GB2352363A

GB2352363A - Satellite communications system with broadcast channel switching

Info

Publication number: GB2352363A
Application number: GB9917123A
Authority: GB
Inventors: Latif Lokman; Sze-Ching Lu
Original assignee: ICO Services Ltd
Current assignee: ICO Services Ltd
Priority date: 1999-07-21
Filing date: 1999-07-21
Publication date: 2001-01-24
Also published as: GB9917123D0; WO2001006678A1

Abstract

A satellite communications system is of the type in which a satellite provides a plurality of spot beams 301 on the earth's surface, a respective main broadcast channel in each of the spot beams carries a signal containing geographical information relating to the boundaries of the corresponding spot beam, and a user terminal 38 is operable to learn its position on the earth's surface and to receive the broadcast channel of the spot beam currently occupied by the user terminal so that the user terminal can confirm that it is located within the spot beam boundaries, in order to maintain camping of the user terminal on the spot beam. If the main broadcast channel of a spot beam has to be turned off, for example to avoid interference with facilities such as radio telescopes or defence installations, this could result in multiple user terminals requesting a location update leading to system overload. To avoid this overload, the satellite transmits a substitute broadcast channel in a spot beam commencing a predetermined time prior to the main broadcast channel of that spot beam being turned off. The substitute broadcast channel carries the same geographical spot boundary information as the main beam, whereby a user terminal in that spot beam will acquire the substitute broadcast channel and remain camped on the same spot beam. The main broadcast channels of contiguous spot beams are all on different frequencies f1 to f7 to one another, and the substitute broadcast channel in a spot beam is on a substitute broadcast frequency which is different to both the main broadcast frequency of that spot beam and the main broadcast frequencies of all the contiguous spot beams. The substitute broadcast channel may initially be at a higher power than the main broadcast channel, but may be reduced in power after the main broadcast channel of the spot beam has been turned off. The satellites may be in medium earth orbit, and the user terminals may be mobile, semi-mobile, or stationary, radio telephones.

Description

2352363 BROADCAST SWITCHING The present invention relates to satellite

communications systems where a satellite provides a plurality of spot beams on the surface of the earth.

In a satellite communications system, a user terminal, in the general form of a radio telephone handset, is operative to communicate with an earth station through the medium of an orbiting satellite. The earth station then provides connectivity with the terrestrial telephone system. Such satellite systems are operated in a general manner io reminiscent of a terrestrial cellular telephone networks, by the satellite providing radio coverage to the earth in the form of a plurality of contiguous spot beams which have the general function of cells in a cellular telephone network. In addition, the position of the user terminal, on the surface of the earth, is found by any of a number of known ranging techniques, for example, as shown in GB-A-2 320 385. 15 One network known as the IRIDIUM' satellite cellular system is described in EP-A-0 365 885 and US Patent No. 5 394 561 (Motorola), which makes use of a constellation of so- called low earth orbit (LEO) satellites, that have an orbital radius of 780 km. Mobile user terminals such as telephone handsets establish a link to an 20 overhead orbiting satellite, from which a call can be directed to another satellite in the constellation and then typically to a ground station which is connected to conventional land-based networks. Alternative schemes which make use of so-called medium earth orbit (MEO) satellite 25 constellations have been proposed with an orbital radius in the range of 10-20,000 kin. Reference is directed to the lCOTm satellite cellular system described for example in GB-A-2 295 296. With this system, the satellite communications link does not permit communication between adjacent satellites. Instead, a signal from a mobile user terminal such as a mobile handset is directed firstly to the satellite and then directed to 3o a ground station or satellite access node (SAN), connected to conventional land-based telephone networks. This has the advantage that many components of the system are compatible with known digital terrestrial cellular technology such as GSM. Also 2 simpler satellite communication techniques can be used than with a LEO network. Reference is directed to "New Satellites for Personal Communications", Scientific American, April 1998, pp. 60 - 67, for an overview of LEO/MEO satellite networks.

Each satellite spot beam includes a broadcast channel, which, for example, as described in EP-A-0 892 506, includes geographical data indicating the boundaries of the particular spot beam. If the user terminal, receiving the broadcast channel, from the spot beam and knowing its position, determines that it does, indeed, lie within the boundaries of the spot beam from which it is receiving the broadcast channel, the user io terminal remains "camped" on that spot beam, provided it has not moved. If, however, the user terminal determines that it is outside the boundaries of the spot beam on which it is camping, it decides that the user terminal has moved and immediately seeks a location update so that it can re-establish itself within a spot beam.

There are occasions when a broadcast channel has to be switched off. When a satellite is non-geo-stationary, its array of spot beams sweeps across the surface of the earth, Occasionally, an area will be entered, by a spot beam, where particular frequencies cannot be used. If the broadcast channel happens to be one of these frequencies, it must be switched off. Even in a geo-stationary situation, facilities (for example, radio telescopes) can come into operation and limit the available frequencies.

When the broadcast channel is switched off, each user terminal, in the spot beam, starts to hunt around a predetermined set of frequencies whereon a broadcast channel can be located. In general, a user terminal will detect the broadcast channel from a neighbouring spot beam (usually the nearest). However, when the user terminal reads the signal in the broadcast channel from the neighbouring spot beam, the user terminal immediately learns that it is outside of the boundaries, indicated in the broadcast channel from the neighbouring spot beam. The user terminal assumes that it has moved, and immediately seeks a location update, involving new range-finding, registration and so on. This produces a heavy demand on the system resources, because should a broadcast channel be switched off, there is the possibility of 3 hundreds or thousands of user terminal, in the same spot beam, all simultaneously requesting a location update. Under such circumstances, the system can overload. The present invention seeks to provide a way of avoiding such a situation.

According to a first aspect, the present invention provides a satellite communications system wherein a user terminal is operable to communicate with an earth station via a communications satellite, wherein said satellite is operable to provide a plurality of spot beams on the surface of the earth, wherein said satellite is operable to provide a main broadcast signal on a respective main broadcast frequency in each of said i o plurality of spot beams, wherein said main broadcast signal in said each spot beam contains geographical information relating to the boundaries of said each spot beam, wherein said user terminal is operable to learn its position on the surface of the earth, and wherein said user terminal is operable to receive the main broadcast signal in the spot beam, currently occupied by said user terminal and confirm that said user terminal is located within said boundaries, said system being characterised by: from the commencement of a predetermined period before said main broadcast signal in said current spot beam is abandoned, said satellite being operable to provide a substitute broadcast signal on a substitute broadcast frequency in said current spot beam, said substitute broadcast signal containing the same geographical information as said main broadcast signal; and said user terminal being operable to receive said substitute broadcast signal and to confirm the location of said user terminal within said boundaries.

According to a second aspect, the present invention provides a method for use in a satellite communications system wherein a user terminal is operative to communicate with an earth station via a communications satellite, wherein said satellite is operative to provide a plurality of spot beams on the surface of the earth, wherein said satellite is operative to provide a main broadcast signal on a respective main broadcast frequency in each of said plurality of spot beams, wherein said main broadcast signal 9 in said each spot beam contains eographical information relating to the boundaries of said each spot beam, wherein said user terminal is operative to learn its position on the surface of the earth, and wherein said user terminal is operative to receive the 4 main broadcast signal in the spot beam, currently occupied by said user terminal and confirm that said user terminal is located within said boundaries, said method being characterised by said satellite, from the commencement of a predetermined period before said main broadcast signal in said current spot beam is abandoned, providing a substitute broadcast signal on a substitute broadcast frequency in said current spot beam, said substitute broadcast signal containing the same geographical information as said main broadcast signal; and said user terminal receiving said substitute broadcast signal and confirming the location of said user terminal within said boundaries.

According to a third aspect, the present invention provides a user terminal for use in a satellite communications system wherein said user terminal is configured to communicate with an earth station via a communications satellite, wherein said satellite is operable to provide a plurality of spot beams on the surface of the earth, wherein said satellite is operative to provide a main broadcast signal on a respective main broadcast frequency in each of said plurality of spot beams, wherein said main broadcast signal in said each spot beam contains geographical information relating to the boundaries of said each spot beam, wherein said user terminal is operable to determine its position on the surface of the earth, and wherein said user terminal is operable to receive the main broadcast signal in the spot beam, currently occupied thereby and confirm that said user terminal is located within said boundaries, the user terminal further being operable so that in response to the satellite providing a substitute broadcast signal on a substitute broadcast frequency in said current spot beam within a predetermined period of abandoning said main broadcast signal, said substitute broadcast signal containing the same geographical information as said main broadcast signal; said user terminal receives said substitute broadcast signal and determine whether the location of said user terminal is within said boundaries.

In a fourth aspect, the invention provides a ground station for use in a satellite telecommunications system wherein the satellite is operable to provide a plurality of spot beams onto the surface of the earth, each of the spot beams including a main broadcast signal containing information relating to boundaries of the spot beam, the ground station being operable to command the satellite to cease broadcasting the main broadcast signal and to provide a substitute broadcast signal for the spot beam on a substitute broadcast frequency and containing the same geographical information as said main broadcast signal.

The main broadcast signal may be provided at a first signal strength, and the substitute broadcast signal provided at a second signal strength, greater than the first signal strength.

io The plurality of spot beams can in motion relative to the surface of the earth, and the communications satellite need not be geo-stationary.

The geographical information may define a hexagon which fits within the current spot beam.

The various aspects of the invention may provide that no main broadcast frequency is shared with any contiguous spot beam in the plurality of spot beams, and that the substitute broadcast frequency is none of the main broadcast frequency of the current spot beam nor of any main broadcast frequency of any spot beam, contiguous to the current spot beam.

The invention is further explained, by way of example with reference to the following description, read in conjunction with the appended drawings, in which: Figure I shows a planar constellation of satellites disposed about the earth;

Figure 2 illustrates how the satellites are disposed in two orthogonal orbital planes; Figure 3 shows the structure of the cone of radio coverage provided by each satellite; Figure 4 shows the "administrative" cells which are formed by the spot beams; Figure 5 schematically illustrates the radio coverage reality corresponding to the cells of Fig. 4, illustrating how adjacent spot beams have considerable overlap with adjacent spot beams; Figure 6 illustrates overlap of adjacent beams in the vicinity of a radio telescope that requires the broadcast channel to be switched off-, and Figure 7 is a flow charge of method steps for an example of the invention.

6 Figure I shows a planar constellation of satellites disposed about the earth. The plurality of satellites 10 are evenly disposed around a circular orbit 12 above the surface of the earth 14.

Each of the satellites 10 is designed to provide radio communications with apparatus on the surface of the earth 14 when the individual satellite 10 is more than 10 degrees above the horizon. Each satellite 10 therefore provides a cone 16 of radio coverage which intersects with the surface of the earth 14.

Three types of coverage areas are provided on the earth's surface. A first type of area 18 is one which has radio coverage from only one satellite 10. A second type of area 20 is an area where there is radio coverage from more than one satellite 10. Finally, a third type of area 22 receives radio coverage from none of the satellites 10 in the orbit 12 shown.

Figure 2 illustrates how the satellites 10 are disposed in two orthogonal orbital planes.

The first orbit 12 of Figure I is supplemented by a second orbit 12' having satellites 10 disposed there about in a similar manner to that shown in Figure 1. The orbits 12' are orthogonal to one another, each being inclined at 45 degrees to the equator 24 and having planes which are orthogonal (at 90 degrees) to each other.

In the example shown, the satellites 10 orbit above the surface of the earth 14 at an altitude in the region of 10 500km, in a constellation which generally corresponds to the JCOTM System.

Those skilled in the art will be aware that other orbital heights and numbers of satellites 10 may be used in each orbit 12, 12'. This configuration is preferred because the example provides global radio coverage of the entire surface of the earth 14, even to the north 26 and south 28 poles. In particular, the orthogonality of the orbits ensures that the satellites 10 of the second orbit 12' provides radio coverage for the third types of area 22 of no radio coverage for the satellites in the first orbit 12, and the satellites 10 in the first orbit 12 provide radio coverage for those areas 22 of the third type where the satellites 10 of the second orbit 12' provide no radio coverage. Although the two orbits 12, 12' are here shown to be of the same radius, the system will function with orbits 12, 12' of different radii.

7 Figure 2 also shows how an earth station 29 is in bi-directional contact with a satellite 10 which, in turn, is in bi-directional contact with a user terminal 38 on the earth 14. The satellite 10 acts as a relay for signals from the earth station 29 to the user terminal 3 8 and for signals from the user terminal 38 to the earth station 29 Figure 3 shows the structure of the cone 16 of radio coverage provided by each satellite 10. For convenience, the radio coverage cone 16 is shown centred, on a map of the earth, at latitude 0 degrees at longitude 0 degrees. The cone 16 of radio coverage is formed of a plurality of spot beams 30, that are individually produced by means of a corresponding i o plurality of directional antennae on the satellite 10. The satellite 10 is intended for mobile radio telephone communications and each of the spot beams 30 corresponds to a cell in a cellular radio telephone network. In Figure 3, the cone of radio coverage 16 is distorted due to the projection geometry of the map. Figure 3 also shows the extent of interaction of the cone 16 of radio coverage down to the edges of the cone 16 being tangential to the earth's 15 surface, that is, to the point where the cone 16 represents a horizontal incidence at its edges, with the surface of the earth. By contrast, Figure I shows the cone 16 at a minimum of 10 degrees elevation to the surface of the earth. It is to be observed, that because of the curvature of the earth, the spot beams 30 are of near 20 uniform, slightly overlapping circular shape at the centre whereas, at the edges, the oblique incidences of the spot beams 30 onto the surface of the earth 14 causes considerable distortion of shape. Figure 4 shows how, for "administrative" purposes, the spot beams 30 are approximated by 25 hexagonal "cells" 301, while Figure 5 shows the reality of overlapping spot beams 30. Because of the orbital movement of the satellite 10 and the rotation of the earth 14, for the purpose of this example, the array of cells 301 (spot beams 30)moves across the surface of the earth as indicated by the arrow 32. A user terminal 38 is at a fixed point, on the surface of the earth 14 and the array of cells 301 (spot beams) moves, as indicated by the arrow 32, with 30 respect to the user terminal 38.

8 Each spot beam 30 has a broadcast channel allocated thereto. Each broadcast channel has a frequency (fI -f7) allocated thereto such that no cell 301 (spot beam 30) has the same broadcast channel frequency as a neighbouring cell. In the example shown, the user terminal 38 is in a cell 301 (spot beam 30) where the allocated broadcast channel frequency is fl. Its nearest neighbouring cell 301 (spot beam 30) has an allocated broadcast channel frequency R.

At first location update (switch-on) the position of the user terminal 3 8 on the surface of the earth 14 was found using any suitable method known such as that described and explained in io GB-A-2 320 385, where combined signal delay and doppler shift measurements between the user terminal 3 8 and the satellite 10 are employed. The result of the position determination is signalled to the user terminal 38.

The broadcast channel fl in the current cell 301 (spot beam 30) occupied by the user terminal 38 provides it with data indicative of the position of the vertices of the hexagon which form the cell 301. The user terminal 38, knowing its position and the positions of the vertices of the hexagon, can compute whether or not the user terminal 3 8 lies within the cell 3 0 1. If the user terminal 38 discovers it is not within the cell 301, the user terminal 38 requests a location update on the assumption that the user terminal 38 has moved relative to the cell concerned.

Figure 6 show how spot beams 30 overlap, and is illustrative of a portion of Figure 5. The Figure shows the signal strengths of the broadcast channel fl in the current spot beam 30, together with the signal strength of the broadcast channel f7 in the adjacent spot beam 30, nearest to the user terminal 38. It can be seen that the signal strengths fall away, but do not entirely disappear, within adjacent spot beams so that, in principle, the user terminal 38, if required, could listen to and decode the broadcast channel f7 in the adjacent spot beam 30 for a considerable distance from the adjacent spot beam 30.

For the sake of illustration of the operation of the system, the current spot beam 30 is shown having moved over a facility or area where transmission is restricted. For the sake of example, a radio telescope 40 is show, but it might equally be a defence installation, or 9 simply an area where, for regulatory purposes, not all frequencies can be used. Again for the sake of example, in Figure 6, one of the frequencies which cannot be used is fl, being the frequency of the broadcast channel in the current spot mean 30. It will be known that the current spot beam 30 is moving over the restricted area, since the system, through its associated earth station 29, will know the position of the spot beams at any instant.

Figure 7 show the behaviour of the system when a restricted area 40 is encountered.

In a first operation 42, the earth station 29 selects a new frequency f8 for a substitute broadcast channel in the current spot beam. The substitute frequency f8 is chosen so that it is io none of the frequencies of the broadcast channels f2 0 f4 f5 f6 f7 in adjacent spot beams, nor is it identical to the broadcast channel frequency fl in the current spot beam, but the substitute frequency f8 is acceptable in the presence of the restriction 40.

A second operation 44 then has the earth station 29 cause the satellite to transmit the substitute broadcast channel on the substitute frequency f8 at a higher power than the main broadcast channel on the main broadcast frequency fl. This ensures two things:

1) that any user terminal 38, currently scanning for a new broadcast channel, will find the substitute broadcast channel f8 to be the strongest and best. This is measured by the user terminal 38 either as raw signal strength, or as a bit rate error. Either may be used.

2) that the substitute broadcast channel on f8 is guaranteed to be stronger than any of the main broadcast channels on adjacent spot beam 30 frequencies f2 B & f5 f6 P. Thus, a user terminal ') 8. seeking a new broadcast channel, will chose the substitute channel on B as being the best, and will not instigate a location update through selection of an adjacent spot beam on f2 B f4 f5 f6 or f7.

The substitute broadcast channel on f8 carries exactly the same geographical data as the main broadcast channel on fl, namely the position of the vertices of the designated hexagon for the current cell 301. Thus, a user terminal 38, acquiring the substitute broadcast channel on f8, will remain camped on the spot beam 30 (or cell 301) since it has no indication that it has moved.

A third operation 46 has the earth station 29 waits a predetermined time, for preference sixteen seconds, but other periods can apply, where the earth station 29 has the satellite 10 simultaneously transmit both the main broadcast channel on fl. and the substitute broadcast channel on f8.

At the end of the predetermined period, a fourth operation 48 has the earth station 29 cause the satellite 10 to cease transmitting the main broadcast channel on fl. The system then carries on with whatever business it may have to hand.

At the end of the fourth operation, any user terminals 38, not yet having acquired the substitute broadcast channel on f8, rapidly make the necessary acquisition. The satellite 10 can then reduce the power of the substitute channel on f8 and continue, now using f8 as the main broadcast channel frequency, until another change may be required.

Although the invention has been described in relation to the ICOTMsystem, it will be appreciated that it could be equally well applied to any of the satellite mobile telecommunications networks described in Scientific American supra.

Also, whilst the user terminals UT have been described herein as mobile telephone handsets, it will be understood that they may be semi-mobile e.g. mounted on a ship or aircraft. The UT may also be stationary e.g. for use as a payphone in a geographical location where there is no terrestrial telephone network.

Claims

I. A satellite communications system wherein a user terminal is operable to communicate with an earth station via a communications satellite, wherein said satellite is operable to provide a plurality of spot beams on the surface of the earth, wherein said satellite is operable to provide a main broadcast signal on a respective main broadcast frequency in each of said plurality of spot beams, wherein said main broadcast signal in said each spot beam contains geographical information relating to i o the boundaries of said each spot beam, wherein said user terminal is operable to learn its position on the surface of the earth, and wherein said user terminal is operable to receive the main broadcast signal in the spot beam, currently occupied by said user terminal and confirm that said user terminal is located within said boundaries, said system being characterised by: from the commencement of a predetermined period before said main broadcast signal in said current spot beam is abandoned, said satellite being operable to provide a substitute broadcast signal on a substitute broadcast frequency in said current spot beam, said substitute broadcast signal containing the same geographical information as said main broadcast signal; and said user terminal being operable to receive said substitute broadcast signal and to confirm the location -)0 of said user terminal within said boundaries.

2. A system according to claim I wherein said main broadcast signal is provided at a first signal strength, and said substitute broadcast signal is provided at a second signal strength, said second signal strength being greater than said first signal strength.

3. A system according to claim I wherein said plurality of spot beams are in motion, relative to the surface of the earth.

4. A system according to claim 3 wherein said communications satellite is not geostationary.

12 5. A system according to claim 1, 2, 3 or 4 wherein said geographical information defines a hexagon which fits within said current spot beam.

6. A system according to claim 1, 2, 3. 4 or 5 wherein no main broadcast frequency is shared with any contiguous spot beam in said plurality of spot beams, and wherein said substitute broadcast frequency is none of the main broadcast frequency of said current spot beam nor of any main broadcast frequency of any spot beam, contiguous to said current spot beam.

io 7. A method for use in a satellite communications system wherein a user terminal is operative to communicate with an earth station via a communications satellite, wherein said satellite is operative to provide a plurality of spot beams on the surface of the earth, wherein said satellite is operative to provide a main broadcast signal on a respective main broadcast frequency in each of said plurality of spot beams, wherein said main broadcast signal in said each spot beam contains geographical information relating to the boundaries of said each spot beam, wherein said user terminal is operative to learn its position on the surface of the earth, and wherein said user terminal is operative to receive the main broadcast signal in the spot beam, currently occupied by said user terminal and confirm that said user ten-ninal is located within said boundaries, said method being characterised by said satellite, from the commencement of a predeten-nined period before said main broadcast signal in said current spot beam is abandoned, providing a substitute broadcast signal on a substitute broadcast frequency in said current spot beam, said substitute broadcast signal containing the same geographical information as said main broadcast signal; and said user terminal receiving said substitute broadcast signal and confirming the location of said user terminal within said boundaries.

8. A method according to claim 7, wherein the satellite provides said main broadcast signal at a first signal strength, and said substitute broadcast signal at a second signal strength greater than said first signal strength.

13 9. A method according to claim 7 or 8, wherein said plurality of spot beams are in motion, relative to the surface of the earth.

10. A method according to claim 9 wherein said communications satellite is not geo-stationary.

11. A method according to claim 7, 8, 9 or 10 wherein said geographical information defines a hexagon which fits within said current spot beam.

i o 12. A method according to claim 7, 8, 9, 10 or 11, including configuring the broadcast frequencies so that no main broadcast frequency is shared with any contiguous spot beam in said plurality of spot beams, and said substitute broadcast frequency is none of the main broadcast frequency of said current spot beam nor of any main broadcast frequency of any spot beam, contiguous to said current spot beam.

13. A user terminal for use in a satellite communications system wherein said user terminal is configured to communicate with an earth station via a communications satellite, wherein said satellite is operable to provide a plurality of spot beams on the surface of the earth, wherein said satellite is operative to provide a main broadcast signal on a respective main broadcast frequency in each of said plurality of spot beams, wherein said main broadcast signal in said each spot beam contains geographical information relating to the boundaries of said each spot beam, wherein said user terminal is operable to determine its position on the surface of the earth, and wherein said user terminal is operable to receive the main broadcast signal in the spot beam, currently occupied thereby and confirm that said user terminal is located within said boundaries, the user terminal further being operable so that in response to the satellite providing a substitute broadcast signal on a substitute broadcast frequency in said current spot beam within a predetermined period of abandoning said main broadcast signal, said substitute broadcast signal containing the same geographical information as said main broadcast signal; said user terminal receives said substitute broadcast signal and determine whether the location of said user terminal is within said boundaries.

14 14. A user terminal according to claim 13, for use in a system wherein said main broadcast signal is provided at a first signal strength, and said substitute broadcast signal is provided at a second signal strength, said second signal strength being greater than said first signal strength.

15. A ground station for use in a satellite telecommunications system wherein the satellite is operable to provide a plurality of spot beams onto the surface of the earth, each of the spot beams including a main broadcast signal containing information relating to boundaries of the spot beam, the ground station being operable to command the satellite to cease broadcasting the main broadcast signal and to provide a substitute broadcast signal for the spot beam on a substitute broadcast frequency and containing the same geographical information as said main broadcast signal.

16. A telecommunications system substantially as hereiribefore described with reference to the accompanying drawings.