JP3518757B2 - Orthogonal frequency division multiplex signal transmission apparatus and orthogonal frequency division multiplex signal transmission method - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention relates to an OFDM (Orthogonal Frequency Division Multiplexing Orthogonal Frequency).
cy Division Multiplexing)
The present invention relates to a signal transmitting apparatus, and more particularly to an OFDM signal transmitting apparatus and an OFDM signal transmitting method for receiving an OFDM signal suitable for digital mobile communication.

[0002]

2. Description of the Related Art A conventional OFDM signal transmitting apparatus will be described with reference to FIG. First, the digital information data signal is supplied to the serial-parallel conversion circuit 70 via the input terminal,
An error correction code is added as needed. The output signal of the circuit 70 is supplied to the IFFT circuit 71, and the output signal thereof is supplied to the D / A converter 73 via the guard interval circuit 72 for reducing multipath distortion. Here, it is converted into an analog signal and the next LPF 7
4 allows only the components in the required frequency band to pass. The output signal of the real, imaginary part of analog value is supplied to the quadrature modulator 75, and the OFDM signal is output.

This OFDM signal is frequency-converted into a frequency band to be transmitted by a frequency converter 76 and supplied to the next transmitting section 77, and is transmitted via a linear amplifier and a transmitting antenna constituting the same. To be done. The output signal of the intermediate frequency generation circuit 78 and the signal passed through the 90 ° shift circuit 78A are supplied to the quadrature modulator 75, respectively. The output signal of this circuit 78 is supplied to the clock signal generation circuit 79. The output clock signal of the circuit 79 is supplied to the serial / parallel conversion circuit 70, the IFFT circuit 71, the guard interval circuit 72, and the D / A converter 73, respectively.

Next, a conventional OFDM signal receiving apparatus for receiving the OFDM signal transmitted with FIG. 6 will be described. The receiving section 80 amplifies the signal from the transmitting section 77 obtained by the receiving antenna constituting the receiving section 80 by a high frequency amplifier, and through a frequency converter 81 for converting a carrier frequency to an intermediate frequency, an intermediate frequency amplifier circuit. 82 and then to the quadrature demodulator 83. The output signal of the circuit 82 is supplied to the intermediate frequency generation circuit 89 via the carrier detection circuit 90. The output signal of the circuit 89 and the signal passed through the 90 ° shift circuit 89A are supplied to the quadrature demodulator 83, respectively, and the output signals of the real and imaginary reparts are decoded. The output signal of the quadrature demodulator 83 is the LPF 84.
Is supplied to the A / D converter 85 via the, and converted into a digital signal, and the output signal of the quadrature demodulator 83 is also supplied to the synchronization signal generation circuit 91.

The output of the A / D converter 85 is passed through the next guard interval circuit 86 to the FFT / QAM decoding circuit 8
7 is supplied. The FFT / QAM decoding circuit 87 is based on the supplied synchronization signal of the synchronization signal generation circuit 91,
Performs a complex Fourier operation and outputs the real and imaginary parts of the input signal for each frequency (real part, imaginary part)
, The level of the quantized digital signal transmitted by the carrier for transmitting digital information is determined, and the digital information is decoded. The output signal of the FFT / QAM decoding circuit 87 is output via the parallel-serial conversion circuit 88. Here, if the intermediate frequency of the transmitting device and the intermediate frequency of the receiving device are completely the same, only the modulation component is obtained, and there is no problem, but the intermediate frequency generating circuit, the local oscillator of the frequency converter (not shown). If the frequency stability is not high, or if there is a phase error between both output signals, it will affect the subsequent demodulation operation and the probability of occurrence of symbol error will increase.

[0006]

In transmitting and receiving an OFDM signal, it is very difficult to completely reproduce the phases of all carriers on the receiving side without having a time-axis fluctuation component. , In order to reduce multipath distortion, the guard interval circuit is set on the transmission side. Therefore, when receiving a transmission signal under such conditions, the effective symbol period part and the guard interval part There is a problem that it is more difficult to reproduce the phase in exactly the same state as the transmitting side. The present invention has been accomplished in view of the above problems, setting the OFDM specific carrier as a pilot signal for carrier, thereby, transmission OFDM signal that is to be able to hold a recipient of the synchronization related constant signal Device and OFDM signal transmission
The purpose is to provide a communication method .

[0007]

The present invention includes the following 1) or
Or the means described in 2) . That is, 1) an IFFT and a pilot signal generation circuit which are supplied with a digital information signal and generate a multilevel QAM modulated signal;
A guard interval setting circuit configured to repeatedly transmit a part of the modulated signal for a predetermined time, and a clock signal generation circuit for generating a clock signal for driving both circuits, and the IFFT and pilot signal generation The circuit keeps the phase at the start point of multiple effective symbol sections in phase with each other in adjacent effective symbol sections
And the amplitude is held constant , and a pilot signal of a higher frequency having an integer frequency ratio relationship with the clock signal is generated as a discontinuous waveform at the head in the guard interval section set by the guard interval setting circuit.
And to be present as a signal of a single frequency after being multiplied by 4 so as to be continuous over a plurality of the symbol intervals.
1. An orthogonal frequency division multiplex signal transmission device characterized in that it is configured to transmit the signals in an orthogonal manner. 2) The supplied digital information signal is converted into a predetermined clock
Multi-valued QA including pilot signal by IFFT based on signal
M-modulated signal is generated, and a part of the multilevel QAM-modulated signal is generated.
Generate a guard interval signal repeatedly for a predetermined time,
The generated guard interval signal is converted into the multilevel QA.
Orthogonal frequency division multiplexing signal transmitted before M-modulated signal
Signal transmission method, start of multiple effective symbol intervals
The phases at the points are mutually adjacent in the effective symbol section.
In-phase, the amplitude is kept constant, and
Higher frequency that has an integer frequency ratio relationship with the clock signal
A first step of generating a wave number pilot signal, and
The pilot signal generated in the first step is
The head does not move within the set guard interval section.
Exist as a continuous waveform and multiply by 4 to obtain a single frequency
As a signal of the plurality of symbol
The second step of continuously sending over the
Orthogonal frequency division multiplexing signal transmission characterized by
Belief method.

[0008]

DETAILED DESCRIPTION OF THE INVENTION Orthogonal Frequency Division Multiplexed Signals of the Invention
Applied to transmitter and orthogonal frequency division multiplexing signal transmission method
For example the OFDM signal transmitting and receiving device that is, with reference to FIGS. 1 to 4 of the accompanying will be described below. Figure 1
It is an embodiment of an OFDM signal transmitting apparatus, and the digital data transmitted here is compressed audio, video signal, and the like. The OFDM signal transmission device arranges a large number of carriers orthogonally and transmits independent digital information in each carrier. Since the carriers are orthogonal to each other, the spectrum of adjacent carriers is the frequency position of the carrier. It becomes zero. IFFT circuit technology is used to create this orthogonal carrier. Inverse DF with N complex numbers during a time interval T, which is a window interval in IFFT
An OFDM signal can be generated by executing T (Discrete Fourier Transform), and each point of the inverse DFT corresponds to the output of the modulation signal.
The N is also called an IFFT or FFT cycle. For details, see pages 74-75 of "Signal Processing Engineering" by Sei Imai, published by Corona Publishing (published on May 20, 1993). It is explained in.

The basic specifications of the apparatus according to this embodiment shown in FIGS. 1 and 2 are as follows. (a) Central carrier frequency ... 100 MHz (b) Number of carriers for transmission ... 248 waves (c) Modulation method ... 256QAM OFDM (d) Number of carriers used ... 257 waves (e) Transmission bandwidth ... 100 kHz, Bandwidth used ... 99 k
Hz (f) Transfer rate ... 750 kbps (g) Guard interval ... 60.6 μsec As shown in FIG. 1, for example, a digital information signal, which is an audio or video signal whose information signal is compressed by an encoding system such as MPEG, It is supplied to the serial-parallel conversion circuit 2 via the input terminal 1, and an error correction code is added as necessary. In this circuit 2, the input signal is arranged and output as a 256QAM modulation signal. This 256QA
M-modulation defines 16 levels in the amplitude direction and 16 levels in the angle direction for each carrier to transmit information.
This is a method of specifying and transmitting 6 × 16 256 values.
In this embodiment, of the 257 wave carriers, 248 waves are used to transmit information, and the remaining 9 waves are used for calibration and for transmitting other auxiliary signals.

The serial-parallel conversion circuit 2 is configured to output 248 bytes of digital data in one symbol period, that is, 248 sets of parallel data of 4 bits each in one symbol period. The output signal of the serial-parallel conversion circuit 2 is IF
It is supplied to the FT and pilot signal generation circuit 3. This circuit 3 operates according to the clock signal output from the clock signal generation circuit 10, and operates on a 248-wave carrier with 2
56QAM modulation is performed and each output signal is output as a real and imaginary component. Further, the IFFT / pilot signal generation circuit 3 uses an IFFT circuit having a cycle N, and discrete frequency point information corresponding to N discrete frequency points (sample points) in each effective symbol period set by this IFFT circuit. Is output from the IFFT / pilot signal generation circuit 3. The Nyquist frequency is 1 of the sample clock frequency in the IFFT having the cycle N.
Corresponding to / 2, and the pilot signal is transmitted as information that the Nyquist frequency has, that is, Nyquist frequency information. Since the Nyquist frequency is 1/2 of the sample clock frequency, it is possible to generate a sampling position signal (sample clock signal) for operating the FFT circuit by decoding and multiplying the Nyquist frequency information by the receiving device. . This Nyquist frequency information is the IFFT, real part input terminal R of the IFFT of the pilot signal generation circuit 3.
It is obtained by applying a constant level signal to the N / 2th frequency terminal in the (imaginary part input terminal I).

The output signals of these IFFT and pilot signal generation circuits 3 are supplied to a guard interval setting circuit 4 having a next RAM (random access memory) 4A. A guard interval gi of a predetermined section for reducing distortion is set as shown in FIG. The guard interval setting circuit 4 operates according to the clock signal output from the clock signal generation circuit 10, and sets the last part in the window section (effective symbol period ts) obtained from the IFFT and pilot signal generation circuit 3 to the window section. Place it just before. In order to set the guard interval, the guard interval setting circuit 4 reads the signal from the IFFT / pilot signal generation circuit 3 fetched in the RAM (4A) of the guard interval setting circuit 4 at the end of the effective symbol period ( (this period is set equal to gi.), the process returns to the beginning of the effective symbol period, the data of the effective symbol period ts is read, and the signal of the symbol period ta is transmitted. The Nyquist frequency information (pilot signal) is
It is transmitted even within the guard interval, but before and after IFF
In order to maintain continuity with the T-window section signal, the transmitted pilot signal has an integral wavelength within the guard interval.

Although the case where the Nyquist frequency is used as the pilot signal has been described, the Nyquist frequency does not necessarily have to be the Nyquist frequency as long as it has a simple integer ratio with the sample clock signal. You may use the thing. When considering an IFFT having a period M, a pilot signal is arranged at a position of 1/2 of the Nyquist frequency, that is, at the M / 4th frequency, and the carriers transmitted by OFDM are M / th to M / th in the IFFT.
The signals output up to the 4th and from the 3rd M / 4 to the Mth are used. Thus, the cycle M = 2N
The IFFT output signal equivalent to that when the IFFT having the cycle N is used can be obtained by using the IFFT. Therefore, it is possible to transmit a continuous pilot signal including the guard interval, and to decode this pilot signal,
A sample clock signal can be obtained by multiplying by 4. If the FFT window section signal information can be decoded separately, the FFT operation of the OFDM signal can be performed by combining with the sample clock signal obtained in this embodiment, and the OFD
It is possible to decode the M signal.

Next, the symbol period set by the guard interval setting circuit 4 will be described with reference to FIG. First, the used bandwidth is 99 kHz and the IFFT cycle is N = 25.
When it is 6, the effective symbol frequency fs and the effective symbol period ts are as follows, respectively. fs = 99,000 / 256 = 387 Hz ts = 1 / fs = 2586 μsec Further, when the guard interval period gi, which is a section for multipath distortion removal, is determined to be three wavelengths of the pilot signal,
gi is set as follows. gi = (1 / 49,500) × 3 = 60.6 μsec The symbol period ta and the symbol frequency fa at this time are as follows. ta = ts + gi = 2586 + 60.6 = 2646.6
μsec fa = 1 / ta = 378 Hz

The output signals of these guard interval setting circuits 4 are supplied to the D / A converter 5, where they are converted into analog signals, and only the necessary frequency band components are passed through by the next LPF 6. Real analog value,
The imaginary output signal is supplied to the next quadrature modulator 7,
Further, the modulator 7 is supplied with the output signal of the 10.7 MHz intermediate frequency generation circuit 9 and the signal through the 90 ° shift circuit 8, respectively, and outputs an OFDM signal. This OF
The DM signal is frequency-converted into a frequency band to be transmitted by the frequency converter 11 and supplied to the next transmission unit 12, and is transmitted via the linear amplifier and the transmission antenna which configure the DM unit. The output signal of the 10.7 MHz intermediate frequency generation circuit 9 is also supplied to the clock signal generation circuit 10. In the clock signal generating circuit 10, the common clock signal supplied from the intermediate frequency generating circuit 9 is used as the clock signal for driving the IFFT / pilot signal generating circuit 3 and the clock signal for driving the guard interval setting circuit 4. It is generated based on. Incidentally, 248
Since a set of 4 + 4 bits of parallel data is transmitted by a carrier of 248 waves, the transmission rate of this device is 248 bytes per symbol period. Therefore, the transmission rate per second is approximately 750 Kbits.

Next, the phase relationship between the guard interval, the symbol period and the synchronizing signal (pilot signal) will be described below with reference to the drawings. Here, FIG. 7, FIG. 8 and FIG.
The description regarding 0 is described as a reference example of the present embodiment. In FIG. 7 shown as a reference example, a synchronization signal (pilot signal) of the same phase is generated in each symbol period,
A case where a synchronization signal having an integral wavelength exists in the guard interval will be described. (This is a first example of generating a continuous synchronizing signal without inverting the polarity.) IF shown in FIG.
FT is synonymous with the effective symbol period and the IFFT period, and one cycle at the end (right part) of the IFFT period is used as the signal of the guard interval G immediately before (left part) the IFFT period. In this example, the synchronization signal (pilot signal) having the same phase is generated for each IFFT period, and since the synchronization signal (pilot signal) is an integral wave in the guard interval section, the pilot signal is generated over a plurality of symbol periods. It is continuously generated.
The case of FIG. 3 already described is the same as the case of FIG. 7, and since the synchronization signal (pilot signal) is an integer wave in the guard interval section, the pilot signal is continuously generated over a plurality of symbol periods. There is.

Referring to FIG. 8 shown as a reference example, a case will be described in which a synchronizing signal (pilot signal) having the same phase is generated in every other symbol period and a synchronizing signal having an odd number of half wavelengths exists in the guard interval. . (This is a second example of generating a continuous sync signal without inverting the polarity.) IFFT is synonymous with an effective symbol period and an IFFT period, and is 1/2 cycle of the end portion (right part) of the IFFT period. Is directly used as the signal of the guard interval in the front part (left part) of the IFFT period. In this example, a sync signal (pilot signal) having an opposite polarity is generated every IFFT period, and a sync signal having an odd multiple of a half wavelength also exists in the guard interval section, so that a plurality of symbol sections (symbol periods) are included. The pilot signal is continuously generated.

In FIG. 9 according to the embodiment of the present invention, the case where the synchronizing signal exists in the guard interval G in an odd multiple of half the wavelength will be described. (This is a first example of generating a synchronization signal with inverted polarity.) In this case, the polarity of the pilot signal is inverted at the start point of the guard interval, and the phase of the pilot signal in each symbol period is in phase. . That is, the terminal voltage corresponding to the frequency for generating the synchronizing signal of the IFFT for generating the frequency division multiplexed signal is constant for each symbol, and the synchronizing signal of the same phase is always generated. Therefore, when the guard interval is an odd multiple of half the wavelength, the sync signal becomes a continuous signal when the polarity of the sync signal is inverted every other symbol period on the receiving device side. In this case, the synchronization signal can be detected by using the PLL circuit in the phase synchronization circuit as shown in FIG.

Referring to FIG. 10 shown as a reference example, a case will be described in which the synchronizing signal (pilot signal) exists in the guard interval at an even multiple of a half wavelength. (This is a second example of generating a sync signal with the polarity reversed.) As shown in FIG. 10, when the sync signal (pilot signal) present in the guard interval is an integer wave (even multiple of half wavelength) Even if there is a sync signal, as in the case of FIG. 9, if the sync signal is inverted and output every other symbol period, a sync output in which the polarity is inverted for each symbol is obtained. Also in this case, the PLL circuit as shown in FIG. 11 can be used to detect the synchronization signal.

FIG. 11 shows a phase synchronization circuit for detecting a synchronization signal which is inverted every other symbol period. This phase synchronization circuit includes a phase comparator PD2 (112), Amp
This is a configuration in which a signal switcher 116 composed of an exclusive OR is inserted in the VCO output of the PLL circuit composed of the (amplifier 113), the LPF (114) and the VCO circuit (115). Phase comparator PD1 (111)
Constitute a synchronous detection circuit which receives the VCO output of the phase locked loop as an input. The frequency division division signal including the synchronization signal applied to the input terminal 110 is input to both the phase synchronization circuit and the synchronization detection circuit PD1 (111). This phase lock circuit includes a phase comparator PD2 (112), an amplifier (1
13), a LPF (114), a VCO (115), and a signal switching device (116). According to the output of the PD1 (111) which is synchronously detected, the signal switch (1
Although the output of the VCO circuit 115 of the PLL is inverted in 16), the synchronization signal whose polarity is inverted for each symbol is detected by the synchronous detection circuit and the phase comparator PD2 (112) forming the PLL. The polarity is inverted to V
Since the CO output is supplied, the lock operation is continuously performed even for the sync signal whose polarity is inverted.

FIG. 12 shows output waveforms of the terminals B and A in FIG. The output A is a sync signal output waveform, and the output B is a symbol sync signal which is transmitted with its polarity inverted every symbol period (symbol period). FIG. 13 shows another circuit example for FIG. 11, in which the signal switch 136 is the phase comparator PD2.
It is inserted between (132) and the amplifier 133. At the same time that the synchronization signal is inverted, it is detected and the polarity of the error signal is inverted, and the operation mode is the same as in FIG. In either case, even if the synchronization signal is inverted every other symbol period (symbol period), it is detected and PL
Since the characteristics of the L loop are inverted, the VCO continues to operate without being inverted. Therefore, the synchronization signal can be decoded normally.

Next, an embodiment of the OFDM signal receiving apparatus applied to the present invention will be described with reference to FIGS. Each component of the receiving device is composed of a circuit that operates in reverse to the transmitting device. The receiving unit 20 amplifies the signal from the transmitting unit 12 obtained by the receiving antenna constituting the receiving unit 20 by a high frequency amplifier and supplies the amplified signal to the frequency converter 21. This output signal is supplied to the intermediate frequency amplifier circuit 22,
The signal is output from the intermediate frequency amplifier circuit 22 as a reception signal of a predetermined level. The output signal of the intermediate frequency amplifier circuit 22 is
Quadrature demodulator 23 and carrier detection (carrier extraction) circuit 2
9 and 9 are supplied respectively. The carrier detection circuit 29 is shown in FIG.
, A phase comparator (multiplier) 41, an LPF 42, V
PLL composed of a CO circuit 43 and a 1/4 frequency dividing circuit 45
The intermediate frequency oscillating circuit 31 having a circuit and supplied with this output signal is a circuit for extracting the center carrier with a small phase error.

In this embodiment, carriers for transmitting information are arranged adjacent to each other at a symbol frequency of 378 Hz to form an OFDM signal. Since the information carrier adjacent to the center carrier is also 378 Hz apart, the center carrier needs to transmit information without being affected by the adjacent information carrier, and a circuit with high selectivity is used. In this embodiment, the central carrier is extracted using the PLL circuit, but the carrier frequency interval between adjacent carriers is approximately 1
Quartz crystal oscillator (V
CXO) is used as the voltage controlled oscillator (VCO) 43,
Activate the circuit. The LPF used in the PLL circuit also has a cutoff frequency sufficiently low with respect to 378 Hz. The output signal of the intermediate frequency generation circuit 31 and the signal passed through the 90 ° shift circuit 30 are multipliers 40,
The output signals of the real and imaginary reparts (real number part, imaginary number part) are supplied to the quadrature demodulators 23 each having 41, and are decoded. The output signals of the real part and the imaginary part are LPF2
4, the signal of the required frequency band transmitted as the OFDM signal information is passed, the input analog signal is sampled, and the output signal is supplied to the A / D converter (sampling circuit) 25. Convert to digital signal.

In the sample synchronizing signal generating circuit 32, the sample clock signal before frequency multiplication is generated by the PLL circuit which is in phase synchronization with the pilot signal, and the analog output signal of the quadrature demodulator 23 is supplied to this circuit.
The PLL is phase-synchronized with the pilot signal transmitted as a continuous signal in each symbol section including the guard interval period, and a demodulated pilot signal is obtained. In the transmitter, the pilot signal is set to a predetermined integer ratio with respect to the sample clock frequency, and frequency multiplication is performed according to the frequency ratio to obtain the sample clock signal. The guard interval processing circuit 26 can obtain the effective symbol period signal of the period ts at an arbitrary timing within the symbol period ta from the transmitted signal, and selects the effective symbol period signal of which the influence of the multipath distortion is small from among them. Then, the output signal is supplied to the FFT / QAM decoding circuit 27.

The symbol synchronization signal generation circuit 33 for detecting the symbol period detects the symbol period. The next FFT, QAM decoding circuit 27 is supplied with the obtained clock synchronization signal and symbol synchronization signal,
Performs a complex Fourier operation and outputs the real and imaginary parts of the input signal for each frequency (real part, imaginary part)
Ask for the level of. The real part of each frequency obtained in this way, the imaginary part signal level, and the real part of each carrier to be transmitted, comparing the demodulation output of the reference carrier for transmitting the reference value of the imaginary part, The level of the quantized digital signal transmitted by the carrier for transmitting digital information is obtained, and the digital information is decoded. This circuit 2
The output signal of No. 7 is output via the parallel-serial conversion circuit 28.

Next, together with FIG. 4, the carrier detection circuit 29,
The sample synchronization (sample clock) signal generation circuit 32 will be described below. This circuit is intended to extract a pilot signal transmitted at a constant level and generate an accurate sample synchronization (sample clock) signal based on this. First, the VCO circuit 43 forming the carrier detection circuit 29 is oscillated at a frequency of 42.8 MHz which is four times the intermediate frequency of 10.7 MHz. VCO circuit 4
The output signal of 3 is supplied to the multipliers 40 and 41 via the 1/4 frequency dividing circuits 44 and 45, respectively. One multiplier 41
Is supplied to the LPF 42, components below the symbol frequency are extracted, and the output signal controls the VCO circuit 43. A loop formed by the multiplier 41, the LPF 42, the VCO circuit 43, and the frequency dividing circuit 45 constitutes a PLL circuit.

The intermediate frequency amplified signals are applied to the input terminals of the multipliers 40 and 41, and orthogonal decoding is performed by this circuit to obtain the output signals of the real number part and the imaginary number part. The sample synchronization signal generation circuit 32 is supplied with the real part output signal from the quadrature demodulator 23 and detects the Nyquist frequency component transmitted as a pilot signal. Variable division ratio circuit (V
The output signal of the VCO circuit 43 is supplied to the CO circuit) 50, and the division ratio is set to 1/426 to 1/438. The multiplier 52 in the sample synchronization signal generating circuit 32 is supplied with the output signal from the quadrature demodulator 23 and the signal from the VCO circuit via the 1/2 frequency dividing circuit 51, and operates as a phase comparator. To do.

As the output signal of the multiplier 52, only the error signal related to frequency control is passed by the LPF circuit 53. The delay circuit 54 and the adder circuit 55 are circuits for attenuating adjacent carrier components and have a symbol frequency of 387.
It has the characteristic of having a dip in Hz. VCO circuit
In the PLL circuit including the (division ratio variable circuit) 50, the multiplier 52, and the LPF 53, the VCO output signal synchronized with the continuous pilot signal included in the real part output signal of the quadrature demodulator 23 of the carrier extraction unit Oscillated, 99 kHz
Is output as the sample clock output signal of. In the above-described embodiment, the case where the IFFT having the cycle of 256 is used to generate the carrier of 257 waves has been described, but as another embodiment, an example of using the IFFT having the cycle of 512 will be described below. In the embodiment using the IFFT with a period of 512, the Nyquist frequency is not used as the pilot frequency, but a high-order frequency having a simple integer ratio relationship with the sample clock signal is used.

That is, when considering the IFFT of the period M, the pilot signal is arranged at the position of 1/2 of the Nyquist frequency, that is, the M / 4th frequency, and the carrier transmitted by OFDM is the first carrier in the IFFT. The signals output up to the M / 4th and from the 3rd M / 4th to the Mth are used. In this way, IF with cycle M = 2N
Even if the FT is used, an output signal of the IFFT equivalent to that when the IFFT of the cycle N is used can be obtained. Therefore, a continuous pilot signal including the guard interval can be transmitted, and a sample clock signal can be obtained by decoding this pilot signal and multiplying it by four.

In the sample synchronizing signal generating circuit used at this time, the frequency of the pilot signal is the same as that of the embodiment in which the cycle N is 256, but the FF shown in FIG.
The sample clock frequency for driving the T / QAM decoding circuit 27 is double that when the cycle N is 256. Accordingly, the doubled 198 kHz sample clock signal is output. Therefore, this sample sync signal generation circuit
The frequency division ratio of the frequency division ratio variable circuit 50 is half that of the above embodiment.
13 to 1/219, and the point that the frequency dividing ratio of the 1/2 frequency dividing circuit 51 is 1/4, and the other configurations are the same as those in FIG. 4, and the description thereof will be omitted. .

[0030]

According to the present prior invention of the claims according to the present invention, there is a discontinuous waveform in the head to guard interval
And a signal of a single frequency that is multiplied by 4 and exists next to
In phase with each other at the start points of the effective symbol intervals
A pilot signal of a higher frequency that is held and has a constant amplitude.
Signal is set to be continuously transmitted, and the frequency spectrum of a signal obtained by multiplying an actually transmitted pilot signal by 4 has a single frequency spectrum. Therefore, can decode a jitter-free clock signal on the receiving side, it makes it easy to set the same time relationship with the FF T operating in IFF T and the receiver operating at the transmission side, when performing IFFT operation It is possible to perform the FFT operation in a form close to, and more accurate information transmission becomes possible . In addition, an effective symbol adjacent to the pilot signal transmitted as an information signal is used.
Signal with a discontinuous waveform at the start of the
Valid symbol because the symbol synchronization information is inserted.
Even if they are held in phase with each other at the beginning of the interval
Regardless, etc. can be decoded synchronization signal before division synchronization signal incoming time at the receiving side has the effect of such a short time in such time Ji Yan'neru switched it is possible to perform decoding of a frequency division multiplex signal ing.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of an OFDM signal transmission apparatus of the present invention.

FIG. 2 is a block diagram of an embodiment of an OFDM signal receiving apparatus applied to the present invention.

FIG. 3 is a diagram showing a relationship between a symbol period and a guard interval of an OFDM signal according to the present invention.

FIG. 4 is a block diagram of a carrier extraction unit and a sample synchronization signal generation unit of an OFDM signal receiving apparatus according to an embodiment applied to the present invention.

FIG. 5 is a block diagram of a conventional OFDM signal transmitter.

FIG. 6 is a block diagram of a conventional OFDM signal receiving apparatus.

FIG. 7 is a diagram showing a relationship between a synchronization signal and a symbol period.

FIG. 8 is a diagram showing a relationship between a synchronization signal and a symbol period.

FIG. 9 is a diagram showing a relationship between a synchronization signal and a symbol period according to the present invention.

FIG. 10 is a diagram showing a relationship between a synchronization signal and a symbol period.

FIG. 11 is a diagram showing an example of a phase synchronization circuit.

FIG. 12 is a diagram showing an output waveform of a phase locked loop circuit.

FIG. 13 is a diagram showing another example of the phase synchronization circuit.

[Explanation of symbols]

2 serial-parallel conversion circuit 3 IFFT, pilot signal generation circuit 4 Guard interval setting circuit 4A RAM (random access memory) 5 D / A converter 6,24,42,53,114,134 LPF 7 Quadrature modulator 8,30 90 ° shift circuit 9,31 Intermediate frequency generation circuit 10 Clock signal generation circuit 11,21 Frequency converter 12 Transmitter 20 Receiver 23 Quadrature demodulator 25 A / D converter (sampling circuit) 26 Guard interval processing circuit 27 FFT, QAM decoding circuit 28 Parallel-serial conversion circuit 29 Carrier detection circuit 32 sample sync signal generator 33 Symbol synchronization signal generation circuit 40, 41, 52 Multiplier (phase comparator) 43, 50, 115, 135 VCO circuit 44,45 1/4 divider circuit 51 1/2 divider circuit 111, 112, 131, 132 Phase comparator (PD) 116, 136 signal switch

─────────────────────────────────────────────────── --Continued from the front page (56) Reference JP-A-56-134862 (JP, A) JP-A-56-158545 (JP, A) JP-A-60-52147 (JP, A) JP-A-6- 141020 (JP, A) JP-A-7-273741 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H04J 11/00

Claims (2)

(57) [Claims] 1. A multilevel QAM supplied with a digital information signal.
IFFT and pilot signal generation circuit for generating a modulation signal, guard interval setting circuit configured to repeatedly transmit a part of the modulation signal for a predetermined time, and clock signal for generating a clock signal for driving both circuits A generator circuit, the IFFT and pilot signal generator circuits keep the phases at the start points of a plurality of effective symbol sections in phase with each other in adjacent effective symbol sections, and
Is held constant , and a pilot signal of a higher frequency having an integer frequency ratio relationship with the clock signal is present as a discontinuous waveform at the head in the guard interval section set by the guard interval setting circuit , and Four
An orthogonal frequency division multiplex signal transmission device, characterized in that it is multiplied so as to be present as a signal of a single frequency, and is continuously transmitted over a plurality of the symbol intervals . 2. A digital information signal supplied to a predetermined clock.
IFFT based on the lock signal
Of the multilevel QAM modulated signal by generating a multilevel QAM modulated signal.
Generate a guard interval signal by repeating a part of the operation for a predetermined time.
The guard interval signal generated and
Orthogonal frequency division that is transmitted before the value-QAM modulated signal
In the method of transmitting multiple signals, The phases at the start points of multiple effective symbol intervals are
Are held in phase with each other in the effective symbol section
Both have a constant amplitude and are aligned with the clock signal.
The pilot signals of higher frequencies that are related to the frequency ratio of the numbers
A first step of generating, The pilot signal generated in the first step is
In the predetermined guard interval section that is set for
Exist as a discontinuous waveform, and multiply by 4 to obtain a single frequency
A plurality of said syn.
A second step of continuously sending out over the bol section, Orthogonal frequency division multiplexed signal characterized by having
How to send.