JP3047665B2 - Satellite communication system by orbiting satellite - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a satellite communication system using orbiting satellites, and more particularly to a satellite communication system of this type in which a communication channel is expanded by using a carrier frequency shift due to the Doppler effect.

[0002]

2. Description of the Related Art The development of satellite communication systems has so far relied heavily on geostationary satellites. Geosynchronous satellites require large-scale launch vehicles to launch to an altitude of 36,000 km above the equator, and are expensive to launch. Also, as the number of geostationary satellites launched by each country so far has increased,
Available geosynchronous orbital positions are running low. Further, since the distance from the ground station is very large, and thus the propagation loss is large, a large-scale transmitting / receiving device is required. For these reasons, conventional satellite communications cannot meet the demand for personal communications, where demand has grown significantly. Against this background, satellite communication systems using low-altitude orbiting satellites have attracted attention. The first advantage of satellite communication using low-altitude orbiting satellites is that the launch vehicle is relatively small,
The cost of one launch can be kept low. Therefore, the risk of launch failure has been reduced, and many small orbiting satellites can be launched.
The narrow service area due to the low altitude of the low-altitude orbiting satellite communication satellite can also be eliminated by increasing the number of launch satellites. The second advantage is that when real-time communication is not required (for example, communication between two points with a 180 degree longitude difference, that is, a time difference of 12 hours), low cost due to a small number of satellite configurations is achieved. The ability to provide a store-and-forward service. In addition, the third advantage is that
Since the distance between the ground and the satellite is small and the propagation loss is small as compared with the case of the geostationary satellite, the scale of the ground station can be reduced and the demand for personal communication can be easily met. In order to provide the above-mentioned low-cost storage and transfer service using a small number of orbiting satellites, it is advantageous to directly connect individual ground stations and satellites by communication lines. Including a relay ground station such as a gateway increases the cost accordingly. An example of such a low cost satellite communication system is an A including a contention control means for enabling random access from a ground station to a satellite in a packet system.
LOHA method (translated by Hideyoshi Tominaga et al., "Computer Networks and Protocols", Corona, pp. 93-117, 19
86. ). As an example of an orbiting satellite communication system based on store-and-forward including the same contention control means as the ALOHA system, JAS-1 (JAPAN Amateur Sa
tellite-1), JAS-1b, U0Sat of Surrey University, UK (literature, JM Radbone, “U0SAT: A Decade of Experience Pio
neering Microsatellites ”Symposium International S
mall Satellite Systems and Service29, June-3, July; 1
992, Arcachon France).

In the above-mentioned storage and transfer system using the ALOHA system, communication concentrates on a satellite visible time (approximately one hour / day) in a certain area as the orbiting satellite orbits into a service area of the satellite. It must have a larger number of communication channels compared to conventional satellite-based real-time communication systems. That is, if the number of communication requests is the same, the number of communication channels to be provided in the storage and transfer system is 24 times that in the case of the geostationary satellite-based real-time communication system.

In this storage and transfer system,
High variation rate of satellite distance to ground (6 km / sec)
Therefore, the Doppler shift occurs in the frequency of the received carrier in both the ground station and the satellite, and therefore, it is necessary to allocate a frequency channel including an allowance for the Doppler shift. For example, carrier frequency 2500 MHz, altitude 70
In a communication system using orbiting satellites in a circular orbit of 0 km, the Doppler shift is ± 50 kHz as shown in FIG.
Reach Therefore, this communication system provides 9.6
The frequency bandwidth required for transmitting a signal at a bit rate of kbps is about 20 KHz when there is no Doppler shift, but if the allowance for the Doppler shift is included, 1
It must be expanded to about 20 KHz, that is, six times. Therefore, the number of available communication channels in this communication system is reduced. As a measure against this problem, US Pat. No. 4,191,923 proposes a communication system using a reference pilot signal for Doppler shift correction. In this communication system, each of the slave ground stations detects the Doppler shift amount based on the frequency and phase of the reference pilot signal from the master ground station, and corrects the carrier wave frequency so that the shift amount becomes zero. This eliminates the need for the Doppler shift, thereby increasing the number of available communication channels.

[0005]

The above-mentioned conventional communication system using a reference pilot signal requires the installation of a large number of master ground stations. That is, since the orbiting satellites need master ground stations set for each visible range corresponding to the satellite visible time, a large number of master ground stations need to be installed around the world. Accordingly, there is a drawback that the hardware for correcting the Doppler shift in the ground station increases as the scale of the communication system increases, and the cost of the entire system increases.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an economical orbiting satellite-based storage and transfer system that enhances the use efficiency of carrier frequencies without causing a substantial increase in hardware for both orbiting satellites and ground stations. It is in.

[0007]

SUMMARY OF THE INVENTION The present invention is directed to a downlink of a communication channel in which an orbiting satellite and a plurality of ground stations are spread over a frequency band centered on a specific frequency, and a plurality of communication channels are spread over a frequency band wider than the frequency band. And one of the ground stations responds to a first instruction from the orbiting satellite by the downlink, and one of the plurality of communication channels indicated by the first instruction is idle. A communication line is formed with the orbiting satellite in accordance with a packet protocol through a channel to supply transmission data to the orbiting satellite, and in response to a second instruction from the orbiting satellite by the downlink, the communication between the earth station and the orbiting satellite is established One forms a communication line with the orbiting satellite according to the protocol through a free channel among the plurality of communication channels indicated by the second instruction. In a satellite communication system based on orbiting satellites that provide storage and transfer services by receiving the transmission data from the orbiting satellites, the orbiting satellites determine a frequency band per one communication channel of the uplink with the terminal and the terminal. Division means for dividing into a plurality of sub-channels based on the amount of Doppler shift of the frequency due to a change in the distance to the orbiting satellite; demodulation means for demodulating the transmission data in sub-channel units; demodulation in sub-channel units Data processing means for processing the transmitted data according to a predetermined procedure.

According to the present invention, the frequency band allocated to the uplink communication channel of the orbiting satellite can be substantially increased.

[0009]

Next, the present invention will be described with reference to the drawings.

Referring to FIG. 2, which schematically illustrates an orbiting satellite-based satellite communication system according to the present invention, a user 2 located on the ground in front of the traveling direction a of the orbiting satellite 1 and a user located on the ground behind. 3 transmits and receives voice signals and other data using a communication device described later mounted on the satellite 1.

When both users 2 and 3 are transmitting at carrier frequency f ^a , the reception frequency at satellite 1 changes due to the Doppler effect, and carrier frequency f ^a from user 2 changes.
Is f ^a1 (> f ^a ), and that from user 3 is f
^a2 to shift, respectively (<f ^a). Similarly, if the user 2 both satellites 1 being received at a carrier frequency f ^b, the frequency f ^b at the user 2 is f ^b1 (> f ^b) next, it in the user 3 to f ^b2 (<f ^b) Shift each. These frequency shifts are called Doppler shifts. If the communication device mounted on the satellite 1 can demodulate the carrier frequency f ^a and the carrier frequencies f ^a1 and f ^a2 including the Doppler shift separately from each other, the uplink transmission bandwidth from the ground surface to the satellite 1 is as shown in FIG. As shown, frequency bands A, B and C can be divided and these bands can be assigned to three communication channels CH3, CH2 and CH1, respectively. In FIG. 3, the third channel CH
3 (frequency band A) is a band centered on frequency f ^a2 ,
Second channel CH2 (frequency band B) in the band centering on the frequency f ^a, the first channel CH1 (frequency band C) correspond respectively to a band around the frequency f ^a1. The frequency of the carrier wave from the satellite 1 observed at one point on the surface of the ground immediately below the flight route of the orbiting satellite 1 is shown in FIG.
As shown in the figure, the temperature is high at the beginning of the visible time, that is, at the time when the satellite 1 starts to be seen from that point, and becomes lower as the satellite 1 moves away from above the point. It is clear that if such changing frequencies are assigned to the first to third communication channels, signals can be transmitted and received for each channel. Further, the relationship between the frequency allocation using the Doppler shift accompanying the passage of the orbiting satellite and the service area of this satellite is described in the second channel C.
Referring to FIG. 5, which shows the time point located above the center of the use area of H2, the area in front of the flight direction a of the satellite is the use area of the first channel CH1, and the area behind it is the use area of the third channel CH3. .

In the satellite communication system based on the orbiting satellite of the present invention shown in FIG. 2, the satellite 1 includes communication channels ch0, ch1, ch2,.
The storage and transfer service is provided to the ground stations 2 and 3 through the network.
Here, the communication channel ch0 is a channel for line management used only for the downlink, and is always in a transmission state. That is, the channel ch0 transmits a channel number of an empty channel among the communication channels ch1, ch2,... To the ground station, and transmits a transmission permission signal for permitting the ground station to transmit on the empty channel. Used for management. When receiving the transmission permission signal via the channel ch0, the communication terminal device of the ground station starts transmitting the transmission message through the empty channel indicated by the signal. Note that the ground station stops transmitting until the transmission permission signal is received. The line connection procedure with the satellite when the ground station actually transmits the signal to be transmitted is AL
According to the packet protocol of the OHA system standard. Each ground station that has received the transmission permission signal starts transmitting at the same time according to this packet protocol, while the satellite 1 uses the Doppler shift to the use area of the channel ch2 including the point immediately below the satellite (FIG. 5). Lines are connected to a total of three ground stations, one station, one station in the channel ch1 use area ahead of the satellite in the traveling direction, and one station in the channel ch3 use area behind the satellite in the traveling direction. this is,
In this type of communication system according to the prior art, the uplink is occupied by signals from ground stations near the point directly below the satellite, in contrast to extremely low frequency utilization in both the uplink and the downlink. It is. When the line connection between the satellite 1 and the ground stations 2 and 3 is completed through the above-described steps, each of the data with the communication partner information from the ground stations 2 and 3 is written into the memory provided in the satellite by the on-board processor of the satellite 1. It is accumulated. The onboard satellite processor manages the transfer destination of the data in the memory, and when the satellite 1 reaches the sky above the target area, calls the transfer destination through the channel management channel ch0.

When the transfer destination ground station receives this transfer destination information via channel ch0, the connection to the satellite 1 is established in accordance with the packet protocol through the vacant channel in the same procedure as in the case of receiving the above-mentioned transmission permission signal. , Satellite 1
Receive data from After the processor of the satellite 1 recognizes the completion of the data transmission at the end of the packet protocol,
Control is performed to erase the data from the memory. Referring also to FIG. 6, which is a block diagram of a satellite-mounted communication device according to an embodiment of the present invention, an input signal 4-1-1 from an antenna 4 of the satellite device 1 passes through a diplexer 4-2 to a low noise amplifier ( LOW NOISE AMPLIFIER) Input to 4-3. The output signal of the low noise amplifier 4-3 is separated into channels ch1, ch2,... Of a predetermined frequency band by a multiplexer 4-4, and the mixers 4-5-1, 4-5-2, 4-5-3 Is output to each. These mixers are local oscillators (LO
CAL OSCILLATOR) 4-6-1, 4-6-2, 4-6-3..., Respectively, and supplies the local oscillation output, and converts the line separation output into an intermediate frequency signal. These mixers 4-5-1,4-5-
The outputs of 2,4-5-3… are further intermediate frequency demodulators 1, 2, 3,
... These intermediate frequency demodulators 1, 2, 3, ...
Consists of three demodulation circuits 4-7, 4-8, and 4-9,
Outputs separated into three channels ch1-1, ch1-2, and ch1-3 by Doppler shift are supplied to a memory 4-14 via a processor 4-10 and stored. The read output signal of the memory 4-14 read via the processor 4-10 is modulated by the modulator (MOD) 4-11 and then received by the local oscillator (LOCAL OSCILLATOR) 4-13 to receive the oscillation output of the mixer. The frequency is converted to the frequency of channel ch-0 by 4-12 and supplied to output amplifier (TX AMP) 4-16 via multiplexer 4-15. Output amplifier 4-16
Output signal 4-1-2 amplified to a predetermined output level by
Is radiated from the antenna 4-1 to the ground station via the diplexer 4-2 which is a coupling / separation circuit. Referring to FIG. 7, which is a block diagram of a communication terminal device of a ground station,
The received signal 5-1-1 received at 1 is supplied to the multiplexer 5-3 via the diplexer 5-2. Multiplexer
5-3 converts this input signal into a mixer 5-ch for channel ch0.
Separate into 4-2 and mixer for free channels 5-4-1. Mixer 5-4-1 is a local oscillator (LOCAL OSCILLATOR) that produces a local oscillation output at the frequency specified by processor 5-8.
The local oscillation output from 5-5-1 is received and the frequency of the received signal is converted into a predetermined intermediate frequency and supplied to a demodulator (DEM) 5-6-1. The signal demodulated by the demodulator (DEM) 5-6-1 is written to the reception data memory 5-7 through the processor 5-8. On the other hand, the mixer 5-4-2 receives a local oscillation output of the frequency of the channel ch0, converts the input frequency from the channel ch0 to a predetermined intermediate frequency, and demodulates the demodulator (DEM) 5-6-2.
Output to The signal demodulated by the demodulator (DEM) 5-6-2 is output to the processor 5-8.

On the other hand, the transmission data input from the input / output terminal 5-9 is transmitted to the transmission data memory 5- via the processor 5-8.
Written to 10. Processor for sending and receiving satellite 1 signals
The output data read from 5-8 is the modulator (MOD) 5-1
Modulated by 1 and output to mixer 5-12. Mixer 5-
Since the local oscillation output of the specified frequency is supplied from the oscillator (LOCAL OSCILLATOR) 5-13 to the processor 12, the output data is subjected to the frequency conversion corresponding to the local oscillation frequency and then output to the output amplifier (TX). AMP) 5-14 is input. The output signal 5-1-2 amplified to a predetermined level by the output amplifier 5-14 passes through the diplexer 5-2 to the antenna 5-1.
Is emitted toward the satellite 1. The satellite communication system of the present invention including the satellite communication device and the ground station device shown in FIGS. 6 and 7 operates as follows.

That is, when the satellite 1 starts accepting the storage and transfer service to the ground station, the processor 4-10
Transmits the transmission permission signal indicating the empty channel to the mixer.
After the frequency is converted to the frequency band of channel ch0 by 4-12, it is emitted from the antenna 4-1 via the multiplexer 4-15, the output amplifier 4-16, and the diplexer 4-2. Each ground station
Upon receiving this transmission permission signal, the mixer 5-4-2 converts the frequency to the intermediate frequency band of channel ch0 and then demodulates the signal to the demodulator 5-6-
Demodulate by 2 and input to processor 5-8. Processor 5-8
Receives this transmission permission signal on channel ch0,
The vacant channel number indicated by the transmission permission signal is supplied to a local oscillator (LOCAL OSCILLATOR) 5-13. Next, the processor 5-8 frequency-converts the connection request signal to the frequency band of the vacant channel in the mixer 5-12, and then outputs the signal to the output amplifier.
5-14, Emitted from antenna 5-1 via diplexer 5-2.

The satellite 1 transmits the connection request signal to the antenna 4
After receiving at 1, diplexer 4-2, multiplexer 4
-4, the local oscillation output corresponding to the vacant channel specified is converted to the intermediate frequency band by the mixer 4-5-1, 4-5-2 ...
The signal is separated and demodulated by an intermediate frequency demodulator (IF / DEM) and supplied to a processor 4-10. If the vacant channel is channel ch1, the connection start signal is demodulated by one of the three demodulation circuits 4-7, 4-8 and 4-9. Which demodulation circuit is used for demodulation depends on whether the ground station is immediately below the satellite or in the area in front of or behind the satellite.

[0017] The processor of the satellite receiving the connection request signal.
4-10 and the processor 5-8 of the ground station exchange signals according to the packet protocol through the same route as when transmitting and receiving the transmission permission signal thereafter, and connect the lines.

Even if the ground stations in the frequency-corresponding area (FIG. 5) of the vacant channel designated by the transmission permission signal start transmitting the connection request signal all at once, the signal is transmitted and received in accordance with the packet protocol. One communication terminal in each of the three areas 5 is connected to the satellite 1.

When the line connection between the satellite and the ground station is completed,
The data with the communication destination information from the ground station is stored in the memory via the satellite processor 4-10 along the same route as the connection request signal.
It is written and accumulated in 4-14.

The processor 4-10 of the satellite manages the transfer destination of the data in the memory. When the satellite arrives at the target area, the channel ch is transmitted in the same manner as when the transmission permission signal is transmitted.
0 outputs a transfer destination calling signal. The processor 5-8 of the transfer destination ground station receives this transfer destination call information through the channel ch0 in the same manner as when receiving the transmission permission signal, and performs the same procedure and route as in the case of the above-described transmission permission signal via the vacant channel. After connecting the line with the satellite in accordance with the packet protocol, data is received from the satellite and the received data is stored in the received data memory 5-7.

[0021]

As described above, in the communication system according to the present invention, the efficiency of use of the uplink frequency is improved, the number of channels can be increased, and the number of access terminals at the time of random access to the orbiting satellite can be increased. It has the effect of being able to do.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a carrier frequency change due to the Doppler effect according to the present invention.

FIG. 2 is a schematic diagram of an orbiting satellite-based satellite communication system according to the present invention.

FIG. 3 is a diagram illustrating an example of frequency assignment of a communication channel in the communication system of the present invention.

FIG. 4 is a diagram showing a relationship between a change in carrier frequency due to the Doppler effect and a communication channel.

FIG. 5 is a diagram showing a relationship between the communication channel and a service area of an orbiting satellite.

FIG. 6 is a block diagram of a satellite-mounted communication device according to an embodiment of the present invention.

FIG. 7 is a block diagram of a communication terminal installed in a ground station according to an embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Satellite 2,3 Earth station 4-1,5-1 Antenna 4-1-1,5-1-1 Input signal 4-1-2,5-1-2 Output signal 4-2,5-2 Diplexer 4 -3 Low noise amplifier 4-4,5-3 Multiplexer 4-5-1,4-5-2,4-5-3,5-4-1,5
-4-2 Mixer 4-6-1, 4-6-2, 4-6-3, 5-5-1,5
-5-2 Local oscillator 4-7,4-8,4-9 Demodulation circuit 4-10,5-8 Processor 4-11,5-11 Modulator 4-12,5-12 Mixer 4-13,5- 13 Local oscillator 4-14 Memory 4-15 Multiplexer 4-16, 5-14 Output amplifier 5-6-1, 5-6-2 Demodulator 5-7 Received data memory 5-9 I / O terminal 5-10 Transmission data memory

Claims (1)

(57) [Claims] An orbiting satellite and a plurality of ground stations form a downlink of a communication channel extending over a frequency band centered on a specific frequency and an uplink of a plurality of communication channels extending over a frequency band wider than the frequency band. One of the ground stations in response to a first indication from the orbiting satellite by the downlink according to a packet protocol over a free channel of the plurality of communication channels indicated by the first indication. And forming a communication line with the orbiting satellite to supply transmission data to the orbiting satellite. In response to a second instruction from the orbiting satellite by the downlink, another of the ground stations receives the second instruction from the orbiting satellite. A communication line is formed between the orbiting satellite and the orbiting satellite according to the protocol through an empty channel among the plurality of communication channels shown from the orbiting satellite. In a satellite communication system using an orbiting satellite that provides a store-and-forward service by receiving transmission data, the orbiting satellite communicates with one of the uplinks.
Dividing means for dividing a frequency band per communication channel into a plurality of sub-channels based on a Doppler shift amount of a frequency resulting from a change in the distance between the terminal and the orbiting satellite, and the transmission data in units of the sub-channels. A satellite communication system using an orbiting satellite, comprising: demodulation means for demodulating; and data processing means for processing the transmission data demodulated for each sub-channel in accordance with a predetermined procedure.