Method of traffic switching in a low orbit satellite communication system with terminals as destination and communication system using this method.

p0001A traffic switching process in a system of communications satellites in low orbit to mobile terminals or not, transceivers or receivers only, connecting a terminal to a terrestrial communications network being accomplished means of a connection station.

p0002The invention also relates to a communications system implementing such a method.

p0003Satellite communications with mobile terminals studied so far used two types of orbits: the orbit of geostationary satellites or highly inclined elliptical orbits having both the property of being located, on average, above areas the high concentration of particles space, called "Van Allen belts". Recently lower orbits have been considered. Their altitude is between 800 and 2000 km. One of the characteristics of communications satellite systems that use such orbits is the possibility to communicate with a large number of mobile devices, eg portable kind. The difference between the higher altitude orbits the "Van Allen belts" and those of lower altitude lies in a space attenuation decreases as the satellite is closest to the earth.

p0004A report by the CCIR (document number US IWP 8 / 14-52; 1st August 1990) entitled "Technical characte- ristics of a personnal satellite mobile communication system" describes a system of communications satellites in low orbit with multibeam antennas. The constellation of satellites and performed includes 77 satellites to provide global coverage of the Earth. It includes 7 shots of 11 satellites each having a circular polar orbit. But such a system to maintain communication when a terminal leaves the coverage of a first satellite, com.prend of inter-satellite links. This solution therefore requires a demodulation aboard the first satellite, referral information, transmission to a second satellite and a reference to the connection station. It is very complicated and expensive.

p0005The object of the present invention is to minimize the cost of the space segment using transparent satellites, and removing any inter-satellite link.

p0006According to the present invention the tilting of the terminal-satellite links and satellite connection-station is possible only within a specific area; the decision to switch the traffic being made based on the degradation of the quality of service and the radio environment of a group of terminals of said area.

p0007In a first variant the traffic switching is a switching from a first satellite to a second having sufficient elevation over a specific area which is a satellite cell.

p0008In a second variant the traffic changeover is that of a first to a second beam of the same satellite.

p0009In a first traffic failover of the first beam to the second is progressive; the establishment of any new communications being carried through the same satellite; the decision is made to switch the remaining traffic to the second beam according to the degradation of the quality of service provided in the first beam and the radio environment of a group of terminals.

p0010In a second failover traffic of the first beam to the second is global; switch the order of the first beam to the second being sent from the connection station to all the terminals of the respective area when all parameters of the radio environment of the terminals reaches a certain limit.

p0011A particularly advantageous system satellite communication implementing the method of the invention uses two constellations WALKER (1389 km, 47 °, 24/08/3) and Walker (1389 km, 55 °, 24/08/3) . Such a system is a global land cover system that complements existing sytèmes communications. It can offer a variety of services. But its main feature is the ability to adapt to situations of trafficking and different from region to region service.

p0012The market analysis has indeed shown that the main communications needs were regional and that there are niche markets to global. The architecture of such a system is designed to accommodate this situation.

p0013These uses comprise both closed networks that public networks. The basic service is digital telephony with all applications that may arise regarding the data. Other services can also be provided, which use the effect "Doppler" from the scroll satellites. These services are radio-navigation service, or radio-location. It is also expected to be able to use the radio-location to perform some of the functions of attachment to a base station.

p0014The invention will be described in more detail hereinafter with the aid of the description and the accompanying drawings.

p0015In these drawings:<ul><li>- Figure 1 shows traffic failover process in a satellite system of the prior art;</li><li>- Figures 2 to 6 illustrate the traffic changeover method according to the invention.</li></ul>

p0016The satellite system or "constellation" is considered a global coverage system which complements existing terrestrial communications systems. In this system, a mobile or non-terminal, transceiver or receiver only, is identified, located and connected by a satellite of the constellation through a connection station in the fixed public network to access it at all the services of the public telephone network or the future ISDN.

p0017Radio resources for all the terminals are divided into cells satel- tary, each corresponding to a small radius zone from the satellite coverage and large radius compared to a terrestrial cell of a cellular network by Land example of the "Groupe Special Mobile" (GSM). This cell is attached to a connecting station and the terminals belonging geographically to this area are attached to this station.

p0018The system has to know, in real time, all active terminals (communication established or being established) and terminals in "sleep"; Out Communications times a terminai, the latter being considered idle or inactive. In this system the terminals therefore belong to a satellite cell attached to a docking station that connects to the cellular network of earth.

p0019The terminals receive in "sleep" mode a signal over a special frequency identifying the cell where they are and allow them to know the parameters assigned to that cell. Any new terminal becoming active in this cell emits its identification code. The update procedure of the terminal positioning data base is identical to that used in the terrestrial mobile radio infrastructure.

p0020Compared to cellular networks, the satellite system does not impose a significant number of data exchange between the terminal positioning databases and the connection station. This is due :<ul><li>- In large radius of coverage of the satellite cell.</li><li>- The principle that in the SA- tellitaires cell array, the passage of a terminal of one cell to another is considered to be very low.</li></ul>

p0021For a terminal, the network access details consist of the given radio resources and channel frequency band. For the connection station in addition to the specific data terminal, a satellite number is assigned to liaise with the terminals in the cell attached to the docking station.

p0022In a given cell, the location and registration of terminals as well as call processing are identical to those used in the cellular land network. At the call processing, additional data relating to the number of the satellite having the best elevation on the cell in question is taken into account by the connection station. Indeed a station from the connection stations includes additional amenities to the location of satellites providing the radio coverage of the cell. These measures transmitted to ground control segment used to calculate the satellite ephemeris and reduce the antenna pointing tabs connecting stations.

p0023In a cell, the connection of a terminal to the terrestrial network is accomplished by means of a connection station. This station has the following functions:<ul><li>- It runs a lot of pre-assigned radio resources and a lot of resources in case of overflow.</li><li>- It supports the procedures for establishing and breaking the bonds with the terminals.</li><li>- It passes the traffic on the satellite having the best elevation. This results in tracking satellites and failover of a satellite traffic to another depending on the elevation presented by each of them.</li><li>- It provides connection to a land exchange.</li></ul>

p0024To provide these functions, the cradle consists of two separate entities:<ul><li>- A radio communication unit including the antenna and radio subsystems (RF), a signal processing subsystem (modems, ...),</li><li>- A connection and control unit comprising sub-management of the radio part of the station (Allocation management of radio resources, preparation and execution of satellite failover, ...) and the subsystem management exchanges with the terrestrial transmission segment.</li></ul>

p0025Each docking station is equipped with three antennas with their associated RF subsystem. Two antennas of the station are in pursuit. The third antenna serves as a backup to the other two. Each antenna is equipped with a programmable tracking system on ephemeris. These pointing tables are provided at regular intervals by the satellite control segment of the system.

p0026On a general point of view and in relation to a cellular network of land, the system does not introduce any changes in the overall architecture of the terrestrial network. The specificity is at the connection station in choosing the satellite having the best elevation of the area and in the management of a satellite traffic to be tilting at.

p0027To minimize the cost of the space segment, satellites are transparent and there are no inter-satellite links.

p0028Of these two options, it follows that to maintain communication simultaneously rocking the mobile-satellite links and connecting satellite station of a satellite on another. This mode implies that:<ul><li>- The location registration principle (roaming) is the attachment of a terminal to the nearest docking station,</li><li>- There is a distance limit below which the tilting of the movable SAT- connections, connection-satellite station is possible.</li></ul>

p0029The estimate of this distance must also consider:<ul><li>- An acceptable elevation to ensure sufficient quality of service,</li><li>- The feasibility for the terminal and the connection station to quickly acquire the carrier synchronization and the new selected channel.</li></ul>

p0030To highlight the benefits of such a solution in Figures 1, 2 and 3 respectively illustrate a failover requiring intersa- link tellite and tilting of the invention that requires no connection of this type.

p0031In Figure 1A the cover 10 of the first carrier 11 includes a docking station 12 and a terminal 13.

p0032Figure 1 B satellite 11 has moved but the connection station 12 and the terminal 13 remain in satellite visibility.

p0033Figure 1C satellite coverage 10 11 no longer includes the connection station 12. The connection terminal 12 of connection 13-station must then go through a second satellite 14 whose coverage includes 15 said station 12.

p0034In a first variant of the method of the invention, we have the same first two cases shown in Figures 2A and 2B. Against by Figure 2C there was tilting of first (11) on the second (14) satellite. The connection terminal 13 - connecting station 12 passes only the second satellite 14.

p0035A second variant of the process of the invention is shown in Figure 3. In Figure 3A the cover of the first beam 17 includes a connection station 12 and a terminal 13. In Figure 3B the beam coverage 17 no longer involved the connection station 12 and the terminal 13. the terminal connection 13 - connection station 12 must pass through the beam 16 of the satellite 11 whose coverage includes said station and said terminal.

p0036The feasibility of switching based on the respect of a set of constraints on the establishment of a link with a new satellite or with a new beam. The values ​​of the timing delay, the offset of carrier frequency must be maintained within the limits of the performance of the signal processing units. The relative positions of the access and visibility satellite station are known from the ephemeris with sufficient accuracy. Only the position of the terminal relative to the connection station is unknown and it is therefore it introduces a hazard on the rise, the synchronization, the frequency of the received carrier.

p0037In the following description it will be considered as an example the first variant.

p0038If we take the example of the Doppler shift. A connection station knows exactly at a given satellite Dopplerdu the incoming shift and that of the outgoing satellite. This station can communicate with the terminal Doppler she observes the switching time and the terminal also observed near a small uncertainty. In this case, the terminal is capable of precorrect its frequency of reception and transmission during tilting. The uncertainty of the Doppler shift offset is then reduced to that resulting from the uncertainty of the position of the terminal.

p0039For a terminal in a given cell attached to a ground station, the radio resources allocated to it is composentd'un number range transmission / reception (combination of a TDMA access (TDMA) and CDMA), and a "beam hopping" or "jump brushes," wherein N coverage spots in the ground are illuminated successively and sequentially forming groups of spots selected among the P N spots) and a frequency channel band.

p0040A satellite changeover corresponds to the passage of a given state (allocation of radio resources, docking station, satellite number) to a new state. Such a changeover can happen by:<ul><li>- Better elevation of the satellite appearing on the fixed cell,</li><li>- A connection terminal to another connection station (terminal portion of a considered cell to another),</li><li>- Radio environment of a more favorable satellite beam.</li></ul>

p0041The decision to conduct a satellite switching is linked to the development of a satellite to better elevation on the geographic area where the terminal and the connection station is located.

p0042Switching is made from the satellite ephemeris and knowledge of the radio environment of the mobile. It must be as short as possible so as not to affect the established communications.

p0043When the satellite switching is performed. A series of synchronization information is sent in to restore a nominal communication. The synchronization procedure is identical to that used in the terrestrial cellular network.

p0044In the case of failure of the resynchronization device automatically resumes the radio resources were allocated to it before the satellite switching and the operation is renewed.

p0045Two execution scenarios: a gradual shift or a global traffic failover.

p0046<ul><li>- When a gradual shift from one satellite to the other traffic is used when a satellite has sufficient elevation on a satellite cell, the establishment of all new communications takes place through this new satellite. Then, depending on the degradation of the quality of service provided by the first satellite and the radio environment on a group of mobile terminals, the decision is made to switch the traffic to the new satellite. This procedure promotes a gradual transfer of a satellite traffic to another. We must continuously monitor the radio environment of the terminals (monitoring beacon to predict or direct exchange with the terminal which then perform measurement series) and quality assured service. Thus, as shown in FIG 4, there is a connection station 20 equipped with three antennas 21,22, and 23, and an associated satellite cell 24. Two terminals 25 and 26 are connected to this station 20 via a first satellite 28. the quality of service provided by the satellite 28 is degrading the cell becomes 24 24 'and 24 ". the connection of the new terminal with 27 calling the station 20 is established directly through the second satellite 29. the decisions to switch terminals 25 and 26 of the first (28) to the second (29) satellite, as defined above, are subsequently taken (arrows 30 and 31).</li><li>- When a failover all traffic from one satellite to the other is used when the new satellite has a favorable angle of elevation of the fixed satellite cell, and when all the parameters of the radio environment of the mobile reaches a certain limit, the order of switching from one satellite to another is sent from the connection station to all the terminals as shown in Figure 5 (arrow 32). The procedure is comprehensive and affects all terminals in a given area. However, all terminals must synchronize before continuing transmission of their information and the radio resource allocations for all the terminals must be taken into account instantly when switching. Figure 6 describes the preparation and execution of a failover.</li></ul>

p0047From the ephemeris prediction of the satellite constellation and a given land area, the satellite having the best elevation in this area is selected. This selection allows the connection station to acquire and track the new satellite and preparing radio resource allocations.

p0048The satellite failover execution order is decided by the connection station and controlled by the Link control subsystem. The implementation of satellite failover causes the joint realization and perfectly synchronized between the cradle and the terminals of the following:<ul><li>- The mobile, a change of transmission and reception time. This results in a translation in time of the initial configuration,</li><li>- For connecting station, a failover traffic to another antenna, the latter pointing to the new satellite.</li></ul>

p0049The satellite uses tilting the range of GSM procedures without creating a new process. In the case of the system considered, this procedure is governed by the connection station.

p0050A particularly advantageous system implementing the method described above comprises at least one constellation of satellites belonging to the group of constellations known and recorded under the name "symmetrical constellations Walker". (On this subject see the article by JG Walker entitled "Continuous whole earth coverage by satellites circular-or- bit" appeared in "Satellite communications systems per mobile and monitoring", IEEE conference publication 95; 1973). These constellations are symmetrical both the even distribution of satellites in the same orbit as the distribution of satellites in the same orbit as the distribution and equal inclination of the orbital planes in space. They were chosen because they help minimize the number of satellites for a given coverage, and are particularly effective in covering a band of latitude.

p0051A WALKER constellation is characterized by five parameters:<ul><li>- Altitude, here 1389 km (for reasons of life).</li><li>- The gradient.</li><li>- The parameter triplet T / P / F:<ul><li>. T is the total number of satellites,</li><li>. P is the number of orbital planes,</li><li>. F is the phasing parameter that indicates the relative position of satellite orbital plane to the next.</li></ul></li></ul>

p0052To maximize coverage of inhabited areas, that is to say between the Equator and 65 ° latitude (North or South), it is a constellation of Walker (1389 km, 52 °, 48/8/1) . This constellation has the advantage of allowing optimum area coverage, especially from the perspective of the elevation, but it has the disadvantage, like all significant number of satellite constellations to ask at least two years to put in square. The system considered therefore uses two constellations, which can be set up consecutively.

p0053Deductions constellations are: WALKER (1389 km, 47 °, 24/08/3), which adequately covers the CONUS and Southern Europe (typically up to the latitude of Lille), but which has large gaps of coverage below 30 ° latitude.

p0054WALKER (1389 km, 55 °, 24/08/3), which covers the rest of the world and to optimize coverage, especially elevation, in countries of latitude between 10 ° and 60 °.

p0055It may be noted that the first constellation to 24 satellites includes areas where the minimum elevation is less than that requested. These areas are mobile and in fact low latitude. The time during which no satellite is visible at a given point, is relatively low. For higher latitude areas, the average elevation is much higher and coverage is no longer holes. These defects are corrected by the launch of the second constellation.

p0056It is understood that the invention has been described and shown by way of preferred example and that we can replace its components by equivalent parts without thereby going beyond the scope of the invention. The explanations given above for the first variant of the method of the invention are of course valid for the second variant: the tilting of the first beam to the second can thus be progressive or global.