AU2003248464B2

AU2003248464B2 - Satellite communication method, and mobile station and gateway station for use with the satellite communication method

Info

Publication number: AU2003248464B2
Application number: AU2003248464A
Authority: AU
Inventors: Toshihiro Shikama; Yoshiko Yamada
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 2002-09-30
Filing date: 2003-09-30
Publication date: 2005-03-17
Anticipated expiration: 2023-09-30
Also published as: JP2004128631A; KR100576666B1; AU2003248464A1; KR20040029280A; JP3743412B2

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant: MITSUBISHI DENKI KABUSHIKI KAISHA Invention Title: SATELLITE COMMUNICATION METHOD, AND MOBILE STATION AND GATEWAY STATION FOR USE WITH THE SATELLITE COMMUNICATION METHOD The following statement is a full description of this invention, including the best method of performing it known.to us: 2 SATELLITE COMMUNICATION METHOD, AND MOBILE STATION AND GATEWAY STATION FOR USE WITH THE SATELLITE COMMUNICATION

METHOD

BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a satellite communication method of providing bidirectional satellite communications between a gateway station and an earth station by way of a communications satellite that forms multiple beams on a ground.

Description of Related Art There have been provided a single-beam method and a multiple-beam method as a method of providing bidirectional satellite communications between a gateway station and a mobile station by way of an Earth-orbiting communications satellite. In accordance with the singlebeam method, though the effective radiation power on the ground is reduced because a wide range on the ground must be covered by a single beam, communication line control within the single beam can be conducted. In contrast, in accordance with the multiple-beam method, though the effective radiation power can be increased, there is a necessity to perform switching between communication lines when the mobile station moves between beam areas because a communication line frequency is assigned to each of multiple beams.

For example, Japanese patent application publication (TOKKAIHEI) No.09-214414 discloses a prior art satellite communication method using such a multiple-beam method.

The prior art satellite communication method includes the steps of a mobile station determining whether or not there is a necessity to perform switching between beams based on \\melb-files\hme$\teresab\keep\Specifications\P50878_03060.doc 30/09/03 3 the line qualities of multiple beams or GPS positional information, and, when there is a necessity to perform switching between beams, the mobile station transmitting a beam switching request signal to a communications satellite. In more detail, the prior art method includes the steps of, when the line quality, such a reception level, decreases, determining that there is a necessity to switch to an adjacent beam oriented to a communication area to which the mobile station is moving, the direction being measured in advance by using a GPS receiver and so on, transmitting a beam switching request signal from the mobile station to the communications satellite, and switching to the beam. This prior art method thus makes it possible to eliminate an additional receiver intended for evaluation of the line quality of an adjacent beam, and to switch between communication lines in a short time, thereby almost preventing interruptions of communication services from taking place.

A problem with a prior art multi-beam satellite communication system using the above-mentioned method is that when the line quality, such as a received signal strength, of a communication line being currently used degrades, for example, when either shielding by a building or tunnel causes temporary degradation in the line quality or when a deterioration of the weather condition causes temporary degradation in the line quality dependent on the frequency band of transmitted signals, the mobile station performs switching between communication lines even though the mobile station is not moving to an adjacent beam area.

To overcome this problem, there can be provided a method of measuring the quality of a communication line associated with an adjacent beam by using an additional receiver, determining whether the communication line has a \\melb-files\home$\eresab\keep\Spefihcations\P50878OgO6Odo 30/09/03 O good quality, and performing switching to the communication 3 line according to this determination result. A problem with Sthis method is however that the mobile station increases in 00 size. A further problem is that when the mobile station performs switching to an adjacent beam, losses of received

\O

signals can be caused until the mobile station sweeps its 00 carrier frequency so that the carrier frequency matches that M of the adjacent beam.

SUMMARY OF THE INVENTION According to one aspect of the present invention there is provided a satellite communication method of carrying out communications between a gateway station and a mobile station by way of a communications satellite that forms multiple beams on a ground, said method comprising the steps of: said gateway station transmitting a measurement allowance information packet to said mobile station; said mobile station measuring a quality of a communication line associated with an adjacent communication area that is adjacent to a communication area in which said mobile station is staying in response to the measurement allowance information packet; said mobile station transmitting a switching request packet indicating a request for switching to the communication line to said gateway station according to a result of the measurement; said gateway station continuously transmitting a switching instruction packet to said mobile station in H:\mavism\keep\specis\P50878_03060.doc 17/02/05 O response to the switching request packet; and 3 said mobile station switching to the communication line while the switching instruction packet is being continuously 00 transmitted thereto.

According to another aspect of the present invention IND there is provided a gateway station comprising: 00 a transmitting unit for communicating with a mobile Sstation by way of a communications satellite that forms Smultiple beams on a ground; a data packet generating unit for generating data packets from contents data applied thereto; a packet sending unit for sending either the data packets generated by said data packet generating unit, a measurement allowance information packet, or a series of switching instruction packets for instructing said mobile station to switch between communication lines to said transmitting unit; and a line switching controlling unit for instructing said packet sending unit to send the series of switching instruction packets to said transmitting unit in response to a switching request packet which is transmitted from said mobile station when there is a necessity to perform switching between communication lines after the mobile station measures quality of a communication line associated with an adjacent communication area.

According to another aspect of the present invention there is provided a mobile station comprising: a receiving unit for receiving data packets including a measurement allowance information packet, transmitted thereto H:\mavism\keep\specis\P50878_03060.doc 17/02/05 3 from a gateway station by way of a communications satellite Sthat forms multiple beams on a ground; 00 a received signal processing unit for reproducing contents data from the data packets received by said receiving unit, and for detecting a switching instruction packet oo 00 transmitted thereto from said gateway station; and Mf a line switching controlling unit for controlling O switching between communication lines according to a movement of said mobile station from a communication area in which said mobile station is staying to an adjacent communication area that is adjacent to the communication area, wherein said line switching controlling unit determines whether or not there is a necessity to perform switching between communication lines after measurement of quality of a communication line associated with an adjacent communication area, transmits a switching request packet to said gateway station by way of a transmitting unit when determining that there is a necessity to perform switching between communication lines, and causes said receiving unit to switch between communication lines within a time period during which said received signal processing unit detects a switching instruction packet that said gateway station has transmitted thereto in response to the switching request packet.

According to another aspect of the present invention there is provided a satellite communication method of carrying out communications between a gateway station and a mobile station by way of a communications satellite that H:\mavism\keep\specis\P50878_03060.doc 17/02/05 forms multiple beams on a ground, said method comprising the 3 steps of: Ssaid gateway station transmitting a measurement 00 allowance information packet to said mobile station; said mobile station measuring a quality of a O communication line associated with an adjacent communication 00 0area that is adjacent to a communication area in which said (cf mobile station is staying in response to the measurement Sallowance information packet; said mobile station transmitting a switching instruction packet indicating an instruction for switching to the communication line to said gateway station according to a result of the measurement; said gateway station switching between receive lines to receive data packets from said mobile station in response to the switching instruction packet from said mobile station, and continuously transmitting a switching instruction packet to said mobile station; and said mobile station switching between receive lines to receive data packets from said gateway station while the switching instruction packet is being continuously transmitted thereto.

According to another aspect of the present invention there is provided a gateway station comprising: a transmitting unit for communicating with a mobile station by way of a communications satellite that forms multiple beams on a ground; H:\mavism\keep\specis\P50878_03060.doc 17/02/05 a data packet generating unit for generating data packets 3 from contents data applied thereto; Sa packet sending unit for sending either the data packets 00 generated by said data packet generating unit, a measurement allowance information packet, or a switching instruction O packet for instructing said mobile station to switch between 00 communication lines to said transmitting unit; and Sa line switching controlling unit for switching between receive lines to receive data packets from said mobile station and instructing said packet sending unit to send a switching instruction packet to said transmitting unit in response to a switching instruction packet from said mobile station.

According to another aspect of the present invention there is provided a mobile station comprising: a receiving unit for receiving data packets including a measurement allowance information packet, transmitted thereto from a gateway station by way of a communications satellite that forms multiple beams on a ground; a received signal processing unit for reproducing contents data from the data packets received by said receiving unit, and for detecting a first switching instruction packet continuously transmitted thereto from said gateway station; and a line switching controlling unit for controlling switching between communication lines according to a movement of said mobile station from a communication area in which said H:\mavism\keep\specis\P50878_03060.doc 17/02/05 mobile station is staying to an adjacent communication area 3 that is adjacent to the communication area, wherein Ssaid line switching controlling unit determines whether 00 or not there is a necessity to perform switching between communication lines after measurement of quality of a O communication line associated with an adjacent communication 00 0area, transmits a second switching request packet to said Sgateway station by way of a transmitting unit when determining Sthat there is a necessity to perform switching between communication lines, and instructs said receiving unit to switch between receive lines within a time period during which said received signal processing unit detects a first switching instruction packet that said gateway station has transmitted thereto in response to the second switching request packet.

According to another aspect of the present invention there is provided a mobile station comprising: a receiving unit for receiving data packets including a measurement allowance information packet, transmitted thereto from a gateway station by way of a communications satellite that forms multiple beams on a ground; a received signal processing unit for reproducing contents data from the data packets received by said receiving unit, and for detecting a switching instruction packet continuously transmitted thereto from said gateway station; and a line switching controlling unit for controlling switching between communication lines according to a movement H:\mavism\keep\specis\P50878_03060.doc 17/02/05 1 of said mobile station from a communication area in which said 3 mobile station is staying to an adjacent communication area Sthat is adjacent to the communication area, wherein 00 said line switching controlling unit identifies the communication area in which said mobile station is staying based on positional information obtained from a GPS receiver, oo 00 determines whether or not there is a necessity to perform M switching between communication lines after measurement of Squality of a communication line associated with an adjacent communication area, transmits a switching request packet to said gateway station by way of a transmitting unit when determining that there is a necessity to perform switching between communication lines, and instructs said receiving unit to switch between receive lines within a time period during which said received signal processing unit detects a switching instruction packet that said gateway station has transmitted thereto in response to the switching request packet.

H:\mavism\keep\specis\P50878_03060.doc 17/02/05 BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing communication lines from a gateway station to a mobile station for use with a satellite communication method in accordance with embodiment 1 of the present invention; Fig. 2 is a block diagram showing communication lines from the mobile station to the gateway station for use with a satellite communication method in accordance with embodiment 1 of the present invention; Fig. 3 is a schematic diagram showing an example of a track that is formed on a ground by a communications satellite for use with the satellite communication method in accordance with embodiment 1 of the present invention; Fig. 4 is a schematic diagram showing transmitted and received contents when switching between communication lines is carried out in accordance with the satellite communication method of embodiment 1 of the present invention; Fig. 5 is a block diagram showing the structure of the gateway station in accordance with embodiment 1 of the present invention; Fig. 6 is a block diagram showing the structure of the mobile station in accordance with embodiment 1 of the present invention; Fig. 7 is a schematic diagram showing transmitted and received contents when switching between communication lines is carried out in accordance with a satellite communication method of embodiment 2 of the present invention; and Fig. 8 is a schematic diagram showing transmitted and received contents when switching between communication lines is carried out in accordance with a satellite \\melb-fies\home$\teresab\eep\Specifications\P50878o306Odoc 30/09103 11 communication method of embodiment 3 of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1.

A satellite communication method, and a mobile station and a gateway station for use with the satellite communication method in accordance with embodiment 1 of the present invention will be explained with reference to Figs. 1 to 6. Fig. 1 is a block diagram showing communication lines extending from the gateway station to the mobile station for use with the satellite communication method in accordance with embodiment 1 of the present invention. In Fig. i, reference numeral 1 denotes a communications satellite, reference numerals 2a and 2b denote communication areas formed by multiple beams transmitted from the communications satellite i, respectively, reference numeral 3 denotes the gateway station, and reference numeral 4 denotes the mobile station. Furthermore, reference numerals 5a and 5b denote communication lines (having carrier frequencies fl and f2, respectively) used for transmission of packets from the gateway station 3 to the communications satellite 1, respectively, reference numeral 6a and 6b denote communication lines (having carrier frequencies f3 and f4, respectively) corresponding to the communication areas 2a and 2b, respectively, the communication lines 6a and 6b being respectively used for transmission of packets from the communications satellite 1 to the communication areas 2a and 2b. Furthermore, the two communication lines and 5b are associated with the two other communication lines 6a and 6b, respectively, so that the communication line 5a pairs up with the communication line 6a and the \\melb files\home$\teresab\keep\Specfications\P50878 03060.doc 30/09/03 communication line 5b pairs up with the communication line 6b. In a variant of this embodiment, the communication lines 6a and 6b respectively extending from the communications satellite 1 to the communication areas 2a and 2b are associated with the communication line extending from the gateway station 3 to the communications satellite 1 (in this case, the communication line 5b is replaced by the communication line 5a from here on) Fig. 2 is a block diagram showing communication lines extending from the mobile station to the gateway station for use with the satellite communication method in accordance with embodiment i. In the figure, reference numerals 7a and 7b denote communication lines (having carrier frequencies f5 and f6, respectively) corresponding to the communication areas 2a and 2b, respectively, the communication lines 7a and 7b being used for transmission of packets from the mobile station 4 to the communications satellite 1, reference numeral 8a and 8b denote communication lines (having carrier frequencies f7 and f8, respectively) used for transmission of packets from the communications satellite 1 to the gateway station 3.

Furthermore, the two communication lines 7a and 7b are associated with the two other communication lines 8a and 8b, respectively, so that the communication line 7a pairs up with the communication line 8a and the communication line 7b pairs up with the communication line 8b.

For example, the communications satellite 1 is a nonstationary communications satellite that can fly in about the zenith direction of a specific region, and forms multiple beams on a communication area including this specific region. For example, this communications satellite 1 can be one communications satellite included in a group of three communications satellites. In this \\melbfies\hcme$\teresab\keep\Specifications\P5087803060doc 30/09/03 case, the three communications satellites stay one after another in a range having a predetermined elevation angle within a specific region for eight hours, and provide communication services one after another. As a result, the three communications satellites can ensure communications between gateway stations and mobile stations at all times. The above-mentioned range having a predetermined elevation angle can be an elevation angle range having elevation angles of 70 to 90 degrees (zenith), for example, and is defined depending upon an individual communication satellite system. Furthermore, the communications satellite 1 that is roughly flying in about the zenith direction can be an elliptic orbit satellite having a periodicity of 24 hours, an orbit tilt angle of 45 degrees, an eccentricity of 0.139, an apogee altitude of 41,650 km, and a perigee altitude of 29,930 km, for example. Fig. 3 shows an example of a track which is formed on the ground by a point located directly under the satellite that is flying in such an orbit, and the satellite might be called a semi-zenith satellite. The number of communications satellites included in an individual communication satellite system can be determined independently.

In Fig. 1 or 2, the gateway station 3 establishes a communication link with a client by way of a communication network and communicates with the mobile station 4 by way of the communications satellite 1. As a usage pattern of such a communication system, there is a case in which the client is a broadcaster, the mobile station 4 is a vehicle-mounted earth station an SNG station: Satellite News Gathering station), and communications are bidirectionally carried out in such a manner that data about video and voice from the vehicle-mounted earth \\melbjles\home$\teresab\keep\Specifications\P50878oao60.d, 30/09/03 14 station are transmitted through the communications satellite 1 and are then transmitted to the broadcaster by way of the gateway station 3, and data about voice and so on are transmitted from the broadcaster to the vehiclemounted earth station. Furthermore, there can be various usage patterns such as a pattern in which the client is a hospital and the mobile station 4 is an emergency vehicle, and communications are bidirectionally carried out in such a manner that data about video and voice from the emergency vehicle that is a mobile station are transmitted through the communications satellite 1 and are then transmitted to the hospital by way of the gateway station 3, and data about voice and so on are transmitted from the hospital to the emergency vehicle by way of the communications satellite 1.

Fig. 4 is a schematic diagram showing transmitted and received contents when switching between communication lines is carried out in accordance with the satellite communication method of embodiment 1 of the present invention. In Fig. 4, reference numeral 9 denotes a data packet including data about voice, video and so on, and reference numeral 10 denotes a measurement allowance information packet. Within a time period during which the gateway station 3 transmits a series of measurement allowance information packets 10 to the mobile station 4, no data packet including data about significant video, voice and so on is transmitted from the gateway station to the mobile station and the mobile station 4 temporarily switches between receive lines so as to measure the line quality of a beam (referred to as an adjacent beam from here on) that is adjacent to a current beam being currently used thereby. Reference numeral 11 denotes a switching instruction packet. The mobile station 4 can \\melb-flles\hone$\teresab\keep\Specifications\p5087803060.dc 30/09/03 switch between receive lines within a time period during which the mobile station 4 receives a series of switching instruction packets 11 from the gateway station 3 (referred to as the GW station in Fig. 4 and also in Figs.

7 and Reference numeral 12 denotes a switching request packet that is transmitted from the mobile station 4 to the gateway station 3 when the mobile station 4 makes a request for beam switching after the mobile station 4 has evaluated the-qualities of the two receive lines, and reference numeral 13 denotes a switching instruction packet that is continuously transmitted from the mobile station 4 to the gateway station 3 when the mobile station 4 instructs the gateway station 3 to switch between receive lines.

The gateway station 3 transmits data packets P1 and P2 to the communications satellite 1 by way of the communication line 5a. The communications satellite 1 receives the data packets from the gateway station 3, and transmits the received data packets P1 and P2 to the communication area 2a by way of the communication line 6a.

The mobile station 4 is staying in the communication area 2a and receives the data packets P1 and P2 by way of the communication line 6a. Assume that the mobile station 4 is now moving to the communication area 2b (referred to as an adjacent communication area in this specification) that is adjacent to the communication area 2a, as shown in Fig.

1.

Then the gateway station 3 transmits a series of measurement allowance packets 10 at predetermined intervals. Each measurement allowance packet 10 only has to include an identifier indicating that the packet itself transmitted from the gateway station 3 is not a data packet 9 and a time period during which the mobile station \\melb-fies\home$\teresab\keep\Speifications\P5087SO306Odo 30/09/03 16 4 measures the line quality of an adjacent beam has come.

Each measurement allowance packet 10 is transmitted, by way of the communication line 5a, to the communications satellite la, and is further transmitted to the mobile station 4 by way of the communication line 6a. The mobile station 4 receives each measurement allowance packet 10 by way of the communication line. 6a, changes the current receive line being currently used thereby to the other communication line 6b associated with the adjacent communication area 2b, and measures the line quality of the communication line 6b, such as a received signal strength or a bit error rate. As shown in Fig. 4, a series of measurement allowance packets 10 from the gateway station 3 is also transmitted to the communications satellite la by way of the communication line 5b so that the mobile station 4 can receive this series of measurement allowance packets 10 by way of the communication line 6b. Based on the measurement of the line quality of the communication line 6b, the mobile station 4 determines if the communication line 6b has a better quality than the communication line 6a, or if the quality of the communication line 6b increases while the quality of the communication line 6a decreases so as to determine whether to switch between the two communication lines 6a and 6b. Instead of a series of measurement allowance information packets 10, a data packet, such as the immediately preceding data packet P2 transmitted by way of the communication line 6a, can be transmitted to the mobile station 4 by way of the communication line 6b.

A common communication line intended for a measurement of the line quality can be provided, as a communication line via which a series of measurement allowance information packets 10 are transmitted, for the communication area 2b.

\\melbfiles\hoe$\teresab\keep\Spefications\P50878o306odoc 30/09/03 In this case, when receiving a series of measurement allowance information packets 10, the mobile station 4 switches to the common communication line intended for a measurement of the line quality and then receives a packet so as to measure the line quality of the communication line associated with the adjacent communication area to which the mobile station is moving.

When determining that there is a necessity to switch to the communication line 6b according to a result of the measurement of the line quality of the communication line 6b, the mobile station 4 transmits a switching request packet 12 to the gateway station 3 (by way of the communication line 7a). This switching request packet 12 includes information about the adjacent communication area 2b associated with the communication line 6b to which the mobile station 4 desires to switch and information about the communication line 6b. When receiving the switching request packet 12, the gateway station 3 continuously transmits a switching instruction packet 11 to the mobile station 4 by way of the communication lines 5a and 5b. As shown in Fig. 4, the gateway station 3 transmits the first half of a series of switching instruction packets 11 to the mobile station 4 by way of the communication lines in the first half of a time period T, and transmits the second half of the series of switching instruction packets 11 to the mobile station 4 by way of the communication lines 5b in the second half of the time period T. As an alternative, the gateway station 3 can transmit the series of switching instruction packets 11 to the mobile station 4 by way of both of the communication lines 5a and throughout the time period T. When receiving the series of switching instruction packets 11, the mobile station 4 switches between the two communication lines 6a and 6b \\melb-fles\hme$\teresab\keep\Specifications\P5087803060.doc 30/09/03 18 based on information included in each switching instruction packet or switches to the communication line 6b that has been determined to be a target of switching by carrying out a line quality evaluation. In other words, the earth station 4 switches from the communication line 6a to the other communication line 6b (having a frequency of f3), and receives the next data packets P4 and P5 by way of the communication line 6b.

On the other hand, the mobile station 4 performs switching between the communication lines 7a and 7b via which the mobile station 4 transmits data packets to the communications satellite 1 as follows. The mobile station 4 measures and evaluates the quality of the receive line associated with. the adjacent communication area 2b to which the mobile station is moving, as previously mentioned, and then transmits a switching request packet 12 to the gateway station 3. The mobile station 4 further needs to switch between the two transmission lines, i.e., switch from the communication line 7a to the other communication line 7b. At that time, as shown in Fig. 4, the mobile station 4 transmits a series of switching instruction packets 13 to the gateway station 3 by way of the communication line 7a, switches from the communication line 7a to the other communication line 7b, and transmits subsequent data packets to the gateway station 3 by way the other communication line 7b. When receiving the series of switching instruction packets 13, the gateway station 3 switches between the two receive lines, i.e., switch from the communication line 8a to the other communication line 8b, and receives the subsequent data packets from the mobile station 4 by way of the other communication line 8b.

While the gateway station 3 can transmit a series of \\melbftes\hme$\teresab\keep\Specifications\j'5087803060doc 30/09/03 switching instruction packets 11 to the mobile station 4 by way of both the communication line 5a and the communication line 5b, the mobile station 4 can receive the series of switching instruction packets 11 after switching to the communication line 6b. When receiving a series of switching instruction packets 11 immediately after switching to the communication line 6b, the mobile station 4 can determine that there are no losses of received data packets. throughout a time period during which it performs the switching to the communication line 6b. Furthermore, when receiving a series of measurement allowance information packets 10, the mobile station 4 can determine that the line quality of the communication line 6a degrades due to trouble that occurs in electromagnetic shielding or the like and does not perform switching between the two communication lines 6a and 6b if the received signal strengths of the communication line 6a associated with the communication area 2a in which the mobile station is staying and the other communication line 6b associated with the adjacent communication area 2b that is adjacent to the communication area 2a are equal to or less than a predetermined level or if the bit error rates of those communication lines are equal to or greater than a predetermined level.

Next, the structure of the gateway station will be explained. Fig. 5 is a block diagram showing the structure of the gateway station. In Fig. 5, reference numeral 14 denotes a transmitting antenna, reference numeral 15 denotes a receiving antenna, reference numeral 16 denotes a transmitting unit for transmitting packets to the mobile station by communicating with the communications satellite by way of the transmitting antenna 14, reference numeral 17 denotes a packet sending \\melbfles\home$\teresab\keep\Specications\P50878-03060.doc 30/09/03 unit for sending data packets to the transmission unit 16, reference numeral 18 denotes a switching control packet generating unit for generating a switching instruction packet 11 and a measurement allowance information packet 10, reference numeral 19 denotes an interface for interfacing with a communication network, reference numeral 20 denotes a data packet generating unit for generating data packets from communication data transmitted thereto by way of the communication network, reference numeral 21 denotes a data packet storage unit for temporarily storing data packets to be sent out by the packet sending unit, reference numeral 22 denotes a receiving unit for receiving a signal transmitted thereto from the mobile station 4 by way of the communications satellite i, reference numeral 23 denotes a received signal processing unit for converting the received signal from the mobile station 4 into a transmission signal according to a protocol used by the communication network, and reference numeral 24 denotes a line switching controlling unit for instructing each of the transmitting unit 16 and the receiving unit 22 to switch between communication lines.

Next, an operation of the gateway station 3 will be explained with reference to Fig. 5. Communication data from each client is input to the network I/F unit 19 by way of the communication network. The network I/F unit 19 sequentially delivers the received communication data to the data packet generating unit 20. The data packet generating unit 20 generates data packets from the received communication data and delivers them to the packet sending unit 17. When the format of packets transmitted via the communication network is the same as that of packets transmitted via the satellite \\melb-iles\hcme$\teresab\keep\Specifations\P5087803060d 30/09/03 21 communication system, the data packet generating unit can make the received communication data to pass therethrough and output them just as they are.

Furthermore, when the communication data received from the network I/F unit 19 are stream data or the like, the data packet generating unit 20 generates data packets by timebased compressing the communication data when necessary.

The data packets generated by the data packet generating unit 20 and either a measurement allowance information packet 10 or a switching instruction packet 11 from the switching control packet generating unit 18 are input to the packet sending unit 17. While a measurement allowance information packet 10 or a switching instruction packet 11 is input, the packet sending unit 17 delivers the packet to the transmitting unit 16 and delivers the data packets generated by the data packet generating unit 20 to the data packet storage unit 21. In contrast, while neither a measurement allowance information packet 10 nor a switching instruction packet 11 is input, the packet sending unit 17 delivers the data packets generated by the data packet generating unit 20 to the transmitting unit 16. When data packets are stored in the data packet storage unit 21, the packet sending unit 17 gives a higher priority to the data packets and then delivers them to the transmitting unit 16. The transmitting unit 16 transmits the generated data packets to the mobile station one by one. In contrast, in the case of transmission of a measurement allowance information packet 10 or a switching instruction packet 11, the transmitting unit 16 continuously transmits a series of measurement allowance information packets 10 or a series of switching instruction packets 11 to the mobile station. As previously explained with reference to Fig. 4, the \\melbiles\home$\teresab\keep\Spefications\P5087803060.doe 30/09/03 22 transmitting unit 16 can transmit either of a series of measurement allowance information packets 10 and a series of switching instruction packets 11 by way of the communication lines 5a and 5b. In the case of transmission of a switching instruction packet 11, the transmitting unit 16 can transmit the first half of a series of switching instruction packets 11 by way of the communication lines 5a in the first half of a time period T, and transmits the second half of the series of switching instruction packets 11 by way of the communication lines 5b in the second half of the time period T. As an alternative, the transmitting unit 16 can transmit the series of switching instruction packets 11 by way of both of the communication lines 5a and throughout the time period T. The line switching controlling unit 24 determines whether the mobile station needs to perform switching between communication lines, and, when determining that the mobile station needs to perform switching between communication lines, sends an instruction for switching between communication lines to the transmitting unit 16, and instructs the switching control packet generating unit 18 to generate and output a measurement allowance information packet 10 or a switching instruction packet 11. At that time, the line switching controlling unit 24 instructs the switching control packet generating unit 18 to generate and deliver a measurement allowance information packet 10 at predetermined intervals. On the other hand, the line switching controlling unit 24 instructs the switching control packet generating unit 18 to generate and deliver a switching instruction packet 11 in response to a switching request packet 12 received and output by the signal processing unit 23.

\\melbffles\home$\teresab\keep\Speifications\P5087803060doc 30/09/03 23 The receiving unit 22 of the gateway station 3 receives a transmission signal transmitted thereto from the mobile station 4 by way of the receiving antenna The receiving unit 22 then converts the received RF signal into a signal having a lower frequency and delivers it to the received signal processing unit 23. The received signal processing unit 23 extracts data packets, a switching request packet 12, or a series of switching instruction packets 13 from the signal having a lower frequency while distinguishing them from each other. The received signal processing unit 23 then delivers the data packets to the network I/F unit 19, the switching request packet 12 or the series of switching instruction packets 13 to the line switching controlling unit 24.

Furthermore, the received signal processing unit 23 timebased expands compressed stream data or the like when necessary so as to reproduce stream data, and delivers them to the network I/F unit 19. On the other hand, when receiving a switching request packet 12 from the received signal processing unit 23, the line switching controlling unit 24 instructs the switching control packet generating unit 18 to generate and deliver a switching instruction packet 11 and instructs the transmitting unit 16 to transmit a series of switching instruction packets 11 to the mobile station, as previously mentioned. Furthermore, when receiving a switching instruction packet 13, the line switching controlling unit 24 instructs the receiving unit 22 to perform switching between communication lines.

Next, the structure of the mobile station 4 will be explained. Fig. 6 is a block diagram showing the structure of the mobile station 4. In Fig. 6, reference numeral 25 denotes a switching control packet generating unit for generating a switching request packet 12 and a \\melb61es\hcme$\teresab\keep\Specifications\5087803060.doc 30/09/03 24 switching instruction packet 13, reference numeral 26 denotes a received signal processing unit for converting a signal transmitted thereto from the gateway station 3 to a transmission signal according to a protocol used by the communication network, the received signal processing unit performing a time-based expanding process on stream data or the like, reference numeral 27 denotes a line switching controlling unit for instructing each of a transmitting unit 16 and a receiving unit 22 to perform switching between communication lines, and reference numeral 28 denotes an input and output I/F unit for inputting or outputting data about voice and video stored in the mobile station 3, or control data. In Fig. 6, circuit components designated by the same reference numerals as shown in Fig.

5 are the same as or similar to those of Fig. An operation of the earth station 4 will be explained with reference to Fig. 6. The receiving unit 22 receives data packets P1 and P2 from the gateway station 3 by way of the communication line 6a, and then receives a series of measurement allowance information packets 10. The series of measurement allowance information packets 10 is identified by the received signal processing unit 26 and is then delivered to the line switching controlling unit 27. The line switching controlling unit 27 instructs the receiving unit 22 to temporarily switch to the other communication line 6b associated with the adjacent communication area 2b to which the mobile station is moving and which is adjacent to the communication area 2a in which the mobile station is staying and to receive subsequent packets by way of the other communication line 6b. The receiving unit 22 then measures the line quality of the other communication line 6b, such as a received signal strength or a bit error rate, and delivers data \\melb.files\home$\teresab\keep\Specifications\P5087803060.doc 30/09/03 about the line quality of the other communication line to the line switching controlling unit 27. The line switching controlling unit 27 determines whether to switch between the two communication lines 6a and 6b according to a result of the line quality measurement. When switching to the other communication line, as previously explained with reference to Fig. 4, the line switching controlling unit 27 instructs the switching control packet generating unit 25 to generate and output a switching request packet 12 and also instructs the transmitting unit 16 to transmit the switching request packet 12 to the gateway station 3.

A series of switching instructing packets 11 is transmitted to the mobile station by the gateway station 3 in response to this switching request packet 12, and is received by the receiving unit 22. These switching instruction packets 11 are identified by the received signal processing unit 26, and are delivered to the line switching controlling unit 27. The line switching controlling unit 27 then instructs the receiving unit 22 to switch from the communication line 6a to the other communication line 6b based on the reception of those switching instruction packets 11.

Next, a process of switching between transmission lines which is performed by the mobile station 4 will be explained. The mobile station 4 measures and evaluates the quality of the receive line associated with the adjacent communication area, as previously mentioned, and transmits a switching request packet 12 to the gateway station 3.

The mobile station 4 also needs to switch between the two transmission lines, switch from the communication line 7a to the other communication line 7b. At that time, the line switching controlling unit 27 instructs the switching control packet generating unit 25 to generate \\melb-fles\homeS\teresab\keep\Speffication\P5O878_0306doc 30/09/03 26 and output a switching instruction packet 13 to the packet sending unit 17, and also instructs the transmitting unit 16 to transmit a series of switching instruction packets 13 to the gateway station by way of the communication line 7a. After the transmitting unit 16 has transmitted these switching instruction packets 13 to the gateway station, the line switching controlling unit 27 causes the transmitting unit 16 to switch to the communication line 7b so that the transmitting unit 16 transmits subsequent data packets to the gateway station by way of the communication line 7b. When receiving the series of switching instruction packets 13, the gateway station 3 switches between the two receive lines, switch from the communication line 8a to the other communication line 8b, and receives subsequent data packets from the mobile station 4 by way of the other communication line 8b.

As previously mentioned, in accordance with embodiment 1, because the mobile station can switch between communication lines to receive data packets from the gateway station within a time period during which the mobile station receives a series of switching instruction packets, the mobile station can prevent losses from being produced in packet data received thereby. Furthermore, because the mobile station determines whether to switch between communication lines according to a result of a measurement of the quality of a communication line associated with an adjacent communication area, the mobile station can surely capture packet data transmitted by way of the communication line after performing switching to the communication line.

Embodiment 2.

A satellite communication method in accordance with \\melbfiles\home$\teresab\keep\Specifications\P50878 03060.doc 30/09/03 27 embodiment 2 of the present invention and a mobile station and a gateway station for use with the method will be explained hereafter. Fig. 7 is a schematic diagram showing transmitted and received contents when switching between communication lines is carried out in accordance with the satellite communication method of embodiment 2 of the present invention. In accordance with above-mentioned embodiment 1, the mobile station 4 transmits a switching request packet 12 and a series of switching instruction packets 13' to the gateway station 3 (see Fig. 4) In contrast, though a mobile station 4 in accordance with embodiment 2 transmits a series of switching instruction packets 13 to the gateway station, the mobile station 4 transmits no switching request packet 12 to the gateway station.

In Fig. 7, circuit components designated by the same reference numerals as shown in Fig. 4 are the same as or similar to those of Fig. 4. As shown in Fig. 7, when determining that there is a necessity to perform line switching according to a result of a line quality measurement, the mobile station 4 transmits a series of switching instruction packets 13 to the gateway station 3.

When receiving the series of switching instruction packets 13, the gateway station 3 switches between two receive lines, switches from a communication line 8a to another communication line 8b, and then transmits a series of switching instruction packets 11 to the mobile station 4. The mobile station 4 performs processing on this series of switching instruction packets 11 in the same way that that of embodiment 1 does. After transmitting a series of switching instruction packets 13, as previously mentioned, the mobile station 4 switches between two transmission lines, switches from a communication \\melb-ffles\home\teresab\keep\Speificatfions\P587O306.doc 30/09103 line 7a to another communication line 7b and then transmits subsequent data packets to the gateway station by way of the other communication line 7b. Those switching processes carried out in the base and gateway stations are actually carried out by a line switching controlling unit 24 of the gateway station 3 as shown in Fig. 5 and a line switching controlling unit 27 of the mobile station 4 as shown in Fig. 6, respectively.

As previously mentioned, in accordance with embodiment 2, because the gateway station performs switching between receive lines to receive data packets from the mobile station in response to a switching instruction packet that the mobile station has transmitted thereto when making a request for switching between receive lines, the gateway station can reduce the time required for performing the switching process.

Embodiment 3.

A satellite communication method in accordance with embodiment 3 of the present invention and a mobile station and a gateway station for use with the method will be explained hereafter. Fig. 8 is a schematic diagram showing transmitted and received contents when switching between communication lines is carried out in accordance with the satellite communication method of embodiment 3 of the present invention. In accordance with above-mentioned embodiment 1, the mobile station 4 receives a series of measurement allowance information packets 10 and performs a measurement of the quality of a communication line associated with an adjacent communication area so as to determine whether or not there is a necessity to perform switching to the communication line. In contrast, in accordance with embodiment 3, the mobile station 4 \\melb.files\homeS\teresab\keep\Specfications\P5878_03060.doe 30/09/03 identifies a communication area in which the mobile station 4 itself is staying based on positional information from a GPS receiver, and determines whether or not there is a necessity to perform switching to a communication line associated with an adjacent communication area. No measurement allowance information packet is transmitted to the mobile station 4, as shown in Fig. 8. When determining that there is a necessity to perform switching to a communication line associated with an adjacent communication area, the mobile station 4 transmits a switching request packet 12 to the gateway station 3. After that, the mobile station 4 and the gateway station 3 carry out their respective processings in the same way that those of embodiment 1 do as previously explained with reference to Fig. 4. Although the mobile station 4 and the gateway station 3 have the same structures as shown in Fig. 5 and Fig. 6, respectively, the gateway station 3 doesn't perform a process of transmitting a series of measurement allowance information packets to the gateway station 4, and the mobile station 4 doesn't perform a process of measuring the line quality of a communication line associated with an adjacent communication area in response to a series of measurement allowance information packets. The mobile station 4 can identify a communication area in which the mobile station 4 itself is staying by using a positional relationship between the positional information obtained from GPS satellites and the locations of communication areas (each of which is determined with the latitude and longitude of the center of each communication area, the radius of each communication area, etc.).

As previously mentioned, in accordance with embodiment 3, because the mobile station identifies a \\melb-ffles\home$\teresab\keep\Speifications\P5087803060doc 30/09/03 communication area in which the mobile station 4 itself is staying based on positional information from a GPS receiver, the mobile station can omit a measurement of the quality of a communication line associated with an adjacent communication area and therefore reduce the time required for performing the switching process.

As previously mentioned, the mobile station 4 in accordance with embodiment 2 does not transmit any switching request packet 12 to the gateway station 3.

Similarly, after determining that there is a necessity to perform switching to a communication line associated with an adjacent communication area, the mobile station 4 in accordance with embodiment 3 can transmit a switching instruction packet 13 to the gateway station 3 without transmitting any switching request packet 12 to the gateway station 3.

Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.

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 \\melbfiles\home$\teresab\keep\Specifications\P5087803060.doc 30/09/03 31 of the common general knowledge in the art, in Australia or any other country \\melb-ffes\home$\teresab\keep\Speifications\P5O87836.doc 30109/03

Claims

2. The satellite communication method according to Claim i, wherein after transmitting the switching request packet to said gateway station, said mobile station transmits a switching instruction packet for instructing said gateway station to switch between receive lines to receive packets from said mobile station. \\melbfes\home$\teresab\keep\Spefications\P5087803060.doc 30/09/03
3. A gateway station comprising: 3 a transmitting unit for communicating with a mobile Sstation by way of a communications satellite that forms 00 multiple beams on a ground; a data packet generating unit for generating data packets O from contents data applied thereto; 00 0a packet sending unit for sending either the data packets Sgenerated by said data packet generating unit, a measurement Sallowance information packet, or a series of switching instruction packets for instructing said mobile station to switch between communication lines to said transmitting unit; and a line switching controlling unit for instructing said packet sending unit to send the series of switching instruction packets to said transmitting unit in response to a switching request packet which is transmitted from said mobile station when there is a necessity to perform switching between communication lines after the mobile station measures quality of a communication line associated with an adjacent communication area.
4. The gateway station according to Claim 3, wherein said line switching controlling unit instructs said packet sending unit to send a measurement allowance information packet for allowing said mobile station to measure a quality of a communication line to said transmitting unit. H:\mavism\keep\specis\P50878_03060.doc 17/02/05 A mobile station comprising: 3 a receiving unit for receiving data packets including a Smeasurement allowance information packet, transmitted thereto 00 from a gateway station by way of a communications satellite that forms multiple beams on a ground; O a received signal processing unit for reproducing 00 contents data from the data packets received by said receiving unit, and for detecting a switching instruction packet transmitted thereto from said gateway station; and a line switching controlling unit for controlling switching between communication lines according to a movement of said mobile station from a communication area in which said mobile station is staying to an adjacent communication area that is adjacent to the communication area, wherein said line switching controlling unit determines whether or not there is a necessity to perform switching between communication lines after measurement of quality of a communication line associated with an adjacent communication area, transmits a switching request packet to said gateway station by way of a transmitting unit when determining that there is a necessity to perform switching between communication lines, and causes said receiving unit to switch between communication lines within a time period during which said received signal processing unit detects a switching instruction packet that said gateway station has transmitted thereto in response to the switching request packet. H:\mavism\keep\specis\P50878_0306.doc 17/02/05 D 6. The mobile station according to Claim 5, wherein said Sline switching controlling unit instructs said receiving unit 00 to measure a quality of a communication line associated with an adjacent communication area that is adjacent to a O communication area in which said mobile station is staying oo00 within a time period during which said gateway station Stransmits a measurement allowance information packet to said mobile station, and determines whether to switch to the communication line according to a result of the measurement.
7. A satellite communication method of carrying out communications between a gateway station and a mobile station by way of a communications satellite that forms multiple beams on a ground, said method comprising the steps of: said gateway station transmitting a measurement allowance information packet to said mobile station; said mobile station measuring a quality of a communication line associated with an adjacent communication area that is adjacent to a communication area in which said mobile station is staying in response to the measurement allowance information packet; said mobile station transmitting a switching instruction packet indicating an instruction for switching to the communication line to said gateway station according to a result of the measurement; said gateway station switching between receive lines to receive data packets from said mobile station in response to the switching instruction packet from said mobile station, and H:\mavism\keep\specis\P50878_03060.doc 17/02/05 I continuously transmitting a switching instruction packet to 3 said mobile station; and Ssaid mobile station switching between receive lines to 00 receive data packets from said gateway station while the switching instruction packet is being continuously O transmitted thereto. 00 M 8. A gateway station comprising: Sa transmitting unit for communicating with a mobile station by way of a communications satellite that forms multiple beams on a ground; a data packet generating unit for generating data packets from contents data applied thereto; a packet sending unit for sending either the data packets generated by said data packet generating unit, a measurement allowance information packet, or a switching instruction packet for instructing said mobile station to switch between communication lines to said transmitting unit; and a line switching controlling unit for switching between receive lines to receive data packets from said mobile station and instructing said packet sending unit to send a switching instruction packet to said transmitting unit in response to a switching instruction packet from said mobile station.
9. A mobile station comprising: a receiving unit for receiving data packets including a measurement allowance information packet, transmitted thereto from a gateway station by way of a communications satellite that forms multiple beams on a ground; H:\mavism\keep\specis\P5087803060.doc 17/02/05 a received signal processing unit for reproducing 3 contents data from the data packets received by said receiving Sunit, and for detecting a first switching instruction packet 00 continuously transmitted thereto from said gateway station; and \O a line switching controlling unit for controlling 0 00 switching between communication lines according to a movement Mf of said mobile station from a communication area in which said Smobile station is staying to an adjacent communication area that is adjacent to the communication area, wherein said line switching controlling unit determines whether or not there is a necessity to perform switching between communication lines after measurement of quality of a communication line associated with an adjacent communication area, transmits a second switching request packet to said gateway station by way of a transmitting unit when determining that there is a necessity to perform switching between communication lines, and instructs said receiving unit to switch between receive lines within a time period during which said received signal processing unit detects a first switching instruction packet that said gateway station has transmitted thereto in response to the second switching request packet. A mobile station comprising: a receiving unit for receiving data packets including a measurement allowance information packet, transmitted thereto from a gateway station by way of a communications satellite that forms multiple beams on a ground; a received signal processing unit for reproducing H:\mavism\keep\specis\P50878_03060.doc 17/02/05 contents data from the data packets received by said receiving 3 unit, and for detecting a switching instruction packet Scontinuously transmitted thereto from said gateway station; 00 and a line switching controlling unit for controlling O switching between communication lines according to a movement 00 0of said mobile station from a communication area in which said Smobile station is staying to an adjacent communication area Sthat is adjacent to the communication area, wherein said line switching controlling unit identifies the communication area in which said mobile station is staying based on positional information obtained from a GPS receiver, determines whether or not there is a necessity to perform switching between communication lines after measurement of quality of a communication line associated with an adjacent communication area, transmits a switching request packet to said gateway station by way of a transmitting unit when determining that there is a necessity to perform switching between communication lines, and instructs said receiving unit to switch between receive lines within a time period during which said received signal processing unit detects a switching instruction packet that said gateway station has transmitted thereto in response to the switching request packet.
11. A satellite communication method as claimed in any one of claims i, 2 or 7, and substantially as herein described with reference to the accompanying drawings. H:\mavism\keep\specis\P50878_03060.doc 17/02/05 I 0 12. A gateway station as claimed in any one of claims (N f 3,4 or 8, and substantially as herein described with reference Sto the accompanying drawings. 00
13. A mobile station as claimed in any one of claims P 5, 6, 9 or 10, and substantially as herein described with CO i00 reference to the accompanying drawings. 0 Dated this 17th day of February 2005 MITSUBISHI DENKI KABUSHIKI KAISHA By their Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and Trade Mark Attorneys of Australia H:\mavism\keep\specis\P50878_03060.doc 17/02/05