JP2007312000A - Orthogonal frequency division multiplexing modulation digital transmission device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an orthogonal frequency division multiplexing modulation digital transmission device capable of obtaining a stable decision on a peak level even in the presence of an adjacent wave. <P>SOLUTION: An adjacency state detection part 16 and a variable BPF 17 are provided to the reception side R of the OFDM digital transmission device. When the adjacent wave is present for an OFDM modulated wave input through a transmission system M, the adjacency state detection part 16 detects it and then band characteristics of the variable BPF 17 and a threshold for peak level detection by a synchronization detection part 4 are switched and controlled according to the state of the adjacent wave, for example, which of an upper adjacent wave and a lower adjacent wave the adjacent wave is. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an orthogonal frequency division multiplexing modulation digital transmission apparatus, and more particularly to an orthogonal frequency division multiplexing modulation digital transmission apparatus for relay transmission.

  In recent years, OFDM digital transmission devices have been used for mobile relay of television broadcasts, etc., and contributed to the enhancement of relay broadcast programs by stable transmission characteristics. Here, this OFDM (Orthogonal Frequency Division Multiplex) This is a kind of multi-carrier modulation system in which a large number of digital modulation waves are added, and is called orthogonal frequency division multiplexing modulation.

  And the signal by this OFDM is represented by numerical formula as follows.

  First, each carrier is a QPSK signal expressed by the following equation (1).

αk (t) = ak (t) ・ cos (2πkft) + bk (t) ・ sin (2πkft) ………… (1)
Here, k indicates a carrier number, ak (t) and bk (t) are data of the k-th carrier, and take a value [−1] or [1].

Therefore, if the number of carriers is N, the OFDM signal is a combination of N carriers, and is therefore expressed by the following equation (2).

  The OFDM signal is composed of the above signal units, and the signal unit symbol at this time is composed of 1056 samples in which, for example, 1024 samples are used as effective samples and guard interval data 32 samples are added thereto.

  A total of 400 symbols obtained by adding four more synchronization symbols to the 396 sets is called a frame, and an OFDM signal is formed by repeating the stream unit by this frame.

  As a result, according to the OFDM transmission apparatus, it is resistant to signal level fluctuations, and stable transmission can be obtained even while moving. Therefore, as described above, it has been conventionally used for mobile relay such as television broadcasting. (See, for example, Patent Document 1).

  Therefore, hereinafter, this OFDM transmission apparatus will be described with reference to the prior art of FIG.

  At this time, in FIG. 5, the upper side is the transmission side T and the lower side is the reception side R, and these are connected by a transmission system M using microwaves. Then, data Din such as an image to be relayed is continuously input to the rate conversion unit 1 on the transmission side T as shown in FIG.

  The data Din at this time is, for example, in units of a frame of 204 symbols. In the IFFT unit, information such as main data and synchronization CP and TMCC is added for each information symbol. 428 and a total of 672 sample periods from timing 594 to timing 1023.

  When the data Din is input to the rate conversion unit 1, data Dii shown in FIG. 6 (b) is output from it, and at the same time, an FST signal indicating the start of the synchronization symbol period is output every 204 symbols which are the frame period. And supplied to other parts. Therefore, the encoding unit 2T encodes the input data Dii, outputs data Rf and data If mapped to the two axes of the I axis and the Q axis, and outputs to the IFFT (Inverse Fast Fourier Transform) unit 3A. Supply.

  At this time, the IFFT unit 3A regards the input data Rf and data If as frequency components, and converts them into a time axis signal R and a time axis signal I having 1024 points, as shown in FIG.

  The guard adding unit 3B adds, for example, the first 32 points of the waveform of the start period of the time axis signal R and the time axis signal I after 1024 points to serve as a guard, and the time waveform Rg and time including a total of 1152 points. The waveform Ig is output. This signal is supplied to a quadrature modulation processing unit 8 including a DA converter 81, a quadrature modulator 82, and an oscillator 83.

  Here, the DA converter 81 DA-converts the real part signal Rsg of the time waveform Rg and the imaginary part signal Isg of the time waveform Ig, and the quadrature modulator 82 applies the frequency from the transmitter 83 to the real part signal. Quadrature modulation is performed by the carrier signal of fc, and quadrature modulation is performed on the imaginary part signal by a signal obtained by shifting the carrier signal of the frequency fc of the oscillator 83 by 90 °.

  Then, these signals are combined to obtain an OFDM modulated wave having a frame configuration as shown in FIG. 6 (d), and then, for example, placed on a microwave transmission system M and transmitted toward the receiving side R. At this time, a clock CK having a frequency SHz required for the operation of each unit is supplied from the clock oscillator 11 to each unit.

  The transmitted OFDM modulated wave transmitted to the receiving side R is input to an orthogonal demodulation processing unit 9 including an orthogonal demodulation unit 91, an AD converter 92, and an oscillator 93, where it is orthogonally demodulated into a baseband signal, The data R′sg and data I′sg are output by digitally converting. Therefore, the processing here is opposite to the transmitting side T, and the signal demodulated by the carrier signal of the voltage controlled oscillator 93 by the quadrature demodulator 91 is used as a real part signal, and the carrier signal is phase-shifted by 90 ° and demodulated. Is extracted as an imaginary part signal.

  Data R′sg and data I′sg output from the orthogonal demodulation processing unit 9 are input to the FFT unit 3C and the synchronization detection unit 4.

  Therefore, the FFT unit 3C performs OFDM demodulation on these data R′sg and data I′sg by dividing a symbol from the time waveform signal based on the FSTr pulse and performing Fourier transform, and each of the data is converted from the time waveform signal to the frequency. Converted to component signals R′f and I′f.

  The synchronization detection unit 4 detects a synchronization symbol group, generates a frame pulse FSTr from this, supplies it to each unit, and inputs the generated control voltage VC to a VCO (Voltage Controlled Oscillator) 12, where The clock CKr generated in (1) is supplied to each unit.

The frequency component signals R′f and I′f output from the FFT unit 3C are input to the decoding unit 2R, where they are identified by, for example, the ROM table technique and become data D′ o, and the rate conversion unit on the transmission side T 1 is input to the rate reverse conversion unit 7 having an inverted version of 1. The operation timing is determined by the clock CKr and the frame pulse FSTr, and is returned to a continuous signal, thereby being output as a relayed signal Dout. Will be used.
JP 2002-247004 A

  In OFDM transmission, normally, as described above, a guard is added and synchronization is performed from the correlation result of the guard. In this case, control is performed so that the peak position of the guard correlation is brought to the time position assumed on the receiving side R. However, in the case of mobile transmission using microwaves, although it is extremely short, it is sometimes complete. In some cases, the transmission is unforeseen, and the level of the received OFDM signal may be greatly reduced, or there may be almost no OFDM signal.

  Here, when the input signal level is low, the effect of AGC is moderate. Therefore, in the state where there is almost no OFDM signal, the AGC of the high frequency part becomes ineffective and the noise is amplified and becomes dominant. Therefore, a peak with a smaller level value appears every symbol everywhere in the OFDM signal.

  Therefore, in the prior art, as shown in FIG. 7, in the determination of the peak level in the synchronization detection unit 4, a threshold value (threshold value) Vth is set so that an excessive level is not adopted. The operation is stabilized.

  Here, FIG. 7A shows a received wave signal, but the signal component at this time includes an increase due to noise as shown in FIG. 7B. Therefore, as shown in the figure, the threshold value Vth is set at a level slightly above the noise level so that the influence of noise can be eliminated.

  However, the conventional technology does not consider the presence of adjacent waves, and if there are adjacent waves, the input signal level becomes inappropriate, the threshold value is set incorrectly, and the operation is unstable. There was a problem of becoming. That is, as shown in FIG. 8 (a), when there is an adjacent wave in the received wave signal, as shown in FIG. 8 (b), a leakage component due to the adjacent wave is further added to the raised portion due to noise. As a result, the threshold value Vth falls within the noise level, and the operation becomes unstable.

  An object of the present invention is to provide an orthogonal frequency division multiplex modulation digital transmission apparatus capable of obtaining a stable peak level determination even in the presence of adjacent waves.

  The purpose is to transmit at least a clock oscillating unit, a rate converting unit, an encoding unit, an IFFT unit, a guard adding unit, and an orthogonal modulation processing unit, at least an orthogonal demodulation processing unit, a synchronization detection unit, an FFT unit, and a decoding unit. In the orthogonal frequency division multiplex modulation digital transmission apparatus in which the transmission side is connected to the reception side provided with the transmission side, the rate inverse conversion unit, and the reception clock oscillation unit, the reception side is provided with adjacent state detection means and band switching means, The adjacent state detecting unit detects a state of an adjacent wave present in a signal transmitted from the transmitting side to the receiving side, generates a mode signal according to a detection result, and the band switching unit is configured to detect the orthogonal demodulation. This is achieved by controlling the band of the signal input from the processing unit to the synchronization detection unit in accordance with the mode signal.

  Similarly, the above object is to provide at least a clock oscillation unit, a rate conversion unit, an encoding unit, an IFFT unit, a guard addition unit, and a transmission side including an orthogonal modulation processing unit, at least an orthogonal demodulation processing unit, a synchronization detection unit, an FFT unit, In an orthogonal frequency division multiplex modulation digital transmission apparatus in which a decoding unit, a rate inverse conversion unit, and a reception side including a reception clock oscillation unit are connected by a transmission system, adjacent state detection means is provided on the reception side, and the adjacent state detection is performed. The means detects the state of an adjacent wave present in the signal transmitted from the transmission side to the reception side, generates a threshold correction value according to the detection result, and the synchronization detection unit This is achieved by controlling the threshold for level determination according to the threshold correction value.

  At this time, the detection result by the adjacent state detection means may be four types of “no adjacent”, “upper adjacent”, “lower adjacent”, “both adjacent”, and “no signal”. The above objective is achieved.

  According to the present invention, a stable peak level determination can always be obtained even when an adjacent wave is present in the relayed OFDM modulated wave. Therefore, the present invention can be applied to mobile relay and provide more stable transmission characteristics. It can greatly contribute to the enhancement of broadcasting programs.

  Hereinafter, an OFDM digital transmission apparatus according to the present invention will be described in detail with reference to embodiments shown in the drawings.

  FIG. 1 shows an embodiment of an OFDM digital transmission apparatus according to the present invention. In this case, as shown in the figure, except for the fact that an adjacent state detector 16 and a variable BPF unit 17 are provided, the block configuration is as follows. 5 is the same as the prior art shown in FIG. 5. Therefore, in this embodiment as well, continuous data Din such as an image to be relayed input to the rate conversion unit 1 on the transmission side T is orthogonally modulated. Is transmitted to the receiving side R, is orthogonally demodulated and returned to the continuous signal Dout, and is used for broadcasting as a relayed signal.

  That is, in the embodiment of FIG. 1 as well, the transmission side T has the same configuration as the prior art of FIG. 5 and performs the same operation. Accordingly, when the data Din is continuously input to the rate converter 1, the OFDM modulated wave is transmitted to the receiving side R.

  On the receiving side R, the configuration is roughly the same as that of the prior art, and a continuous signal Dout can be obtained from the received OFDM modulated wave. As a result, power is generated in relay broadcasting with stable transmission characteristics. There is no change in the point that can be.

  However, at this time, in this embodiment, when the OFDM modulated wave that appears on the receiving side R is input to the orthogonal demodulation processing unit 9, it is also captured by the adjacent state detector 16, and the state of the received OFDM modulated wave Accordingly, a correction value α of the mode signal BPFode and the threshold value Vth is generated, the mode signal BPFode is input to the variable BPF unit 17, and the correction value α is input to the synchronization detection unit 4.

  The characteristics of the variable BPF unit 16 are controlled by the mode signal BPFMode, and the threshold value Vth of the synchronization detection unit 4 is controlled by the correction value α, which is different from the prior art. .

  Therefore, the operation of the embodiment of FIG. 1 will be described in more detail with emphasis on differences from the conventional technique of FIG.

  First, the control of the variable BPF unit 16 by the mode signal BPFMode will be described.

  In this embodiment, as described above, the variable BPF unit 17 is provided, so that when the data R′sg and the data I′sg output from the orthogonal demodulation processing unit 9 are taken into the synchronization detection unit 4, R′sg and data I′sg are input via the variable BPF unit 17 as shown in the figure.

  At this time, in the variable BPF unit 17, the passband characteristic is controlled by the mode signal BPFMode supplied from the adjacent state detector 16, and as shown in FIG. 2, the characteristic WB, the characteristic LP, the characteristic HP, and the characteristic NB 4 It is configured to be set to any one of the pass band characteristics.

Here, as shown in FIG. 2, the OFDM-modulated received wave of the own channel received through the transmission system M is centered on the frequency f ₀ , the high frequency side is the frequency f _U , and the low frequency side is the frequency f _L. Suppose that it has the band characteristic PB.

In this case, first, the characteristic WB means a wideband characteristic. As shown in FIG. 2A, centering on the frequency f ₀ , the characteristic WB extends to a frequency higher than the frequency f _U on the high frequency side, and the frequency f _L on the low frequency side. It is set as a wideband characteristic that extends to a lower frequency and has a gentle slope cutoff characteristic in a wider band than the band characteristic PB.

Next, characteristics LP means low range characteristic, as shown in FIG. 2 (b), the center frequency f _0, has a cut-off characteristic which sharply attenuates near the frequency f _U in the high-frequency side, the low-frequency On the side, it is set to a low frequency characteristic that extends to a frequency lower than the frequency f _{L and} has a gentle slope cutoff characteristic.

The characteristic HP means a high frequency characteristic. As shown in FIG. 2 (c), the characteristic HP extends to a frequency higher than the frequency f _U on the high frequency side with the frequency f ₀ as the center, and has a gentle slope cutoff characteristic. On the low frequency side, a high frequency characteristic having a cutoff characteristic that sharply attenuates in the vicinity of the frequency f _L is set.

The characteristic NB means a narrow band characteristic. As shown in FIG. 2 (d), the high frequency side and the low frequency side are steep in the vicinity of the frequency f _U and the frequency f _L around the frequency f _0. Therefore, it is set to have a narrow passband characteristic that approximates the band characteristic PB of the OFDM modulated wave.

  Next, the adjacent state detection unit 16 will be described. As shown in FIG. 3A, the adjacent state detection unit 16 includes a high frequency BPF 161 and a low frequency BPF 162, a U detection unit 163, a D detection unit 164, an A detection unit 165, and a level. The determination unit 166 is configured.

First, as shown in the figure, the high-frequency BPF 161 is composed of a band-pass filter having a certain frequency width as a passband UW on the high frequency side of the center frequency f ₀ of the OFDM modulated wave at a position away from the high frequency end. The band characteristic PB of the own channel OFDM modulated reception wave mainly serves to extract a signal appearing at a portion away from the high frequency end, that is, the upper adjacent wave U shown in FIG.

Next, as shown in the figure, the low-frequency BPF 162 is composed of a band-pass filter whose passband DW is a certain frequency range at a position away from the low-frequency end on the low-frequency side of the center frequency f ₀ of the OFDM modulated wave. Then, the band characteristic PB of the own channel OFDM modulated reception wave mainly serves to extract a signal appearing at a portion away from the low frequency end, that is, the lower adjacent wave D shown in FIG.

  Therefore, the U detector 163 receives the signal component U and detects the signal X corresponding to the level of the signal component U. Next, the D detector 164 inputs the signal component D, and It functions to detect a signal Y corresponding to the level of the signal component D.

  On the other hand, the received OFDM modulated wave of the band characteristic PB is input to the A detector 165. This serves to detect a signal Z corresponding to the level of the OFDM modulated wave.

  Next, the level determination unit 166 inputs the above three types of signals X, Y, and Z, performs an operation according to the level of each signal, and according to the rules shown in FIG. α is generated.

  At this time, as shown in FIG. 3B, the mode signal BPFMode has four types of mode “W”, mode “L”, mode “H”, and mode “N”. ”Is mode“ W ”,“ upper adjacent ”is mode“ L ”,“ lower adjacent ”is mode“ H ”,“ both adjacent ”is mode“ N ”, and“ no signal ”is mode“ N ”. N ".

  Further, there are three correction values α, “0”, “A”, and “2A”, “0” for “no adjacency”, “A”, “ “Lower adjacent” is “A”, “Both adjacent” is “2A”, and “No signal” is “2A”.

  Therefore, the mode signal BPFMode is input to the variable BPF unit 16, and the correction value α is supplied to the synchronization detection unit 4. As a result, the characteristic of the variable BPF unit 16 changes to the mode according to the state of the received wave. The threshold value Vth of the synchronization detecting unit 4 is controlled to be changed by the correction value α. Therefore, according to this embodiment, even when an adjacent wave is present in the received OFDM modulated wave, it is stable. This will be described in more detail below.

  As described above, the synchronization detection unit 4 receives the data R′sg and the data I′sg output from the orthogonal demodulation processing unit 9, detects a synchronization symbol group from these, determines the peak level, and determines the frame pulse. FSTr is generated.

  Therefore, at this time, if there is an adjacent wave in the received wave, the input signal level of the synchronization detector 4 becomes inappropriate, and a stable peak level cannot be determined with the threshold value Vth set there. End up.

  Therefore, in this embodiment, the state of the adjacent wave in the received OFDM modulated wave is determined by the adjacent state detector 16, the band characteristic of the variable BPF unit 16 is switched from the determination result, and the synchronization detection unit 4 The threshold value Vth for peak level determination is corrected to eliminate the influence of adjacent waves. The following five types of band characteristic switching control and peak level determination threshold correction at this time will be described below. This will be explained in the following cases.

Case 1: In the case of FIG. 2A in which there is no adjacent wave in the received wave In this case 1, as shown in FIG. 3B, the mode signal BPFMode output from the adjacent state detector 16 is a mode. Become W. Therefore, as shown in FIG. 2A, the band characteristic of the variable BPF unit 16 is switched to the wideband characteristic WB having a gentle slope cutoff characteristic in a wider band than the band characteristic PB with the frequency f ₀ as the center. The correction amount α is “0”.

  Therefore, at this time, the data R′sg and the data I′sg output from the orthogonal demodulation processing unit 9 are directly input to the synchronization detection unit 4 without any band limitation, and the peak level is determined and the frame is determined. The normal operation state in which the pulse FSTr is generated, and there is no adjacent wave at this time, so there is no particular problem, and a stable peak level determination is obtained under a preset threshold value Vth. become.

  At this time, as shown in FIG. 2A, the wide band characteristic WB has a considerably wider band than the band characteristic PB for the following reason.

  Although the OFDM modulated wave is as small as about 50 dB, a power component is also generated outside the target carrier band, which is also useful as a signal component for synchronization pull-in. The capture limit will be improved if it is also captured.

  Therefore, in this case 1, that is, when there is no adjacent wave in the received wave, the wideband characteristic WB has a considerably wider band than the band characteristic PB. Therefore, according to this embodiment, There is no possibility that the synchronization acquisition limit is lowered when in the operating state.

Case 2: In the case of FIG. 2B where the upper adjacent wave U exists in the received wave In this case 2, as shown in FIG. 3B, the mode signal BPFode output from the adjacent state detector 16 Becomes mode L. Therefore, as shown in FIG. 2B, the band characteristic of the variable BPF unit 16 is switched to a low-frequency characteristic LP having a steep cutoff characteristic in the vicinity of the frequency f _U on the high-frequency side, and the threshold correction amount α Becomes “A”.

  Therefore, at this time, the upper adjacent wave U is effectively suppressed by the steep high-frequency cutoff characteristic of the low-frequency characteristic LP, and the value A of the threshold correction amount α is set in advance in FIG. ), The threshold value Vth is set to a value corresponding to the adjacent leakage component at this time, so that an appropriate threshold value Vth + A corresponding to the adjacent leakage component at this time is set. Therefore, even when the upper adjacent wave U exists in the received wave, a stable peak level determination can be reliably obtained.

Case 3: In the case of FIG. 2 (c) where the lower adjacent wave D exists in the received wave In this case 3, as shown in FIG. 3 (b), the mode signal BPFMode output from the adjacent state detector 16 Becomes mode H. Therefore, as shown in FIG. 2C, the band characteristic of the variable BPF unit 16 is switched to a high frequency characteristic HP having a steep cutoff characteristic in the vicinity of the frequency f _L on the low frequency side. α becomes “A”.

  Therefore, at this time, the lower adjacent wave D is effectively suppressed by the steep low-frequency cutoff characteristic of the low-frequency characteristic HP, and the threshold value Vth is set as shown in FIG. Therefore, an appropriate threshold value Vth + A corresponding to the adjacent leakage component at the time is set, and therefore, even when the lower adjacent wave D exists in the received wave, a stable peak level can be reliably determined. .

Case 4: In the case of FIG. 2D in which both the lower adjacent wave D and the upper adjacent wave U exist in the received wave In this case 4, as shown in FIG. 3D, the adjacent state detector 16 The mode signal BPFMode that is output from is set to mode N. Therefore, as shown in FIG. 2 (d), the band characteristic of the variable BPF unit 16 is a narrow band characteristic NB that sharply attenuates in the vicinity of the respective frequencies f _U and f _L on both the high frequency side and the low frequency side. The threshold value correction amount α is “2A”.

  Therefore, at this time, both the lower adjacent wave D and the upper adjacent wave U are effectively suppressed due to the steep out-of-band cutoff characteristic of the narrow band characteristic NB, and the threshold Vth is set as the threshold value Vth. A larger threshold value Vth + 2A is set in response to an increase in leakage components due to the presence of adjacent waves.

  Therefore, even when both the lower adjacent wave D and the upper adjacent wave U exist in the received wave, a stable peak level determination can be reliably obtained.

Case 5: When there is no received wave signal or the level is very low In this case 5, the mode signal BPFMode output from the adjacent state detector 16 is in mode N as shown in FIG. Therefore, as shown in FIG. 2 (d), the band characteristic of the variable BPF unit 16 is a narrow band characteristic NB that sharply attenuates in the vicinity of the respective frequencies f _U and f _L on both the high frequency side and the low frequency side. The threshold value correction amount α is “2A”.

  In this case, since the level is extremely low even if there is no received wave, demodulation of the received wave cannot be expected anyway, and even if a peak level is detected, it is only a result due to noise. A large threshold value Vth + 2A is set as Vth so that at least only malfunction due to noise can be suppressed.

  Therefore, according to this embodiment, stable peak level determination can be obtained even when there is an adjacent wave. As a result, it can be applied to mobile relay and can provide more stable transmission characteristics. It can greatly contribute to the enhancement of broadcasting programs.

It is a block block diagram which shows one Embodiment of the OFDM digital transmission apparatus by this invention. It is explanatory drawing of the frequency characteristic setting operation | movement by one Embodiment of this invention. It is detailed explanatory drawing of the adjacent detection part in one Embodiment of this invention. It is explanatory drawing of the threshold value setting operation | movement by one Embodiment of this invention. It is a block block diagram which shows an example of the OFDM digital transmission apparatus by a prior art. It is explanatory drawing of an OFDM signal. It is explanatory drawing of the suitable threshold value by a prior art. It is explanatory drawing of the inappropriate threshold value by a prior art.

Explanation of symbols

1: Rate conversion unit 2T: Coding unit 3A: IFFT unit (inverse Fourier transform unit)
3B: Guard addition unit 7: Rate inverse conversion unit 8: Quadrature modulation processing unit 9: Quadrature demodulation unit 10: Clock rate conversion unit 11: Clock oscillator 12: VCO (voltage controlled oscillator)
16: Adjacent state detector 17: Variable BPF

Claims (3)

At least a clock oscillation unit, a rate conversion unit, an encoding unit, an IFFT unit, a guard addition unit, and a transmission side including an orthogonal modulation processing unit, at least an orthogonal demodulation processing unit, a synchronization detection unit, an FFT unit, a decoding unit, and a rate inverse In an orthogonal frequency division multiplex modulation digital transmission apparatus in which a conversion unit and a reception side including a reception clock oscillation unit are connected by a transmission system,
Providing adjacent state detection means and band switching means on the receiving side,
The adjacent state detection means detects a state of an adjacent wave present in a signal transmitted from the transmission side to the reception side, and generates a mode signal according to a detection result,
The orthogonal frequency division multiplex modulation digital transmission apparatus characterized in that the band switching means controls a band of a signal input from the orthogonal demodulation processing unit to the synchronization detection unit in accordance with the mode signal. At least a clock oscillation unit, a rate conversion unit, an encoding unit, an IFFT unit, a guard addition unit, and a transmission side including an orthogonal modulation processing unit, at least an orthogonal demodulation processing unit, a synchronization detection unit, an FFT unit, a decoding unit, and a rate inverse In an orthogonal frequency division multiplex modulation digital transmission apparatus in which a conversion unit and a reception side including a reception clock oscillation unit are connected by a transmission system,
Providing adjacent state detection means on the receiving side,
The adjacent state detection means detects a state of an adjacent wave present in a signal transmitted from the transmission side to the reception side, generates a threshold correction value according to the detection result,
The quadrature frequency division multiplex modulation digital transmission apparatus characterized in that the synchronization detector controls a threshold for peak level determination by the synchronization detector according to the threshold correction value. In the orthogonal frequency division multiplex modulation digital transmission apparatus according to any one of claims 1 and 2,
Quadrature frequency characterized in that the detection result by the adjacent state detecting means is four types of “no adjacent”, “upper adjacent”, “lower adjacent”, “both adjacent”, and “no signal”. Division multiplex modulation digital transmission device.

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