JPH05316008A - Satellite communication system - Google Patents

Abstract

PURPOSE:To improve the slot utilizing efficiency while keeping counter- interference performance by adopting a single spread code so as to reduce a synchronizing signal for communication. CONSTITUTION:A demodulated data string is inputted to a frame synchronization circuit 4, in which frame synchronization is established. A frame pattern is in use to activate a VSAT station slot timing generating circuit 6, which generates a slot timing. The slot timing is inputted to a spread code generator 3, in which the generation of a spread code is initialized to a prescribed value at the start point of the slot at all times in the slot timing. The spread signal reset for each slot is inputted to a spread device 2, which diffuses the modulation signal. Thus, the VSAT station sends the spread signal to a center station in the intermittent mode. A demodulation means in the center station uses same spectrum spread to demodulate the signal from a start point of a time slot of a data signal thereby reproducing in-band data.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used for multiple access communication by a communication satellite. In particular, it relates to simplification of ground station equipment by using a small diameter antenna.

[0002]

2. Description of the Related Art VSAT (Very Small Aperture Termin
al) The communication system provides a satellite communication system using an antenna with a diameter of about 1.2 m, and has recently been rapidly developing because its system cost is lower than that of the conventional satellite communication.

However, in the VSAT system which generally constitutes a star network, the cost borne by the user is as follows: the central station and the satellite line usage fee are equally shared by all stations, and the cost per station is borne. The forehead is small. On the other hand, the equipment cost and the installation cost of the VSAT installed in each station are the burden of one station as they are, and thus the cost per station is controlled.

The installation cost, especially the installation cost of the antenna, is dominant in this cost. For example, the antenna is 1.2
Even in the m-caliber class, the opening area is quite large, and it must be installed so that it can withstand the wind pressure applied by typhoons and the like. For this reason, it is necessary to make sufficient foundation work to install the antenna. Further, when the antenna diameter is large, an appropriate installation space is required, which may be a hindrance to installation in a general company or a construction company such as VSAT.

[0005]

In the actual communication, the signal-to-noise ratio obtained on the line must be kept above a certain value in consideration of the characteristics guaranteed by the communication equipment. Since the antenna diameter and the antenna gain are in a proportional relationship, when the antenna diameter is reduced, it is necessary to increase the power supplied to the antenna according to the degree to which the antenna gain is reduced.

As for the quality of the communication line, the effect of reducing the antenna diameter can be prevented to some extent by this operation, but the following problems occur. With the recent development of satellite communication, the number of satellites for communication has increased dramatically. Moreover, most of the communication satellites are geostationary orbiting satellites and are located on the equator. Therefore, the space between satellites has become extremely small. For example, in Japan, private satellites are arranged at 4 ° intervals, and in the United States, they are arranged at 2 ° intervals.

Under such an environment, the interference of the signal transmitted from the VSAT station with other satellites becomes a problem. 1.
In the 2 m aperture class antenna, the gain in the off-axis direction of the antenna drops rapidly, so the amount of interference with adjacent satellites is not so great, but in the 75 cm aperture class antenna, the off-axis gain does not decrease so much. Therefore, since the antenna aperture is reduced and the antenna feeding power is correspondingly increased, the effective radiation power (e
Since irp) does not change, the interference signal power radiated toward the adjacent satellite increases. For this,
In terms of cost, it was difficult to adopt a small-diameter antenna even though it was clear that the cost could be reduced. In order to reduce this off-axis radiated power, spread spectrum multiple access method (hereinafter SS
One of the effective means is to reduce the unit power density of the radiated power by using the MA (Spread Spectrum Multiple Access) method. Regarding this method, US Patent 4455651, SATELLITE COMMUNICATION SYST
EM AND APPARATUS, Inventor Paul Baran, JUN
There are E, 19, 1984. This patent is based on S
SMA system is used, but SSMA is used as a communication path.
CDMA (Code Division Multip
le access) method, in which a large number of VSAT stations access satellites at the same time, and the stations are separated by a spreading code. Generally, in order to receive an SSMA type signal, FIG. As shown in 3, the chip synchronization (103) is established (despreading), and after that, a modulated signal equivalent to that before spreading is obtained, so this is demodulated (68) again.

In order to go through such a process, in the SSMA system, the sum of the despreading synchronization time and the demodulation synchronization time becomes the time required for establishing the synchronization. Therefore, in the satellite access method that uses time multiplexing, such as the simple and efficient slotted Aloha method used in the star-type satellite network, data cannot be reproduced until synchronization is correctly established. This is not preferable because the time required is wasted and the slot usage efficiency is significantly deteriorated.

The present invention has been made against such a background, and the SSMA which can be used by a small antenna and is resistant to interference is used.
In the system, it is an object to provide a satellite communication system that shortens the synchronization time for communication and improves slot use efficiency.

[0010]

SUMMARY OF THE INVENTION The present invention is directed to a plurality of peripheral stations which spread a data signal by a modulation means using a spread spectrum code and send it out to a communication satellite, and a spread spectrum data signal from the peripheral stations. In a satellite communication system including a single central station for despreading the data signal by means of a spread spectrum code demodulation means, and a plurality of peripheral station modulation means, Means for performing the spreading for a station from the start of the time slot of the data signal using the same spread spectrum code, the demodulation means of the central station, the despreading from the start of the time slot of the data signal It is characterized in that it includes means for using the same spread spectrum code. Further, the start point may be a start point of a frame of the data signal.

[0011]

The time of arrival of each slot at the central station is known because it is generated on the basis of the out-band signal. Furthermore, the starting point of the spreading code at the central station is clear, and basically, time division is performed. Paying attention to the fact that there is no problem even if each VSAT station uses the same spreading code, the central station and each VSAT station have one same spreading code in one system, and each VSAT station has a slot. From the start point of the frame or the start point of the frame in which a plurality of slots are collected, the modulated wave is spread with the same start code, and the central station starts the despreading code with the start point of each slot or the start point of the frame as a reference.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The configuration of the first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a block diagram of a VSAT station according to the first embodiment of the present invention. FIG. 2 is a block diagram of the central station of the first embodiment of the present invention. FIG. 3 is an overall configuration diagram of the first embodiment of the present invention.

According to the present invention, a VSAT station 22 as a plurality of peripheral stations which spreads a data signal by a spreader 2 as a modulation means using a spread spectrum code and sends it to a communication satellite 20, and from this VSAT station 22. A single central station 2 that receives a spread spectrum data signal via a communication satellite 20 and despreads the data signal with a despreader 16 as demodulation means using a spread spectrum code.
1 is a satellite communication system, the modulation means of the plurality of VSAT stations 22 is a plurality of VSAT stations 22 that are peripheral stations.
VSAT station slot timing generation circuit 6 as means for performing the above-mentioned spreading from the start point of the time slot of the data signal by using the same spread spectrum code.
The demodulation means of the central station 21 includes a central station slot timing generation circuit 18 as means for performing the despreading from the start point of the time slot of the data signal using the same spread spectrum code. To do. Further, since the starting point may be configured to start from the starting point of the frame of the data signal, this matter will be described as the second embodiment of the present invention.

Next, the line format used in the first and second embodiments of the present invention will be described with reference to FIG.
FIG. 6 is a diagram showing a line format used in the star network. In FIG. 6, the out-band line is transmitted from the central station 21 to all VSAT stations 22 in the broadcast mode. In this, a TDM (Time Division Multiplexer) in which a synchronization word repeated in a frame cycle is inserted.
ng) format. The line transmitted from the VSAT station 22 to the central station 21 is called a slotted Aloha line and is generated based on the frame timing of the outband signal. FIG. 6 shows a case where five slots are present in one frame. The starting point of each slot in the first embodiment of the present invention is determined based on the frame timing obtained from the outband signal.
The signal transmitted by the VSAT station 22 is transmitted in one of the slots # 1 to # 5 depending on the arrival of the transmission data of the VSAT station 22. In this way, the in-band line is time-multiplexed.

Next, the VSAT station 22 according to the first embodiment of the present invention will be described with reference to FIG. The VSAT station 22 is used by the outband demodulator 5 to demodulate the outband signal transmitted from the central station in the broadcast mode.
The demodulated data string is input to the frame synchronization circuit 4 to detect the frame pattern of the outband signal and establish frame synchronization. By using this frame pattern, VS is generated with a frame pulse having a fixed delay set for each station.
The AT station slot timing generation circuit 6 is operated to generate slot timing. This slot timing is always generated regardless of whether or not the VSAT station 22 is transmitting. This slot timing is input to the spreading code generator 3 and the generation of the spreading code is initialized to a fixed value at the slot start point at the slot timing. The value of this fixed delay is set by the VSAT station delay circuit 7 so that the signal of each VSAT station 22 that reaches the central station 21 enters a predetermined slot. The spread signal that is reset for each slot is input to the spreader 2 and spreads the modulated wave that is the other input. The data to be transmitted by the VSAT station 22 is modulated by the modulator 1. In the slotted Aloha communication system, the modulator 1 has a transmission output inhibition circuit because it is necessary to inhibit the transmission signal at times other than the transmission time. In this way, the spread signal is transmitted from the VSAT station 22 toward the central station in the intermittent mode (burst form).

Next, the central station 21 of the first embodiment of the present invention will be described with reference to FIG. The frame timing reference of the out-band signal is created by the frame generation circuit 11, and the UW (Unique Word) signal is multiplexed with the out-band signal by the multiplexing circuit in order to notify all the VSAT stations 22 of the frame information and transmitted. To be done. VSAT station 22
As shown in FIG. 1, since all stations are receiving the out-band line, all stations are synchronized with the frame of the central station. Then, the VSAT station 22 has its existing position (latitude, longitude)
Since it is given a fixed peculiar fixed delay in accordance with the above, it is received at the reception point of the central station in a state in conformity with the in-band signal format of FIG. The frame timing reference generated by the frame generation circuit 11 is the central station delay circuit 14
And is input to the central station slot timing generation circuit 18. The purpose of the central station delay circuit 14 is to allow the outband signal transmitted from the central station 21 to pass through the satellite line to VSAT.
This is to provide a delay time corresponding to the delay time until the in-band signal which reaches the station 22 and is sent from each VSAT station 22 to the central station 21 with a fixed delay is received by the central station 21. The generated slot signal is the despreading code generator 1
5 is reset every slot timing. Therefore, the start position of the despread signal matches the received in-band spread signal. The output of the despreading code generator 15 is input to the despreader 16 and the received spread signal is despread. The despreader 16 is usually realized by a multiplier as in the transmitting side. Since the spread signal corresponds to the modulated wave of the output of the modulator 1 of the VSAT station 22, the in-band data is reproduced by the in-band demodulator.

Next, the VSAT station 22 of the second embodiment of the present invention will be described with reference to FIG. FIG. 4 is a block diagram of the VSAT station 22 according to the second embodiment of the present invention. VSAT station 22
Is used by the outband demodulator 5 to demodulate the outband signal transmitted from the central station 21 in the broadcast mode. The demodulated data string is input to the frame synchronization circuit 4 to detect the frame pattern of the outband signal and establish frame synchronization. Using this frame pattern, the spreading code generator 3 is initialized with a frame pulse having a fixed delay set for each station. The fixed delay is set so that the signal of each VSAT station 22 reaching the central station 21 enters a predetermined slot. The spread signal that is reset for each frame is input to the spreader 2 and spreads the modulated wave that is the other input. VSAT station 22
The data to be transmitted by is modulated by the modulator 1. In the slotted Aloha communication system, it is necessary to prohibit the transmission signal except at the time of transmission, so that the modulator 1
Has a transmission output inhibition circuit. In this way, V
The spread signal is transmitted from the SAT station 22 to the central station 21.

Next, the central station 21 of the second embodiment of the present invention will be described with reference to FIG. FIG. 5 is a block diagram of the central station 21 of the second embodiment of the present invention. The frame timing reference of the out-band signal is created by the frame generation circuit 11, and the UW (Unique Word) signal is multiplexed with the out-band signal by the multiplexing circuit in order to notify all the VSAT stations 22 of the frame information and transmitted. To be done. VSAT station 22
As shown in FIG. 4, since all stations are receiving the out-band line, all stations are synchronized with the frame of the central station. Then, the VSAT station 22 has its existing position (latitude, longitude)
Since the fixed point is given a fixed peculiar fixed delay, the reception point of the central station 21 receives the signal in conformity with the in-band signal format of FIG. Frame generation circuit 11
The frame timing reference generated in 1 is input to the despreading code generator 15 via the central station delay circuit 14. The purpose of the central station delay circuit 14 is that the outband signal transmitted from the central station 21 reaches the VSAT station via the satellite line,
Each VSAT station 22 has a delay time corresponding to the delay time until the in-band signal sent to the central station 21 with a fixed delay is received by the central station, and the start position of the despread signal is set to the received in-band spread signal. Used to match.
The despread signal is reset for each frame by the delayed frame signal and input to the despreader 16. The despreader 16 is usually realized by a multiplier as in the transmitting side. Since the spread signal corresponds to the modulated wave output from the modulator of the VSAT station 22, the in-band demodulator reproduces the in-band data.

As described above, since the code synchronization for unifying the spreading codes of each VSAT station 22 and for performing the despreading is performed by adjusting the timing between the stations, the synchronization time in the despreading process can be omitted. The spreader 2 and the despreader 16 are the same as those described in FIG. 3 of 25
And FIG. It corresponds to one multiplier in 65 of 4. In the cited patent, FIG. 4 of 65
Although eight multipliers are required as shown in FIG. 3, only one multiplier is required in the device of the present invention.

[0020]

In the SSMA system, which is resistant to inter-satellite interference and can miniaturize the antenna of the VSAT station, the spreading code is unified and the synchronization time for communication is shortened, so that the interference resistance is maintained. The slot use efficiency can be improved.

Further, in the CDMA system, when the spread code length is a value of 1000 bits which is generally used, the number of stations which can be simultaneously accessed is around 50 stations due to the restriction of the code distance. In the present invention, since the basic configuration is the slotted Aloha system, there is no such limitation.

That is, while maintaining the anti-interference characteristic of the SSMA system, the communication efficiency of the slotted Aloha system and the simplicity of the equipment can be realized. That is, it is possible to realize a satellite communication system that adopts the respective advantages of the SSMA system and the slotted Aloha system.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a VSAT station according to a first embodiment of the present invention.

FIG. 2 is a configuration diagram of a central station according to the first embodiment of the present invention.

FIG. 3 is an overall configuration diagram of a first embodiment of the present invention.

FIG. 4 is a block diagram of a VSAT station according to a second embodiment of the present invention.

FIG. 5 is a block diagram of a central station of a second embodiment of the present invention.

FIG. 6 is a diagram showing a line format used in a star network.

[Explanation of symbols]

 1 modulator 2 spreader 3 spreading code generator 4 frame synchronization circuit 5 out-band demodulator 6 VSAT station slot timing generation circuit 7 VSAT station delay circuit 11 frame generation circuit 12 multiplex circuit 13 out-band modulator 14 central station delay circuit 15 Despread Code Generator 16 Despreader 17 In-Band Demodulator 18 Central Station Slot Timing Generation Circuit 20 Communication Satellite 21 Central Station 22 VSAT Station

Claims (2)

[Claims] 1. A plurality of peripheral stations that spread a data signal by a modulation means using a spread spectrum code and send it to a communication satellite, and receive a spread spectrum data signal from the peripheral station via the communication satellite. In a satellite communication system provided with a single central station that despreads the data signal with a demodulation means using a spread spectrum code, the modulation means of a plurality of the peripheral stations uses the spreading of the data signal for the plurality of peripheral stations. The center station demodulation means includes means for performing the same spread spectrum code from the starting point of the time slot, wherein the demodulation means of the central station uses the same spread spectrum code from the starting point of the time slot of the data signal. A satellite communication system characterized by including means for performing. 2. The satellite communication system according to claim 1, wherein the start point is a start point of a frame of a data signal.