AU2004201012B8

AU2004201012B8 - Satellite communication system, base station and mobile station

Info

Publication number: AU2004201012B8
Application number: AU2004201012A
Authority: AU
Inventors: Seiya Inoue
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 2003-03-28
Filing date: 2004-03-10
Publication date: 2005-11-24
Anticipated expiration: 2024-03-10
Also published as: JP2004304287A; KR20040085054A; AU2004201012B2; AU2004201012A1; JP4037302B2

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant(s): MITSUBISHI DENKI KABUSHIKI KAISHA Invention Title: SATELLITE COMMUNICATION SYSTEM, BASE STATION AND MOBILE STATION The following statement is a full description of this invention, including the best method of performing it known to me/us: 2 SATELLITE COMMUNICATION SYSTEM, BASE STATION AND MOBILE STATION BACKGROUND OF THE INVENTION i. Field of the Invention The present invention relates to a satellite communication system, a base station and a mobile station, and particularly to an improvement in beam handover in the mobile communication system in which multi-beams are formed by using a communication satellite, for example, a quasi-zenith satellite.

2. Description of the Related Art Fig. 12 is a schematic view showing an example of a conventional satellite communication system. This satellite communication system is constructed of a communication satellite i, mobile stations 2 and a base station 3, and is a mobile communication system in which wireless communications between the mobile stations 2 and the base station 3 are relayed by using the communication satellite 1. A communication line between the communication satellite 1 and each of the mobile stations 2 is called a service link SL, and a communication line between the communication satellite 1 and the base station 3 is called a feeder link FL.

The communication satellite 1 includes plural directional antennas, and forms beams as areas in which wireless signals can be transmitted and received for the respective directional antennas. Fig. 12 shows a case where the number of the beams is three (beams #1 to The communication satellite 1 forms the plural beams (multi-beams) in this way, and provides wireless communications in which the areas covered by all the beams #1 to #3 are service area. That is, the service link SL is provided by each beam of the multi-beams.

Since the multi-beam system is a system in which a cover area of each beam is narrow and transmission power is concentrated on this beam, there is a merit that an H \Valma\Keep\Specifications\P52397.F1054. doc 10/03/04 3 antenna diameter of the mobile station 2 can be made small and its transmission power can be reduced. Thus, in recent years, the multi-beam system is often applied to a satellite communication system having a high frequency band of 10 GHz or higher. On the other hand, the system is constructed such that the antenna diameter and transmission power in the base station 3 become large as compared with the mobile station 2.

Communication channels are dynamically assigned to the mobile stations 2 belonging to the beams #1 to #3, and each of the mobile stations 2 uses the assigned communication channel to perform wireless communication with the communication satellite 1. Besides, the base station 3 performs multiplex communication with the communication satellite 1 in which the communication channels for the respective mobile stations 2 are multiplexed. Here, it is assumed that in the service link SL, three frequency channels fl, f2 and f3 are assigned to the respective mobile stations 2, and in the feeder link FL, the frequency division multiple access (FDMA) of these is performed.

It is necessary that the frequency channel assigned to the mobile station 2 is made to correspond to the beam to which the mobile station 2 belongs, and this correspondence is performed in the communication satellite 1. Thus, a transmission signal from the base station 3 is transmitted through the communication satellite 1 to the beam in which the mobile station 2 exists. On the contrary, a transmission signal from the mobile station 2 is multiplexed and is transmitted to the base station 3.

The assignment of the frequency channel to the mobile station 2 is maintained even if the mobile station 2 moves between the beams, and the mobile station 2 can continue using the same frequency channel during the communication. In this case, it is necessary to switch the correspondence relation between the frequency channel and the beam when the mobile station 2 moves between the H.\Valma\Keep\Specifications\P52397.F1O54.doc 10/03/04 4 beams. This switching process is called a beam handover and is performed in the communication satellite i.

Fig. 13 is a block diagram showing a structural example of the communication satellite 1 of Fig. 12. This communication satellite 1 includes antennas 6 forming the respective beams of the service link SL, an antenna 7 used in the feeder link FL, a down transponder 4 for relaying down transmission signals from the base station 3 to the mobile station 2, and an up transponder 5 for relaying up transmission signals from the mobile station 2 to the base station 3. The antennas 6 and the antenna 7 are shared between the down transponder 4 and the up transponder Fig. 14 is a block diagram showing the detailed structure of the down transponder 4 of Fig. 13. The down transponder 4 is constructed of a matrix switch 400, plural frequency converters 401A, 401B and 401C, a distributor 402, plural frequency converters 403A, 403B and 403C, band pass filters (BPFs) 411, 412 and 413, a command receiver 420 and a matrix switch controller 421.

The frequency converters 401A, 401B and 401C are provided for the beams #1 to respectively, and are up converters for the service link SL used for transmission to the mobile station 2. On the other hand, the frequency converters 403A, 403B and 403C are provided for the frequency channels fl to f3, respectively, and are down converters for the feeder link FL used for reception from the base station 3. Besides, the BPFs 411, 412 and 413 are also provided for the frequency channels fl, f2 and f3, respectively, limit bands of reception signals and extract signals of the corresponding frequency channels, respectively.

An RF (Radio Frequency) signal transmitted from the base station 3 is received by the antenna 7, and is distributed to the respective frequency converters 403A, 403B and 403C and the command receiver 420 by the distributor 402. The reception signals converted into IF (Intermediate Frequency) signals by the respective H \Valma\Keep\Specifications\P52397.FO54.doc 10/03/04 5 frequency converters 403A, 403B and 403C become signals for the respective frequency channels fl, f2 and f3 in the BPFs 411, 412 and 413, and the signals are inputted to the matrix switch 400.

The matrix switch 400 is a beam switching unit for causing the frequency channel to correspond to the beam, and the respective frequency channels are made to correspond to the beam the beam #2 and the beam #3 in which the mobile stations 2, to which the frequency channels are assigned, exist. Accordingly, the reception signals for the respective frequency channels fl, f2 and f3 outputted from the BPFs 411, 412 and 413 are distributed to the frequency converters 401A, 401B and 401C for the respective beams by the matrix switch 400, and are transmitted to the mobile stations 2 from the antennas 6 after they are converted to RF signals.

Besides, the command receiver 420 receives a satellite command from the base station 3, and the matrix switch controller 421 switches the matrix switch 400 on the basis of the satellite command, so that the beam handover is performed.

For example, in the case where the mobile station 2 existing in the beam #1 uses the frequency channel fl to communicate with the base station 3, the RF signal from the base station 3 is converted to the IF signal by the frequency converter 403A, and after the band is limited by the BPF 411 of the frequency channel fl, the signal is inputted to the matrix switch 400. When the mobile station 2 exists in the beam a switch Saa indicated by a white dot mark in the matrix switch 400 is on. Thus, the IF signal of the frequency channel fl is outputted to the frequency converter 401A of the beam and is transmitted from the antenna 6 to the beam #1 after it is converted to the RF signal.

In this state, when the mobile station 2 is moving from the beam #1 to the beam the mobile station 2 requests the base station 3 to switch the beam. On the H:\Valma\Keep\Specifications\P52397.F1O54.doc 10/03/04 6 basis of the beam switch request, the base station 3 generates a satellite command to instruct switching of the matrix switch 400 so that the frequency channel fl is made to correspond to the beam and transmits it to the communication satellite i. As a result, a switch Sba of the matrix switch 400 is turned on, the switch which is to be turned on shifts from the white dot to a black dot, and the beam handover is completed.

Fig. 15 is a block diagram showing the detailed structure of the up transponder 5 of Fig. 13. The up transponder 5 is constructed of a matrix switch 500, plural frequency converters 501A, 501B and 501C, synthesizer 502, plural frequency converters 503A, 503B and 503C, and band pass filters (BPFs) 511, 512 and 513.

The frequency converters 501A, 501B and 501C are provided for the beam the beam #2 and the beam #3, respectively, and are down converters for the service link SL used for signal reception from the mobile station 2.

On the other hand, the frequency converters 503A, 503B and 503C are provided for the frequency channels fl, f2 and f3, respectively, and are up converters for the feeder link FL used for signal transmission to the base station 3. Besides, the BPFs 511, 512 and 513 are also provided for the frequency channels fl, f2 and f3, respectively, and limit bands of reception signals to extract signals of the corresponding frequency channels, respectively.

Incidentally, here, for convenience, although the same symbols fl, f2 and f3 are used in the description for the down frequency channels and the up frequency channels, the frequency may be made different between the down channel and the up channel. For example, the down channel and the up channel indicated by the same symbol fl may have different frequencies.

The RF signals transmitted from the mobile stations 2 are received by the antennas 6 and are inputted to the matrix switch 500 after they are converted to IF signals by the respective frequency converters 501A, 501B H\Valma\Keep\Specifications\P52397.F1054.doc 10/03/04 7 and 501C. The matrix switch 500 is a beam switch unit quite similar to the matrix switch of Fig. 14. The reception signals of the respective beams outputted from the frequency converters 501A, 501B and 501C are distributed to the BPFs 511, 512 and 513 for the respective frequency channels fl; f2 and f3 by the matrix switch 500, and signals of the respective frequency channels are extracted. Then, the signals are converted to RF signals by the frequency converters 503A, 503B and 503C, and the RF signals from the respective frequency converters 503A, 503B and 503C are synthesized by the synthesizer 502 and are transmitted from the antenna 7 to the base station 3. Incidentally, the switching of the matrix switch 500 in the beam handover is also controlled by the matrix controller 421 in the down transponder 4.

For example, in the case where the mobile station 2 existing in the beam #1 uses the frequency channel fl to communicate with the base station 3, in the matrix switch 500, a switch SAA indicated by a white dot mark is turned on, and when the mobile station 2 moves from the beam #1 to the beam a switch SBA indicated by a black dot mark in the matrix switch 500 is turned on, and the switch which is to be turned on shifts from the white dot to the black dot.

Fig. 16 is a block diagram showing the detailed structure of the mobile station 2 of Fig. 12. This mobile station 2 is constructed of a transmission part, a reception part, a beam switching request generator 200 and a transmission/reception antenna 210. The transmission part includes a multiplexing processing part 201, a baseband processing part 202, a modulator 203, a frequency converter 204, and a high power amplifier (HPA) 205.

Besides, the reception part includes a low noise amplifier (LNA) 221, a frequency converter 222, a demodulator 223 and a baseband processing part 224.

Transmission data TD is a baseband signal made of digital data to be transmitted to the base station 3, and H:\Valma\Keep\Specifications\P52397.F154.doc 10/03/04 8 after an error correction, scrambling, framing and the like are performed in the baseband processing part 202, the signal is modulated by the modulator 203 and becomes an IF signal. After this IF signal is converted to an RF signal by the frequency converter 204, the signal is amplified by the HPA 205 and is transmitted from the antenna 210 to the communication satellite i.

Besides, the RF signal transmitted from the base station 3 and received by the antenna 210 through the communication satellite 1 is amplified by the LNA 221, and is converted to an IF signal by the frequency converter 222. This IF signal is demodulated by the demodulator 223, is subjected to processing, such as an error correction and descrambling, in the baseband processing part 224, and becomes a baseband reception signal, which becomes reception data RD.

The beam switching request generator 200 generates a beam switching request when the station including itself moves between the beams. For example, on the basis of lowering in the signal reception level from the base station 3 or position information of the station including itself measured by a position measurement apparatus such as a car navigation system, the beam switching request is issued. The beam switching request generated by the beam switching request generator 200 is multiplexed with the transmission data TD in the multiplexing processing part 201, and is transmitted to the base station 3.

Fig. 17 is a block diagram showing the detailed structure of the base station 3 of Fig. 12. This base station 3 is constructed of a transmission part, a reception part, a satellite command generator 307, a command transmission part 308, and a transmission/reception antenna 300. The transmission part includes baseband processing parts 310A, 310B and 310C, modulators 311A, 311B and 311C, frequency converters 312A, 312B and 312C, a synthesizer 313, and an HPA 314.

H, \Valma\Keep\Specifications\P52397. F1054 .doc 10/03/04 9 Besides, the reception part includes an LNA 301, a distributor 302, frequency converters 303A, 303B and 303C, demodulators 304A, 304B and 304C, baseband processing parts 305A, 305B and 305C, and switching request separators 306A, 306B and 306C. Incidentally, an identical name is given to both a block in the mobile station 2 and a block corresponding thereto.

Composite transmission processing systems including the baseband processing parts 310A, 310B and 310C, the modulators 311A, 311B and 311C, and the frequency converters 312A, 312B and 313C are respectively provided for the frequency channels, and respectively receive transmission data TDa, TDb and TDc to generate RF signals. These RF signals are synthesized in the synthesizer 313, and the synthesized signal is transmitted to the communication satellite 1 from the antenna 300 after it is amplified by the HPA 314.

Besides, composite reception processing systems including the frequency converters 303A, 303B and 303C, the demodulators 304A, 304B and 304C, the baseband processing parts 305A, 305B and 305C, and the switching request separators 306A, 306B and 306C are respectively provided for the frequency channels, and output reception data RDa, RDb and RDc for the respective frequency channels. The RF signal transmitted from the mobile station 2 and received by the antenna 300 through the communication satellite 1 is amplified by the LNA 301, is distributed to the reception processing systems by the distributor 302, and baseband reception signals for the respective frequency channels are obtained.

The beam switching request inserted into the reception data from the mobile station 2 is separated from other reception data by the switching request separators 306A, 306B and 306C. In the case where the beam switching request is received, the satellite command generator 307 generates a satellite command to instruct beam switching.

This satellite command is converted into an RF signal by Hs\Valma\Keep\Specifications\P52397.F1O54.doc 10/03/04 10 the command transmitter 308, is synthesized with other Stransmission RF signals by the synthesizer 313, and is O transmitted to the communication satellite 1.

As described above, the beam switching in the conventional satellite communication system is performed by the switching of the matrix switches 400 and 500.

Thus, there is a problem that a signal is instantaneously interrupted at the time of switching of the matrix switch, C and part of communication data is lost.

Besides, there is a problem that at the time of Sswitching of the matrix switch, a signal path on the communication satellite 1 is changed, and a delay time is changed, so that out-of-step occurs at the time of demodulation on the reception side, and more communication data is lost.

SUMMARY OF THE INVENTION According to one aspect of the present invention there is provided a satellite communication system including a communication satellite for forming multibeams of a plurality of beams, a plurality of mobile stations and a base station, in which the communication satellite causes a plurality of communication channels, each of the communication cannels is assigned to each of the mobile stations to correspond to each of the beams to which each of the mobile stations belongs, and relays wireless communications between each of the mobile stations and the base station, wherein each of the mobile stations includes a beam switching request means for outputting a beam switching request, the base station includes a switching time specifying means for specifying a beam switching time for the communication satellite on the basis of the beam switching request from each of the mobile stations, and the communication satellite includes a beam switching means for switching a correspondence relations H:\deboram\keep\specifications\2004201012.doc 18/10/05 -11o between each of the communication channels and each of the Sbeams at the beam switching time specified by the base O station.

According to another aspect of the present invention there is provided a base station used in a satellite communication system including a communication satellite for forming multi-beams of a plurality of beams, a plurality of mobile stations and the base station, in C which the communication satellite causes a plurality of communication channels, each of the communication channels Sis assigned to each of mobile stations to correspond to each of the beams to which each of the mobile stations belongs, and communications between the base station and each of the mobile stations are performed through the communication satellite, the base station including: a switching time specifying means for specifying a beam switching time for the communication satellite on the basis of a beam switching request from each of the mobile stations; and a dummy data insertion means for inserting dummy data, which is relayed by the communication satellite at the beam switching time, into down transmission data.

According to another aspect of the present invention there is provided a mobile station used in a satellite communication system including a communication satellite for forming multi-beams of a plurality of beams, plurality of the mobile stations and a base station, in which the communication satellite causes a plurality of communication channels, each of the communication channels is assigned to each of the mobile stations to correspond to each of beams to which each of the mobile stations belongs, and communications between each of the mobile stations and the base station are performed through the communication satellite, the mobile station including: a beam switching request means for outputting a beam switching request to the base station; a timing control means for obtaining an insertion H:\deboram\keep\specifications\2004201012.doc 18/10/05 12- O timing of dummy data on the basis of a beam switching time Sspecified from the base station and orbit information of O the communication satellite; and a dummy data insertion means for inserting the dummy data into up transmission data on the basis of the insertion timing.

According to another aspect of the present invention there is provided a mobile station used in a C satellite communication system including a communication satellite for forming multi-beams of a plurality of beams, plurality of the mobile stations and a base station, in which the communication satellite causes a plurality of communication channels, each of the communication channels is assigned to each of the mobile stations to correspond to each of the beams to which each of the mobile stations belongs, and communications between each of the mobile stations and the base station are performed through the communication satellite, the mobile station including: a beam switching request means for outputting a beam switching request to the base station; and a dummy data insertion means for inserting dummy data into up transmission data on the basis of an insertion timing specified from the base station.

H:\deboram\keep\specifications\2004201012.doc 18/10/05 13 BRIEF DESCRIPTION OF THE DRAWINGS SFig. 1 is a block diagram showing a structural O example of a main part of a satellite communication system according to embodiment 1 of the invention and shows a structure of a base station 3.

Fig. 2 is a block diagram showing a structural example of a main part of a communication satellite 1 according to the embodiment 1 of the invention and shown a C structure of a down transponder 4.

Fig. 3 is a sequence diagram showing an example Sof an operation of the satellite communication system according to the embodiment 1 of the invention.

Fig. 4 is a block diagram showing a structural example of a main part of a satellite communication system according to embodiment 2 of the invention.

Fig. 5 is a block diagram showing a structural example of a mobile station 2 according to the embodiment 2 of the invention.

Fig. 6 is a sequence diagram showing an example of an operation of the satellite communication system according to the embodiment 2 of the invention.

Fig. 7 is a block diagram showing a structural example of a main part of a satellite communication system according to embodiment 3 of the invention.

Fig. 8 is a block diagram showing a structural example of a mobile station 2 according to the embodiment 3 of the invention.

Fig. 9 is a sequence diagram showing an example of an operation of the satellite communication system according to the embodiment 3 of the invention.

Fig. 10 is a block diagram showing a structural example of a main part of a satellite communication system H:\deboram\keep\specifications\2004201012.doc 18/10/05 14 according to embodiment 4 of the invention and shows a structure of a mobile station 2.

Fig. 11 is a schematic view showing a structural example of a positioning system using GPS satellites.

Fig. 12 is a schematic view showing an example-of a conventional satellite communication system.

Fig. 13 is a block diagram showing a structural example of a communication satellite 1 of Fig. 12.

Fig. 14 is a block diagram showing a detailed structure of a down transponder 4 of Fig. 13.

Fig. 15 is a block diagram showing a detailed structure of an up transponder 5 of Fig. 13.

Fig. 16 is a block diagram showing a detailed structure of a mobile station 2 of Fig. 12.

Fig. 17 is a block diagram showing a detailed structure of a base station 3 of Fig. 12.

DETAILED DESCRIPTION OF THE INVENTION Embodiment 1 Fig. 1 is a block diagram showing a structural example of a main part of a satellite communication system according to embodiment 1 of the invention and shows a structure of a base station 3. When this base station 3 is compared with the case of Fig. 17 (related art), the difference is such that a time generator 321, a satellite orbit predictor 322, a timing controller 323 and a dummy data insertion part 324 are provided.

The time generator 321 generates accurate time data and outputs it to the timing controller 323. It is desirable that this time data has such accuracy that an error from time data in an after-mentioned communication satellite 1 becomes sufficiently short as compared with an instantaneous interruption time at the time of beam switching. Here, it is assumed that the time data generated by the time generator 321 has sufficiently high accuracy as compared with the instantaneous interruption time at the time of beam switching.

H;\Valma\Keep\Specifications\P52397.F1O54.doc 10/03/04 15 The satellite orbit predictor 322 accurately predicts orbit information of the communication satellite 1 by calculation and outputs it to the timing controller 323. It is assumed that the orbit information has such accuracy that a signal propagation time between the communication satellite 1 and the base station 3 obtained based on that is sufficiently short as compared with the instantaneous interruption time at the time of beam switching.

In the case where a beam switching request is made by the mobile station 2, the timing controller 323 decides a beam switching time in the communication satellite 1 on the basis of the time data and the satellite orbit information, and decides a timing when dummy data is inserted into transmission data of the base station 3. The dummy data insertion parts 324A, 324B and 324C insert the dummy data into the transmission data on the basis of the insertion timing.

Fig. 2 is a block diagram showing a structural example of a main part of the communication satellite 1 according to the embodiment 1 of the invention and shown a structure of a down transponder 4. When this down transponder 4 is compared with the case of Fig. 14 (related art), the difference is such that a time generator 422 is provided.

The time generator 422 generates accurate time data and outputs it to a matrix switch controller 421. It is assumed that this time data has sufficiently high accuracy as compared with the instantaneous interruption time at the time of beam switching. The matrix switch controller 421 receives a satellite command including a beam switching time from a command receiver 420. Then, when the time data from the time generator 422 coincides with the beam switching time, matrix switches 400 and 500 are switched.

Fig. 3 is a sequence diagram showing an example of an operation of the satellite communication system H,\Valma\Keep\Specifications\P52397.FlO54.doc 10/03/04 16 according to the embodiment 1 of the invention and shows the respective operations of the communication satellite 1, the mobile station 2 and the base station 3. In the case where the mobile station 2 detects its own inter-beam movement at step Sl, it outputs a beam switching request BSR to the base station 3. That is, the beam switching request BSR is multiplexed with the transmission data and is transmitted to the base station 3 through the communication satellite 1. This beam switching request BSR is separated from the reception data by the switching request separator 306 of the base station 3, and is outputted to the timing controller 323.

On the basis of the beam switching request BSR, the timing controller 323 first decides a beam switching time BST at step S2. The beam switching time BST is the time when the matrix switches 400 and 500 are switched in the communication satellite 1, and is decided in view of a propagation delay time of a satellite command and the like. A satellite command generator 307 generates a satellite command SC including the beam switching time BST decided in this way. This satellite command SC is converted to an RF signal by a command transmitter 308, and is transmitted from an antenna 300 to the communication satellite 1 through a synthesizer 313 and an HPA 314.

Next, the timing controller 323 decides an insertion timing of dummy data DD at step S3 on the basis of the beam switching time BST. This timing is decided so that the inserted dummy data DD is relayed by the communication satellite 1 at the beam switching time BST.

That is, on the basis of the satellite orbit information, a time of signal propagation from the base station 3 to the communication satellite 1 is obtained by calculation, and the time of signal propagation, a delay time in the base station 3 and a delay time in the communication satellite 1 are calculated back from the beam switching time BST, so that the insertion timing can be obtained.

H.\Valma\Keep\Specifications\P52397.F1O54.doc 10/03/04 17 The dummy data insertion part 324 inserts the dummy data DD into the transmission data on the basis of this insertion timing at step S4. Similarly to normal transmission data, the transmission data into which the dummy data is inserted is modulated by a modulator 311, and is converted to an RF signal by a frequency converter 312, and then, the signal is transmitted from the antenna 300 to the communication satellite 1 through the synthesizer 313 and the HPA 314. In this way, the transmission signal passing through the matrix switch 400 in the down transponder 4 of the communication satellite 1 can be made the dummy data DD at the beam switching time

BST.

Here, the instantaneous interruption by the switching of the matrix switch 400 has a predetermined time width (that is, instantaneous interruption time).

Thus, when the length of the dummy data DD to be inserted is made longer than the instantaneous interruption time, since all transmission signals transmitted from the base station 3 and passing through the communication satellite 1 during the instantaneous interruption time can be made the dummy data DD, it is possible to prevent the communication data of the down link from being lost.

According to this embodiment, the base station 3 decides the beam switching time BST, the communication satellite 1 performs the beam switching at this beam switching time BST, and the base station 3 inserts the dummy data DD into the transmission data. Besides, this insertion timing is decided on the basis of the beam switching time BST and the satellite orbit information.

Thus, the transmission signal transmitted from the base station 3 and relayed by the communication satellite 1 at the beam switching time BST can be made the dummy data DD, and it is possible to prevent the communication data of the down link from being lost by the beam switching.

Embodiment 2 In the embodiment 1, the description has been H.\Valma\Keep\Specifications\P52397 .Fl54.doc 10/03/04 18 given to the satellite communication system in which the dummy data DD is inserted into the transmission signal from the base station 3, and the communication data of the down link is not lost at the time of beam switching. On the other hand, in this embodiment, a description will be given to a satellite communication system in which dummy data DD is inserted into a transmission signal from a mobile station 2 so that communication data of an up link is not lost at the time of beam switching.

Fig. 4 is a block diagram showing a structural example of a main part of a satellite communication system according to embodiment 2 of the invention and shows a structure of a base station 3. When this base station 3 is compared with the case of Fig. 1 (embodiment the difference is such that a timing controller 330 and switching information insertion parts 331A, 331B and 331C are provided instead of the timing controller 323 and the dummy data insertion part 324.

In the case where a beam switching request BSR is issued from the mobile station 2, the timing controller 330 decides a beam switching time BST in the communication satellite 1 similarly to the case of the embodiment i. In this embodiment, further, this beam switching time BST is outputted as switching information to the switching information insertion parts 331A, 331B and 331C, and the switching information insertion parts 331A, 331B and 331C insert this switching information into transmission data.

Fig. 5 is a block diagram showing a structural example of the mobile station 2 according to the embodiment 2 of the invention. When this mobile station 2 is compared with the case of Fig. 16 (related art), the difference is such that a switching information separator 230, a time generator 231, a timing controller 232, a dummy data insertion part 233 and a satellite orbit predictor 234 are provided.

The switching information separator 230 separates the beam switching time as the switching information, H \Valma\Keep\Specifications\P52397.F154.doc 10/03/04 19 which is inserted into reception data from the base station 3, from other reception data, and outputs it to the timing controller 232 and the satellite orbit predictor 234.

The time generator 231 generates accurate time data and outputs it to the timing controller 232. It is desirable that this time data has such accuracy that an error from time data in the communication satellite 1 is sufficiently short as compared with the instantaneous interruption time at the time of beam switching. Here, it is assumed that the time data generated by the time generator 231 has sufficiently high accuracy as compared with the instantaneous interruption time at the time of beam switching.

The satellite orbit predictor 234 accurately predicts orbit information of the communication satellite 1 at the time of beam switching, and outputs it to the timing controller 232. It is assumed that this orbit information has such accuracy that a time of signal propagation between the communication satellite 1 and the mobile station 2 obtained on the basis of that becomes sufficiently short as compared with the instantaneous interruption time at the time of beam switching.

In the case where the beam switching time is transmitted as switching information SI from the base station 3, the timing controller 232 decides a timing when dummy data DD is inserted into the transmission data of the mobile station 2 on the basis of the time data, the satellite orbit information and the beam switching time.

The dummy data insertion part 233 inserts the dummy data DD into the transmission data on the basis of this insertion timing.

Fig. 6 is a sequence diagram showing an example of an operation of the satellite communication system according to the embodiment 2 of the invention, and shows the respective operations of the communication satellite i, the mobile station 2 and the base station 3. In the H \Valma\Keep\Specificationa\P52397.F1O54.doc 10/03/04 20 case where the mobile station 2 detects its own inter-beam movement at step SI, it outputs a beam switching request BSR to the base station 3, and the beam switching request BSR is separated from reception data by the switching request separator 306 of the base station 3, and is outputted to the timing controller 330.

The timing controller 330 decides a beam switching time on the basis of the beam switching request BSR at step S2. A satellite command generator 307 generates a satellite command SC including this beam switching time, and this satellite command SC is transmitted from an antenna 300 to the communication satellite 1. Besides, the switching information insertion part 331 inserts switching information SI including the beam switching time into transmission data, and it is transmitted to the communication satellite 1 from the antenna 300 similarly to the transmission data.

This switching information SI is separated from reception data by the switching information separator 230 of the mobile station 2, and is outputted to the timing controller 232. The timing controller 232 decides an insertion timing of dummy data DD on the basis of the beam switching time BST included in the switching information SI. This timing is decided so that the inserted dummy data DD is relayed by the communication satellite 1 at the beam switching time BST. That is, a time of signal propagation from the mobile station 2 to the communication satellite 1 is obtained by calculation on the basis of the satellite orbit information, and the time of signal propagation, a delay time in the mobile station 2 and a delay time in the communication satellite 1 are calculated back from the beam switching time BST, so that the insertion timing is obtained.

The dummy data insertion part 233 inserts the dummy data DD into the transmission data at step S4 on the basis of this insertion timing. Similarly to normal transmission data, the transmission data into which the H:\Valma\Keep\Specifications\P52397.Fl054.doc 10/03/04 21 dummy data DD is inserted is modulated into an RF signal by a modulator 203, and is converted to an RF signal by a frequency converter 204, and then, the signal is transmitted from an antenna 210 to the communication satellite 1 through an HPA 205. In this say, at the beam switching time BST, the transmission signal passing through the matrix switch 500 in the up transponder 5 of the communication satellite 1 can be made the dummy data

DD.

Here, the instantaneous interruption by the switching of the matrix switch 500 has a predetermined time width (that is, instantaneous interruption time).

Thus, when the length of the inserted dummy data is made longer than this instantaneous interruption time, since all transmission signals transmitted from the mobile station 2 and passing through the communication satellite 1 during the instantaneous interruption time can be made the dummy data DD, it is possible to prevent the transmission data of the up link from being lost.

Embodiment 3 In the embodiment 2, the description has been given to the satellite communication system in which the insertion timing is decided in the mobile station 2, and the dummy data is inserted into the transmission signal.

On the other hand, in this embodiment, a description will be given to a satellite communication system in which a base station 3 decides an insertion timing of dummy data in a mobile station 2, and issues an instruction to the mobile station 2.

Fig. 7.is a block diagram showing a structural example of a main part of a satellite communication system according to embodiment 3 of the invention and shows a structure of the base station 3. When this base station 3 is compared with the case of Fig. 4 (embodiment the difference is such that a timing controller 340 and timing instruction insertion parts 341A, 341B and 341C are provided instead of the timing controller 330 and the H, \Valma\Keep\Specifications \P52397. F1054. doc 10/03/04 22 switching information insertion part 331.

Similarly to the cases of the embodiments 1 and 2, in the case where a beam switching request BSR is issued from the mobile station 2, the timing controller 340 decides a beam switching time BST in the communication satellite i. In this embodiment, further, the timing controller 340 decides a timing when dummy data DD is inserted into transmission data in the mobile station 2, and outputs timing instruction data, which instructs the mobile station 2 on this insertion timing, to the timing instruction insertion parts 341A, 341B and 341C. The timing instruction insertion parts 341A, 341B and 341C insert the timing instruction data into the transmission data.

Fig. 8 is a block diagram showing a structural example of the mobile station 2 according to the embodiment 3 of the invention. When this mobile station 2 is compared with the case of Fig. 5 (embodiment the differences is such that the time generator 231 and the satellite orbit predictor 234 are not provided. Besides, the difference is such that a timing instruction separator 240 and a timing controller 241 are provided instead of the switching information separator 230 and the timing controller 232.

The timing instruction separator 240 separates the timing instruction data, which is inserted into the reception data from the base station 3, from other reception data, and outputs it to the timing controller 241.

In the case where the timing instruction data is transmitted from the base station 3, on the basis of this timing instruction data, the timing controller 241 decides the timing when the dummy data DD is inserted into the transmission data of the mobile station 2. The dummy data insertion part 233 inserts the dummy data DD into the transmission data on the basis of this insertion timing.

At this time, the timing controller 241 decides H;\Valma\Keep\Specifications\52397.F P1054.doc 10/03/04 23 the insertion timing, without using accurate time data, on the basis of the time when the timing instruction data is received. For example, when the timing controller 241 receives the timing instruction data, it causes the dummy data insertion part 233 to insert the dummy data DD immediately, or to insert the dummy data DD after timer measurement of the passage of a definite time or the passage of a specified time included in the timing instruction data.

Fig. 9 is a sequence diagram showing an example of an operation of the satellite communication system according to the embodiment 3 of the invention and shows the respective operations of the communication satellite 1, the mobile station 2 and the base station 3. The beam switching request from the mobile station 2 is separated from the reception data by the switching request separator 306 of the base station 3 at step SI, and is outputted to the timing controller 340.

On the basis of this beam switching request BSR, the timing controller 340 decides the beam switching time BST at step S2. The satellite command generator 307 generates the satellite command SC including this beam switching time BST, and this satellite command SC is transmitted from the antenna 300 to the communication satellite i.

Besides, the timing controller 340 decides the insertion timing of the dummy data DD in the mobile station 2 on the basis of the beam switching time BST.

This timing is decided so that the dummy data DD inserted by the mobile station 2 is relayed by the communication satellite 1 at the beam switching time BST. That is, a time of signal transmission from the mobile station 2 to the communication satellite 1 is obtained by calculation on the basis of the satellite orbit information and the position information of the beam to which the mobile station 2 belongs, and the time of signal transmission, a delay time in the mobile station 2 and a delay time in the H;\Valma\Keep\Specifications\P52397.F1OS4.doc 10/03/04 24 communication satellite 1 are calculated back from the beam switching time BST, so that the insertion timing is obtained.

Further, on the basis of this insertion timing, the timing controller 340 generates timing instruction data and outputs it to the timing instruction insertion part 341. The instruction insertion part 341 inserts this timing instruction data into the transmission data.

Here, the insertion position of the timing instruction data in the transmission data becomes a reference time for the insertion of the dummy data by the mobile station 2. Thus, the timing controller 340 decides the insertion timing of the timing instruction data, and outputs the insertion timing of the timing instruction data to the timing instruction insertion part 341. On the basis of the satellite orbit information and the position information of the beam to which the mobile station 2 belongs, this timing is decided in view of a time from the point when the timing instruction data is inserted into the transmission data in the base station 3 to the point when the timing instruction data is separated in the mobile station 2.

This timing instruction data is separated from the reception data by the timing instruction separator 240 of the mobile station 2, and is outputted to the timing controller 241. The timing controller 241 decides the insertion timing of the dummy data at step S3 on the basis of the timing instruction data. That is, based on the reception time of the timing instruction data, the insertion timing of the dummy data is decided without using time data separately. The dummy data insertion part 233 inserts the dummy data into the transmission data at step S4 on the basis of the timing information TI from the timing controller 241 and on the basis of the insertion timing.

According to this embodiment, the base station 3 specifies the insertion timing of the dummy data in the H:\Valma\Keep\Specifications\P52397.F1054.doc 10/03/04 25 mobile station 2, and the mobile station 2 inserts the dummy data into the transmission data on the basis of this timing instruction. Thus, even if the mobile station 2 does not include an accurate time generator, similarly to the case of the embodiment 2, it is possible to prevent the communication data of the up link from being lost by the beam switching.

Embodiment 4 In the embodiment 3, the description has been given to the satellite communication system in which the base station 3 obtains the insertion timing of the dummy data in the mobile station 2 on the basis of the position information of the beam to which the mobile station 2 belongs. On the other hand, in this embodiment, a description will be given to a satellite communication system in which a base station 3 obtains an insertion timing of dummy data in a mobile station 2 on the basis of position information measured by the mobile station 2.

Fig. 10 is a block diagram showing a structural example of a main part of a satellite communication system according to embodiment 4 of the invention, and showing a structure of the mobile station 2. When this mobile station 2 is compared with the case of Fig. 8 (embodiment the difference is such that a position detector 250 is provided.

The position detector 250 is a position measurement unit which receives signals from plural GPS satellites and specifies a position of the station including itself, and outputs the measured position information to a beam switching request generator 200.

The beam switching request generator 200 generates a beam switching request BSR including this position information.

Thereafter, an operation in which the beam switching request is multiplexed with transmission data and is transmitted to the base station 3 is similar to the cases of the respective embodiments.

A structure of the base station 3 in this H.\Valma\Keep\Specifications\P52397.F154.doc 10/03/04 26 embodiment is similar to the case of Fig. 7 (embodiment and the beam switching request BSR is separated from other reception data by a switching request separator 306 of the base station 3 and is outputted to a timing controller 340. The timing controller 340 generates timing instruction data on the basis of the position information of the mobile station 2 included in this beam switching request BSR, and outputs it to a timing instruction insertion part 341. The subsequent operation is quite identical to the case of the embodiment 3.

In the embodiment 3, the position information of the beam to which the mobile station 2 belongs is substituted for the position information of the mobile station 2. In this case, at the maximum, an error corresponding to the beam width can occur. Thus, a signal delay time between the mobile station 2 and the communication satellite 1 can not be obtained with accuracy. However, in the case where the mobile station 2 can measure its own position with higher accuracy than the beam width, insertion of the dummy data DD can be performed with higher accuracy by using the measured position information. According to the embodiment, as compared with the case of the embodiment 3, the length of the dummy data DD inserted at the time of beam switching can be shortened, and transmission efficiency can be improved.

Embodiment In this embodiment, a specific example of a satellite communication system suitable for the embodiments 1 to 4 will be described. Fig. 11 is a schematic view showing a structural example of a positioning system using GPS satellites. In the drawing, reference numeral 10 denotes a quasi-zenith satellite; 2, a mobile station; 3, a base station; 11 to 14, GPS satellites; and 20, an electronic reference point.

The quasi-zenith satellite 10 is the communication satellite for relaying wireless Hi\Valma\Keep\Specifications\P52397.F1l54.doc 10/03/04 27 communication between the mobile station 2 and the base station 3. A conventional communication satellite is a geostationary satellite or an orbiting satellite having a low orbit, an antenna elevation angle in the mobile station 2 is small in any of them, and shadowing in which buildings, trees and the like become obstacles to communication is apt to occur. On the other hand, the quasi-zenith satellite is the satellite (figure-of-eight satellite, etc.) orbiting along an extended elliptical orbit having a height higher than the geostationary satellite, and a large antenna elevation angle can be ensured by causing the satellite to pass through the quasi-zenith. Thus, by using the quasi-zenith satellite as the communication satellite, the mobile communication system in which shadowing does not easily occur can be provided.

Each of the GPS satellites 11 to 14 has a builtin time generator outputting accurate time data, for example, an atomic clock, and is a mobile satellite for sending electric waves on the basis of this time data.

The mobile station 2 receives transmission signals from the three or more GPS satellites 11, 12, 13 and 14 so that its own position can be measured.

That is, in the case where the mobile station 2 includes an accurate time generator, when electric waves from the three GPS satellites 11, 12 and 13 are received, distances RI, R2 and R3 from the mobile station 2 to the GPS satellites 11, 12 and 13 can be obtained on the basis of the reception times of these. Thus, when orbit information of the respective GPS satellites 11, 12 and 13 is already known, its own position can be obtained by calculation.

Besides, even in the case where the mobile station 2 does not include the accurate time generator, when electric waves from the four GPS satellites 11 to 14 can be received, its own position can be accurately obtained. In general, the mobile station 2 does not often H,\Valma\Keep\Specifications\P52397.F1054.doc 10/03/04 28 include the accurate time generator because of miniaturization and reduction in weight, and positioning is generally performed using the four GPS satellites 11, 12, 13 and 14. However, the mobile station 2 can not always receive the electric waves from the four GPS satellites 11, 12, 13 and 14.

Here, when the quasi-zenith satellite 10 has an accurate built-in time generator and sends electric waves on the basis of this time data, it can be used as one of the GPS satellites. Since the quasi-zenith satellite hardly causes the shadowing, a possibility that the mobile station 2 can receive electric waves from more GPS satellites becomes high. That is, the quasi-zenith satellite 10 supplements the conventional GPS satellites 11, 12, 13 and 14, so that it is possible to provide the positioning system in which accurate positioning can be always performed.

In this way, the quasi-zenith satellite 10 is used as the communication satellite and can be used as one of the GPS satellites. In other words, by using the same quasi-zenith satellite 10, a multiple service including a communication service and a positioning service can be provided.

The electronic reference point 20 is fixedly installed on the ground and is a receiver for receiving the electric waves from the GPS satellites 11 to 14.

Since the accurate position of the electronic reference point 20 is already known, as to position information obtained by receiving the electric waves from the GPS satellites 11, 12, 13 and 14, its error can be measured.

When the mobile station 2 can use this error information, positioning with higher accuracy can be performed by compensating the error of the GPS positioning.

Thus, the error information measured at the electronic reference point 20 is transmitted from the base station 3 to the mobile station 2 through the communication satellite 1. On the basis of the error H \Valma\Keep\Specifications\P52397. F1054.doc 10/03/04 29 information, the mobile station 2 compensates the error of the position information obtained using the GPS satellites 11 to 14 and the quasi-zenith satellite 10, and can measure the position information with higher accuracy.

In this way, the quasi-zenith satellite provides the multiple service including the communication service and the positioning service, so that positioning accuracy can be improved as the synergistic effect. In the case where the quasi-zenith satellite 10 as stated above is used as the communication satellite 1 of the embodiments 1 to 4, the time generator for GPS can also be shared in the beam handover.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

H \Valma\Keep\Specifications\P52397.F1054. doc 10/03/04

Claims

1. A satellite communication system including a communication satellite for forming multi-beams of a plurality of beams, a plurality of mobile stations and a base station, in which the communication satellite causes a plurality of communication channels, each of the communication cannels is assigned to each of the mobile stations to correspond to each of the beams to which each of the mobile stations belongs, and relays wireless communications between each of the mobile stations and the base station, wherein each of the mobile stations includes a beam switching request means for outputting a beam switching request, the base station includes a switching time specifying means for specifying a beam switching time for the communication satellite on the basis of the beam switching request from each of the mobile stations, and the communication satellite includes a beam switching means for switching a correspondence relations between each of the communication channels and each of the beams at the beam switching time specified by the base station.

2. The satellite communication system according to claim 1, wherein the base station includes a dummy data insertion means for inserting dummy data, which is relayed by the communication satellite at the beam switching time, into down transmission data.

3. The satellite communication system according to claim 2, wherein the base station includes a timing control means for obtaining an insertion timing of the dummy data on the basis of the beam switching time and orbit information of the communication satellite, and the dummy data insertion means inserts the dummy data into the down transmission data on the basis of the H.\Valma\Keep\Specifications\P52397.F1OS4.doc 10/03/04 31 insertion timing.

4. The satellite communication system according to claim i, wherein each of the mobile stations includes a dummy data insertion means for inserting dummy data, which is relayed by the communication satellite at the beam switching time, into up transmission data.

The satellite communication system according to claim 4, wherein the mobile station includes a timing control means for obtaining an insertion timing of the dummy data on the basis of the beam switching time and orbit information of the communication satellite, and the dummy data insertion means inserts the dummy data into the up transmission data on the basis of the insertion timing.

6. The satellite communication system according to claim 4, wherein the base station includes an insertion timing instruction means for specifying an insertion timing of the dummy data for each of the mobile stations, and the dummy data insertion means inserts the dummy data into the up transmission data on the basis of the insertion timing.

7. The satellite communication system according to claim 6, wherein the insertion timing instruction means obtains the insertion timing of the dummy data on the basis of position information of each of the beams to which each of the mobile stations belongs.

8. The satellite communication system according to claim 6, wherein each of the mobile stations includes a position information transmission means for transmitting its own position information to the base station, and the insertion timing instruction means obtains the insertion timing of the dummy data on the basis of the position information received from each of the mobile stations. H \Valma\Keep\Specifications\P52397. P1054. doc 10/03/04 1 32

9. The satellite communication system according to claim 6, wherein the insertion timing instruction means specifies the insertion timing of the dummy data in each of the mobile stations on the basis of a signal reception time from the base station.

A base station used in a satellite communication system including a communication satellite for forming multi-beams of a plurality of beams, a plurality of mobile stations and the base station, in which the communication satellite causes a plurality of communication channels, each of the communication channels is assigned to each of mobile stations to correspond to each of the beams to which each of the mobile stations belongs, and communications between the base station and each of the mobile stations are performed through the communication satellite, the base station including: a switching time specifying means for specifying a beam switching time for the communication satellite on the basis of a beam switching request from each of the mobile stations; and a dummy data insertion means for inserting dummy data, which is relayed by the communication satellite at the beam switching time, into down transmission data.

11. The base station according to claim further including a timing control means for obtaining an insertion timing of the dummy data on the basis of the beam switching time and orbit information of the communication satellite, wherein the dummy data insertion means inserts the dummy data into the down transmission data on the basis of the insertion timing.

12. The base station according to claim further including an insertion timing instruction means for specifying an insertion timing of dummy data to up transmission data for each of the mobile stations.

13. A mobile station used in a satellite communication system including a communication satellite H:\Valma\Keep\Specifications\P52397.FlO54.doc 10/03/04 33 for forming multi-beams of a plurality of beams, plurality of the mobile stations and a base station, in which the communication satellite causes a plurality of communication channels, each of the communication channels is assigned to each of the mobile stations to correspond to each of beams to which each of the mobile stations belongs, and communications between each of the mobile stations and the base station are performed through the communication satellite, the mobile station including: a beam switching request means for outputting a beam switching request to the base station; a timing control means for obtaining an insertion timing of dummy data on the basis of a beam switching time specified from the base station and orbit information of the communication satellite; and a dummy data insertion means for inserting the dummy data into up transmission data on the basis of the insertion timing.

14. A mobile station used in a satellite communication system including a communication satellite for forming multi-beams of a plurality of beams, plurality of the mobile stations and a base station, in which the communication satellite causes a plurality of communication channels, each of the communication channels is assigned to each of the mobile stations to correspond to each of the beams to which each of the mobile stations belongs, and communications between each of the mobile stations and the base station are performed through the communication satellite, the mobile station including: a beam switching request means for outputting a beam switching request to the base station; and H:\Valma\Keep\Specifications\PS52397.F1054.doc 10/03/04 -34 C) a dummy data insertion means for inserting dummy data into up transmission data on the basis of an O insertion timing specified from the base station. 0

15. A satellite communication system as claimed in any one of claims 1 to 9, and substantially as herein described with reference to the accompanying drawings.

16. A base station as claimed in any one of Sclaims 10 to 12, and substantially as herein described Ci with reference to the accompanying drawings.

17. A mobile station as claimed in claim 13 or O claim 14, and substantially as herein described with reference to the accompanying drawings. Dated this 19th day of October 2005 MITSUBISHI DENKI KABUSHIKI KAISHA By their Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and Trade Mark Attorneys of Australia H:\deboram\keep\specifications\2004201012.doc 19/10/OS