JPH0369218B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a satellite data transmission system.

One of the methods widely used in satellite communications
SCPC-FM (Single Channel Per Carrier-
Frequency Modulation; single channel perky rear frequency modulation)
It is a method. This is a method in which one carrier is frequency-modulated with one channel's worth of voice band signals and information is transmitted via a satellite RF channel of a predetermined frequency, and is widely used for telephone transmission between stations with low traffic. There is. When transmitting telephone calls between two specific stations using this method, a pair of (round trip) satellites of a specific frequency are used for this transmission.
An RF channel is assigned, and each of these two stations sets the transmitting/receiving frequency of its modulator/demodulator to the assigned frequency and performs transmission/reception. Therefore, this satellite RF channel will be dedicated between these two stations, at least while the call is ongoing between these two stations.

If one attempts to use one satellite channel of the SCPC-FM system to transmit data between a large number of stations, each with a relatively small amount of traffic, the following problem will occur. The data to be sent from each of these many stations generally takes the form of relatively short data that is not limited to a specific station within the system and is generated randomly in time. The most suitable transmission form for this type of data transmission is the so-called packet data transmission system. In this method, data is divided into blocks having a predetermined length, and each block is transmitted as a packet in a fixed format, with a header containing information such as the destination and origin of the data added. When this method is applied to the SCPC-FM method, each station uses this packet to FM-modulate the burst carrier of the satellite channel each time a packet to be transmitted is generated, and transmits it toward the uplink of the satellite. . The satellite receives these packets from each earth station and loops them back on the downlink. Each earth station receives the downlink satellite channel, performs RM demodulation on it and regenerates packets from all earth stations within the system, and selects only the packets with headers addressed to it. This takes the form of transmission to the destination of the own station. but,
If packets are transmitted from each station independently and randomly in time, there is a probability that these packets will overlap each other in time as they pass through the satellite, making the transmitted data invalid. do. To deal with this, there is the so-called Aloha method, which uses some method to detect when a packet overlaps with another packet and retransmits the same packet, but this method is not suitable for increasing line efficiency. It is. Furthermore, each station must demodulate the burst carrier-modulated FM packets transmitted from different stations, thus requiring the use of sophisticated demodulators operating in burst mode at each station. do.

An object of the present invention is to provide a satellite data transmission system that eliminates these problems.

The satellite data transmission system of the present invention includes a plurality of small stations,
Each of the small stations has a large station that transmits data via a communication satellite and relays information transmitted between the small stations via the communication satellite. a first storage means for storing the transmission information to be transmitted to the first storage means; an accumulation state sending means for generating storage state information indicating the amount of information stored in the first storage means; After a predetermined time has elapsed for the local station, the burst carrier is modulated with the accumulated state information and sent to the global station, and the second control signal received from the global station is transmitted to the local station. a first transmitting means for modulating the burst carrier with the transmission information read from the first storage means and transmitting the modulated burst carrier to the large station; and a receiving means for receiving the second control signal as a time-division signal from the large station, and the large station generates the first control signal, which is transmitted from each of the small stations in response to the first control signal. control signal generating means for generating the second control signal based on the accumulated state information; and a second control signal generating means for accumulating the transmission information transmitted from the small station in response to the second control signal.
storage means, and a second transmission for reading out the transmission information stored in the second storage means, converting it into a time-division signal together with the first and second control signals, modulating a continuous carrier, and transmitting the modulated signal to the small station. means.

Next, the present invention will be explained in detail with reference to the drawings.

FIG. 1 is a block diagram showing one embodiment of the present invention. This embodiment consists of a plurality (n) of relatively small-scale earth stations (hereinafter referred to as small stations) 1-1, ..., 1-k,
...1-n, a communication satellite 2, and one relatively large-scale earth station (hereinafter referred to as a large-scale earth station) 3.

Any small station 1-k is a small data terminal station 10,
It is composed of an SCPC-FM modulation transmitter 11, an antenna section 12, and an SCPC-FM receiver and demodulator 13. Each of the small stations 1-1 to 1-n is connected to the first SCPC-1, which is relayed from the large station 3 via the communication satellite 2.
Information is received via the downlink of the FM channel (hereinafter referred to as the global-local downlink channel).

Signals transmitted from the large station via this large station to small station downlink channel are
When the FM reception demodulator 13 demodulates and decodes the reception,
The continuous time-division digital signal shown in Figure 2 (hereinafter
(referred to as TDM signal). That is, this
The TDM signal has a constant frame period T, and a synchronization signal S having a fixed pattern is placed at the beginning of each period. This synchronization signal S is used as a reference for decoding subsequent signals.
Following this synchronization signal S, there is a fixed length command signal field C for transmitting a command signal.
Furthermore, there is a packet signal field P, also of fixed length, for transmitting the packet signal. The first part of this packet signal field P is a fixed length header field H, in which information such as the destination and source of this packet signal is transmitted.

Now, at any small station 1-k, SCPC-
The TDM signal shown in FIG. 2 demodulated and decoded by the FM reception demodulator 13 is supplied to the small data terminal 10 via an output line 1300.

FIG. 3 is a block diagram showing details of the small data terminal 10 of this embodiment.

The TDM supplied via line 1300
The signal is first supplied to the TDM signal receiving circuit 100. This circuit 100 detects the synchronization signal S and establishes frame synchronization for reception. Based on this, data in the command signal field C and data in the packet signal field P are separated, and the former is supplied to the command decoder 102 via an output line 1000, and the latter is supplied to the packet receiving terminal 101 via an output line 1001. supplied to The data in the packet signal field P supplied to the packet receiving terminal 101 is transmitted to the terminal 1.
At 01, the contents of the header field H are monitored, and only packet data having a header addressed to the terminal 101 is selected at the same terminal 101 and output to the land line 1010 after undergoing necessary processing.

On the other hand, data in command field C supplied to command decoder 102 is decoded here.

There are two types of command signals sent from the main station 3 by the command field C, the first of which is a status request command signal. This signal has a predetermined fixed pattern, and when the command decoder 102 detects this fixed pattern (status request command), it immediately generates a status request timing pulse and outputs the output line 10.
20 to the status data sending circuit 106. This command signal is addressed to all small stations, and the operations up to this point are the same for all small stations.

The second type of command signal is a packet sending command signal. This signal contains information specifying a particular station and instructs the specified station to send out one packet. When the command data 102 decodes this command signal and detects a packet sending command directed to the own station, it immediately generates a packet sending timing pulse and sends it to the packet sending control circuit 1 via the output line 1021.
Supply on 05.

Data transmission from the small station 1-k is performed as described above in response to the reception of the above command signal. Data to be sent is supplied to packet transmitting terminal 103 via terrestrial line 1030. This terminal 103 converts this data into a block of a fixed length, adds a header indicating the destination and source, and supplies it to the transmission packet buffer 104 as a packet with the aforementioned fixed length.

This buffer 104 accumulates and stores supplied packets one after another, and also outputs buffer status data indicating how many packets are currently stored in this buffer 104 via a status data output line 1040. It is supplied to the sending circuit 106.

Now, when the status request timing pulse is input to the status data sending circuit 106 via the output line 1020, this circuit 106
The buffer status data supplied via the output line 1040 is converted into a specific format at a time delayed by a certain delay setting time Ts-k (this value differs for each small station) from the request timing pulse. is then encoded as a status signal and sent via output line 1060 to the SCPC-
The signal is sent to the FM modulation transmitter 11. The transmitter 11 is
When this status signal is supplied, the burst carrier is SCPC-FM modulated with this status signal and sent to the second communication satellite of the communication satellite 2 via the antenna section 12.
It is transmitted to the SCPC-FM channel (hereinafter referred to as the small station-large station uplink channel). On the other hand, when the packet sending timing pulse is input via the output line 1021, the packet sending control circuit 105 immediately (in this case, without giving any particular information delay) sends the read clock to the sending packet buffer 104 via the output line 1050. The information for one packet is read out via the output line 1041, and sent to the SCPC via the output line 1051 as a transmission packet signal.
- Send to FM modulation transmitter 11. When the transmitter 11 receives this transmission packet signal, it transmits the burst carrier to SCPC-FM using this transmission packet signal.
The signal is modulated and transmitted via the antenna section 12 to the small station-to-large station uplink channel of the communication satellite 2.

In this way, the burst-like status signal and the packet signal are transmitted to the small station-to-large station uplink channel after receiving SCPC-FM modulation from each of the small stations 1-1 to 1-n.
is looped back into the local-to-global downlink channel via one transponder.

As is clear from the above explanation, any small station 1-
The sending of status signals and packet signals from k is performed in response to command signals from the main station 3. By appropriately selecting this command and the delay setting time Ts-k, which is different for each small station, it is possible to control the burst signals from each small station passing through the transponder so that they do not overlap with each other.

Now, the signal of the small station-big station downlink channel is transmitted through the antenna section 32 at the large station 3.
The received signal is demodulated and decoded by the SCPC-FM receiving demodulator 33. The thus reproduced digital signals from each small station are supplied to the large station data terminal 30 via a line 3300.

FIG. 4 is a block diagram showing the details of the global data terminal 30 of this embodiment. The received digital signal provided via line 3300 is first
The signal is supplied to the status signal/packet signal distribution circuit 300, where it is separated into the status signal and the packet signal using a specific format given to each signal, and the former is supplied to the command signal generation control section 301. The latter is provided to the first packet register 302.

The command signal generation control section 301 generates the data in the command field C shown in FIG. 2, and thereby controls the data transmission of each small station as described above. This function is as follows.

First, the status request command signal of a fixed pattern is generated and stored in an internal output register (not shown). This status request command signal is read by the read clock 3030 supplied from the TDM signal synthesis circuit 303 at the time when the command signal field C of the next transmission frame appears, and is sent as the status request command signal through the path described below. SCPC of all small stations through
The signal is demodulated and decoded by the FM reception demodulator 13. As a result, each of the small stations 1-1 to 1-n, as described above,
The number of packets currently stored in the transmission packet buffer 104 of the local station is sent as a status signal. The status signal sent from each station is
The signals are transmitted so that they do not overlap each other due to the delay setting time Ts-k, and are input to the command signal generation control unit 301 of the main station 3 as described above.

That is, when the control section 301 generates a status request command signal and sends it, the number of packets currently stored in the transmission packet buffer 104 of each small station can be obtained one after another as a response. As will be detailed later, by appropriately selecting the length of one frame T, it is possible to send a status request command to the command signal field of one frame and then send the next command to the command signal field of the next frame. It is possible to obtain all the data regarding the number of accumulated packets from each of the small stations 1-1 to 1-n with sufficient time before the transmission time arrives. The data obtained in this way is sequentially transmitted to the control unit 3.
It is registered in a status table in a read/write memory (RAM: not shown) provided in 01. By scanning this status table, the control unit 301 selects the small station that is currently accumulating the largest number of packets (if two or more small stations are accumulating the same number of packets, a predetermined The small station with the higher priority (hereinafter referred to as small station 1-P) is selected, the packet sending command specifying this small station 1-P is created, and the packet sending command is stored in the output register. This is read out in the command field C of the next transmission frame (the frame after sending the status request command) and transmitted to each small station, but only the specified small stations 1-P as described above. Sends only one packet that has been accumulated.

The control unit 301 modifies the status table according to the sending of this packet sending command. In other words, small station 1- in the status table
Subtract 1 from the number of packets in P. The status table updated in this way is scanned, and the same algorithm as before is used to select only one small station that is currently accumulating the most packets, create a packet sending command that specifies this small station, and send it to the output register. Store. Also, the status table is updated according to this transmission.

In this way, the small stations to which packet data should be sent are specified one after another in the command signal field C of successive frames, and this process is repeated until the number of accumulated packets of all small stations becomes 0 in the status table. I'll give it back. When this status table becomes all 0, the control unit 301 next generates a status request command and stores it in the output register. In this way, new status data from each small station is acquired again, this data is registered in the status table, and the above-described processing cycle is repeated.

As described above, the packet signals sent from each small station are transmitted to the large station 3 without overlapping with each other, separated from the status signal by the distribution circuit 300, and stored in the first packet register 302.

In the large station 3, data to be transmitted from the local station to another station (small station) is supplied to the packet transmitting terminal 304 in the data terminal station 30 via the ground line 3040, and this terminal 304 transmits the data to the small station. Similarly to the packet transmitting terminal 103, the packet is converted into a packet having the same format and sent to the second packet.
It is stored in the packet register 305.

When the storage of one packet data in the packet register 302 or 305 is completed, the packet transfer control circuit 306 immediately decodes the destination of the header of the packet. If this destination is a small station, the entire packet is sent to TDM.
The packet is transferred to and stored in the packet buffer 307, and if the destination is a global destination, it is transferred to the receiving terminal packet buffer 30.
Transfer and store in 8. The packets addressed to the general station transferred and stored in this buffer 308 are sent to the packet receiving terminal 3.
The data is read out via the output line 3080 using the read clock 3090 sent from the terminal 309, subjected to necessary processing at the terminal 309, and sent to the terrestrial line 3091.

On the other hand, packets addressed to the small station stored in the control TDM packet buffer 307 are read out by the read clock 3030 from the TDM signal synthesis circuit 303.
At the time of each frame, the packet signal field P is read out one after another according to the first-in-out rule (FIFO) via line 3070 and is sent to circuit 3.
At 03, the TDM signal is combined with a synchronization signal and a command signal, and the TDM signal is supplied to the SCPC-FM modulation transmitter 31.

This TDM signal undergoes SCPC-FM modulation on a continuous carrier in the transmitter 31, and is sent to the uplink (large station - small station uplink channel) of the first SCPC-FM channel of the communication satellite 2 via the antenna section 32. Sent out.

Communication satellite 2 receives this and sends it to the first SCPC-
It loops back to the downlink of the FM channel, that is, the large-station downlink channel. Thus, the SCPC-FM modulated TDM from Broadcast 3
The signals are received by each of the small stations 1-1 to 1-n, and command signals and packet signals are transmitted to each small station.

Now, in satellite communications, when one earth station 1 sends a pulse to another earth station 2 via a satellite, and this station 2 immediately returns the pulse to station 1 via the satellite, station 1 sends a pulse again to station 1. If the delay time until the pulse is received is D, this delay time D is
It is determined by the position on the earth's surface of 2 and the relative position of the satellite at that time and is not constant. However, the value of D in a particular satellite communication system has a self-determined range. Now, in the satellite communication system of this embodiment, the large station 3 and all the small stations 1-1 to 1-n
It is assumed that the value of the delay time D between the reference value D M and the reference value D _M falls within the range D _M ±ΔD, where the reference value is D M and the range of variation thereof is ΔD. For example, the maximum distance between earth stations via satellite is 42000km x 2, and the minimum distance is
Assuming 35900Km×2, D _M =260msec, ΔD=
It will be 20msec.

Figure 5 shows the large station - small station uplink channel signal (transmitted signal of the large station 3) (Figure 5a) and the small station - large station downlink channel signal (received signal of the large station 3) (the received signal of the large station 3) seen at the large station 3. The relative time relationship with Figure b) is shown. However, in Figure 5b, the value of D is the reference value.
D _M is shown, so each burst of the actual signal is ±
This means that there is variation within the range of ΔD.

Now, suppose that a status request command signal is sent in command field C of frame 1 of signal a, then the time t ₁ , which is delayed by D _M after the end of command field C, is at the small station 1 -
This is the reference time point for starting status signal reception from 1. Actual status signal ST ₁ from small station 1-1
The reception start point is necessarily within the range of ±ΔD around this reference time _t1 . However, the status data sending circuit 10 of the small station 1-1
It is assumed that the delay setting time T _S-1 in 6 is set to 0, and that the delay time for processing can be ignored.

Now, assuming that all the status signal lengths are L, the delay setting time T _S-2 of small station 1-2 is T _S-2 = L +
If set to 2ΔD+G, even in the worst case,
Status signal ST ₁ from small station 1-1 and small station 1-
There is a guard time of G between the status signal ST2 and the status signal _ST2 . Similarly, for small station 1-2, T S _{- 3} = 2(L+2ΔD+
G), the status signal _ST3 from the small station 1-3 can have a guard time of G in the worst case between it and the status signal _ST2 from the small station 1-2. Similarly, for small stations 1-n, T _{So =} (n-1) (L+
2ΔD+G).

Furthermore, if the TDM frame period T is T>D _M + (n-1) (L+2ΔD+G) +L+ΔD+G+Tk+TcF, the command signal generation control unit 301 receives the status signal from the last small station 1-n,
Create the status table, scan it to determine the small station that should send the packet first, create a packet signal sending command that specifies this small station, and send it to the command signal field of the next frame (frame 2). This packet signal sending command can be sent using C. However, _Tk is the processing time required for this calculation, and TcF is the time length of the command signal field C.

Furthermore, if T _Y + TcF > 2ΔD + G (1) is selected, the packet signal Pi from each small station,
It is clear that Pk has a guard time greater than G between each other even in the worst case. however,
T _Y is the time length of the synchronization signal S, and the length of the packet from each small station is the packet length of the large station T _p
It is assumed to be the same as ≡T−T _Y −TcF. Note that the signal from the small station is a burst carrier SCPC-FM signal, and the length of the burst carrier is slightly longer than the actual digital signal, so it is necessary to take this into account in the guard time G mentioned above. be.

As is clear from Fig. 5, the packet transmission efficiency is T _p /(T _p + T _Y
+TcF), and the efficiency of signal b is further degraded by the insertion of a frame for transmitting a status signal. In order to increase efficiency, it is necessary to make T _p as large as possible relative to T _Y +TcF, while T _Y +TcF is calculated as described in (1) above.
From the formula, the minimum value cannot be smaller than the value of 2ΔD+G allowed by the change range ΔD and guard time G. However, this embodiment uses SCPC-FM modulation to transmit low bit rate data (e.g.
9.6 KBPS), it is possible to select T _p that is sufficiently long for the value of 2ΔD+G even if a practical storage capacity of the transmission packet buffer 104 is used. For example, 2ΔD+G=2×20+20=60
If T _p = 1 sec, the efficiency reaches approximately 94%, but even in this case, the buffer capacity required to store one packet is only 1.2K.
It's just a part-time job. Therefore, a buffer having the capacity to store several tens of packets can be easily obtained in practice, and highly efficient transmission can be easily achieved.

In addition, in the received signal of the large station (Figure 5b), the transmission efficiency automatically increases as the number of parts to be transmitted from each small station increases. Because the total number of reported packets increases,
The proportion of frames in which a status request command should be issued decreases and approaches the value of T _p /(T _p +T _Y +TcF) described above. In this case as well, it is easy to use the practical storage capacity of the transmission buffer 104 to prevent overflow from occurring until a sufficiently high transmission efficiency is obtained for data transmission at a low bit rate. be.

As described above, in this embodiment, the signal from the small station is accumulated in the transmission packet buffer of the small station, and the probability that the buffer at the small station will overflow is determined according to the command signal from the large station. are read out in the order in which they are the smallest when viewed in the entire system), and are read out via the local and global channels without overlapping each other, and without the need for complex burst control in response to the movement of the satellite. , transmitted to the big picture with high efficiency. The packet destined for the small station from the small station is further combined with the packet destined for the small station from the large station and transmitted to the small station as a continuous mode TDM signal. Therefore, a packet destined for a small station from a small station is necessarily transmitted to the destination small station via the large station. Although this seems complicated at first glance, it has the following excellent features.

Each small station only needs to receive continuous mode TDM signals containing commands and packets sent from the large station. Therefore, a continuous signal with a stable constant level is received, and a complicated demodulator or the like that operates in burst mode is not required. Also, the requirements for the antenna size (G/T) are reduced accordingly.

On the other hand, in the big picture with few numbers, high G/T
By using an antenna section with a large antenna and a particularly high-grade demodulator, it is possible to easily achieve highly reliable reception of burst signals from each small station. In this way, the economic burden of system integration can be greatly reduced by simplifying the facilities of the small stations, which are overwhelmingly more numerous than the large stations.

Note that packet transmission from one small station to another is a two-hop satellite transmission as described above, but the delay caused by this is not a big problem since it is a data transmission. In particular, data traffic between local areas (small stations) and the central government (large stations) is overwhelmingly greater than traffic between one side (small stations) and another (small stations), such as in administrative data communication networks. In a communication network, one hop, which requires a lot of traffic, is sufficient.

Note that in the above embodiment, the length of the packet is approximately the same as the length of the frame, and one frame per frame.
Although the packets are transmitted at a rate of one packet at a time, the length of the packets can be shortened so that a plurality of packets to different destinations can be transmitted in one frame. In this case, the algorithm of the command signal generation control unit 301 may be made complicated so that a specified number of packets are sent from each of a plurality of different small stations from a specified time point in one frame. can.

In addition, in the above embodiment, a plurality of small stations,
Although a satellite communication system configured with one large station has been described, the present invention can be applied in almost the same way to a case where there are many small stations and a small number of multiple large stations. That is, the entire system is divided into a plurality of subgroups each containing one global view, data is transmitted within each subgroup using the same method as described above, and data transmission between each subgroup is performed using the same method as described above. In this case, an appropriate data transmission line may be provided between the large stations belonging to the subgroup, and data may be transmitted between the small stations belonging to another subgroup via this inter-large station line. In this case, this can be easily realized by using, for example, a pair of SCPC-FM lines as the data transmission line between the stations.

As described above, when the present invention is used, SCPC
- It is possible to provide a practical satellite data transmission system in which data is transmitted in packets between a large station and a plurality of small stations using a pair of channels of an FM system. This increases the flexibility of data transmission in the satellite communication system.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an embodiment of the satellite data transmission system of the present invention, Fig. 2 is a diagram for explaining each field of the TDM signal transmitted from the large station, and Fig. 3 is a small diagram of the same embodiment. FIG. 4 is a block diagram of a data terminal for a local office in the same embodiment. FIG. 5 is a block diagram of a data terminal for a large office in the same embodiment. FIG.
FIG. 3 is a diagram showing a relative time relationship with a signal of a global downlink channel. In the above diagram, 1-1,...,1-k,...1
-n...Small station, 2...Communication satellite, 3...Big station, 1
0...Small data terminal station, 11...SCPC-FM
Modulation transmitter, 12... antenna section, 13... SCPC
-FM reception demodulator, 30... data terminal for general use,
31...SCPC-FM modulation transmitter, 32...Antenna section, 33...SCPC-FM receiver demodulator, 100
...TDM signal receiving circuit, 101...Packet receiving terminal, 102...Command decoder, 103...
... Packet transmission terminal station, 104 ... Transmission packet buffer, 105 ... Packet transmission control circuit, 10
6...Status data transmission circuit, 300...Status signal/packet signal distribution circuit, 301...
...Command signal generation control unit, 302...First packet register, 303...TDM signal synthesis circuit,
304...Packet transmission terminal, 305...Second packet register, 306...Packet transfer control circuit, 307...TDM packet buffer, 308
...Receiving terminal packet buffer, 309...Packet receiving terminal.

Claims (1)

[Claims] 1. A system comprising a plurality of small stations and a large station that transmits data to these small stations via a communication satellite and relays information transmitted between the small stations via the communication satellite. , each of the small stations includes a first storage means for storing the transmission information to be transmitted to the small stations other than the local station, and storage status information indicating the amount of information stored in the first storage means. accumulation state sending means for generating the accumulation state information, and receiving a first control signal from the global station, modulating a burst carrier with the accumulation state information after a predetermined time has elapsed for the local station, and transmitting the modulated burst carrier to the global station; a first sending means for modulating the burst carrier with the transmission information read from the first storage means in response to a second control signal received from the large station, and transmitting the modulated burst carrier to the large station; receiving means for receiving the transmission information from the small station together with the first and second control signals as a time-division signal from the large station, the large station generating the first control signal and transmitting the information to the small station; control signal generating means for generating the second control signal based on the accumulation status information transmitted from each in response to the first control signal; a second storage means for storing the transmitted transmission information; and a second storage means for reading out the transmission information stored in the second storage means and transmitting it as a time-division signal together with the first and second control signals to generate a continuous carrier. a second transmitting means for modulating and transmitting the modulated data to the small station.