JPH09266466A - Digital transmission system - Google Patents

Abstract

There is provided a digital transmission system capable of easily coping with an increase in the number of terminal stations and effectively using a frequency band while being resistant to monotone noise. SOLUTION: In a terminal station 10, an SP converter 11,
The inverse FFT 12, the PS converter 13, and the DA converter 14 modulate a plurality of subchannel carrier signals unique to the terminal station with the symbol sequence to be transmitted to the center station 30, and further, the oscillator 16, the multiplier 17, and the BPF 18.
Thus, the carrier signal is modulated into a transmission signal to be transmitted to the center station 30. The same applies to the terminal station 20. In the center station 30, the arrived combined transmission signal is demodulated by the BPF 31, the oscillator 32, and the multiplier 33, and subchannel demodulated by the AD converter 34, the SP converter 35, and the FFT 36, and the DEM.
The UX 37 and the PS converters 38a and 38b restore the symbol string transmitted from each terminal station.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital signal transmission technique suitably used in a dendritic network using a CATV transmission line, for example.

[0002]

2. Description of the Related Art Conventionally, research and development of large-capacity transmission technology using a CATV transmission line network has been advanced. For example,
The “Research and Development Report” (March 30, 1993) edited by CATV Basic Technology Research Laboratories describes the time division multiplexing (TDMA: Time).
Using the Divison Multiple Access method, the results of research on delay measurement deterioration analysis, suppression stabilization method, relationship between actual transmission line fluctuation and transmission quality, etc. have been reported.

According to this, from the viewpoint of realizing the largest number of communication channels while ensuring a sufficient margin in terms of transmission quality under predetermined preconditions, four-phase phase modulation (neither error correction code nor coded modulation is performed). QPSK: Quadrature
It has been concluded that a method of performing delay detection of a Phase Shift Keying) signal and performing high-precision delay time measurement control is optimal as a TDMA system on a CATV network.

The TDMA method, which is most suitable in this report, uses one QPSK wave in a time division manner, and the carrier frequency is predetermined to one specific wave. Also,
A reference clock is transmitted from the center station to control the symbol transmission timing of each terminal station. Since there is only one carrier frequency, the center station has the feature that only one receiver is required and the data transmission rate is somewhat flexible.

[0005]

However, in the above-mentioned conventional example, there is a problem that the condition of the symbol transmission timing of each terminal station is strict and the control by the center station is not easy. Further, since there is only one carrier frequency and no error correction code is applied, when monotone noise is present, there is a problem that all the data cannot be received due to the noise.

By the way, as a multiplexing technique, in addition to TDMA, frequency division multiplexing (FDM) is used.
lexing) method. In particular, orthogonal frequency division multiplexing (OFDM), which is one type of FDM system, is used.
The uency Division Multiplexing) system modulates a plurality of sub-channel carrier signals with information to be transmitted as a baseband time series signal, further modulates one carrier signal with this baseband time series signal, and outputs the result. This is a multi-carrier transmission method of transmitting to a partner station. This multi-carrier transmission system uses a large number of sub-channel carrier signals, so it is resistant to frequency selective fading in a ghosted transmission line, has a large effect of error correction coding, has a high frequency band utilization efficiency, etc. It solves the above-mentioned problems of the TDMA system (for example, Journal of Television Society Vol.50, No.1, pp.
24-41 (1996)).

However, when this multi-carrier transmission system is applied as it is to a dendritic network for transmitting data between one center station and each of a plurality of terminal stations such as a CATV transmission line network, There is such a problem. In other words, since it is necessary to use different carrier signals for each terminal station, the center station requires demodulators or the like as many as or more than the number of terminal stations, resulting in a large-scale system. Not only that, but when the number of terminal stations increases beyond the number of demodulators provided in the center station, the center station cannot immediately deal with the problem.

Although random errors can be corrected by applying an error correction code when transmitting from the terminal station to the center station, the burst error caused by monotone noise is transmitted by one transmission channel (one terminal station). However, there is a problem that the center station cannot perform error correction decoding on the transmission signal from the terminal station.

The present invention has been made to solve the above problems, and even when a CATV transmission line network of a dendritic network between a plurality of terminal stations and one center station is used. It is an object of the present invention to provide a digital transmission system that can easily cope with an increase in the number of terminal stations and that is resistant to monotone noise while effectively using a frequency band.

[0010]

A digital transmission system according to the present invention is a digital transmission system based on a multi-carrier transmission system between a center station and each of a predetermined number of terminal stations of two or more. Sub-channel modulation means provided in each of the number of terminal stations, a plurality of sub-channel carrier signals unique to each terminal station is modulated with a symbol string to be transmitted to the center station to generate and output a baseband time-series signal, (2) Modulating means provided in each of the predetermined number of terminal stations, which modulates a carrier signal common to all of the predetermined number of terminal stations with a baseband time-series signal and outputs a transmission signal; (3) a predetermined number of terminals The transmission signals generated by each station are combined to form a combined signal, and the combining means for sending this combined signal to the center station and (4) Demodulating means for demodulating the carrier signal and outputting the baseband time-series mixed signal, and (5) the center station is provided with the baseband time-series mixed signal for each of a plurality of sub-channel carrier signals of all the predetermined number of terminal stations. Sub-channel demodulation means for demodulating and outputting a symbol sequence mixed signal, and (6) the symbol sequence mixed signal provided in the center station is separated and output to each symbol sequence transmitted from each of a predetermined number of terminal stations. Separating means, (7) a symbol rate adjusting means for adjusting the transmission rate at which a symbol string is transmitted from each of a predetermined number of terminal stations, and (8) a timing of transmitting a transmission signal from each of a predetermined number of terminal stations. Adjusting the transmission timing so that the transmission signals sent from each of the predetermined number of terminal stations arrive at the center station at a fixed time Means and are provided.

In this digital transmission system, in each of a predetermined number of terminal stations, a plurality of sub-channel carrier signals unique to each terminal station are modulated by the sub-channel modulation means with a symbol string to be transmitted to the center station, The carrier signal that is a band time-series signal and is common to all the predetermined number of terminal stations is modulated by the baseband time-series signal by the modulation means, and the transmission signal is output. The transmission signals generated by each of the predetermined number of terminal stations are combined by the combining means to be a combined signal, and this combined signal is sent out to the center station.

At the center station, the received combined signals are collectively demodulated by the demodulating means with respect to the carrier signal to become a baseband time series mixed signal, and the baseband time series mixed signal is collectively received by the subchannel demodulating means. Then, the plurality of sub-channel carrier signals of all the predetermined number of terminal stations are demodulated into a symbol string mixed signal, and the symbol string mixed signal is transmitted to each of the symbol strings transmitted from each of the predetermined number of terminal stations by the separating means. To be separated.

At this time, the transmission rate at which the symbol string is transmitted from each of the predetermined number of terminal stations is adjusted by the symbol rate adjusting means so that the amount of data reaching the center station does not exceed the receiving capability of the center station. Further, the timings at which the transmission signals are transmitted from the respective predetermined number of terminal stations are adjusted by the transmission timing adjusting means, and the respective transmission signals transmitted from the respective predetermined number of terminal stations arrive at the center station at the same time.

The frequency of each of the plurality of sub-channel carrier signals unique to each of the predetermined number of terminal stations is an integral multiple of the reference frequency, and the sub-channel modulating means is (1-a).
A first serial-parallel converter for converting a symbol string to be transmitted to the center station into a parallel signal, and (1-b) an output signal from the first serial-parallel converter, respectively, for a plurality of sub-channel carrier signals. An inverse Fourier transformer for performing an inverse Fourier transform on the frequency of and a (1-c) first signal for converting an output signal from the inverse Fourier transformer into a serial signal
Parallel-serial converter, and (1-d) a digital-analog converter that converts an output signal from the first parallel-serial converter into an analog signal and outputs a baseband time-series signal, Sub-channel demodulation means (2
-a) An analog-digital converter that converts the baseband time-series mixed signal into a digital signal, and (2-b) a second serial-parallel converter that converts the output signal from the analog-digital converter into a parallel signal. And (2-c) Fourier transforming an output signal from the second serial-parallel converter for each frequency of a plurality of sub-channel carrier signals of each of a predetermined number of terminal stations to output a symbol string mixed signal. A converter, and the separating means is
(3-a) Demultiplexer that separates the symbol string mixed signal into parallel signals corresponding to each symbol string transmitted from each of a predetermined number of terminal stations, and (3-b) Parallel signals separated by the demultiplexer And a second parallel-serial converter for converting to a serial signal and outputting the serial signal. In this case, OF
The symbol string is transmitted from each terminal station to the center station by a method based on the DM method.

When the frequencies of a plurality of sub-channel carrier signals unique to each of a predetermined number of terminal stations are mixed within a certain frequency band, the error correction coding technique is used in combination so that only random errors occur. In addition, it is possible to realize strong transmission against burst errors caused by monotone noise.

The digital transmission system according to the present invention further comprises (1) transmission signal strength adjusting means which is provided in each of a predetermined number of terminal stations and which adjusts the strength of the transmission signal based on the strength control signal; Based on the strength of the combined signal reaching the center station, the strength level of each transmission signal transmitted from each of the predetermined number of terminal stations is obtained, and the strength control signal is generated based on the strength level to generate the corresponding terminal. Transmission signal strength control means for sending to each station. In this case, since the transmission signals transmitted from the respective terminal stations to the center station by the transmission signal intensity adjusting means have substantially equal intensities to each other based on the intensity control signal output by the transmission signal intensity controlling means, noise is reduced. Strong transmission is possible.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description. Note that, for simplicity, the following description will be made on the assumption that there are two terminal stations. FIG. 1 is a configuration diagram of a digital transmission system according to the present embodiment.

The digital transmission system according to the present embodiment performs digital transmission between each of the terminal stations 10 and 20 and the center station 30. Further, each of the terminal station 10 and the terminal station 20 is provided with a similar device. Therefore, one terminal station 10 will be mainly described. A case will be described where a transmission method conforming to the OFDM method is adopted as the multicarrier transmission method.

The terminal station 10 has a serial channel as a sub-channel modulating means 10A for generating a baseband time-series signal by modulating a sub-channel carrier specific to the terminal station 10 with a symbol string to be transmitted to the center station 30. A parallel converter (hereinafter, SP converter) 11, an inverse Fourier transformer (hereinafter, inverse FFT) 12, a parallel-serial converter (hereinafter, PS converter) 13, and a digital-analog converter (hereinafter, DA conversion) 14) is provided.
Further, the terminal station 10 has a sub-channel modulation means 10A.
An oscillator 16, a multiplier 17, and a bandpass filter (hereinafter, referred to as B) are used as a modulation unit that modulates a carrier signal with a baseband time-series signal output from the device and outputs a transmission signal.
PF) 18 is provided.

The symbol string (serial data) to be transmitted from the terminal station 10 to the center station 30 is first of all the SP converter 1
1 converts the data into parallel data having a predetermined data length.
Then, the symbol string made into the parallel data is the inverse FF.
It is input to T12 and subjected to inverse Fourier transform. That is, the symbol string d _k (k = 1,2,3, ...), which is the parallel data, is

[Equation 1] The sub-channel carrier modulated signal x (n · ΔT) is converted according to the following conversion formula. Here, ΔT is a sampling interval, n · ΔT is a sampling point, j is an imaginary unit,
π is the pi, and N is the number of subchannel carrier signals. Also, the sub-channel carrier signal is

[Equation 2] It is. As can be seen from equations (1) and (2), the sub-channel carrier signal is in the form of being modulated with a symbol string. Such a function is performed by a DSP (Digital Sign).
al Processor) can be easily realized.

The waveform of the sub-channel carrier signal expressed by the equation (2) has orthogonality. That is,
When a product of two sub-channel carrier signals having the same n value is time-integrated over one period, it becomes a non-zero finite value, but a product of two sub-channel carrier signals having different n values is obtained over one period. It becomes 0 when integrated over time.

The symbol string thus inverse-Fourier-transformed by the inverse FFT 12 is again converted into serial data by the PS converter 13 and converted into analog data by the DA converter 14 to become a baseband time series signal. . A high-frequency component of the baseband time-series signal is cut by a low-pass filter (hereinafter, LPF) 15, and the carrier signal output from the oscillator 16 and the multiplier 1
The signal is multiplied by 7 and then only the signal of the frequency component in the predetermined band passes through the BPF 18. In this way, the carrier signal is modulated with the baseband time-series signal and becomes a transmission signal to be transmitted to the center station 30.

Similarly, in the terminal station 20, the symbol string (serial data) to be transmitted to the center station 30 is converted into parallel data by the SP converter 21, and the inverse FFT 22 is performed.
Is inverse-Fourier-transformed, converted into serial data again by the PS converter 23, converted into analog data by the DA converter 24, and becomes a baseband time-series signal. A high frequency component of the baseband time series signal is cut by the LPF 25, the carrier signal output from the oscillator 26 is multiplied by the multiplier 27, and B
Only the signal of the frequency component in the predetermined band passes through the PF 28 and becomes a transmission signal to be transmitted to the center station 30. Here, the carrier signal is transmitted to the oscillator 26 in the terminal station 20.
It has the same frequency as the carrier signal output from.

The transmission signals generated by the terminal stations 10 and 20 in this manner are combined and transmitted to the center station 30.
Sent to. At this time, each transmission signal needs to satisfy the following conditions.

First, the frequency f _k (k = 1, 2, 3, ...) Of the sub-channel carrier signal used in the terminal station 10.
And the frequency g _k (k = 1,2,3, ...) Of the sub-channel carrier signal used in the terminal station 20 must not be the same. This is because the transmission signals from the terminal stations 10 and 20 are multiplexed and the center station 3
This is because when they reach 0, they must be separable by the center station 30. So, for example,
As shown in FIG. 2, the frequency f _k (k = 1, 2, 3, ...) Of the sub-channel carrier signal used in the terminal station 10 and the frequency g _k (k = k = k = _k ) of the sub-channel carrier signal used in the terminal station 20. 1,2,3, ...) are alternately and equally spaced. That is, the frequency f _k (k = 1,2,3, ...)

(Equation 3) (FIG. 2 (a)), and the frequency g _k (k = 1,2,3, ...)

(Equation 4) (FIG. 2B). In this way, the transmission signal obtained by multiplexing them is transmitted to the center station 30 as described later.
, It is possible to separate them after collectively demodulating.

Secondly, it is necessary that the lengths (symbol rates) of the symbol sequences transmitted from the terminal stations 10 and 20 per unit time are equal to each other. For this purpose, for example, the symbol rate in each terminal station may be fixedly set in advance. Further, from the center station 30 to each of the terminal stations 10 and 20,
Each of the terminal stations 10 and 20 may transmit the symbol rate information and send a symbol string having a data length per unit time according to the symbol rate information to the center station 30. In this way, when the number of terminal stations connected to the network increases or when the number of terminal stations transmitting to the center station 30 increases, the center station 30 sends the symbol rate to each terminal station. By instructing the symbol rate information to reduce the value, it is possible to prevent the amount of data reaching the center station 30 from exceeding the processing capability of the center station 30.

Thirdly, it is necessary that the transmission signals sent from the terminal stations 10 and 20 arrive at the center station 30 at the same time. This is because, as will be described later, the center station 30 collectively demodulates the multiplexed and transmitted signals transmitted from the terminal stations 10 and 20, respectively.
Therefore, the delay time between each of the terminal stations 10 and 20 and the center station 30 is measured, and the transmission timing of the transmission signal from each of the terminal stations 10 and 20 is adjusted based on this.

Specifically, for example, the center station 30 transmits a predetermined signal to the terminal station 10, the terminal station 10 receives the predetermined signal, and then transmits the predetermined signal toward the center station 30. Then, the center station 30 receives the predetermined signal arriving from the terminal station 10, the center station 30 measures the time between these, and the delay time corresponding to this time is measured by the terminal station 1
Indicate 0. Alternatively, in the case of the CATV network, if the length of the line between the terminal station 10 and the center station 30 is known, the delay time between them can be known. Therefore, the delay time corresponding to the length of this line can be used as the delay time. It may be set in advance. The delay time setting in the terminal station 10 is, for example, an inverse FFT.
A register (not shown) is provided in the subsequent stage of 12, and the inverse FFT is performed.
The 12 output data are held for a predetermined time. Alternatively, a FIFO memory (not shown) may be provided in the subsequent stage of the PS converter 13 so that the output data of the PS converter 13 is delayed by a predetermined time and then output. The same applies to the terminal station 20.

The transmission signals respectively output from the terminal stations 10 and 20 as described above are transmitted to the transmission path 40 of the dendritic network and multiplexed, and the multiplexed transmission signals reach the center station 30. To do. The center station 30 is provided with a BPF 31, an oscillator 32, and a multiplier 33 as demodulation means for collectively demodulating the transmission signals that have been multiplexed and arrived and outputting a baseband time series mixed signal. Further, to the center station 30, a sub-channel demodulation means 3 for demodulating the baseband time series mixed signal and outputting the symbol string mixed signal transmitted from each terminal station.
As 0A, an analog-digital converter (hereinafter, AD converter) 34, an SP converter 35, and a Fourier converter (hereinafter, FFT) 36 are provided. In addition, the center station 3
0 is a demultiplexer (to be referred to as a DEMUX hereinafter) 37, P as demultiplexing means for demultiplexing the symbol sequence mixed signal into each symbol sequence transmitted from each terminal station.
S converters 38a and 38b are provided.

In the multiplexed transmission signal reaching the center station 30, first, only the signal of the frequency component of a predetermined band passes through the BPF 31, and the carrier signal output from the oscillator 32 is multiplied by the multiplier 33. And demodulated, and a baseband time series mixed signal is output. This carrier signal has the same frequency as the carrier signals output from the oscillators 16 and 26 of the terminal stations 10 and 20, respectively. The baseband time series mixed signal is a mixture of the baseband time series signal generated by the subchannel modulation means 10A of the terminal station 10 and the baseband time series signal generated by the subchannel modulation means 20A of the terminal station 20. Signal (FIG. 2 (c)).

This baseband time series mixed signal is demodulated by the subchannel demodulation means 30A for each subchannel carrier. That is, the baseband time series mixed signal is converted into analog data by the AD converter 34 and converted into parallel data by the SP converter 35,
Fourier transformation is performed by the FFT 36, and a symbol string mixed signal is output. Here, the operation performed in the FFT 36 is the Fourier transform corresponding to the equation (1),

(Equation 5) It is represented by This FFT 36 can also be easily realized by the DSP. The symbol sequence mixed signal is a signal in which the symbol sequence transmitted from the terminal station 10 and the symbol sequence transmitted from the terminal station 20 are mixed.

This symbol string mixed signal is a DEMUX3 signal.
7 separates the symbol sequences transmitted from the terminal stations 10 and 20, respectively. This DEMUX3
Reference numeral 7 determines which sub-channel carrier signal each symbol in the symbol sequence mixed signal has transmitted, and based on this, the symbol sequence transmitted from the terminal station 10 and the symbol sequence transmitted from the terminal station 20. And separate. The symbol string transmitted from the terminal station 10 is P
The symbol string transmitted from the terminal station 20 after being converted into serial data by the S converter 38a is converted into serial data by the PS converter 38b and output. As described above, the center station 30 receives each symbol string transmitted from each of the terminal stations 10 and 20.

When the strength of the transmission signal reaching the center station 30 differs depending on the terminal station to which the transmission signal is transmitted, the strength of the transmission signal is controlled so as to keep it at a substantially constant level. Means for adjusting may be provided. For example, in the center station 30, the baseband time-series mixed signal is monitored for each sub-channel carrier signal, the strength of the transmission signal transmitted from each of the terminal stations 10 and 20 is calculated, and the strength of the transmission signal is substantially constant. Feedback is given to each of 10 and 20 to control. Terminal station 1
In each of 0 and 20, an amplifier or an attenuator is provided after the DA converters 14 and 24, and the intensity level of the transmission signal sent to the transmission line 40 is adjusted according to the instruction from the center station 30. By doing so, it is possible to realize a digital transmission system that is more resistant to noise.

As described above, this system adopts the multi-carrier system, makes the frequency of the sub-channel carrier signal unique to each terminal station, and makes the frequency of the carrier signal common to all terminal stations. Therefore, the symbol trains from all the terminal stations can be transmitted to the center station with one carrier signal, and moreover, the center station needs only one demodulation means and sub-channel demodulation means. Easy and cheap. Also, when the number of terminal stations connected to the network increases, or when the number of terminal stations simultaneously transmitting to the center station increases, the symbol rate of each terminal station is reduced by the instruction from the center station. This can be easily handled by reducing the data size handled by the inverse FFT of the terminal station.

Since the frequency of the sub-channel carrier signal of one terminal station and the frequency of the sub-channel carrier signal of the other terminal station are set alternately,
In particular, even if ingress noise or a radio interference wave which is a problem in a CATV transmission network is mixed in the transmission line, the influence of the noise does not concentrate on the transmission signal transmitted from a certain terminal station, and It is distributed to each transmission signal sent from the terminal station. Therefore, since a random error occurs in the transmission signal transmitted from each terminal station, it is possible to perform error correction by performing error correction coding. Furthermore, by combining time interleaving or frequency interleaving with error correction coding, it is possible to further realize transmission that is more resistant to burst errors. In a transmission path where noise is not a problem, the frequency of the sub-channel carrier signal unique to each terminal station is
As shown in (c), the terminals corresponding to the respective terminal stations do not have to be arranged alternately, but may be grouped for each terminal station.

The present invention can be applied to a multi-point image acquisition system. For example, one transmission channel is 20
In the case of Mbps, 6 Mbps is assigned to the video from each of the three terminal stations to transmit a fine video, while the video from the other one terminal station is 2 Mbps if the video is a coarse video. Can be assigned to transmit a video with a small amount of data. In this way, it is possible to transmit images having different data amounts to the center station depending on the purpose.

The present invention is not limited to the above embodiment, but various modifications can be made. For example, the multi-carrier transmission system is not limited to the OFDM system, but other multi-carrier transmission systems such as DMT.
A transmission method based on the (Digital Multi-tone) method may be used. The number of terminal stations is not limited to 2,
It may be 3 or more. In addition, the transmission line network is CATV
The present invention is not limited to the above, but can be applied to transmission line networks of other dendritic networks.

The setting of the frequency of the sub-channel carrier signal of each terminal station is not limited to equal intervals.
The frequency of the sub-channel carrier signal of one terminal station and the frequency of the sub-channel carrier signal of another terminal station may not be set alternately at equal intervals. As long as they are mutually orthogonal and the same frequency is not set between different terminal stations, the frequency of any subchannel carrier signal can be used.

The frequency of the carrier signal of each terminal station and the center station may be preset and fixed, or the oscillator of each terminal station may be controlled by an instruction from the center station. The frequency may be changed. In this case, a voltage-controllable crystal oscillator (VCXO) is preferably used as the oscillator. Also, the frequency of the sub-channel carrier signal in the sub-channel modulation means of each terminal station may be preset and fixed,
The frequency of the sub-channel carrier signal may be changed by changing the calculation parameter in the inverse FFT of each terminal station according to an instruction from the center station.

[0040]

As described above in detail, according to the present invention, in each terminal station, a plurality of sub-channel carrier signals unique to each terminal station are modulated with a symbol string to be transmitted to the center station, Band time-series signals, and further, carrier signals common to all terminal stations are modulated with baseband time-series signals to form transmission signals, and the transmission signals generated by each terminal station are combined and combined signals And the combined signal is sent out to the center station.

In the center station, the received combined signals are collectively demodulated by the demodulating means with respect to the carrier signal to become a baseband time series mixed signal, and the baseband time series mixed signal is further subchannel demodulated. By the means, the plurality of sub-channel carrier signals of all the predetermined number of terminal stations are collectively demodulated to become a symbol string mixed signal, and the symbol string mixed signal is transmitted from each of the predetermined number of terminal stations by the separating means. It is separated into each symbol string.

At this time, the transmission rate of the symbol string transmitted from each of the predetermined number of terminal stations is adjusted so that the amount of data reaching the center station does not exceed the reception capability of the center station. Further, the timing of transmitting the transmission signal from each of the predetermined number of terminal stations is adjusted so that the transmission signals transmitted from each of the predetermined number of terminal stations reach the center station at the same time.

With this configuration, since the carrier signal has only one frequency, only one set of demodulating means and sub-channel demodulating means is required at the center station, which simplifies the structure and reduces the cost. . Even if the number of terminal stations connected to the center station increases or the number of terminal stations simultaneously transmitting to the center station increases, these situations can be easily dealt with without adding a new device.

When the frequencies of a plurality of sub-channel carrier signals unique to each terminal station are mixed within a certain frequency band, the error correction coding technique is used in combination so that only random errors occur. In addition, it is possible to realize strong transmission against burst errors caused by monotone noise.

Further, if the strengths of the transmission signals transmitted from the respective terminal stations are adjusted so as to be substantially equal to each other, it is possible to perform the transmission more resistant to noise.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a digital transmission system according to an embodiment.

FIG. 2 is an explanatory diagram of frequencies of subchannel carrier signals.

[Explanation of symbols]

10 ... Terminal station, 10A ... Sub-channel modulation means, 11
... serial-parallel converter (SP converter), 12 ... inverse Fourier transformer (inverse FFT), 13 ... parallel-serial converter (PS converter), 14 ... digital-analog converter (DA converter), 15 … Low-pass filter (LP
F), 16 ... Oscillator, 17 ... Multiplier, 18 ... Bandpass filter (BPF), 20 ... Terminal station, 20A ... Subchannel modulation means, 21 ... Serial-parallel converter (S
P converter), 22 ... Inverse Fourier transformer (inverse FFT), 2
3 ... Parallel-serial converter (PS converter), 24 ...
Digital-analog converter (DA converter), 25 ... Low pass filter (LPF), 26 ... Oscillator, 27 ... Multiplier, 28 ... Band pass filter (BPF), 30 ... Center station, 30A ... Sub-channel demodulation means, 31 ... band pass filter (BPF), 32 ... oscillator, 33 ... multiplier, 34 ... analog-digital converter (AD converter),
35 ... Serial-parallel converter (SP converter), 36
... Fourier transform (FFT), 37 ... Demultiplexer (DEMUX), 38a, 38b ... Parallel-serial converter (PS converter), 40 ... Transmission path.

Claims (4)

[Claims] 1. A digital transmission system according to a multi-carrier transmission system between a center station and each of a predetermined number of terminal stations of 2 or more, wherein the digital transmission system is provided in each of the predetermined number of terminal stations and is unique to each terminal station. Subchannel modulation means for modulating a plurality of subchannel carrier signals with a symbol string to be transmitted to the center station to generate and output a baseband time series signal; and a predetermined number provided for each of the predetermined number of terminal stations. A modulation means for modulating a carrier signal common to all terminal stations with the baseband time-series signal and outputting a transmission signal; and multiplexing and multiplexing the transmission signals generated by each of the predetermined number of terminal stations. A signal, and a multiplexing unit that sends the multiplexed signal toward the center station; and the arrived multiplexed signal for the carrier signal, which is provided in the center station. And a demodulation means for outputting a baseband time-series mixed signal, and demodulating the baseband time-series mixed signal provided in the center station for each of the plurality of sub-channel carrier signals of all the predetermined number of terminal stations. And a sub-channel demodulation means for outputting a symbol sequence mixed signal, and a separation unit provided in the center station for separating and outputting the symbol sequence mixed signal for each symbol sequence transmitted from each of the predetermined number of terminal stations. Means, a symbol rate adjusting means for adjusting a transmission rate at which the symbol string is transmitted from each of the predetermined number of terminal stations, and a timing of transmitting each of the transmission signals from each of the predetermined number of terminal stations, The time when each of the transmission signals transmitted from each of the predetermined number of terminal stations reaches the center station is Digital transmission system comprising: the transmission timing adjusting means for a constant, a. 2. The frequency of each of the plurality of sub-channel carrier signals unique to each of the predetermined number of terminal stations is an integral multiple of a reference frequency, and the sub-channel modulating means is a symbol string to be transmitted to the center station. To a parallel signal, and an inverse Fourier transformer for performing an inverse Fourier transform on the output signal from the first serial-parallel converter for each frequency of the plurality of sub-channel carrier signals. A first parallel-serial converter for converting an output signal from the inverse Fourier transformer into a serial signal; and an output signal from the first parallel-serial converter for converting into an analog signal by the baseband A digital-analog converter that outputs a time-series signal; An analog-digital converter for converting the baseband time-series mixed signal into a digital signal, a second serial-parallel converter for converting an output signal from the analog-digital converter into a parallel signal, and the second Fourier transforming the output signal from the serial-parallel converter for each of the plurality of sub-channel carrier signals of each of the predetermined number of terminal stations, and outputting the symbol string mixed signal. The demultiplexing unit demultiplexes the symbol sequence mixed signal into parallel signals corresponding to respective symbol sequences transmitted from each of the predetermined number of terminal stations, and parallel signals demultiplexed by the demultiplexer. Second parallel-converted into serial signal and output
The digital transmission system according to claim 1, further comprising: a serial converter. 3. The digital transmission system according to claim 1, wherein the frequencies of the plurality of sub-channel carrier signals unique to each of the predetermined number of terminal stations are mixed in a certain frequency band. . 4. A transmission signal strength adjusting means provided in each of the predetermined number of terminal stations, for adjusting the strength of the transmission signal on the basis of a strength control signal, and strength of the combined signal reaching the center station. Based on the strength level of each transmission signal transmitted from each of the predetermined number of terminal stations, the strength control signal is generated based on the strength level, and the transmission signal strength is transmitted to each corresponding terminal station. The digital transmission system according to claim 1, further comprising: control means.