally used. That is,
In this communication method, on the transmitting side T, a carrier wave having a frequency f ₀ is generated by an oscillator 1, and information data D to be transmitted is modulated by a multiplier 2 using the carrier wave, and the modulation is performed using a multiplier 3. A signal is converted into pseudo noise of a required bit length (hereinafter, PN
The information data D is spread-spectrum by multiplying and modulating the information data in a sequence, and then transmitted to the receiving side R via a wireless communication path.

On the other hand, on the receiving side R, the local oscillator 4 generates a local signal of the frequency f _L , and the PN sequence generator 5 generates the same PN sequence as used on the transmitting side T, thereby obtaining the transmitting side. The frequency spread of the spread spectrum signal sent from T and the local signal is performed by the mixer 6 so that the spread spectrum signal is converted to a frequency f.
After being converted into an intermediate frequency signal of _0- f _L (hereinafter, referred to as an intermediate frequency f _I ), it is output to each of a data demodulation unit 7 and a delay lock loop circuit (hereinafter, referred to as a DLL) 8. The data demodulation unit 7 despreads the intermediate frequency signal with the PN sequence using the multiplier 9 and, based on the despread signal, a bandwidth based on the transmission speed of the information data D, and the center frequency is f Bandpass filter 1 for _I signal component
After extraction with 0, envelope detection by the detection unit 11 the extracted signal, demodulates the original information data D by removing the carrier component of the frequency f _I. The receiving side R obtains the absolute value of the amplitude of the information data by using the level detector 12, integrates the absolute value signal for each data cycle of the information data, and determines whether the level exceeds a predetermined value. And the phase of the PN sequence generated by the PN sequence generator 5 is shifted by one bit until the level exceeds a predetermined value, so that the PN sequences used on the transmitting side T and the receiving side R are synchronized with each other. .

Further, after synchronizing the PN sequence of the transmitting side T with the PN sequence of the receiving side R by this method, the DLL 8 uses a multiplier 13 so that the PN sequence generator 5 PN delayed by 1 bit from the supplied PN sequence
After the intermediate frequency signal is despread in a sequence and a signal component having a bandwidth based on the transmission rate of the information data D and having a center frequency of f _I is extracted from the despread signal by the bandpass filter 14, A demodulated signal obtained by envelope-detecting the extracted signal by the detector 15 is supplied to one end of a subtractor 16. Further, the intermediate frequency signal is despread with a PN sequence that is one bit ahead of the PN sequence supplied from the PN sequence generator 5 to the data demodulation unit 7 using the multiplier 17 and despread. Of the information data D from the transmitted signal, and the center frequency is f _I
Is extracted by the band-pass filter 18, the extracted signal is envelope-detected by the detector 19, and the demodulated signal is supplied to the other end of the subtracter 16. 19 is supplied to a shift clock generator of the PN sequence generator 5 (not shown) so that its clock frequency is reduced by the subtractor 16.
Is controlled in accordance with the polarity of the output signal to maintain the synchronization state between the PN sequence of the transmitting side T and the PN sequence of the receiving side R.

However, in the above-described demodulation method, when the transmitting side, the receiving side, or both move at high speed, the frequency of the received signal fluctuates due to the Doppler effect. The frequency of the spread signal is out of the pass band of the band-pass filter, and as a result, the original information data cannot be demodulated.
In the case of L, there is a problem that the shift clock frequency of the PN sequence generator is erroneously controlled although the transmission side and the reception side are normally synchronized. A similar problem also occurs when the frequency of a carrier generated on the transmitting side or the frequency of a local signal generated on the receiving side fluctuates. In particular, the transmission speed of information data is slow and the frequency of the carrier is low. Was extremely high. Further, as a method for solving this problem, it is conceivable to widen the pass band width of the band-pass filter by taking into account the frequency fluctuation of the carrier wave due to the Doppler effect or the like. Absent.

Conventionally, as a method for solving this problem, as shown in FIG. 5, a transmitting side T transmits a continuous wave CW separately from a carrier for transmitting information data. On the other hand, the receiving side R is provided with a Doppler detector 20 for observing the reception frequency of the continuous wave CW, and corrects the frequency of the local signal generated by the local oscillator 4 based on the difference between the observation frequency and the specified frequency. The frequency of the signal despread with the PN sequence is always the center frequency f _{I of the} bandpass filter 10.
I was trying to be. However, the above spread spectrum signal communication method requires a transmitter and a receiver for measuring the influence of the Doppler effect and the like in addition to the transmitter and the receiver for communicating information data, so that the apparatus is very complicated and expensive. In addition, there is a disadvantage that the frequency use efficiency is deteriorated.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in order to solve the problems of the spread spectrum communication system in the case where the Doppler effect as described above or the fluctuation of the carrier frequency on the transmission side is involved. It is an object of the present invention to provide a demodulator for a spread spectrum signal capable of obtaining original information data without accompanying the frequency utilization efficiency.

[0008]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention takes the following measures. For example, in a device for demodulating a spread spectrum signal superimposed on a carrier wave by despreading with the same PN sequence as the transmitting side, a digital filter is provided at a stage subsequent to the despreading means, and the maximum level of the despread signal is provided. Means for varying the center frequency of the digital filter based on the frequency value of the spectrum component.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the illustrated embodiments. FIG. 1 is a block diagram showing a demodulator for a spread spectrum signal according to an embodiment of the present invention. Here, in order to facilitate understanding of the operation, a description will be given including the transmitting side. In the figure, T is a transmitting side, and frequency 1.
An oscillator 1 for generating a carrier wave of 575 [GHz];
A PN sequence generator 21 for generating a 10-stage M-sequence code having a bit rate of 1.023 [MHz];
The bit rate is 50 [bit / s] using the multiplier 2
After modulating the carrier with the information data D to be transmitted, and using a multiplier 3 to multiply and modulate the modulated signal with the M-sequence code to spread the data D, and To the demodulator W.

On the other hand, W is a demodulator according to the present invention, which is a local oscillator 22 for generating a first local signal having a frequency of 1.512 [GHz], and a PN sequence for generating the same M-sequence code as that of the transmitting side T. Generator 23 and frequency 63
A local oscillator 24 for generating a second local signal of [MHz] is provided. A low-noise amplifier 25 and an image removing filter 26 transmit a spread spectrum signal of a carrier frequency of 1.575 [GHz] transmitted from the transmitting side T to the local oscillator 24. The signal extracted through the first local signal and the signal extracted through the mixer 2
7, the bandpass filter 28 having a center frequency of 63 [MHz] and a pass bandwidth of 10 [MHz].
Is used to convert the spread spectrum signal to a frequency of 63.
The signal is converted into a first intermediate frequency signal of [MHz], and the signal is connected to the data demodulation unit 30 and the DLL 31 via the amplifier 29 so as to be respectively supplied. The data demodulation unit 30 uses a multiplier 32 to despread the first intermediate frequency signal with the M-sequence code, and converts the despread signal to a center frequency 63
A signal generated through a band-pass filter 33 and an amplifier 34 having a pass band width of 2 [MHz] at [MHz] and the second local signal are frequency-mixed by a mixer 35 to convert the despread signal into a baseband signal. After the conversion, a low-pass filter 36 for removing aliasing based on the sampling theorem, an A / D converter 37 for converting the baseband signal into a digital signal, a passband of 100 [Hz] and a center frequency The original information data is demodulated via a digital filter 38 and a detector 39 which can change the data.

The DLL 31 has a third frequency 63 [MHz].
A local oscillator 40 for generating a local signal is provided, and the PN sequence generator 23 uses a multiplier 41 to generate a first PN sequence that is delayed by one bit from the PN sequence supplied to the data demodulation unit 30 by the first PN sequence. A signal generated by despreading the intermediate frequency signal through a band-pass filter 42 and an amplifier 43 having a center frequency of 63 [MHz] and a pass bandwidth of 2 [MHz], and the third local signal Are frequency-mixed by a mixer 44 to convert the despread signal into a baseband signal, and the baseband signal is passed through a low-pass filter 45, an A / D converter 46, and a passing filter for removing aliasing based on a sampling theorem. The detector 4 has a bandwidth of 100 [Hz] via a digital filter 47 whose center frequency can be changed.
After that, the output signal is supplied to one end of a subtractor 49. Further, the PN sequence generator 23 uses the multiplier 50 to reduce the PN sequence supplied to the data demodulation unit 30 by 1
The first intermediate frequency signal is despread with a PN sequence whose bit phase has advanced, and the despread signal is converted to a center frequency of 63 [MH].
z] and a signal generated through the band-pass filter 51 and the amplifier 52 having a pass band width of 2 [MHz] and the third
The despread signal is converted into a baseband signal by frequency mixing the local signal with a mixer 53, and the baseband signal is converted to a low-pass filter 54 for removing aliasing.
After the signal is supplied to a detector 57 via an A / D converter 55 and a digital filter 56 having a pass band width of 100 [Hz] and capable of changing its center frequency, the output signal is supplied to the other side of the subtracter 49. A signal obtained by subtracting the output signal of the detector 57 from the output signal of the detector 48 to the input terminal of the timing generator 58 is supplied to the input terminal of the timing generator 58 so that the timing generator 58 shifts the PN sequence generator 23. It is configured to control the clock frequency.

The demodulation device W supplies the output signal of the A / D converter 37 to a synchronization acquisition unit 59, and the synchronization acquisition unit 5
9 to the data input terminals of the digital filters 38, 47 and 56 via the correction circuit 60, and the detection signal from the synchronization acquisition section 59 to the timing generation circuit 5
8 so as to be supplied to input terminals of a PN sequence generator 23, respectively. Further, it is connected so that a predetermined sampling signal is supplied from the timing generation circuit 58 to the input terminals of the A / D converters 37, 46 and 55, respectively. The synchronization acquisition section 59 converts the output signal of the A / D converter 37 into a Fast signal.
Fourier Transform (hereinafter FFT)
The control signal and the detection signal are output by supplying the control signal and the detection signal to the determination unit 62 via the control unit 61.

The demodulating device W thus configured operates as follows. That is, since the bit rate of the information data D is 50 [bit / s],
The frequency shift Δf of the baseband signal caused by the influence of the Doppler effect or the like by the FFT 61 every 0 [s] period
The frequency spectrum of the baseband signal is obtained from the output signal of the A / D converter 37 in a frequency band B _x [Hz] where _d [Hz] can be allowed, and the maximum value of the level of the frequency spectrum is a predetermined value. Determining section 6 whether or not
2 and outputs a detection signal to the timing generation circuit 58 until the level exceeds a predetermined value, whereby the phase of the PN sequence generated by the PN sequence generator 23 becomes 1
The bit shift is performed, and the PN sequence of the transmitting side T and the PN sequence of the PN sequence generator 23 are synchronized with each other. The determination unit 62 supplies the frequency value of the spectrum exceeding the predetermined value as a control signal to the correction circuit 60, and sets the center frequency of the digital filters 38, 47 and 56 to the frequency of the spectrum exceeding the predetermined value. Are output from the correction circuit 40. Note that the baseband signal frequency shift △ may allow f _d frequency band B _x
Is the baseband frequency f _b [Hz] corresponding to the frequency of the despread signal when the demodulation device W despreads the spread spectrum signal normally transmitted from the transmission side T.最大_max [m /
s], and the speed of light is c [m / s].

[0014] _{B x ≧ 2 · υ max ·} f b / c [Hz]

Therefore, according to the demodulation device W, even if the reception frequency fluctuates under the influence of the Doppler effect or the like, the frequency of the baseband signal fluctuates within the frequency band B _x [Hz]. If the maximum value of the despread signal converted to the baseband signal can always be detected, the PN sequence on the transmitting side and the receiving side can be synchronized with each other, and the pass band of the digital filter 38 can be expanded. Instead, the center frequency is changed to the frequency of the despread signal converted to the baseband signal, so that the original information data can be demodulated without deteriorating the S / N. Furthermore, since the center frequency of the digital filters 47 and 56 is changed to the frequency of the despread signal converted to the baseband signal without expanding the pass band,
The synchronization state between the PN sequence on the transmitting side and the PN sequence on the receiving side can be normally maintained without deteriorating the S / N.

FIG. 2 shows a modified embodiment of the present invention.
Local oscillator 63 for generating a fourth local signal having a frequency of 63 [MHz], mixers 64 and 65, phase shifter 66, low-pass filters 67 and 68 for aliasing removal, and A
The / D converters 69 and 70 are connected to the synchronization acquisition unit 5 shown in FIG.
9, a fourth local signal and an output signal of the amplifier 34 are frequency-mixed by a mixer 64 to convert an output signal of the amplifier 34, that is, a despread signal into a baseband signal. The digital signal generated through the A / D converter 69 is supplied to one input terminal of the FFT 61, and the phase of the fourth local signal is delayed by π / 2 [rad] using the phase shifter 66. After the signal and the despread signal are frequency-mixed by a mixer 65 to convert the despread signal into a baseband signal, the digital signal generated via a low-pass filter 68 and an A / D converter 70 is converted to the other signal of the FFT 61. The control signal and the detection signal are output by supplying the control signal and the detection signal to the input terminal. According to the demodulation device W, the real signal component and the imaginary component of the output signal of the amplifier 34, that is, the signal obtained by despreading the first intermediate frequency signal are obtained, and each is converted into a digital signal. Since the frequency spectrum is obtained by calculating the value, for example, in the embodiment shown in FIG. 1, since the phase of the second local signal of the local oscillator 24 and the phase of the despread signal are different by π [rad], the mixer 35 originally outputs Without setting the level of the baseband signal to be zero to zero, that is, in the embodiment shown in FIG. 2, the level of the frequency spectrum component is kept constant regardless of the phase difference between the fourth local signal and the despread signal. And the determination by the determination unit 61 can be made accurate.

In the above-described embodiment, after the spectrum component of the despread signal is obtained by using the FFT, it is determined whether or not the maximum level exceeds a predetermined value by using the determination unit. Although the detection signal is supplied to the timing generation circuit and the control signal is supplied to the correction circuit by obtaining the maximum level frequency value of the despread signal, the present invention is not limited to this.
For example, the synchronization acquisition section 59 shown in the embodiment of FIG. 1 has a pass bandwidth of 100 [Hz] and center frequencies of 450, 450.1, 450.2,.
9, 460 [KHz] band pass filters F _{1 to} F
_A local oscillator 71 for generating a fifth local signal of ₁₀₁ and a frequency of 62.545 [MHz] is provided. After the fifth local signal and the output signal of the amplifier 34 are frequency-mixed by a mixer 72, the center frequency is 455 [MHz]. The output signal of the amplifier 34, that is, the despread signal is converted into a fifth intermediate frequency signal having a frequency of 455 [KHz] using a band-pass filter 73 having a pass bandwidth of 100 [Hz] and a bandwidth of 455 [KHz]. The signal may be supplied to each of the band-pass filters F _{1 to} F ₁₀₁ via the amplifier 74 and the output signal of each filter may be supplied to the determination unit 62.

In the demodulation device W, the synchronization acquisition section 59 uses the band-pass filters F _{1 to} F ₁₀₁ to set the frequency to 4
50, 450.1, 450.2, ..., 459.9, 46
A signal of 0 [KHz] is extracted from the fifth intermediate frequency signal, a maximum value of a signal level of each frequency is obtained for each bit period of the information data, and a determination unit determines whether the maximum value exceeds a predetermined value. At 62, a detection signal is output until the level exceeds a predetermined value, and the phase of the PN sequence generated by the PN sequence generator 23 is shifted by one bit, so that the PN between the transmitting side T and the PN sequence generator 23 is changed. Synchronize the phases of the sequences. The determination unit 62 determines the bandpass filter that has output a signal exceeding the predetermined value, and supplies the value of the center frequency of the filter to the correction circuit 60 as a control signal. A predetermined parameter corresponding to the frequency value of
8, 47 and 56 to change the center frequency of each filter to the baseband frequency value corresponding to that frequency value. Therefore, according to the demodulation device W, even if the reception frequency fluctuates due to the influence of the Doppler effect on the spread spectrum signal and the frequency of the baseband signal deviates from the pass band of the digital filter 38, the fifth intermediate frequency signal If the frequency deviation is ± 5 [KHz], the maximum value of the despread signal converted into the fifth intermediate frequency signal can be detected from any one of the output terminals of the band-pass filters F _{1 to} F _101. Since the PN sequence of the transmitting side and the receiving side can always be synchronized with each other, and the center frequency of the digital filter 38 can be changed to the frequency of the despread signal converted into the baseband signal without expanding the pass band. , And the original information data can be demodulated without deteriorating the S / N ratio. Further, since the center frequency of the digital filters 47 and 56 is changed to the frequency of the despread signal converted into the baseband signal without widening the pass band, the PN sequence on the transmitting side and the reception are reduced without deteriorating the S / N. The synchronization state with the PN sequence on the side can be normally maintained.

In the above-described embodiment, the pass band width is 100 [Hz] and the center frequencies are 450, 450.1, 4
450.2,... 459.9, 460 [KHz] band pass filters are provided in the synchronization acquisition unit. The pass band width, the center frequency of each band pass filter, and the number of band pass filters provided in the synchronization acquisition unit are as follows. There is no need to be limited to this. That is, the center frequency of the signal supplied to the synchronization acquisition section f _x [H
z], the maximum frequency shift of the signal caused by the influence of the Doppler effect or the like is f _d [Hz], and the bit rate of the information data to be transmitted is f _r [bps]. Each pass bandwidth B [H
z] may be determined from the formula B ≧ 2f _r [Hz], also by obtaining the step frequency f _s [Hz] from the equation 0 <f _s ≦ B, the center frequency of the bandpass filter are each f _x -
_{f d, f x -f d +} f s, f x -f d + 2f s, f x -
_{f d + 3f s, ...,} f x + f d -2f s, f x + f d -
If f _s and f _x + f _d [Hz], the synchronization acquisition unit always keeps the detection signal and the control signal even if the center frequency of the signal supplied to the synchronization acquisition unit changes under the influence of the Doppler effect or the like. Can be output.

[0020]

As described above, according to the present invention, a digital filter is provided at a stage subsequent to the means for despreading a spread spectrum signal, the spectrum component of the despread signal is obtained, and the maximum value of the spectrum component exceeds a predetermined value. And synchronizes the PN sequence between transmission and reception, and performs inverse multiplexing based on the frequency value of the spectral component exceeding the predetermined value or by using a plurality of band-pass filters having different center frequencies and equal pass bandwidths. A signal of a predetermined frequency is extracted from the spread signal, and it is detected that the maximum value of the output signal level of each of the bandpass filters exceeds a predetermined value to synchronize the PN sequence between transmission and reception. The center frequency of the digital filter is changed to a predetermined frequency based on the center frequency value of the band-pass filter that has output the output signal. Therefore, even if the reception frequency fluctuates due to the influence of the Doppler effect or the oscillation frequency of the local oscillator fluctuates, it is possible to obtain information data to be demodulated without deteriorating the S / N. This is very effective in providing a spread spectrum signal demodulation device.

[0022]

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing one embodiment of the present invention.

FIG. 2 is a configuration diagram showing a modified embodiment of the present invention.

FIG. 3 is a configuration diagram showing a modified embodiment of the present invention.

FIG. 4 is a diagram illustrating a conventional spread spectrum communication system.

FIG. 5 is a diagram for explaining another conventional spread spectrum communication system.

[Explanation of symbols]

 W demodulation device 22 and 24 local oscillator 23 PN sequence generator 25 and 29 amplifier 26 and 28 filter 27 mixer 30 data demodulation unit 31 DLL 58 timing generation circuit 59 synchronization acquisition unit 60 correction circuit

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H04J 13/00

Claims (4)

(57) [Claims] An apparatus for demodulating a spread spectrum signal superimposed on a carrier by despreading with the same PN sequence as that of a transmitting side, wherein a digital filter is provided at a stage subsequent to the despreading means, and the despread signal is provided. Of the spectrum
A spread spectrum signal demodulation apparatus, wherein a center frequency of the digital filter is varied based on a frequency value of a spectrum component at a maximum level. 2. A device for demodulating a spread spectrum signal superimposed on a carrier by despreading with the same PN sequence as that of a transmitting side, wherein a digital filter is provided at a stage subsequent to the despreading means, and center frequencies differ from each other. A predetermined signal is extracted from the despread signal using a plurality of bandpass filters having the same pass band width, and whether or not the maximum value of the output signal level of any of the bandpass filters exceeds a predetermined value A spread-spectrum signal demodulator characterized in that the center frequency of the digital filter is varied based on the value of the center frequency of the band-pass filter that has output a signal exceeding the predetermined value. 3. A first signal obtained by despreading a spread spectrum signal superimposed on a carrier with the same PN sequence as that on the transmitting side, and despreading the spread spectrum signal with a PN sequence having a phase different from that of the PN sequence. Based on the difference between the PN sequence used to generate the first signal and the second signal despread with a PN sequence having a different phase from the same PN sequence as the transmitting side. PN between sending and receiving
In a device for demodulating the spread spectrum signal while maintaining sequence synchronization, a digital filter is provided at each subsequent stage of despreading means for generating the first and second signals, and A spread- spectrum signal demodulator, wherein a spectrum component of a signal despread by the same PN sequence is obtained, and a center frequency of each of the digital filters is varied based on a frequency value of a spectrum component at a maximum level. 4. A first signal obtained by despreading a spread spectrum signal superimposed on a carrier with the same PN sequence as the transmitting side, and despreading the spread spectrum signal with a PN sequence having a phase different from that of the PN sequence; Based on the difference between the PN sequence used to generate the first signal and the second signal despread with a PN sequence having a different phase from the same PN sequence as the transmitting side. PN between sending and receiving
In a device for demodulating the spread spectrum signal while maintaining sequence synchronization, a digital filter is provided at each subsequent stage of the despreading means for generating the first and second signals, and the center frequencies are mutually different. A predetermined signal is extracted from a signal despread with the same PN sequence as the transmitting side by using a plurality of bandpass filters having different and equal passbands, and a maximum of the output signal level of any one of the bandpass filters is obtained. Determining whether the value has exceeded a predetermined value, and varying the center frequency of each of the digital filters according to the value of the center frequency of the band-pass filter that has output the signal exceeding the predetermined value. Spread spectrum signal demodulator.