JPH09252276A - Digital communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain up-link in a prescribed time slot by taking synchronization of plural up-link stations conducting up-link processing by the time division multiplex system without using a pilot signal. SOLUTION: A slot setting console 19 sets a time slot to be up-linked and enters video data of a transmission object to a buffer device 11. A timing controller 18 outputs a high frequency carrier to a carrier controller 14 through a time slot assigned for transmission and outputs a bit stream of a transmission object to a bit stream controller 12. The control operation of the timing controller 18 is conducted based on a high precision clock according to time information of the GPS. A modulator 15 modulates the high frequency carrier with the bit stream and provides an output.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital communication system for delivering data from an uplink station via a transponder, and more particularly to delivering data from a plurality of uplink stations via transponders in a time division multiple access system. The present invention relates to a digital communication system that enables the above.

[0002]

2. Description of the Related Art Digital communication systems utilizing broadcasting artificial satellites (broadcasting satellites) such as satellite data broadcasting communication systems and satellite digital moving image transmission systems have been developed. In this type of broadcast satellite digital communication system, a transponder for transmitting received data is provided in a satellite arranged in outer space. The earth station sends a data signal to the transponder at a predetermined frequency. The transponder transmits the received data signal toward the earth. The data signal transmitted from the transponder is
It can be received even in areas away from the earth station.

In this type of broadcast satellite digital communication system, when there is a material to be distributed to the hub station, the uplink is performed as it is. For example, as shown in FIG. 8, when the hub station 4 itself includes the camera video source 1 or the recorded video source 2 and transmits these video signals,
The video signal is encoded into digital video data by the digital video encoder 3 and transmitted from the hub station 4 to the satellite transponder.

If the material does not exist in the hub station 4, for example, as shown in FIG. 9, the station in which the material exists transmits data to the hub station 4 via the communication network 5, and the hub station 4 transmits the data. Uplink. In this way, when the image material is not in the hub station 4, the communication network 5
You will need to pay a line fee to use this service.

In recent years, it is often the case that a plurality of earth stations hold different material data and commonly distribute these material data via satellite transponders. In such a case, an SC that forms one channel for each carrier frequency
It is conceivable that a frequency division multiple access method based on a PC (single channel per carrier) method is adopted and uplink is carried out to a transponder at a frequency different for each earth station to perform multi-channel distribution. In this system, for example, as shown in FIG.
a transmits the transmission data to the satellite transponder 7 at the first carrier frequency, and the second earth station 6b transmits the transmission data to the satellite transponder 7 at the second carrier frequency different from the first carrier frequency. .

However, the frequency division multiplexing requires a wider frequency band as a whole as the number of channels increases. In addition, it is necessary to install a modulation device for individual transmission for each data transmission rate required by the receiving station, etc.
Large-scale equipment and facilities are required. Also,
In order to make maximum use of the satellite transponder, it is necessary to variably control the antenna power depending on the states of carriers of other frequencies in the same transponder. For this reason,
The equipment configuration of each earth station becomes complicated.

Therefore, conventionally, MCPC has been used for multi-channel transmission in a broadcast satellite digital communication system.
(multi channel par carrier) method is used.
In this system, video data to be distributed is collected in one earth station, and the one earth station performs time division multiplexing (TDM:
Multi-channelization is performed by time division multiplex) method, and uplinks to satellite transponders are made on a large number of channels using different time slots.

For example, the earth station 6 shown in FIG. 11 obtains transmission data at its own station and collects transmission data from other stations via the network 5. Earth station 6
The transmission data of its own station and that of another station are assigned to different time slots, that is, channels, time-division multiplexed with carriers of the same frequency, and uplinked to the satellite transponder 7. The satellite transponder 7 transmits the received data to the earth. The signals from the satellite transponder 7 are received by the other earth stations 8a and 8b.

In the case of the MCPC system, it is necessary to collect digital video data from a plurality of earth stations to be multiplexed into one earth station and then allocate the time-division multiplex to different time slots. In this case, between earth stations,
In order to transmit the transmission data, a line usage fee is required, which increases the cost.

In addition, time divisional multiple access (uplink) from multiple earth stations to the same satellite transponder in different time slots of the same frequency.
The multi access method is, for example, VSAT (very small).
aperture terminal) Used for two-way communication. In this manner, for example, as shown in FIG. 12, the earth stations 9a and 9b each uplink to the satellite transponder 7 using a predetermined time slot having the same frequency and different timing. Satellite transponder 7
Transmits the received digital video data to the earth. The signals from the satellite transponder 7 are received by the other earth stations 8a and 8b.

In this system, a plurality of earth stations use the time slots properly for uplinking, so that it is necessary to accurately synchronize the plurality of earth stations. Therefore, one control station among a plurality of earth stations is defined, a pilot signal for synchronous control is transmitted from this control station to another earth station, and this pilot signal is used to make an uplink of each earth station. A slot reservation scheme that controls timing is used. However, in the case of the slot reservation method, it is necessary to set a large guard time between time slots in order to prevent malfunction due to timing shift. Therefore, in this method,
It was difficult to obtain a fast and stable bit stream. Further, it is necessary to secure the frequency band of the control carrier for the pilot signal.

It is also possible to use an asynchronous time division multiple access system. However, in this case, carrier collision occurs when the uplink timings from a plurality of earth stations overlap. Therefore, it is more difficult to extract a stable and high-speed bit stream by this asynchronous method than when synchronous control is performed by a pilot signal.

[0013]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and synchronizes a plurality of uplink stations performing uplink by the time division multiple access system without using a pilot signal. An object of the present invention is to provide a digital communication system capable of uplinking each uplink station to a predetermined time slot at an appropriate timing.

[0014]

In order to achieve the above object, a digital communication system according to a first aspect of the present invention is a transponder for transmitting received data, and data is transmitted / received by uplinking to the transponder. In a digital communication system that includes a plurality of uplink stations and each uplink station uplinks in different time slots to perform communication by a time division multiple access method, each uplink station uses a global positioning system. GPS means for extracting time information and generating a timing signal, and based on the timing signal from said GPS means, using the same predetermined frequency and different time slots as other uplink stations,
Transmission / reception control means for uplinking to the transponder by a time division multiple access method.

According to the digital communication system having such a configuration, each uplink station extracts time information by GPS and generates a timing signal. Therefore, timing control becomes accurate and easy, and a plurality of uplink stations can synchronize and appropriately deliver data without using a pilot signal or the like.

Assigning part of the time slot to control,
Time slot allocation information may be transmitted to each uplink station using the control time slot. In this case, it becomes easy to allocate and change the time slot.

A digital communication system according to a second aspect of the present invention is a slot allocating means for allocating time slots for control and each uplink station, and time information is extracted by a global positioning system to output a timing signal. First uplink control for transmitting allocation information to the transponder by the control time slot according to the allocation by the allocation means based on the first GPS means to be generated and the timing signal from the first GPS means. At least one master station having means and a second GPS for extracting time information and generating a timing signal by a global positioning system
And a control slot discrimination means for discriminating the control time slot based on the timing signal from the second GPS means, and a timing signal from the GPS means and information from the control slot discrimination means. A control slot receiving means for receiving allocation information from the transponder in the control time slot; and an uplink to the transponder using a time slot indicated by the allocation information received by the control slot receiving means. And a plurality of slave stations having two uplink control means.

According to the digital communication system of the second aspect, each uplink station extracts time information from GPS to generate a timing signal. For this reason, timing control becomes accurate and easy, and there is no need to distribute pilot signals and the like. Further, by allocating the time slot to each uplink station using the control time slot, the time slot allocation becomes easy.

A digital communication system according to a third aspect of the present invention comprises a transponder for transmitting received data, a plurality of uplink stations for transmitting / receiving data in different time slots via the transponder,
In a time division multiple access type digital communication system configured by each uplink station, each uplink station collects data received intermittently by a plurality of time slots and data received by the receiving means. Conversion means for converting and outputting to a continuous bit stream, the digital communication system controls allocation of time slots to each uplink station according to a conversion speed of the conversion means of each uplink station. It is characterized by comprising means for performing.

The performance (conversion speed) of the conversion means differs for each uplink station. According to this configuration, since the number and position of time slots are set according to the performance of the conversion means, a continuous bit stream can be obtained regardless of the performance of the conversion means, and the transmission / reception / display of moving images can be performed. Etc. can be performed appropriately.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) The configuration of the digital communication system according to the first embodiment is basically the same as that of the conventional TDM digital communication system shown in FIG. , A satellite transponder 7 and a plurality of receiving stations 8. Each uplink station 9
Is a time slot for transmission assigned to itself,
Uplink data (for example, digital video data) (transmit data). Satellite transponder 7
The data transmitted from the uplink station 9 is received and retransmitted. Each receiving station 8 and the uplink station 9 receive the data retransmitted by the satellite transponder 7 in the receiving time slot assigned to itself.

However, unlike the prior art, each uplink station 9 and each receiving station 8 operate in synchronization with each other by obtaining a timing signal by the global positioning system (GPS).

FIG. 1 shows the configuration of the transmission section 10 of the uplink station 9 which constitutes the digital communication system of the first embodiment. As shown in FIG. 1, the transmission unit 10 includes a buffer device 11, a bitstream control device 12, a high frequency oscillator 13, a carrier control device 14, and a modulation device 1.
5, GPS antenna 16, GPS receiving device 17,
Timing controller 18 and slot setting console 1
9 is provided.

The buffer device 11 is for transmitting data to be transmitted,
For example, digital video data is temporarily stored. In accordance with the timing control signal from the timing control device 18, the bit stream control device 12 sequentially reads the data from the buffer device 11 during the period corresponding to the time slot allocated for the uplink of the uplink station 9 to obtain the digital video. Form a bit stream of data (data stream).

The high frequency oscillator 13 generates a high frequency carrier. The carrier control device 14 outputs the high frequency carrier supplied from the high frequency oscillator 13 according to the timing control signal from the timing control device 18 in the period corresponding to the time slot allocated for the uplink of the uplink station 9. .

The modulator 15 modulates the high frequency carrier output from the carrier controller 14 with the bit stream output from the bit stream controller 12 to obtain an intermediate frequency (IF) output. The intermediate frequency output of the modulator 15 is converted into a high frequency (RF) signal by the frequency converter, amplified by the high frequency amplifier circuit, and transmitted from the transmitting antenna to the satellite transponder 7.

The GPS antenna 16 receives radio waves from GPS satellites. The GPS receiver 17 generates a high-precision clock based on high-precision timing information from the radio waves of GPS satellites received by the GPS antenna 16. Note that G
A high-precision clock generation method itself using a PS satellite is well known.

The slot setting console 19 is a console for manually setting the time slot in which the uplink station 9 transmits data. For example, the slot setting console 19 is provided with a normal input device and a display device, and can be operated manually by an operator. Are allocated for transmission by this uplink station 9.

The timing controller 18 determines the timing according to the high-precision clock from the GPS receiver 17, and activates the bit stream controller 12 and the carrier controller 14 during the time slot set by the slot setting console 19. And Therefore, the high frequency carrier is modulated by the bit stream and transmitted in this transmission time slot.

Therefore, the slot setting console 19 is arranged so that the transmission timings of the respective uplink stations 9 do not overlap.
By setting transmission slots from 1 to 3, uplink by a time division multiple access (TDM) method becomes possible.

FIG. 2 shows the configuration of the receiving unit 20 of the uplink station 9 and the receiving station 8. As shown in FIG.
0 is a demodulation device 21, a slot selection control device 22,
The buffer device 23, the GPS antenna 24, the GPS receiving device 25, the timing control device 26, and the slot setting console 27 are provided.

The demodulator 21 receives the signal from the satellite transponder 7, demodulates the received signal and outputs a bit stream. The slot selection control device 22 outputs the bit stream demodulated by the demodulation device 21 in accordance with the control signal from the timing control device 26 only during the time slot allocated for reception of this station. The buffer device 23 temporarily stores the bit stream intermittently supplied from the slot selection control device 22 in units of time slots, and converts the bit stream into a continuous bit stream at a speed lower than that of the received signal.

The GPS antenna 24 receives radio waves from GPS satellites. The GPS receiver 25 generates a high-precision clock from the signal received by the GPS antenna 24.

The slot setting console 27 is used by the earth station 9
Or, it is a console for manually setting a time slot for receiving data at 8. The timing control device 26 makes the slot selection control device 22 active and outputs the bit stream only during the period of the receiving slot set by the slot setting console 27 in accordance with the high precision clock from the GPS receiving device 25. The timing control device 26 determines the time from the transmission of data by the uplink station 9 to the reception of this station, that is,
The slot selection control device 22 is set on / off in consideration of a constant transmission time (delay time).

According to the digital communication system having such a configuration, the same frequency is used in accordance with the TDM system,
Data can be communicated in different time slots. Moreover, it becomes possible for the earth stations 8 and 9 to operate accurately in synchronization with each other according to the high-precision clock generated by the GPS system. Further, since a plurality of uplink stations 9 are arranged, it is sufficient to send the material to be distributed to the uplink stations 9 in the vicinity, and it is possible to suppress the cost on the terrestrial line.

In the uplink station 9, the GP
The S antennas 16 and 24, the GPS receivers 17 and 25, and the slot setting consoles 19 and 27 are respectively included in the transmitter 1
0 and the receiving unit 20 may share the same.

(Second Embodiment) In the first embodiment, the time slots for transmission and reception are individually set (assigned) manually from the console.
You may make it allocate automatically. A digital communication system according to the second embodiment for automatically setting time slots will be described with reference to FIGS. 3 and 4. FIG.
4 shows how control data for slot allocation is transmitted and received, and FIG. 4 shows how data is communicated using the allocated slot.

As shown in FIGS. 3 and 4, this digital communication system includes a main station (slot allocation station) 30 and 3
It comprises three slave stations 31a, 31b and 31c and a satellite transponder 32. Master station 30 and slave station 3
1a, 31b and 31c have an uplink function and a downlink function, respectively.

In such a configuration, the master station 30 and the slave stations 31a, 31b, 3 and 3 are operated by the operator manually operating the slot setting console in the master station 30.
1c allocates a time slot used for transmission. As shown in FIG. 3, the main station 30 transmits information about the assigned time slot (assignment information) to the satellite transponder 32 by using a preset control slot. The satellite transponder 32 transmits the received allocation information in the control slot. The master station 30 and the slave stations 31a to 31c receive the allocation information transmitted using the control slot from the satellite transponder 32, and set the time slot to be used by the own station for uplink based on the allocation information.

When the time slot has been set, FIG.
As shown in, the master station 30 and the slave stations 31a to 31c are
Each assigned time slot is used to uplink to satellite transponder 32 for data transmission.

The control data transmitted using the control slot may include control information other than the slot allocation information.

As shown in FIG. 5, each of the slave stations 31a, 31b, 31c has a buffer device 11, a bit stream control device 12, a high frequency oscillator 13, and a carrier control device 1.
4, a modulator 15, a GPS antenna 16, a GPS
The receiver 17 includes a timing controller 18, a control slot receiver 41, and a slot allocation controller 42. Here, the same parts as those in FIG. 1 are denoted by the same reference numerals, and detailed description thereof will be omitted.

The control slot receiver 41 receives and demodulates control data transmitted from the main station 30 via the satellite transponder 32 during the control slot in accordance with the timing control signal supplied from the timing controller 18. The slot allocation signal is output. The slot allocation control device 42, based on the slot allocation signal output from the control slot reception device 41, performs timing control device 18 so as to execute uplink in the allocated slot.
Control.

The timing controller 18 turns on the control slot receiver 41 at the timing of receiving the control slot in accordance with the high precision clock supplied from the GPS receiver 17. Also, during the time slot given by the slot allocation control device 42, the bit stream control device 12 and the carrier control device 14 are activated, the bit stream and the output carrier are output, and the data is transmitted.

As shown in FIG. 6, the master station 30 basically has the same configuration as the slave station shown in FIG. 5, and further includes a slot setting console 43 and a control information storage section 44.

In the master station 30 having the configuration shown in FIG. 6, from the slot setting console 43 to the master station 30 and the slave stations 31a,
Transmission slots and reception slots are set in 31b and 31c. The set information is stored in the control information storage unit 44. At a timing of a predetermined control slot, the timing control device 18 turns on the carrier control device 14 and the control information storage unit 44, the carrier control device 14 outputs a high frequency carrier, and the control information storage unit 44 outputs allocation information. . The modulator 15 modulates the high frequency carrier with the allocation information and outputs it.

The control information transmitted from the master station 30 using the control slot is transmitted to the master station 30 and the slave stations 31a, 31b and 31c via the satellite transponder 32. The master station 30 and the slave stations 31a, 31b, 31c receive this control information via the control slot receiver 41 and store it in the slot allocation controller 42. This completes the allocation of the time slots to the master station 30 and the slave stations 31a, 31b, 31c.

The slot allocation control device 42 notifies the timing control device 18 of the data transmission slot according to the stored control information. The timing control device 18 is a GP
The carrier control device 14 and the bit stream control device 12 are turned on at the timing of the transmission slot notified from the slot allocation control device 42 according to the high-accuracy clock from the S reception device 17, and the high frequency carrier modulated by the bit stream is output To do.

In this way, the master station 30 and the slave stations 31
a, 31b and 31c transmit data in the slots set by the main station 30.

According to the structure of this embodiment, the operation timing is controlled by using the high-precision clock generated from the GPS signal, so that a plurality of ground stations can be accurately synchronized and operated. Further, it is only necessary to transmit the distribution material from the station that holds the data to the nearest uplink station, and it is possible to reduce the cost for using the terrestrial line. Also, because it is possible to periodically uplink to a fixed time slot, it is easy for the receiving station to combine it with a data buffer and extract it as a continuous bit stream, which is effective for digital moving image transmission, etc. is there.

Furthermore, by incorporating a control slot in the time slot, each uplink earth station can be controlled by a specific master station. By dynamically allocating the time slots by this control (adjusting the allocation position and the number as needed), the time slots can be efficiently shared by the plurality of uplink earth stations.

The number of timeslots allocated in the uplink earth station is calculated by the demodulator 21 and the buffer 23 on the receiving side.
By controlling according to the operating speed of the, the speed close to the processing speed on the receiving side and the transmission speed according to the user's request,
It can be realized without adding or changing equipment or devices on the uplink side. For example, when transmitting data to a receiving station that can operate at high speed, data is transmitted to 4 time slots in n time slots, and when transmitting data to a receiving station that operates at low speed, it is transmitted in n time slots. The data can be transmitted in one time slot of. Similarly, it is possible to allocate the m-th time slot to a certain station at a certain timing and the k-th time slot to the same station at another timing.

Although the control slot receiver 41 is arranged to receive the control slot in FIGS. 5 and 6, the control information may be reproduced from the data received by the ordinary receiving circuit. A configuration example of such a station will be described with reference to FIG.

The receiving section 50 shown in FIG. 7 includes a demodulation device 21, a slot selection control device 22, a buffer device 23, a GPS antenna 24, and a GPS receiving device 2 similar to those in FIG.
5, the timing control device 26, and the slot control device 2
7, a control slot recognition device 51, and a slot allocation device 52. Here, the same parts as those in FIG. 2 are designated by the same reference numerals, and detailed description thereof will be omitted.

The control slot recognition device 51 is the demodulation device 2
The received bit stream output from 1 is monitored to determine the control slot, and the time slot allocation information is extracted. The slot allocation device 52 uses the control slot recognition device 5
Based on the time slot allocation information extracted in 1, the time slot corresponding to the transmitting station to be received by this station is automatically set. The timing control device 26 turns on the slot selection control device 22 during the set time slot to output a bitstream.

The present invention is not limited to the above embodiment, and various modifications and applications are possible. For example, the configuration of each station can be changed arbitrarily.

[0057]

As described above, in the digital communication system according to the present invention, time information is extracted by GPS from each of a plurality of uplink stations to generate a timing signal, and an operation is performed based on the timing signal. To do. Therefore, it is possible to suppress errors between the earth stations and operate in synchronization. Also, timing control by the pilot signal is unnecessary. Further, by using a part of the time slot for control, it becomes possible to control each uplink station based on the information of the control time slot. In addition, the time slot allocation location and the number of allocations are variable. Therefore, the allocation of each earth station can always be made the best, and the satellite transponder can be used more effectively.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a transmitting unit of an earth station which constitutes a digital communication system according to a first embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a receiving unit of an earth station which constitutes the digital communication system according to the first embodiment.

FIG. 3 is a diagram showing a configuration and slot allocation operation of a digital communication system according to a second embodiment of the present invention.

FIG. 4 is a diagram showing a configuration of the digital communication system of FIG. 3 during data transmission.

5 is a block diagram showing a configuration of a slave station shown in FIGS. 3 and 4. FIG.

6 is a block diagram showing a configuration of a master station shown in FIGS. 3 and 4. FIG.

FIG. 7 is a block diagram showing a modified example of the configuration shown in FIGS. 5 and 6.

FIG. 8 is a diagram for explaining the configuration in the case where the distribution material is in the hub station in the conventional system.

FIG. 9 is a diagram for explaining a configuration in the case where a distribution material is not present in a hub station in a conventional system.

FIG. 10 is a diagram illustrating the configuration of a frequency-multiplexing digital communication system.

FIG. 11 is a diagram illustrating a configuration of a time division multiplexing digital communication system.

FIG. 12 is a diagram illustrating another example of the configuration of a time division multiplexing digital communication system.

[Explanation of symbols]

 10 Transmitter 11 Buffer Device 12 Bit Stream Controller 13 High Frequency Oscillator 14 Carrier Controller 15 Modulator 16 GPS Antenna 17 GPS Receiver 18 Timing Controller 19 Slot Setting Console 20 Receiver 21 Demodulator 22 Slot Selection Controller 23 Buffer Device 24 GPS antenna 25 GPS receiving device 26 Timing control device 27 Slot setting console 30 Main station (slot allocation station) 31a to 31c Slave station 32 Satellite transponder 41 Control slot receiving device 42 Slot allocation control device 43 Slot setting console 44 Control information storage unit 50 Receiver 51 Control slot recognition device 52 Slot allocation device

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Satoshi Masamoto 3-3-3 Toyosu, Koto-ku, Tokyo NTT Data Communications Corporation

Claims (8)

[Claims] 1. A transponder for transmitting received data, and a plurality of uplink stations for transmitting and receiving data by uplinking to the transponder, each uplink station uplinking in a different time slot. In a digital communication system that performs communication by a time division multiple access method, each uplink station extracts GPS time information by a global positioning system to generate a timing signal, and based on the timing signal from the GPS means, A transmission / reception control unit that uplinks to the transponder in a time division multiple access system using the same predetermined frequency and different time slots as those of other uplink stations, and a digital communication system. 2. The transmission / reception control means of one uplink station among the plurality of uplink stations allocates part of the time slot for control, and uses the control time slot to perform slave control of timing control. 2. The digital communication system according to claim 1, further comprising slot control means for transmitting allocation information for time slot allocation to another uplink station. 3. The digital communication system according to claim 2, wherein said slot control means includes means for controlling the number and / or position of time slots assigned to one uplink station. 4. The digital communication system according to any one of claims 1 to 3, wherein the transponder comprises a broadcast satellite transponder arranged in outer space. 5. A slot allocating means for allocating time slots for control and each uplink station, a first GPS means for extracting time information by a global positioning system to generate a timing signal, and the first GPS means. At least one master station having first uplink control means for transmitting allocation information to the transponder in the control time slot according to the allocation by the allocation means based on the timing signal from the GPS means, and global positioning A second GPS means for extracting time information by the system to generate a timing signal;
Based on the timing signal from the second GPS means,
Control slot discriminating means for discriminating the control time slot, and timing information from the GPS means and information from the control slot discriminating means to receive allocation information from the transponder in the control time slot. Control slot receiving means, and a plurality of slave stations having second uplink control means for uplinking to the transponder using the time slot indicated by the allocation information received by the control slot receiving means. A digital communication system characterized by the above. 6. The digital communication system according to claim 5, wherein said slot allocation means includes allocation number control means for variably controlling the number of time slots allocated to one uplink station. 7. The digital communication system according to claim 5, wherein the transponder comprises a broadcast satellite transponder arranged in outer space. 8. A time division multiple access digital communication system comprising: a transponder for transmitting received data; and a plurality of uplink stations for mutually transmitting and receiving data in different time slots via the transponder. In the system, each uplink station includes receiving means for receiving data intermittently supplied in a plurality of time slots, and converting means for converting the data received by the receiving means into a continuous bit stream and outputting the continuous bit stream. The digital communication system comprises slot allocation control means for controlling time slot allocation to each uplink station according to the conversion speed of the conversion means of each uplink station. Digital communication system.