JPH114209A - Orthogonal frequency division/multiplex modulation signal transmitting system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an orthogonal frequency division multiplex modulation signal transmitting system which can precisely detect the shift quantity of a carrier frequency and synchronously pulls it in even if frequency fluctuation exceeding the frequency interval of a carrier exists. SOLUTION: A CW insertion place securing circuit 15 and a CW insertion circuit 16 are provided on a transmission device-side and a CW symbol being a special symbol wherein only one carrier has a signal is inserted at every prescribed period. A CW extraction circuit 17 and an adjusting circuit 10 are provided on a reception device-side. The carrier frequency shift ΔFL of the orthogonal frequency division/multiplex(OFDM) signal of a base band is calculated and the oscillation frequency fr of a reception side local oscillator 9 is controlled by using the values of the complex vector signals Zcw(n) of plural carriers obtained by discrete Fourier transformation. Thus, an inexpensive and small oscillator whose fluctuation width of the oscillation frequency exceeds a carrier frequency interval can be used as the local oscillator on a transmission side and a reception side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transmission system using an orthogonal frequency division multiplex modulation signal, and more particularly, to an orthogonal frequency division system in which a demodulation operation automatically follows a shift of a received carrier frequency. The present invention relates to a multiplex modulation signal transmission system.

[0002]

2. Description of the Related Art Currently, as a multiplex transmission system for mobile or terrestrial digital radio communications, an orthogonal frequency division multiplex system having a feature of being resistant to multipath fading and ghosts.
g: OFDM system). In this system, as shown in FIG. 7, a large number of dozens or hundreds of carriers arranged at the same frequency fs interval are used. An information code is transmitted using a signal digitally modulated at a symbol frequency fsy (= 1 / Tsy), that is, an OFDM signal (orthogonal frequency division multiplex modulation signal). Here, the time interval Tsy is a symbol period of the digital signal.

As a digital modulation method of each carrier in the OFDM method, a QPSK method (four-phase shift keying method), a 16QAM method (16-level quadrature amplitude modulation method), and the like are being studied. FIG. 8 shows that each carrier is Q
FIG. 1 is a block diagram showing an example of a conventional OFDM transmission system in which digital modulation is performed by a PSK system. The upper side of the figure is the transmitter side, and the lower side is the receiver side.

In a transmitting apparatus, an information code to be transmitted is QP
The SK modulation circuit 1 modulates the signal into a complex vector signal of the QPSK method (hereinafter, referred to as a QPSK signal). Q obtained by modulation
After the PSK signal is distributed to each carrier by the distribution circuit 2,
The FFT circuit 3 performs an inverse discrete Fourier transform (IFFT).

[0005] By this IFFT processing, QPSK signals are separated from each other by a frequency interval fs at a symbol period Tsy.
The signal is converted into a baseband OFDM signal multiplexed by an orthogonal frequency division multiplexing modulation method including Ns carrier waves orthogonal to each other.

Next, the OFDM signal is input to a mixer 4 and is, for example, several hundred MHz by a transmission-side local signal having a frequency fr supplied from a high-frequency transmission-side local oscillator 5.
Band, or frequency converted to a high frequency signal of several GHz band,
The power is amplified and transmitted from the transmitting antenna 6.

On the other hand, on the receiving device side, the received signal received by the receiving antenna 7 is amplified and then input to the mixer 8 where the frequency f supplied from the local oscillator 9 on the receiving side is amplified.
The base station OFDM signal that has been frequency-converted and multiplexed by the receiving-side local oscillator signal r is reproduced.

Next, the OFDM signal is supplied to an FFT circuit 10, where it is subjected to a discrete Fourier transform (FFT).
It is separated into baseband complex vector signals Z (n) for each carrier. Here, n represents the number of the separated carrier.

[0009] The complex vector signal Z (n) of each carrier thus separated is input to a coupling circuit 11, where it is rearranged in the original time order by a procedure reverse to that of the distribution circuit 2 on the transmission side. As a result, the signal is returned to a temporally continuous QPSK signal, demodulated by the QPSK demodulation circuit 12, and output as an information code.

By the way, in order to demodulate the received signal in this way, the amount of deviation between the symbol period generated in the receiving apparatus, its timing, and the synchronization of the carrier frequency and the same synchronization of the received signal must be detected. Then, it is necessary to correct the deviation and to properly synchronize.

In other words, it is necessary to reproduce a baseband OFDM signal as a result of correctly obtaining the signal of the frequency fr obtained from the receiving side local oscillator 9 and the synchronous detection by the mixer 8. For this reason, the operation of the mixer 8 in a state where the above-mentioned shift amount remains may be called quasi-synchronous detection.

Therefore, in order to detect this shift, conventionally, for example, in Japanese Patent Application Laid-Open No. 5-503737, a pilot signal using two special carrier waves prepared in advance is transmitted from the transmitting side to a signal to be transmitted. And a method of detecting a deviation of a symbol period, a timing, and a carrier frequency from a phase variation position of a received pilot signal on the receiving side (first related art).
Japanese Patent Laid-Open Publication No. Hei 6-501357 proposes a method (second related art) for detecting a deviation of a symbol period from a phase rotation of a received complex vector signal.

For example, the system shown in FIG. 8 is an application of the above-mentioned first prior art, in which a pilot insertion circuit 13 is provided on the transmitting apparatus side, whereby a pilot signal is continuously inserted into a transmission signal and transmitted. On the receiver side, the pilot detection circuit 14 extracts a pilot signal from the received signal to detect a carrier frequency deviation.

Each of these conventional deviation detection methods has high accuracy, can follow small frequency fluctuations due to the Doppler effect or the like accompanying the movement of the transmission / reception device, and can demodulate a high-quality information code with a small error rate. it can.

[0015]

The above prior art does not take into account the fact that there is a fairly small limit on the frequency fluctuation range that can be followed, and cannot respond to practically possible frequency fluctuations. There was a problem. Hereinafter, the problems of the conventional technology will be described.

First, in this frequency division multiplex modulation system,
The greater the number of carriers to be multiplexed, the higher the efficiency of use of the frequency band, and the advantages of the system can be utilized. In order to increase the number of carrier waves in the ordinary OFDM transmission system, the frequency interval fs of the carrier waves is set to a narrow frequency of several tens of kHz, for example, about 20 kHz or less.

On the other hand, the center frequency of a carrier wave in an apparatus using a space as a transmission path is, for example, 800 in the case of an FPU (Field PickUp) used for television relay or the like.
It is an extremely high frequency of about MHz or 7 GHz. Here, the oscillation frequency of a normal crystal oscillator has a frequency variation of about ± 2 ppm (± 2 × 10 ⁻⁶ times the oscillation frequency) even when an oscillator with a thermostat is used.

As a result, for example, when a carrier of 7 GHz is used, the sum of the frequency fluctuations of the oscillator on the transmitting device side and the oscillator on the receiving device side is approximately ± 28 KHz, which is approximately ± 1.5 in the number of carrier waves. Frequency fluctuation occurs.
Further, when the superheterodyne system using the intermediate frequency after the frequency conversion is used, the fluctuation width further widens and reaches the frequency fluctuation width of about ± 2 in the number of the carrier waves.

Therefore, at the beginning of the reception,
It must be assumed that the carrier frequency of the baseband OFDM signal obtained by the frequency conversion may have such a wide frequency shift. However, in the prior art, a pilot carrier (a carrier transmitting a pilot signal) and an information carrier (another carrier transmitting an information code) are separated only by the frequency interval fs of the carrier.

Therefore, if there is such a frequency shift of several carriers, the position of the pilot carrier shown by the bold line in FIG. 9A is changed to the position of the adjacent information carrier as shown in FIG. 9B. , And even beyond the position of the adjacent information carrier, it becomes impossible to distinguish which carrier signal is the pilot signal. Therefore, in the prior art, if there is a frequency variation of several carriers, the amount of deviation of the carrier frequency cannot be detected, and the carrier cannot be reproduced to synchronize.

On the other hand, in the case of the second method, it is assumed that the information code can be demodulated almost correctly. Therefore, a small frequency variation of about several% to several tens of the carrier frequency interval fs, for example, If the frequency changes due to the Doppler effect or the like, the shift amount can be detected and made to follow. However, even in the second method, as described above, if there is a frequency variation of several carriers, the amount of deviation of the carrier frequency cannot be detected, and the carrier is reproduced to synchronize. You will not be able to do it.

Here, in order to cope with such a problem, for example, a high-precision oscillator having a frequency stability of ± 0.5 ppm or less is used as a local oscillator of the transmitting device and the receiving device, and a frequency fluctuation width is set to a carrier wave. The frequency interval fs should be sufficiently smaller than the frequency interval fs.

However, such a high-precision oscillator itself is not only extremely expensive, but also requires a large thermostatic bath and requires electric power for heating, so that it is difficult to reduce the size of the device. The problem of becoming

It is an object of the present invention to provide an orthogonal frequency division multiplex modulation signal transmission system capable of correctly detecting the amount of deviation of a carrier frequency and pulling it in synchronously even if there is a frequency variation exceeding the frequency interval of the carrier. It is in.

[0025]

According to a first aspect of the present invention, there is provided:
The purpose is to have the same frequency fs or frequency fs
In an orthogonal frequency division multiplexing modulation signal transmission system that transmits information codes using Ns carriers that are orthogonal to each other and have a frequency interval that is an integer multiple of Ns symbols (Nf is a positive integer of 2 or more) of the orthogonal frequency division multiplexed modulation signal, and one symbol out of the Ns carriers at a rate of one symbol (for example, a carrier at the center frequency of the transmission band) A predetermined symbol (hereinafter, referred to as a CW symbol) consisting of a signal containing only a signal component is inserted into the carrier, and on the receiving side, a carrier wave appearing in a baseband orthogonal frequency division multiplex modulation signal obtained by frequency conversion by a local oscillation signal The amount of frequency shift ΔFL is determined by the CW inserted in the signal.
A symbol signal is calculated based on a complex vector signal Zcw (n) of a plurality of carriers obtained by performing a discrete Fourier transform (n is the number of a separated carrier), and the local oscillation signal is calculated based on the calculated shift amount ΔFL. By controlling the frequency, control is achieved such that the deviation ΔFL converges.

Similarly, according to the second aspect of the present invention, the above object is achieved by the first aspect of the present invention, wherein the shift amount ΔF
The means for calculating L is composed of the first means and the second means, and the calculation accuracy of the shift amount ΔFL by the second means is higher than the calculation accuracy of the shift amount ΔFL by the first means. Is achieved.

Similarly, according to the third aspect of the present invention, the above object is achieved by the first aspect of the present invention, wherein the shift amount ΔF
The means for calculating L is the CW inserted on the transmitting side.
From the number n0 of the carrier of the symbol and the number nmax of the carrier having the largest absolute value level of the complex vector signal Zcw (n) of the CW symbol portion and the number of the carrier closest to the number n0, This is achieved by a third means for calculating.

Similarly, according to a fourth aspect of the present invention, the above object is achieved by the first aspect of the present invention, wherein the shift amount ΔFL on the receiving side is provided.
Means for calculating the carrier number n0 of the CW symbol inserted on the transmitting side and the absolute value level of the complex vector signal Zcw (n) of the CW symbol portion are the largest,
And the number of the carrier is the number nmax of the carrier closest to the number n0, the absolute value level Rmax of the complex vector signal Zcw (nmax), and the absolute value level of the complex vector signal among the carriers adjacent to the carrier of the number nmax. Is obtained by the fourth means for calculating the shift amount ΔFL from the number nnext of the carrier having the larger carrier and the absolute value level Rnext of the complex vector signal Zcw (nnext) of the carrier having the number nnext. You.

Similarly, according to the fifth aspect of the present invention, the above object is achieved by the first aspect of the present invention, wherein the shift amount ΔF
The means for calculating L is the CW inserted on the transmitting side.
The carrier number n0 of the symbol, the carrier number nmax whose absolute value level of the complex vector signal Zcw (n) of the CW symbol portion is the largest and the carrier number is closest to the number n0, and the complex vector signal Zcw (nmax),
Absolute value level Rmax of the complex vector signal Zcw (nmax)
The magnitude of the complex vector signal Zcw (nmax-1) of the adjacent carrier having a lower frequency than the carrier of the number nmax;
From the magnitude of the complex vector signal Zcw (nmax + 1) of the adjacent carrier having a higher frequency than the carrier of ax, the deviation ΔFL
This is achieved by the fifth means for calculating.

[0030] Similarly, according to a sixth aspect of the present invention, the above object is achieved by the first aspect of the present invention, wherein C is inserted at the transmission side.
This is achieved by configuring the average power of the W symbol to be equal to the average power of the information symbol portion transmitting the information code.

According to the first aspect of the present invention, a carrier vector deviation ΔFL of a baseband OFDM signal obtained by a receiving apparatus is converted into a complex vector signal Zcw () of a plurality of carriers obtained by performing a discrete Fourier transform on a CW symbol portion. n).

As described above, there is a possibility that the frequency shift width may extend to the frequency width of several carrier waves. In this case, for example, the signal transmitted using the carrier wave of the number n0 is transmitted from the FFT circuit of the receiver. Is output at a carrier wave position shifted by several lines from the carrier wave of the number n0.

Thus, the CW symbol used in the present invention has only the signal of the carrier of the number n0, and the carrier near the carrier of the number n0 has no signal. Therefore, unlike the case of the above-described first conventional technique, the deviation ΔFL of the carrier wave frequency can be reliably calculated without being confused with other information signals. At this time, the width of the frequency shift that can be detected in the present invention is the number n
When a carrier of 0 is set at the center of the transmission frequency band, the frequency width becomes about half of the transmission band.

In the second aspect of the present invention, the first means responds at a high speed, and roughly reduces the amount of deviation ΔFL.
The second means reduces the deviation amount ΔFL with high accuracy, so that high accuracy can be obtained with good responsiveness.

In the third aspect of the present invention, since the third means is used, it is possible to detect a frequency shift over a unit of the number of carrier waves, to calculate the shift amount ΔFL, and to calculate the shift amount ΔFL.
Rough adjustment (hereinafter referred to as coarse adjustment) necessary to reduce the frequency shift at a stroke can be performed up to / 2 lines.

According to the fourth aspect of the present invention, since the fourth means performs a complicated operation as compared with the third means, the circuit scale slightly increases. However, the detection accuracy of the frequency shift is increased by that much, and the carrier wave is reduced to about 1/4.
Therefore, the frequency deviation after the adjustment can be made smaller than the case where the third means is used.

According to the fifth aspect of the present invention, since the fifth means performs a higher-level operation than the third means and the fourth means, the detection accuracy can be further improved. For example, it is possible to detect a frequency shift with an accuracy of at most 1/16 carrier wave, which is the minimum necessary for following the carrier frequency in the 16QAM system, and calculate the shift amount ΔFL with high accuracy.

Here, in order to demodulate an information code having a low error rate, the amount of deviation of the frequency of the carrier is determined by the carrier frequency interval fs.
Needs to be reduced to several percent or less. By the way, the deviation detection method according to the first prior art and the deviation detection method according to the second prior art can also satisfy this condition. However, in these prior arts, the operation cannot be started if there is a frequency shift of several carrier waves at first, so that it is necessary to use a large and expensive oscillator having high oscillation frequency stability as described above.

However, according to the present invention, as described above, the amount of deviation can be sufficiently reduced even if there is a large deviation at first, and the amount of deviation of the carrier frequency should be at least １ ／ or less of the frequency interval fs of the carrier. Can be reduced to

Therefore, by using the present invention in combination with a high-accuracy detection method such as the deviation detection method according to the first conventional technique or the deviation detection method according to the second conventional technique, the frequency deviation corresponding to several carrier waves can be obtained. Even if there is, it is possible to reliably pull in synchronization and to demodulate a high-quality information code.

Therefore, a small and inexpensive oscillator can be used instead of an expensive and large-sized high-performance oscillator, and the transmission device itself can be made small and inexpensive.

In particular, the deviation detection method according to the second prior art has extremely high detection accuracy of the carrier frequency deviation amount, and can demodulate a high-quality information code. However, on the contrary, it is required that the deviation amount of the carrier frequency be about several tens% or less from the beginning. As described above, according to the fifth aspect of the fifth aspect of the present invention, the amount of deviation of the carrier frequency is set to one of the frequency intervals of the carrier.
/ 16 or less.

Therefore, by using the shift detection method according to the fifth invention and the second conventional technique in combination, it is possible to demodulate an extremely high quality information code even with a small and inexpensive oscillator.

Next, in this type of transmission apparatus, in order to make the level of the input signal to the FFT of the reception apparatus substantially constant, an AGC which is usually controlled by the amount of fluctuation of the envelope level of the reception signal
The circuit is used before the FFT circuit.

At this time, according to the sixth invention, the average transmission power of the CW symbol is made equal to the average transmission power of the other information symbols.

As a result, the envelope waveform of the received signal becomes substantially constant irrespective of the symbol position. Therefore, according to the sixth aspect of the present invention, even if the CW symbol is inserted, the AGC
The circuit gain can be prevented from fluctuating and becoming unstable under the influence of a special symbol such as the CW symbol.

The transmission power of one carrier of the CW symbol is Ns of the transmission power of one carrier of the information symbol.
Double. Therefore, the S / N of the FFT circuit output signal of the CW symbol is the S / N of one carrier signal of the information symbol.
/ N is improved by (10 × log (Ns)) dB. For example, if the number of carriers to be multiplexed is set to 1000, the S / N of the output signal of the FFT circuit of the CW symbol is improved by 30 dB, and a highly accurate detection value less affected by noise can be obtained.

[0048]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an orthogonal frequency division multiplex modulation signal transmission system according to the present invention will be described in detail with reference to the illustrated embodiments. FIG. 1 shows the same QPS as the prior art shown in FIG. 8 as a digital modulation method for each carrier.
This embodiment is different from the prior art shown in FIG. 8 in that the KW system is applied, and the CW insertion place securing circuit 1 is replaced with the pilot insertion circuit 13 on the transmission side.
5 and a CW insertion circuit 16, a CW extraction circuit 17 instead of the pilot detection circuit 14 on the receiving side,
An adjustment circuit 20 including a coarse adjustment circuit 18 and a quasi-fine adjustment circuit 19 is provided. Therefore, also in the embodiment of FIG.
This is the same as the prior art in FIG.

First, the CW insertion place securing circuit 15 controls the distribution circuit 2, and when a signal is input from the distribution circuit 2 to the IFFT circuit 3, one symbol time required to insert a CW symbol and an information signal 2 is temporarily stopped, and signal processing for inserting a dummy signal is performed instead, and as shown in FIG. 2A, a function of securing time for inserting a CW symbol is provided.

FIG. 2 shows the carrier waves of the OFDM signal appearing sequentially in the symbol period Tsy with time taken on the horizontal axis and the number n. At this time, the frequency at which the dummy signal, ie, the CW symbol is inserted, is shown. Is set to one symbol for every Nf symbols. Here, the value of Nf is a positive integer of 2 or more and is usually set to 500 to 1000. Therefore, the CW symbol is inserted at a rate of once every 500 to 1000 times. become.

Next, as shown in FIG. 2B, the CW insertion circuit 16 replaces the dummy signal in the signal output from the IFFT circuit 3 with the CW symbol signal shown in FIG. 3A. . Here, the CW symbol signal is a signal having only the carrier itself as information and not including any other information. Then, the output signal of the CW insertion circuit 16 is frequency-converted into a signal of a high frequency band and transmitted from the transmission antenna 6 as in the case of the conventional transmission device.

On the other hand, on the receiving device side, the baseband OFDM signal is reproduced from the received signal received by the receiving antenna 7 in the same manner as in the conventional receiving device, and then the FF is further reproduced.
The T circuit 10 separates each carrier. And this FF
The carrier wave separated by the T circuit 10 is a CW extraction circuit 1
7, and the CW symbol is extracted therefrom.

The detection of the CW symbol by the CW extracting circuit 17 is performed by utilizing the difference between the waveforms of the CW symbol portion and the information symbol portion in each carrier. That is, in each carrier, the information symbol contains various frequency components, and thus has a waveform similar to the waveform of random noise, whereas the CW symbol is a carrier signal composed of a single frequency component. Waveform,
Or it becomes a fixed value direct current. Therefore, by utilizing the difference between the waveforms, the position of the CW symbol can be obtained.

In order to more reliably obtain the detection of the CW symbol, as shown in FIG. 2C, a symbol for synchronization other than the CW symbol, for example, NULL having no signal is used. A symbol may be inserted, and the position of the CW symbol may be obtained by detecting the NULL symbol.

Next, a method of detecting a frequency shift using the CW symbol will be described. As already explained,
Due to the oscillation accuracy of the local oscillators of the transmitting device and the receiving device, the temperature change thereof, or the Doppler effect accompanying the movement of the device, the carrier frequency of the received signal is the original carrier frequency that should be the output signal of the FFT circuit 10 of the receiving device. (Referred to as the receiving carrier frequency).

Since the signal processing by the FFT circuit 10 is a discrete Fourier transform, it is separated into components at discrete frequency points indicated by points in FIG. 3B. For this reason, for example, the frequency distribution near the CW symbol in the reproduced baseband OFDM signal is as shown in FIG. 3B. At this time, the CW symbol position of the received signal is determined by the carrier wave used for transmission. , Ie, the position of the number n0 shown in FIG. 3A, is shifted by the frequency ΔFL.

Therefore, as shown in FIG. 3B, when the frequency position of the received carrier wave is shifted from the discrete frequency points (hereinafter simply referred to as frequency points), FIG. As shown in (1), a low-level component appears not only at the frequency point closest to the frequency of the carrier wave but also at a frequency point adjacent to the frequency point.

The size of the component obtained at this time is shown in FIG.
(c) shows a sinc function (sin (x) / x) centered on the original carrier frequency as indicated by an envelope E formed by a broken line.
The size is defined by That is, the magnitude of the component obtained by each carrier changes depending on the amount of deviation of the received signal frequency from the frequency point.

Therefore, the present invention is characterized in that the deviation of the carrier frequency is detected by using this property. In the embodiment of FIG. 1, the adjusting circuit 20 is a circuit for this purpose. , A coarse adjustment circuit 18 and a sub-fine adjustment circuit 19 are provided separately.

First, the coarse adjustment circuit 18 controls the reproduced OFD
It functions to detect the amount of deviation ΔFL between the carrier frequency of the CW symbol portion of the M signal and the frequency point determined by the FFT circuit 10 with a coarse accuracy, and as shown in FIG.
It comprises a number detection circuit 181, a NEXT number detection circuit 182, and a shift amount calculation circuit 183.

The CW symbol signal extracted by the CW extraction circuit 17
1 and the complex vector signal Zcw (n) at each frequency point n shown in FIG. The number nmax of the frequency point at which the absolute value level of the vector signal Zcw (n) becomes maximum is obtained.

Next, in the NEXT number detection circuit 182,
The complex vector signal Zcw (nmax−n) of the frequency point on both sides of the frequency point of the number nmax detected by the MAX number detection circuit 181
Among 1) and Zcw (nmax + 1), the number nnext of the frequency point having the larger absolute value level is detected.

Further, in the shift amount calculating circuit 183, the number n0 of the carrier wave inserted as a CW symbol by the transmitting device is used.
, The frequency point number nmax detected by the MAX number detection circuit 181, and the complex vector signal Zcw of the frequency point.
(Nmax), the number nnext of the frequency point detected by the NEXT number detection circuit 182, and the absolute value level R of the complex vector signal Zcw (nnext) at that frequency point.
From next, the following formula (1) is used to calculate the carrier frequency deviation Δ
Calculate FL. ΔFL = fs × {(Rmax × nmax + Rnext × next) / (Rmax + Rnext) −n0} (1)

Then, the deviation ΔFL calculated by the coarse adjustment circuit 18 is supplied to the local oscillator 9 such as a VCO (voltage controlled oscillator).
L decreases, and the local oscillation frequency fr is controlled in the direction of convergence.

The calculation of the deviation .DELTA.FL by the coarse adjustment circuit 18 is performed by a considerably simplified calculation process, so that only coarse accuracy can be obtained, but a quick response can be obtained.
As a result, the carrier frequency of the CW symbol portion of the reproduced OFDM signal is reduced to a frequency of about １ ／ or less of the carrier with respect to the frequency point of number n0 defined by the FFT circuit 10 on the receiving device side. It can be approached quickly.

Next, the semi-fine adjustment circuit 19 outputs the reproduced OF
The carrier frequency of the CW symbol part of the DM signal and the FFT
The function of detecting the deviation amount ΔFL from the frequency point determined by the circuit 10 with higher accuracy than that of the coarse adjustment circuit 18 is shown in FIG.
A shift amount detection circuit 192 is provided.

The signal of the CW symbol extracted by the CW extraction circuit 17 is first transmitted to the MAX
The number nmax of the frequency point at which the absolute value level of the complex vector signal becomes maximum is input to the number detection circuit 191.

Next, in the shift amount calculating circuit 192, the number n0 of the carrier wave inserted as a CW symbol on the transmitting device side
, The number nmax of the frequency point detected by the MAX number detection circuit 191, the complex vector signal Zcw (nmax), the conjugate complex vector signal Zcw ^* (nmax), and the absolute value level Rmax () of the complex vector signal Zcw (nmax). nmax) and the number n
The magnitude of the complex vector signal Zcw (nmax-1) of the next frequency point whose frequency is lower than the frequency point of max (the carrier of the receiving device), and the complex vector signal Zcw of the next frequency point whose frequency is higher than the frequency point of number nmax From the value of (nmax + 1), the deviation ΔFL of the carrier frequency is calculated by the following equation (2). ΔFL = fs × [(Zcw ^* (nmax) / Rmax) × {(Zcw (nmax + 1) / Rmax) − (Zcw (nmax−1) / Rmax)} − n0] (2)

The detection of the carrier frequency deviation by the quasi-fine adjustment circuit 19 is configured to be performed after the above coarse frequency adjustment is performed, and the oscillation frequency of the local oscillator 9 is again determined based on the detection result. Is controlled.

Here, the arithmetic processing in the semi-fine adjustment circuit 19 is a more advanced arithmetic processing than the arithmetic processing executed in the coarse adjustment circuit 18, so that a long arithmetic time is required. However, since the coarse adjustment has been performed, the frequency point number nmax matches the carrier number n0 of the CW symbol that has already been inserted on the transmitting device side during the adjustment by the sub-fine adjustment circuit 19.

Therefore, the quasi-fine adjustment circuit 19 can use the complex vector signals of the frequency points on both sides which are previously set to the carrier number n0 = nmax, so that the above operation can be executed immediately. , Carrier wave 1 /
The frequency deviation can be detected with a high accuracy of 16 lines or less, and the oscillation frequency of the local oscillator 9 is controlled again by using the considerably high-precision detection value. Therefore, control with high accuracy can be easily obtained. it can.

Therefore, according to this embodiment, even if there is a frequency shift exceeding the carrier frequency interval fs between the carrier frequency of the reproduced OFDM signal and the frequency point determined by the FFT circuit 10, the carrier wave Frequency deviation ΔF
L can be detected correctly, thereby achieving synchronization and correctly demodulating the information code.

Further, by performing the sub-fine adjustment of the local oscillator 9 in this manner, the deviation of each carrier frequency of the baseband OFDM signal can be reduced by the carrier 1 / the carrier which is the application limit of the above-described deviation detection method according to the second prior art. The accuracy can be suppressed to 16 or less. Therefore, according to this embodiment, it is possible to use together with the second conventional deviation detection method. Even if the interval is longer than fs, it is possible to reliably synchronize and demodulate a high-quality information code having a low code error rate.

As is apparent from the above description, the MAX number detection circuit 19 of the sub-fine adjustment circuit 19 shown in FIG.
1 merely serves to confirm that nmax and n0 match, and is therefore not always necessary and may be omitted, or may be omitted from the coarse adjustment circuit 18 of FIG.
Obviously, the AX number detection circuit 181 may be shared.

By the way, it is preferable that the average transmission power of the CW symbol is equal to the average transmission power of the other information symbols. In the receiving apparatus, in order to make the input signal level of the FFT circuit 10 substantially constant, it is usual to use an AGC circuit controlled by the amount of fluctuation of the envelope level of the received signal at the preceding stage. If the transmission power is set as described above, the envelope waveform of the received signal becomes almost constant irrespective of the symbol position, so that the gain of the AGC circuit is affected by a special symbol such as a CW symbol. It can be prevented from fluctuating and becoming unstable.

If the number of carriers to be multiplexed is set to, for example, 1000, the transmission power of one carrier in the information symbol portion is only 1/1000 of the total transmission power, whereas the transmission power of the CW symbol is less than 1/1000. Since the total transmission power can be concentrated on one carrier signal, a CW symbol signal improved by 30 dB from the S / N of one carrier signal of the information symbol can be obtained. As a result, the influence of noise can be obtained. And a highly accurate detection value can be obtained.

In the embodiment shown in FIG. 1, on the transmitting apparatus side, the CW insertion place securing circuit 15 secures a time for inserting a CW symbol in a stage preceding the IFFT circuit 3, and
Although the CW symbol is inserted after the signal processing by the FFT circuit 3, the number n to be transmitted by the CW symbol is used instead.
A CW symbol may be directly inserted by a method of inputting a signal only to the carrier 0 and not inputting a signal to the other carriers.

However, at this time, as described above, CW
When a method of setting the signal level of a symbol to a value larger than one carrier signal level of an information symbol and transmitting the symbol is applied, the arithmetic processing performed in the IFFT circuit 3 does not exceed the dynamic range of the circuit. You need to be careful.

Next, another embodiment of the coarse adjustment circuit 18 will be described with reference to FIG. The coarse adjustment circuit 18 'according to the embodiment of FIG. 6 omits the NEXT number detection circuit 182 in the coarse adjustment circuit 18 shown in FIG. 4, and further performs the following arithmetic processing in place of the shift amount arithmetic circuit 183. The shift amount calculating circuit 184 is used to calculate the shift amount.

That is, in the shift amount calculating circuit 184, as shown in the following equation (3), the number n0 of the carrier inserted as a CW symbol on the transmitting device side and the frequency point detected by the MAX number detecting circuit 181 are calculated. From the difference between the numbers nmax,
This is to calculate a deviation amount ΔFL of the carrier frequency. ΔFL = fs × (nmax−n0) (3)

The detection accuracy of the carrier frequency shift by the coarse adjustment circuit 18 shown in FIG. 6 is about 1/2 of the carrier wave, so that the shift amount of the carrier frequency is just
When the number of the carrier waves becomes 1/2, the detection may exceed the half of the carrier wave erroneously due to the influence of subtle noise. However, even in this case, since the coarse adjustment is repeatedly executed, the coarse adjustment circuit 18 'of FIG. 6 can obtain an effect that is not inferior to that of the embodiment of FIG. The advantage is that the amount is kept small.

In the above embodiment, the coarse adjustment circuit 1
8 and the semi-fine adjustment circuit 19 are used together.
In this case, the present invention is not limited to the combination of the circuit of FIG. 4 and the circuit of FIG. 5, but may be implemented by combining the circuit of FIG. 6 or by using only one circuit. Nor.

[0083]

According to the present invention, even if there is a frequency shift exceeding the frequency interval of the carrier between the carrier frequency of the OFDM signal reproduced from the received signal and the frequency point determined by the FFT circuit of the receiving apparatus. In addition, the carrier frequency deviation can be correctly detected, the synchronization can be drawn, and the information code can be correctly demodulated.

Therefore, according to the present invention, a small and inexpensive oscillator can be used instead of an expensive and large-sized high-performance oscillator. As a result, the transmission device itself can be configured to be small and inexpensive. it can.

[Brief description of the drawings]

FIG. 1 is a block circuit diagram showing an embodiment of an orthogonal frequency division multiplex modulation signal transmission system according to the present invention.

FIG. 2 is an explanatory diagram showing a method of inserting a CW symbol in one embodiment of the present invention.

FIG. 3 is an explanatory diagram illustrating a frequency distribution of a CW symbol signal according to an embodiment of the present invention.

FIG. 4 is a block diagram of a coarse adjustment circuit according to an embodiment of the present invention.

FIG. 5 is a block diagram of a sub-fine adjustment circuit according to an embodiment of the present invention.

FIG. 6 is a block diagram illustrating another example of the coarse adjustment circuit according to the embodiment of the present invention.

FIG. 7 is a diagram illustrating an arrangement of carrier waves in an orthogonal frequency division multiplex modulation signal transmission system.

FIG. 8 is a block circuit diagram showing an example of a conventional orthogonal frequency division multiplex modulation signal transmission system.

FIG. 9 is a diagram illustrating a problem of a carrier frequency deviation detection method according to the related art.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 QPSK modulation circuit 2 Distribution circuit 3 IFFT circuit 4, 8 Mixer 5 Transmitting local oscillator 6 Transmitting antenna 7 Receiving antenna 9 Receiving local oscillator 10 FFT circuit 11 Coupling circuit 12 QPSK demodulation circuit 13 Pilot insertion circuit 14 Pilot detection circuit 15 CW Insertion location securing circuit 16 CW insertion circuit 17 CW extraction circuit 18 Rough adjustment circuit 19 Semi-fine adjustment circuit 20 Adjustment circuit 21 MAX number detection circuit 22 NEXT number detection circuit 23, 24, 25 Deviation amount calculation circuit

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Seiichi Sano 32 Miyukicho, Kodaira-shi, Tokyo Inside Hitachi Electronics Co., Ltd. Koganei Plant (72) Inventor Nobuo Tsukamoto 32 Miyukicho, Kodaira-shi, Tokyo Koganei Plant (72) Inventor Kenichi Tsuchida 1-10-11 Kinuta, Setagaya-ku, Tokyo Japan Broadcasting Corporation Research Institute (72) Inventor Shigeki Moriyama 1-10-11 Kinuta, Setagaya-ku, Tokyo Japan Broadcasting Corporation Inside the Technical Research Institute

Claims (6)

[Claims] 1. An orthogonal frequency division multiplexing modulation signal transmission system for transmitting an information code by using Ns carrier waves that have mutually equal frequencies fs or integer multiples of the frequency fs and that are orthogonal to each other, On the transmitting side, Ns is used as the ratio of one symbol to Nf symbols (Nf is a positive integer of 2 or more) of the orthogonal frequency division multiplex modulation signal digitally modulated using the time interval Tsy as a symbol period.
A predetermined symbol (CW symbol) consisting of a signal containing a signal component only in one predetermined carrier of the one carrier
On the receiving side, the amount of deviation ΔFL of the carrier frequency appearing in the baseband orthogonal frequency division multiplex modulation signal obtained by frequency conversion by the local oscillation signal is determined by the discrete Fourier transform of the CW symbol signal inserted in the signal. Calculating based on the complex vector signals Zcw (n) of a plurality of carriers obtained by conversion (n is the number of the separated carrier), and controlling the frequency of the local oscillation signal based on the calculated shift amount ΔFL. Wherein the deviation ΔFL is controlled to converge. 2. The apparatus according to claim 1, wherein the means for calculating the shift amount ΔFL on the receiving side includes:
The first means and the second means, the second means being more accurate than the accuracy of calculating the shift amount ΔFL by the first means.
The orthogonal frequency division multiplex modulation signal transmission system is configured to increase the calculation accuracy of the shift amount ΔFL by the means. 3. The method according to claim 1, wherein the means for calculating the shift amount ΔFL on the receiving side comprises: a carrier wave number n0 of the CW symbol inserted on the transmitting side.
A third means for calculating the shift amount ΔFL from the carrier number nmax having the largest absolute value level of the complex vector signal Zcw (n) of the CW symbol portion and the carrier number being closest to the number n0. An orthogonal frequency division multiplex modulation signal transmission system, characterized in that: 4. The invention according to claim 1, wherein the means for calculating the shift amount ΔFL on the receiving side comprises: a number n0 of a carrier of a CW symbol inserted on the transmitting side.
And the absolute value level of the complex vector signal Zcw (n) of the CW symbol portion, and the carrier number nmax whose carrier number is closest to the number n0, and the absolute value of the complex vector signal Zcw (nmax) The level Rmax and the number nma
The deviation amount is determined from the number nnext of the carrier having the greater absolute value level of the complex vector signal and the absolute value level Rnext of the complex vector signal Zcw (nnext) of the carrier having the larger absolute value level of the complex vector signal among the carriers adjacent to the carrier x. Fourth for calculating ΔFL
And an orthogonal frequency division multiplex modulation signal transmission system. 5. The method according to claim 1, wherein the means for calculating the amount of deviation ΔFL on the receiving side comprises: a carrier wave number n0 of a CW symbol inserted on the transmitting side.
The complex vector signal Zcw (nmax), wherein the absolute value level of the complex vector signal Zcw (n) of the CW symbol portion is the largest, and the carrier number is the carrier number nmax closest to the number n0; Complex vector signal Zcw (nma
x) and the complex vector signal Zcw (nmax-nmax) of the adjacent carrier having a lower frequency than the carrier of the number nmax.
Fifth means for calculating the deviation amount ΔFL from the magnitude of 1) and the magnitude of the complex vector signal Zcw (nmax + 1) of the adjacent carrier having a higher frequency than the carrier of the number nmax. Orthogonal frequency division multiplexing modulation signal transmission system. 6. The invention according to claim 1, wherein an average power of a CW symbol inserted on the transmission side is equal to an average power of an information symbol portion for transmitting an information code. Orthogonal frequency division multiplexing modulation signal transmission system.