JP2001077738A - Low orbit satellite communication system constituting cell specific to user - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a system which improves a communication quality between a satellite and a user terminal, is excellent in frequency recycling efficiency, never lowers receiving ability at a user terminal and can effectively utilize satellite power, by constituting a cell specific to a user for irradiating a spot beam so that the position of the user terminal becomes the substantial center of the spot beam. SOLUTION: A cell specific to a user for irradiating a spot beam so that the position of a user terminal becomes the substantial center of the spot beam. It is preferable to have a user positional information gaining means for detecting the position of the user terminal. It is also preferable to additionally have a weight deciding means for deciding the weight coefficient of each position of the user terminal based on information detected by this user positional information gaining means. It is also preferable to have a means for deciding the radiation pattern of the spot beam based on this weight coefficient.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-orbit satellite communication system for realizing a cell configuration in a next-generation satellite communication system.

[0002]

2. Description of the Related Art Conventionally, "Iridium" has started a service as a low-orbit non-geostationary satellite communication system.
Systems such as CO and Globalstar are also under development.
These systems are commonly referred to as "Big-LEOs" and are targeted for voice calls or low bit rate data communications. On the other hand, as a next-generation satellite communication system,
Large-capacity "BroadBand-LE" for high bit rate services for multimedia such as "Teledesic" or "SkyBridge"
A system called "O" has been proposed.

FIG. 1 shows a cell configuration system (hereinafter, referred to as a satellite fixed cell system (SFC: SFC)) used in a conventional Big-LEO.
llite Fixed Cell). Big-
Since the spot beam used in LEO is fixed in the satellite station coordinate system, the irradiation area of the spot beam also moves as the satellite moves.

FIG. 2 is a schematic diagram of the service coverage of the spot beam of the fixed satellite cell system of FIG. Figure 2
It is shown that the area of the hexagonal cell is an example of the traffic accommodation coverage of the spot beam.

[0005] FIG. 3 shows a cell configuration method (hereinafter referred to as a ground fixed cell method (EFC: Ea: Ea) used in the conventional BroadBand-LEO.
rth Fixed Cell). This method is proposed in BroadBand-LEO, divides the ground surface into quadrangular cells, and irradiates the cells with the same spot beam as long as the cells are present in the coverage area in the satellite. In the same satellite, the frequency of handover can be greatly reduced because the spot beam irradiating the table cell in each place is unchanged.

FIG. 4 is a schematic diagram of the traffic accommodating coverage of the fixed ground cell system shown in FIG. In FIG. 4, it is shown that the area of the square cell is the service coverage of the spot beam.

FIG. 5 is a conceptual diagram showing the configuration of a satellite payload for realizing the ground fixed cell system shown in FIG. 3 in the TDMA system. According to FIG. 5, it basically includes an array antenna, a synchronization signal generation unit, a weight coefficient determination unit for each cell, and a DBF (Digital Beamforming) network unit. Here, the expression Ncell is used. For example, in the case of Teledesic, Ncell = 9 because nine ground-fixed cells are sequentially irradiated according to the TDMA method. Further, in the satellite fixed cell system, since the radiation pattern of the satellite spot beam is invariable, the outgoing line from DeMUX (De-multiplexer) in FIG. One for each antenna element. Therefore, there is no need to send a synchronization signal to various places.

[0008]

However, a cell system for improving the communication quality between a satellite and a user terminal more than the conventional cell system described above is being studied.

Therefore, the present invention improves the communication quality between a satellite and a user terminal, is excellent in frequency reuse efficiency, and does not reduce the reception capability at the user terminal, as compared with the conventional cell system. It is another object of the present invention to propose a cell system that effectively uses satellite power, and to provide a low-orbit satellite communication system constituting the system.

[0010]

The low-Earth-orbit satellite communication system according to the present invention constitutes a user-specific cell for irradiating a spot beam such that the position of the user terminal is substantially at the center of the spot beam. . As a result, compared with the conventional cell system, the communication quality between the satellite and the user terminal is further improved, the frequency reuse efficiency is excellent, the reception capability at the user terminal is not reduced, and the satellite power is effectively used. A low-Earth-orbit satellite communication system constituting this cell system can be provided.

[0011] According to another embodiment of the present invention, it is preferable to have a user position information acquiring means for detecting the position of the user terminal. Further, it is preferable that the apparatus further comprises a weight coefficient determining means for determining a weight coefficient for each position of the user terminal based on the information detected by the user position information obtaining means. It is also preferable to have means for determining the radiation pattern of the spot beam based on this weighting factor.

The configuration of the present invention is similar to the configuration used in the conventional cell system in that an array antenna and a DBF network unit are combined to form a spot beam. This is different in that it is dynamically changed for each user. Further, since the processor load of the satellite payload can be designed independently of the number of spot beams, there is no system influence of forming a spot beam unique to traffic from a user.

[0013]

FIG. 6 is a conceptual diagram showing the application of a user-specific cell system according to the present invention. According to this method, the spot beam is emitted from the low-orbit satellite communication system such that the position of the user terminal is substantially at the center of the spot beam.

FIG. 7 is a conceptual diagram of a configuration of a satellite payload for realizing the user-specific cell system of the present invention shown in FIG. FIG. 7 shows a return link (a link from a user terminal to a gateway earth station via a satellite), taking TDMA as an example. In addition, what is necessary is just to make FIG. 7 an opposite direction for a forward link.

According to FIG. 7, the fact that beam forming is performed for the user 1 is indicated by a solid line. The time slots from user 2 to the maximum number of users Nuser serviced by the satellite are not connected because their time slots are different from their own time slots.

The first feature of the configuration shown in FIG. 7 is that user position information is required for beam forming (beam forming). As a second feature, the user location information is converted into a weighting factor Wi required for the DBF network unit.
(I = 1 to Nuser) to determine the value of the weight coefficient for each user. This is because the beam shape differs for each user. This is because a desired beam cannot be formed unless a different weighting factor is given to each antenna element for each beam shape. As a third feature, in the case of TDMA, for example, since the user to be irradiated changes for each time slot, the time (time slot) at which the signal extracted from the DeMUX is multiplied by the weighting factor is determined by the time of the target user. It is necessary to synchronize with time slots. Similarly, it is necessary to synchronize when the multiplied signal is input to the terminal for input to the addition operator.

[0017]

[Table 1]

Table 1 shows the user-specific cell scheme of the present invention,
It is a comparison result of the communication characteristics of the satellite fixed cell system and the ground fixed cell system. For comparison, it is assumed that the system conditions (system parameters such as satellite orbit altitude, spot beam HPBW and gain, and used frequency band) of each cell system are the same.

Regarding points other than the link budget and the frequency utilization efficiency, the difference between the satellite fixed cell and the ground fixed cell and the user specific cell, that is, the spot beam is non-st
The examination results are roughly classified according to the eering method or the steering method. For fixed-surface cells or user-specific cells,
In the same satellite coverage, the user is irradiated with the same spot beam, so that the frequency of handover is reduced. As a result, the amount of broadcast information of the system necessary for the user terminal to communicate is reduced, so that not only the processing load and power consumption of the user terminal but also the terrestrial network system and the satellite can be reduced. Also,
If the frequency bands available in each region are different and frequency allocation is performed as an operation plan for each spot beam,
Frequency allocation processing and management are considerably simplified. On the other hand, in the case of a fixed surface cell or a user-specific cell, the irradiation direction of the beam is time-varying in the satellite coordinate system, so that the management of the Doppler shift amount is also dynamic.

Here, advantages in the link budget of the user-specific cell system will be described. Cell type link budget [C / (N + I): C = desired wave power,
N = noise, I = in-system interference power from other user terminals], consider an average transmission / reception power. Assuming that traffic is uniformly distributed in the coverage, the average power value is obtained by dividing the power value integrated in the coverage area by the area of the area. In the case of the satellite fixed cell system, the expected power is P-1.25 [d
B] (P means electric power at the center of the spot beam).
In the case of the ground fixed cell system, the expected power is P-1.0
[dB]. As the number of users or the number of spot beams increases, the sum of the expected power differences increases.

In the case of the user-specific cell system, since the user terminal is located at the center of the beam, the expected power of each user terminal is equal to the peak power P. Therefore, the user-specific cell system of the present invention becomes more advantageous as the number of user terminals or the number of spot beams increases as compared with the conventional cell system. This not only reduces the load on the reception capability of the user terminal, but also enables effective use of finite resources such as satellite power.

Next, advantageous points in the frequency reuse interval of the user-specific cell system will be described. FIG. 8 is a comparison diagram of frequency reuse intervals between the satellite fixed cell system / ground fixed cell system and the user-specific cell system.

As shown in FIG. 8, in the case of the satellite fixed cell / ground fixed cell, the so-called “Edge of Co.”
Since the conditions of the user terminal existing in "verage" are the strictest, it is necessary to consider the allowable C / I in the Edge of Coverage. Required C / I at each spot beam to reuse the same frequency (ie Single-Entry C / I Isolation)
Is assumed to be 20 dB, for a satellite fixed cell / surface fixed cell, the minimum reuse interval is 1.88 HPBW. However, in the user-specific cell, the required C / I may be considered in the beam center, so the minimum reuse interval is 1.29 HPBW. 1.88HP
In order to satisfy BW, in the case of a satellite fixed cell, 7-cell repetition (= 2.29HPBW) is necessary, and in the case of radiating the same frequency at the same time in a square cell proposed as a ground fixed cell, It is necessary to use 9-cell repetition (= 2HPBW) to satisfy the minimum frequency reuse interval.

For example, FDMA and T using FDMA together
In the case of DMA, the quality of the frequency use greatly affects the line capacity that can be accommodated in the entire system. Therefore, in the user-specific cell system, since the frequency reuse interval R at which the same frequency can be used is narrower than the satellite fixed cell system / ground fixed cell system, excellent frequency use efficiency can be expected.

Here, the relationship between C / (N ₀ + I ₀ ) and the number of spot beams capable of reusing the same frequency carrier (that is, the number of users) around a certain point is obtained. Required C / Isingle
In the case of 20dB, it is necessary to apply at least 7-cell repetition in satellite fixed cells. On the other hand, the minimum repetition interval for a user-specific cell is 1.29 HPBW. The value of `` 1.29 '' is almost the same as (= 1.299), and in the user-specific cell, as well as the satellite fixed cell, assuming a hexagonal cell, the user-specific cell is located at the center of the adjacent cell and the second round of the adjacent cell. Hexagonal cells are located. FIG. 9 is an explanatory diagram of the number of repeating cells of a satellite fixed cell and a user-specific cell. As is apparent from FIG. 9, it is equivalent to "2.25 cell repetition". As described above, since the reuse interval is narrow, use is permitted even when the ground surface area where the same frequency band is obtained (that is, the ground surface area where the license band assigned to the satellite system from each country is common) is narrow. Since the frequency can be reused effectively, it can be said that the frequency resource is effectively used.

Various changes, modifications and omissions of the technical concept and scope of the present invention can be easily made by those skilled in the art in the embodiment described in detail above. Therefore, the above-described embodiment is merely an example, and is not intended to limit the present invention. The invention is limited only as defined by the following claims and equivalents thereof.

[0027]

As described above, according to the low-Earth-orbit satellite communication system constituting the user-specific cell of the present invention, the satellite and the user terminal are advantageous in link budget as compared with other cell construction systems. This has the effect of improving the communication quality between the two. Further, since it is advantageous in terms of frequency reuse as compared with other cell systems, there is an effect that the frequency band of the system can be used effectively. Furthermore, since the expected power of each user terminal is equal to the peak power P, even if the number of user terminals is large or the number of spot beams is large, the reception capability at the user terminal is not reduced, and the effective use of finite resources such as satellite power is not achieved. There is an effect that utilization is aimed at.

Therefore, the low-Earth-orbit satellite communication system constituting the user-specific cell according to the present invention is used to provide a cell configuration system in a next-generation satellite communication system.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing the application of a conventional satellite fixed cell system.

FIG. 2 is a schematic diagram of the traffic accommodation coverage of the fixed satellite cell system of FIG. 1;

FIG. 3 is a conceptual diagram showing an application of a conventional fixed-cell surface method.

FIG. 4 is a schematic diagram of an example of a traffic accommodation coverage of the fixed surface cell system of FIG. 3;

FIG. 5 is a conceptual diagram of a configuration of a satellite payload for realizing the fixed surface cell system shown in FIG. 3;

FIG. 6 is a conceptual diagram illustrating an application of a user-specific cell scheme according to the present invention;

FIG. 7 is a conceptual diagram of a configuration of a satellite payload for realizing the user-specific cell system of the present invention shown in FIG. 6;

FIG. 8 is an explanatory diagram of the number of repeating cells of a satellite fixed cell and a user-specific cell.

FIG. 9 is an explanatory diagram of the number of repeating cells of a satellite fixed cell and a user-specific cell.

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Claims (4)

[Claims] 1. A low-Earth-orbit satellite communication system, wherein a user-specific cell for irradiating a spot beam is configured so that the position of a user terminal is substantially at the center of the spot beam. 2. The low-Earth-orbit satellite communication system according to claim 1, further comprising user position information acquisition means for detecting a position of the user terminal. 3. The apparatus according to claim 2, further comprising a weighting factor determining unit that determines a weighting factor for each position of the user terminal based on the information detected by the user location information obtaining unit. Low orbit satellite communication system. 4. The low-Earth-orbit satellite communication system according to claim 2, further comprising: means for determining a beam radiation pattern of a spot beam based on the weight coefficient.