JPH04330831A - Traveling object satellite communication system - Google Patents

Abstract

PURPOSE:To transmit a correct outbound signal without any omission or repetition of data due to a jitter. CONSTITUTION:This system is constituted of a network control station 12 which generates the data for a satellite communication, and a central station connected through an asynchronous line 14 with this control station 12, which operates a communication through plural traveling object stations and communication satellites. The central station 13 is constituted of a demodulator 23 which demodulates an outbound signal whose one part of frames includes time information transmitted from the network control station 12, PLL circuit 22 which generates a clock signal based on a timing signal indicating a frame timing transmitted from the network control station 12, frame timing generating circuit 24 which counts one frame of the clock signal, and reproduces a frame timing signal, and data receiving circuit 21 provided at the previous stage of the demodulator 23, which operates the buffering of the outbound signal, and transmits a reset signal which resets the time information to the frame timing generating circuit 24 at the time of finding the time information in the content of the outbound signal.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mobile satellite communication system with improved data transfer methods.

0002

2. Description of the Related Art A mobile satellite communication system consists of a large number of mobile stations, a central station, and a network control station. The central station communicates with individual mobile stations via communication satellites. The network control station has the role of supervising and controlling all these communications.

By the way, the central station transmits various information to the mobile station in broadcast mode. This signal is called an outbound signal. Although the type of information differs depending on the system, time information such as universal time is generally incorporated into this outbound signal and is often used for various purposes such as displaying time on mobile stations.

FIG. 2 shows an example of a state in which time information is incorporated into an outbound signal. Generally, a control field C is provided at the beginning of one frame F on the time axis, and various information other than data D is inserted therein. Time information T may be inserted into this control field C.

[0005] The central station can be functionally divided into a transmitter/receiver section for satellite communication, a modem section for satellite communication, and a section that controls these. Here, we will focus on the modulation/demodulation section and the control section.

The outbound signal modulator in the modulation/demodulation section modulates and outputs the outbound signal containing time information sent from the control section. At this time, the timing difference from the input to the output of the signal can be reduced to an almost negligible level, or even if it is slightly larger, it becomes a fixed value.

The control section requires a frame timing signal to create time information. For this reason, the control section is equipped with a frame timing generator. The control unit creates an outbound signal containing time information using the frame timing signal as a time reference, and sends it to the modulation/demodulation unit. Taking into account the delay time in the modulator mentioned above and the approximately 240 millisecond propagation time to the communication satellite, if the frame timing signal is offset by that amount in the earlier direction, the outbound signal can be transmitted to the mobile station. The timing of reception at the station can be roughly aligned with standard time such as universal time.

In such a conventional mobile satellite communication system, the central station can provide time information with sufficient precision for practical use to each participating mobile station, so there are no problems in this regard. will not occur.

By the way, due to the system configuration, conventionally a mobile satellite communication system has been proposed in which a control unit is installed in a network control station, a central station is made up of a modulation/demodulation unit and a transmitting/receiving unit, and both stations are connected via an asynchronous line. It had been. In such a mobile satellite communication system, all control systems are integrated within the network control station, so the configuration of control system equipment can be rationalized, and the economic efficiency and reliability of the system can be improved. Therefore, the introduction of such mobile satellite communication systems is likely to increase in the future.

0010

[Problem to be solved by the invention] However, in such a mobile satellite communication system, the outbound signal and clock signal input to the modulator have jitter, and errors such as data dropout and repetition occur in the output of the modulator. There is a risk of Therefore, failures such as loss of frame synchronization occur in all mobile stations, and instantaneous line interruptions tend to occur frequently, which poses a major problem in terms of system configuration.

SUMMARY OF THE INVENTION An object of the present invention is to provide a mobile satellite communication system that can transmit correct outbound signals without data loss or repetition.

0012

[Means for Solving the Problems] The present invention includes a control unit that generates data for satellite communication, and a center that is connected to the control unit via an asynchronous line and communicates with a plurality of mobile stations via a communication satellite. In a satellite communication system that consists of a central station, a modulator that modulates an outbound signal that includes time information sent from the control unit as part of the frame, and a frame that is also sent from the control unit. A PLL circuit that generates a clock signal based on a timing signal that indicates timing, a frame timing generation circuit that counts clock signals for one frame and reproduces a frame timing signal, and a frame timing generation circuit that is placed before the modulator and is sent from a control section. The data receiving circuit buffers the received outbound signal and sends a reset signal to the frame timing generating circuit when time information is found in the content of the signal.

That is, in the present invention, a PLL circuit is provided in the central office to generate a clock signal based on a signal indicating frame timing sent from the control unit, and an asynchronous connection format is adopted for outbound signals. . Then, a clean clock without jitter is regenerated at the central station, and the outbound signal 18 input from the control unit is modulated using this clock, thereby making it possible to transmit a correct outbound signal without data loss or repetition. .

[0014]

EXAMPLES The present invention will be explained in detail with reference to Examples below.

FIG. 1 shows the configuration of a mobile satellite communication system in one embodiment of the present invention. This mobile satellite communication system 11 has a configuration in which a network control station 12 and a central station 13 are connected via an asynchronous line 14. The network control station 12 includes a frame timing generation circuit 15 as before. A frame timing signal 16 output from the frame timing generation circuit 15 is input to a data transmission circuit 17, and an outbound signal 18 is created based on this signal.

The central station 13 includes a data receiving circuit 21 that receives data, a PLL circuit 22 that generates a clock based on frame timing, a modulator 23 that modulates received data, and a frame circuit that sets frame timing. A timing generation circuit 24 is provided.

In the mobile satellite communication system having such a configuration, at the beginning of each frame of the outbound signal 18 sent from the network control station 12, as in the conventional frame configuration shown in FIG. time information is created and inserted based on frame timing. The frame timing signal is determined by delays occurring in the modulator 23 and data receiving circuit 21 in the central station, and
Taking into account the round-trip delay from to the mobile station, the offset is made earlier by that amount of time.

The outbound signal 18 is not sent as is to the central office 13, but is sent using an asynchronous data transfer method such as start-stop synchronization. The data transmission circuit 17 within the network control station 12 performs this.

Asynchronous data outputted from the data transmission circuit 17 is inputted to the data input circuit 21 of the central station via the asynchronous line 14. The data input circuit temporarily buffers the input asynchronous outbound signal 18 while checking its contents. Then, when the time information at the beginning of the frame is found in the content of the received outbound signal 18, a reset signal is sent to the frame timing generation circuit 24.

The frame timing generation circuit 24 has a PL
One frame length is counted by the clock signal sent from the L circuit 22, and a frame timing signal is generated. This frame timing signal is input to the data input circuit 21. Note that the reset signal is accepted by the frame timing generation circuit 24 only once, and is not accepted after the second time. Otherwise, the timing of the frame timing signal would be shifted by the reset signal containing jitter from the signal passed through the asynchronous line 14. If something like this happens,
This will cause errors such as omissions and repetitions in the outbound signal being transmitted.

The data receiving circuit 21 sequentially reads out the buffered data starting from the first data of the frame according to the frame timing signal outputted from the frame timing generating circuit 24 and sends the data to the modulator 23 . For reading at this time, a clock signal reproduced by the PLL circuit 22 is used.

On the other hand, the clock signal is reproduced as follows. That is, P located at the central station 13
A clock component is sent to the LL circuit 22 from the network control station 12 via the asynchronous line 14. Specifically, for example, data in the format "1, 0, 1, 0, . . ." has a frequency component that is half the frequency of the clock signal. Therefore, if such a signal is input to the central station 13, even if it is an asynchronous signal and contains jitter,
The PLL circuit 22 can sufficiently reproduce the original clock signal.

Once a clean clock signal is recovered in this manner, it can be used as a read clock for the data receiving circuit 21, a clock signal for the modulator 23, and a clock signal for the frame timing generating circuit 24. Can be done.

[0024]

[Effects of the Invention] As explained above, according to the present invention, it is possible to transmit correct outbound signals without jitter even if the modulation/demodulation section for satellite communication and the control section for generating outbound signals are connected via an asynchronous line. can. Therefore, it is possible to configure a mobile satellite communication system that does not cause problems such as instantaneous interruptions to mobile stations.

[Detailed explanation of drawings]

FIG. 1 is a schematic configuration diagram showing the configuration of a mobile satellite communication system in an embodiment of the present invention.

FIG. 2 is a frame configuration diagram of an outbound signal in which time information is incorporated at the leading end of the frame.

[Explanation of symbols]

11 Mobile satellite communication system 12 Network control station 13 Central station 14 Asynchronous line 24 Frame timing generator

Claims (1)

[Claims] Claim 1: Consisting of a control unit that generates data for satellite communication, and a central station that is connected to the control unit via an asynchronous line and communicates with a plurality of mobile stations via a communication satellite. generates a clock signal based on a modulator that modulates an outbound signal that includes time information sent from the control unit as part of the frame, and a timing signal that indicates frame timing that is also sent from the control unit. a frame timing generation circuit that counts the clock signal for one frame and reproduces a frame timing signal; and a frame timing generation circuit that is placed before the modulator and buffers the outbound signal sent from the control section. a data receiving circuit that sends a reset signal to reset the frame timing generating circuit when time information is found in the content of the frame timing generating circuit.