JP2000138647A - Digital transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To avoid disturbances on other adjacent communication services in a transmission system adopting the orthogonal frequency division multiplexing, to attain highly accurate recovery of a sampling clock and to enhance the transmission rate even in a transmission system with a narrow transmission bandwidth. SOLUTION: In the digital transmission system adopting the orthogonal frequency division multiplex demodulation, a transmission signal from a transmission 1T has a frame configuration consisting of a synchronizing symbol group having at least a reference symbol and a plurality of data symbols that follow the synchronizing symbol group. A receiver 1R is provided with a means that controls a sampling clock of the receiver so that it matches the sampling clock of the transmitter by using the reference symbol data.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an orthogonal frequency division multiplex (OFD).
M) The present invention relates to a transmission device and a reception device of a digital transmission device using a digital modulation system represented by a modulation system.

[0002]

2. Description of the Related Art In recent years, orthogonal frequency division multiplexing (OFDM), which is characterized by being resistant to multipath fading and ghost, as a modulation method suitable for digital audio broadcasting for mobile objects and terrestrial digital television broadcasting.
The modulation scheme is receiving attention. The OFDM system is a type of multi-carrier modulation system, and is a transmission system in which n (n is several tens to several hundreds) carrier waves orthogonal to each other are digitally modulated. A discrete code is assigned to each of the I-axis component and the Q-axis component of each carrier as a modulated signal, and the code is updated every symbol period (several tens μsec). Then, as shown in FIG. 5, a large number of these digitally modulated waves are added, and an OFDM modulated signal obtained by orthogonally modulating the I-axis and Q-axis signals is transmitted. Here, as the digital modulation method of the carrier wave, a four-phase differential phase shift keying method (DQPSK: Differential Quadrature Pha
Although se shift keying is generally used, 16 quadrature amplitude modulation (16QAM) is used.
ion) or a multi-level modulation method such as 32QAM. Further, the symbol configuration of the OFDM signal is configured by adding a guard interval to the effective data symbol to reduce the influence of the delayed wave. The guard interval is a signal added so that the modulation symbol signal is cyclic. With the addition of the guard interval, for the delay wave with the delay time in the guard,
Since deterioration due to the inter-symbol interference can be avoided, it has strong resistance to multipath fading.

On the other hand, in the OFDM system, since the frequency interval between subcarriers is narrow, interference between subcarriers due to a carrier frequency error between transmitting and receiving apparatuses and a sampling clock frequency error of a demodulation system is likely to occur, and these frequencies have high precision. Need. Therefore, in order for the receiving device to continue receiving the OFDM signal correctly, it is necessary to perform a sampling clock regeneration process for matching the sampling clock frequency of the receiving device with the sampling clock frequency of the transmission signal. Further, when the frame period or the symbol period of the received signal fluctuates with time, it is necessary to make the sampling clock frequency of the receiving device follow the fluctuation.
Therefore, on the transmitting side, a transmission frame is composed of an OFDM transmission signal from valid data symbols and several types of synchronization symbols. The receiving side performs synchronization pull-in processing based on these synchronization symbols, synchronizes the sampling clock frequency between the transmitting side and the receiving side, and
The demodulation of the DM signal is performed.

FIG. 6 shows an example of the structure of a transmission frame.
FIG. 7 shows a transmitter for transmitting the OFDM signal of the transmission frame, and the operation will be described. The first symbol constituting the transmission frame shown in FIG.
(Hereinafter, a null symbol), and the null symbol is I,
The amplitude level of both Q axes is 0, and is a symbol for roughly detecting a specific position on the time axis. As shown in FIG. 6, during the null symbol generation period, the null symbol period pulse NP becomes LOW, the selector 13 switches so that the signal from the null symbol generator 16 is output, and outputs a null symbol. . The second symbol is a symbol whose amplitude is constant and whose frequency changes from the lower limit frequency to the upper limit frequency in the transmission band at a fixed rate with time over a symbol period (hereinafter, a sweep symbol). Since the autocorrelation function of this sweep symbol has a sharp peak value, it is possible to know a specific time point on the time axis with higher accuracy by using this property as compared with the null symbol. In the generation period of the sweep symbol, the sweep symbol period pulse SP shown in FIG. 6 becomes LOW, the selector 13 switches so that the signal from the sweep symbol generator 61 is output, and outputs the sweep symbol. In the third symbol, a reference symbol (hereinafter, referred to as a reference symbol) indicating a phase reference necessary for differential detection and demodulation is arranged.

In the reference symbol generation period, the reference symbol generation period pulse RP shown in FIG.
And the switch is switched so that the switch of the selector 32 outputs the signal from the reference signal generator 31. The reference signal generator 31 outputs a random sequence signal having the same sequence value for each frame, such as a spread code (PN code) reset by a frame synchronization pulse or the like. The random sequence signal from the reference signal generator 31 is sequentially assigned to each carrier, and constellation mapped (signal point arrangement) on polar coordinates by the I and Q axis mapping units 11. The phase of the signal at the time of the constellation mapping is used as an initial phase reference for demodulation by differential detection. Here, the reason for performing constellation mapping of the random sequence signal is to avoid as much as possible the modulated signal having regularity.

[0006] The constellation-mapped signal is transmitted to the IFFT processing unit 12 in the same manner as the data symbol.
In the IFFT (Inverse Fast Fourie Transform:
Inverse fast Fourier transform) is performed, converted into a signal on the time axis, and transmitted as a reference symbol. Therefore, the reference symbol converted to the time axis signal is
Like a data symbol, it becomes a signal like Gaussian noise. The fourth and subsequent symbols are composed of data symbols for transmitting valid data,
Dozens to hundreds of data symbols are continuous. As a data symbol, an information data sequence D from the outside is input to the selector 32, and during a data symbol generation period, as shown in FIG.
The state becomes OW, and the selector 32 switches so as to output the external input signal D. The output from the selector 32 is I, Q
It is input to the axis mapping unit 11 and subjected to constellation mapping.
The signal is input to the processing unit 12 and converted from a signal on the frequency axis into a signal on the time axis. A transmission frame is composed of the synchronization symbol and the data symbol, and the OFDM baseband signal composed of the frame is output from the selector 13. This signal is subjected to digital / analog conversion by the D / A converter 14 and then converted to baseband / IF (hereinafter referred to as BB).
The signal is up-converted into an IF frequency band by the (/ IF) conversion unit 15 and transmitted as an OFDM transmission signal in transmission frame units.

[0008] In the receiving apparatus shown in FIG.
After the signal is converted to a frequency signal, the IF / BB converter 21
The signal in the F frequency band is converted into a signal in the baseband frequency band to obtain an OFDM baseband signal. Then, the OFDM baseband signal is sampled by the A / D converter 22 using the sampling clock of the VCO 27 to obtain a received sample value sequence signal. Next, A / D
A series of operations of the sampling clock regeneration process for matching the sampling clock frequency at the time of sampling by the conversion unit 22 with the sampling clock frequency of the transmission signal will be described. First, as a first step, the null symbol start point is detected by the null symbol detection section 23 from the received sample value sequence signal from the A / D conversion section 22, and a rough symbol start point on the time axis is detected.

FIG. 9 shows a specific example of the configuration of the null symbol detecting section 23, and the procedure for detecting a null symbol will be described. First, the received sample value sequence signal is converted to an absolute value by the absolute value device 231. After that, by attenuating the high frequency components by the low pass filter 232, during the data symbol period,
Obtain a signal near a certain level. Here, when the transition from the data symbol period to the null symbol occurs, since the absolute value level of the null symbol is 0, the output from the low-pass filter 232 also approaches the 0 level. Therefore, a level difference occurs between the null symbol level and the data symbol level. The comparator 234 provides a threshold value for determining the magnitude level between the level differences, compares the magnitude of the data symbol level with the magnitude of the null symbol level, and indicates to the counter 235 the number of signals whose level is lower than the threshold value. Measure. The counter 235 counts the number of the low-level signals, and when the number exceeds a predetermined number, determines the start point of the null symbol and outputs a null symbol start pulse NS. In this way, the transition point on the time axis of the data from the last data symbol of the transmission frame to the null symbol is roughly detected, and the start position of the null symbol on the time axis is roughly confirmed.

In the second stage, a time window having a certain length is provided on the time axis based on the null symbol start position obtained in the first stage, and a synchronous reproduction process of the sampling clock is performed. First, the sweep symbol correlation calculator 71 calculates the cross-correlation between the sample value sequence signal included in the time window and the previously stored sweep symbol signal while moving the time window on the time axis. The process is performed for each window position, a cross-correlation value is calculated, and the correlation peak position detection unit 25 detects the peak value of the cross-correlation value within the time window. Then, from the position of the detected cross-correlation peak value, the time interval of the synchronization symbol in the baseband signal is counted in clock units of the sampling clock, and error information of the count value with respect to a predetermined value is calculated. V
In the CO control unit 26, based on the error information, the VCO
The VCO 27 outputs a signal for variably controlling the output clock frequency, and the VCO 27 outputs a variably controlled sampling clock based on a control signal from the VCO control unit 26. In this way, the sampling clock on the receiving side can always be synchronized with the clock on the transmitting side.

Next, a demodulation process after the synchronization between the transmitting device and the receiving device is established will be described. The sample value sequence signal from the A / D conversion unit 22 is input to the FFT processing unit 28, where it is subjected to FFT (Fast Fourie Transform) processing, and is converted from a time axis signal to a frequency axis signal. The output from the FFT processing unit 28 is a complex signal having I and Q axis components, and has an amplitude component and a phase component. The demodulation processing section 29 performs demodulation processing on the output signal from the FFT processing section 28, and the demodulation processing mainly uses synchronous detection demodulation and delay detection demodulation. In the synchronous detection demodulation, the received signal is demodulated by detecting the amplitude level and the absolute phase obtained by the FFT processing unit 28. Synchronous detection is characterized by being more resistant to code errors than asynchronous detection in a stable transmission path. On the other hand, since it is necessary to reproduce an absolute phase, there is a disadvantage that carrier reproduction processing, clock reproduction processing, and the like become complicated.
On the other hand, in the delay detection demodulation, the phase of the received signal one symbol before is regarded as the reference phase, and the phase difference from the current received signal is detected, whereby the original signal is obtained from the current received signal and the difference signal. This is a demodulation method.

In the example of the transmission frame shown in FIG.
In the reference symbol of the symbol, a signal serving as an initial phase reference for differential detection is transmitted. The fourth data symbol detects the phase difference from the signal of the reference symbol, and the fifth symbol detects the phase difference from the signal of the fourth symbol, thereby performing delay detection demodulation. Differential detection is suitable for mobile transmission in which the transmission path condition fluctuates every moment, and demodulation is possible even when carrier reproduction or clock reproduction is difficult. However, the differential detection has a disadvantage that it is vulnerable to a code error because noise is included not only in the received signal but also in the reference signal.

[0012]

In the prior art, when an OFDM signal and another communication service are used adjacent to each other on the frequency axis, the OFDM signal should not interfere with other communication services. is necessary. For example, if the adjacent communication service is an analog video signal, interference from the OFDM signal will appear as image disturbance. Here, the data symbol and the reference symbol are signals such as Gaussian noise, and even if they give an interference to the analog video signal band, they are not so disturbing as image disturbance. However, the sweep symbol in the frame configuration is a regular signal whose amplitude is constant over the symbol period and whose frequency changes from the lower limit to the upper limit of the transmission band, and which appears at a fixed time interval every frame period. It is. When such a signal interferes with an adjacent analog video signal, there is a drawback that band-like noise appears on a scanning line of an image, and the image becomes noticeable.

As a feature of OFDM modulation, a transmission bandwidth to be used can be determined by arbitrarily setting the number of subcarriers. For example, FIG.
When the transmission band is wide as shown in (a), the sweep symbol is also a signal having a wide band frequency component. The autocorrelation waveform of the sweep symbol in this case is shown in FIG.
0 (b). In this case, as is apparent from the figure, there is a sharp peak value of the correlation result at the center of the waveform, and the difference between the correlation value and the correlation peak value at the time when one sample is shifted is large.
Therefore, the start point of the sweep symbol can be detected with high accuracy, and the sampling clock can be reproduced with high accuracy.

On the other hand, when the transmission band is narrow, the sweep symbol is a signal having only narrow band frequency components as shown in FIG. In this case, the autocorrelation waveform of the sweep symbol is as shown in FIG. That is, although this waveform also has a peak value at the center, the peak value is not sharp as compared with the case of a wide band, and the difference between the peak value and the time point shifted by one sample is small. Therefore, in such a correlation waveform, the correlation value at the time when one sample is shifted due to the influence of noise or multipath may exceed the original correlation peak value. As a result, the correlation peak position detection unit 25 sets the sweep symbol position to 1
The detection is performed at the time when the sample is shifted, and the position of the sweep symbol cannot be accurately detected. Therefore, there is a disadvantage that the sampling clock cannot be correctly reproduced and demodulation cannot be performed correctly. Therefore, the present invention eliminates these disadvantages, and in the orthogonal frequency division multiplexing transmission device, does not interfere with other adjacent communication services, and also in a transmission device with a narrow transmission bandwidth,
It is an object of the present invention to enable high-precision sampling clock reproduction and to improve a transmission rate.

[0015]

According to the present invention, in order to achieve the above object, in a digital transmission apparatus using an orthogonal frequency division multiplexing modulation method, a transmission signal from a transmission apparatus is converted into a synchronization symbol group having at least a reference symbol. A signal having a frame configuration including a plurality of data symbols following the signal symbol, and a receiving device provided with means for controlling a sampling clock of the receiving device using the received reference symbol data so as to match a sampling clock of the transmitting device. It is a thing. Further, in a digital transmission apparatus of the orthogonal frequency division multiplexing modulation scheme, a transmission signal from the transmission apparatus is converted into a signal having a frame configuration including a synchronization symbol group having at least a reference symbol and a plurality of data symbols following the synchronization symbol group. Means for performing a cross-correlation operation between the signal corresponding to the set reference symbol data and the received signal of the received sample value sequence, and matching the sampling clock of the transmission device based on the peak position of the obtained correlation operation value Thus, means for controlling the sampling clock of the receiving apparatus is provided. Further, the receiving apparatus is provided with means for detecting a no-signal synchronization symbol period of the synchronization symbol group from a reception signal, and operating the means for performing the cross-correlation calculation processing based on the detected no-signal synchronization symbol period. It was made. Further, as the reference symbol data, a signal of a random value sequence different according to a modulation scheme used in the transmission apparatus is used.

That is, the present invention performs a cross-correlation operation between a signal of a received sample value sequence and a reference symbol when reproducing a sampling clock. Here, the reference symbol transmits a constant signal waveform for each frame. However, since the reference symbol is a signal like Gaussian noise like the data symbol, it has an effect on other communication services as compared with the conventional technology using the sweep symbol. The effect is small. In addition, the correlation peak value of the autocorrelation function of the reference symbol is extremely sharp as shown in FIG. 12, and the difference between the correlation peak value and the correlation value at the time of one sample shift becomes large. Unlike the sweep symbol, the difference between the correlation peak values is always large regardless of the transmission bandwidth. Therefore, the present invention has a feature that it is more resistant to noise and multipath as compared with the process of pulling in the sweep symbol even during narrowband transmission.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a digital transmission device according to the present invention will be described in detail with reference to the illustrated embodiments. FIG. 1 shows the overall configuration of the transmission device of the present invention.
1T is a transmission device having a modulation unit 1, a synchronization symbol insertion unit 2, and a transmission unit 3 described later, 4 is an antenna, and 1R is
A receiving device 5 having a receiving unit 6, a synchronization detecting unit 7, a sampling clock reproducing unit 8, and a demodulating unit 9 described later is an antenna. FIG. 2 is a block diagram of a transmitting apparatus 1T according to the present invention. In the prior art transmitting apparatus described with reference to FIG. 7, a reference symbol generator, a selector 32 and a sweep symbol generator 61 are newly added. 17 and the other configuration is the same as that of the prior art shown in FIG. In the present invention, the frame configuration of the OFDM signal is composed of at least a synchronization symbol group having a reference symbol and a plurality of data symbols (in the present embodiment, as shown in FIG. 13, a first symbol is a null symbol, a second symbol is a second symbol). , A reference symbol and a data symbol after the third symbol).

Next, the operation of the transmitting apparatus 1T will be described. First, in the first symbol of the frame, the switch of the selector 13 is set to the null symbol generator 1 as in the prior art.
The selector 13 switches so as to output the signal, and the selector 13 outputs null-symbol non-signal data. After the null symbol is transmitted, the selector 1
3 is switched, and the signal from the reference symbol generator 17 is output. Next, the configuration of the reference symbol generator 17 is shown in FIG. 14, and its operation will be described. The above-mentioned reference symbol waveform is represented by R
A non-volatile memory such as OM172 is recorded and a waveform is output by reading out the data. Therefore, it is necessary to create a reference symbol waveform in advance.

The creation of reference symbol waveform data to be recorded in the ROM 172 will be described.
The reference symbol indicates the initial phase reference of the frame as described above, and for all carriers,
Set the initial phase reference in advance. It is desirable that the initial phase reference for each carrier be as random as possible in order to avoid the modulation signal having regularity. Constellation mapping of these initial phase references to a polar coordinate system, like data symbols,
It is composed of I and Q axis components. Also, it is desirable that the average power at this time be the same as the average power of the data symbols following the reference symbol. This is because AGC (Aut) for keeping the reception power at a constant value in the receiving apparatus.
o Gain Control) circuit.
In this way, the IFFT processing is performed on the data whose amplitude and random phase are indicated by the I and Q axis components in each carrier, and the data on the frequency axis is converted into the data on the time axis. This data becomes a signal having a waveform like Gaussian noise, like the data symbol. This signal is used as reference symbol data, and is recorded in a nonvolatile memory such as the ROM 172 in advance.

Next, the operation of these circuits will be described. The address counter 171 outputs an address for reading data from the ROM 172, and is input to the ROM 172 as a signal indicating a specific address of the ROM 172. The ROM 172 outputs the data recorded at the address as a reference symbol signal in accordance with the signal from the address counter 171. After transmitting the reference symbol, the selector 13 sets the IF as in the prior art.
The switch is switched so as to output a signal from the FT processing unit 12. The signal from the selector 13 is D /
After passing through the A conversion unit 14 and the baseband / IF (BB / IF) conversion unit 15, it is transmitted as a transmission signal of the OFDM system in transmission frame units.

Next, an embodiment relating to the receiving apparatus 1R will be described with reference to FIG. This is because in the prior art receiving apparatus described with reference to FIG.
1 except for the reference symbol correlation calculator 24
The other configuration is the same as that of the prior art. First, the received signal is converted to IF / baseband (IF / B
B) The signal is passed through the conversion unit 21 and the A / D conversion unit 22 to obtain a signal of a received sample value sequence. Thereafter, a synchronization pull-in process such as a clock recovery process for matching the sampling clock frequency of the A / D converter 22 to the sampling clock frequency of the transmission signal is performed as described later.

Next, the operation of the synchronization pull-in process according to the present invention will be described. The synchronization pull-in processing of the present invention is basically the same as the clock recovery from the cross-correlation value of the sweep symbol of the prior art, but the present invention performs the synchronization pull-in processing using the reference symbol instead of the sweep symbol. . FIG. 15 shows the configuration of the reference symbol correlation calculator 24 for performing the cross-correlation calculation of the reference symbols, and the synchronization pull-in operation using the reference symbol correlation calculator 24 will be described below.
First, a null symbol start point is roughly detected by a null symbol detection section 23 from a signal of a received sample value sequence output from the A / D conversion section 22. A time window corresponding to a range for calculating the correlation value of the reference symbol signal is provided based on the start point of the detected null symbol.

Next, the signal of the sample value series included in the time window provided on the time axis is stored in a specific address of the RAM 242 designated by the RAM address generator 241. At this time, the range of the time window to be provided is set based on the calculation processing capability of the correlation calculator 245 and the range in which the sampling clock between the transmitting and receiving devices can be pulled in.
In general, a time window is provided for several samples before and after the time. Then, after the sample value sequence signal within the provided time window is fetched into the RAM 242, the cross-correlation calculation of the reference symbol signal is started. The correlation calculator 245 performs a cross-correlation calculation between the signal of the sample value sequence stored in the RAM 242 and the reference symbol signal stored in the ROM 243 in advance within the time window, and sets the time window on the time axis. This is performed for each time window position while moving, and the respective correlation values are calculated.

The reference symbol waveform stored in the ROM 243 is the same as that of the ROM 1 of the transmitting apparatus shown in FIG.
The waveform is the same as the reference symbol waveform stored in 72 or a waveform similar to the amplitude direction. After the cross-correlation calculation, the correlation peak position detection unit 25 detects the peak value of the cross-correlation value within the time window from the correlation value signal obtained by the reference symbol correlation calculation unit 24, as in the related art. Then, error position information of the correlation peak value is calculated. The VCO control unit 26 outputs a signal for variably controlling the output clock frequency of the VCO 27 based on the error position information. As a result, the VCO 27 outputs a sampling clock having a frequency corresponding to the control signal. In this way, the sampling clock of the receiving device is synchronized with the sampling clock of the transmitting device.
Here, the operation of the demodulation unit 9 is exactly the same as the operation described in the related art.

Next, another embodiment of the present invention will be described with reference to FIG. The embodiment shown in FIG. 4 relates to the transmitting apparatus of the present invention, and is the same as the conventional apparatus described with reference to FIG. 7, except that the sweep symbol generator 61 is omitted. It is. Also in the frame configuration in the present embodiment, as shown in FIG. 13, a null symbol is arranged in the first symbol, a reference symbol is arranged in the second symbol, and a data symbol is arranged in the third and subsequent symbols. Next, the operation of the transmitting device will be described. First, in the first symbol of the frame, the switch of the selector 33 is switched to the input side from the null symbol generator 16 as in the related art, and a null symbol signal is output. After the null symbol is transmitted, the selector 33
Is switched so that the signal from the IFFT processing unit 12 is output. At the same time, the selector 32 switches so that the signal from the reference signal generator 31 is output, and as described in the related art,
A random sequence signal having the same sequence value every frame, such as a PN pattern reset by a frame synchronization pulse or the like, is output as a reference signal.

Here, for example, the modulation system used in this transmitting apparatus is a DQPSK system or an 8-phase differential phase shift keying system (D8PSK: Differential 18 Phase Shift Keying).
In this case, the reference symbol signal also needs to output a random sequence signal corresponding to each modulation scheme. The signal from the selector 32 is input to the I- and Q-axis mapping unit 11, is subjected to constellation mapping, is input to the IFFT processing unit 12, and is converted from a signal on the frequency axis to a signal on the time axis to generate a reference symbol signal. . After sending the reference symbol,
The selector 32 switches so as to output the information signal D from the outside, and sends out a data symbol signal. The signal from the selector 33 passes through the D / A conversion unit 14 and the BB / IF conversion unit 15 and then passes through the OF of the transmission frame configuration described above.
It is output as a DM signal.

Next, an embodiment of the reference symbol correlation calculator 24 of the receiving apparatus corresponding to the transmitting apparatus of FIG. 4 will be described. The configuration of the reference symbol correlation operation unit 24 is shown in FIG. 16, and the synchronization pull-in operation using the reference symbol correlation operation unit 24 will be described below. First, similarly to the above-described embodiment, after a signal of a sample value series included in a predetermined time window is stored in the RAM 242, a correlation operation of a reference symbol is started. The correlation calculator 245 performs a cross-correlation calculation between the sample value sequence signal stored in the RAM 242 and the reference symbol signal stored in the RAM 242 while moving the time window on the time axis. The calculation is performed for each time window position.

In this embodiment, since the reference symbol waveform differs depending on the modulation method such as DQPSK or D8PSK, the reference symbol waveform which is the correlated operation signal is prepared for each modulation method, and these are stored in the ROM 246 in advance. Stored in the ROM 248. The reference symbol waveforms stored in the ROMs 246 to 248 are exactly the same data sequence signals as the random symbol reference symbol signals different for each modulation scheme as described in the transmission apparatus. This is a reference symbol waveform on the time axis by performing the IFFT processing. The selector 249 selects one of the ROMs 246 to 248 in which the corresponding reference symbol waveform is stored according to the modulation scheme used here, outputs a corresponding reference symbol waveform, and uses the selected reference symbol waveform as a correlated calculation signal. Input to the container 245. here,
The correlation calculation and the control of the sampling clock frequency based on the calculation result are the same as those of the prior art and the above-described embodiment. In this way, the sampling clock of the receiving device is synchronized with the sampling clock of the transmitting device.

[0029]

As described above, in the present invention, the sweep symbol is removed from the transmission frame and the synchronization is established using the reference symbol.
Even if the signal is used adjacent to other communication services,
Even when transmitting over a narrow transmission band, there is little effect on other communication services, and it is possible to establish synchronization that is more resistant to noise and multi-pulses than the conventional method using sweep symbols. It is possible. Furthermore,
Since data symbols can be allocated to the frame structure excluding the sweep symbols, the transmission rate of valid data can be improved.

[Brief description of the drawings]

FIG. 1 is a block diagram showing the overall configuration of a digital transmission device according to the present invention.

FIG. 2 is a block diagram showing an embodiment of a transmission device of the present invention.

FIG. 3 is a block diagram showing one embodiment of a receiving apparatus of the present invention.

FIG. 4 is a block diagram showing another embodiment of the transmitting apparatus of the present invention.

FIG. 5 is a waveform chart showing a waveform of an OFDM signal.

FIG. 6 is a diagram illustrating a frame configuration of a transmission signal according to the related art.

FIG. 7 is a block diagram illustrating a configuration of a transmission device according to a conventional technique.

FIG. 8 is a block diagram showing a configuration of a receiving apparatus according to the related art.

FIG. 9 is a block diagram illustrating an embodiment of a null symbol detection unit.

FIG. 10 is a waveform diagram showing a sweep symbol waveform and its autocorrelation waveform in wideband transmission.

FIG. 11 is a waveform diagram showing a sweep symbol waveform and its autocorrelation waveform in narrowband transmission.

FIG. 12 is a waveform chart showing a reference symbol waveform and its autocorrelation waveform.

FIG. 13 is a diagram illustrating an example of a frame configuration of a transmission signal according to the present invention.

FIG. 14 is a block diagram showing one embodiment of a reference symbol generator of the present invention.

FIG. 15 is a block diagram showing an embodiment of a reference symbol correlation calculator according to the present invention;

FIG. 16 is a block diagram showing another embodiment of the reference symbol correlation calculator of the present invention.

[Explanation of symbols]

1T: transmitting apparatus, 1R: receiving apparatus, 1: modulation section, 2: synchronization symbol insertion section, 3: transmission section, 6: reception section, 7: synchronization detection section, 8: sampling clock reproduction section, 9: demodulation section, 11: I and Q axis mapping unit, 12: IFFT processing unit, 13: selector, 14: D / A conversion unit, 15: BB
/ IF converter, 16: null symbol generator, 17: reference symbol generator, 21: IF / BB converter, 2
2: A / D converter, 23: null symbol detector, 24:
Reference symbol correlation calculator, 25: correlation peak position detector, 26: VCO controller, 27: VCO, 28:
FFT processing unit, 29: demodulation processing unit, 31: reference signal generator, 32, 33: selector, 231: absolute value device, 232: low-pass filter, 233: threshold value output device, 2
34: comparator, 235: counter, 171: address counter, 172, 243, 246 to 248: ROM, 2
41: RAM address generator, 242: RAM, 24
4: ROM address generator, 245: correlation calculator, 24
9: Selector.

Claims (5)

[Claims] 1. A digital transmission apparatus using an orthogonal frequency division multiplexing modulation method, wherein a transmission signal from a transmission apparatus is a signal having a frame configuration including a synchronization symbol group having at least a reference symbol and a plurality of data symbols following the synchronization symbol group. A digital transmission device provided with means for controlling the sampling clock of the reception device so as to match the sampling clock of the transmission device using the received reference symbol data in the reception device. 2. A digital transmission apparatus using an orthogonal frequency division multiplexing modulation method, wherein a transmission signal from a transmission apparatus is a signal having a frame configuration including a synchronization symbol group having at least a reference symbol and a plurality of data symbols following the synchronization symbol group. Means for performing a cross-correlation calculation process between a signal corresponding to the reference symbol data set in advance and a received signal of a received sample value sequence, and transmitting the signal based on a peak position of the obtained correlation calculation value. A digital transmission device comprising means for controlling a sampling clock of the receiving device so as to match a sampling clock of the device. 3. The digital transmission apparatus according to claim 2, further comprising: means for detecting a no-signal synchronization symbol period of the synchronization symbol group from a received signal, wherein the receiving apparatus further includes means for detecting the no-signal synchronization symbol period. A digital transmission device for operating the means for performing the cross-correlation calculation process based on the digital transmission device. 4. The digital transmission apparatus according to claim 1, wherein a signal of a random value sequence different according to a modulation scheme used in the transmission apparatus is used as the reference symbol data. . 5. The digital transmission apparatus according to claim 1, wherein the reference symbol data transmitted from the transmission apparatus is used as a reference signal for an initial phase and a reference signal for sampling clock recovery at the time of differential detection and demodulation. A digital transmission device characterized by the following.