Satellite communication system

Rogulohion 10 1P/00/001Solion 29AUSTRALIAPatents Act 1990PATENT REQUEST: STANDARD PATENT/PATENT OF ADDITIONWe being the persons(s) identified below as the Applicant, request the grant of a patent to lie person identified below as the NominatedPerson for an invention descrlhed in the accompanying standard complete specificationFull application details follow(71) Applicant ALCATEL ofBurgemeester Elsenlaan 170, 2288 BH Rijswijk, TheNetherlands.Nominated Person ALCATEL N.V.Address Burgemeester Elsenlaan 170, 2288 BH Riswijk, The Netherlands.(54) Invention Title SATELLITE COMMUNICATION SYSTEM (72) Name(s) of actual inventor(s): Denis ROUFFET; Fr6d6ric BERTHAULT; MichelMAZZELLA; Yannick TANGUY:rr(74) Address for service in Australia Patent Department, Alcatel Australia Limited,280 Botany Road, Alexandria, 2015, AustraliaBASIC CONVENTION APPLICATION(S) DETAIL(2P Application Number 9112119 (33) CountryFRCountry Code (32) Date of ApplicationFRANCE 2 OCTOBER1991rDIVISIONAL APPLICATION DETAILS [62] Original Application No. 26029/92of Addition requests only)PARENT INVENTION DETAILS (Patent (61) Application Number Drawing number recommended to accompany the abstract Patent Number 645905ALCATEL N. V.18 July 1996P.M. Conrick(Authorized Signatory)DateX01 3070 22079Ill llli lilllllllili llllllllAU9660662(12) PATENT ABSTRACT (54) (19) AUSTRALIAN PATENT OFFICETitleSATELLITE COMMUNICATION SYSTEMInternational Patent Classification(s)H04B 007/185 H04B 007/212 Application No 60662/96 Priority Data(31) (43) (61) (62) (71) (72) (74) (57)Number 91 12119 (32) Date 02.10.91 (33) CountryFR FRANCE(11) Document No AU-A-60662/96H04B 007/216(22) Application Date 22.07.96(21) Publication Date 03,10,96Related toAddition(s) :17137/92Related to Division(s) Applicant(s)ALCATEL N.V.26029/92Inventor(s)DENIS ROUFFET; FREDERIC BERTHAULT; MICHEL MAZZELLA; YANNICK TANGUYAttorney or AgentALCATEL AUSTRALIA LIMITED BOX 525 SYDNEY N.S.W. 2001This invention relates to a communications system between satellites in loworbit and terminals, in which the beams (12) of each coverage area (14) are switchedon in a spatial scan referred to as \"beam-hopping\". For each satellite and terminal,transmission and reception are separated over time and synchronised, a time-divisionduplexing being used.P 004) 11AUSTRALIAS4* 4SPatentg Act 1990ORI GINALCOMPLETE SPECIFICATIONSTANDARD PATENTInvention Title:\"SATELLITE COMMUNICATION SYSTEM\"Thle following statemnent is a full description ofthig inlventionl, inludinlg the best method ofperforming it known to LIS:-S. This invention relates to a communications system between satellites in loworbit and mobile or fixed terminals, which are transmittor-rocoivers or only receivers.Communications between satellites and mobile terminals designed up untilnow used two types of orbits: geostationary satellite orbits or sharply inclined ellipticalorbits each having the property of being situated, on average, above the region ofspace with a high concentration of particles, called \"Van Allen belts\".Recently lowerorbits have been considered, with orbits between 800 and 2000 km.Onecharacteristic of satellite communication systems which use such orbits is the possibilityof communicating with a large number of mobile terminals, for example portabletypes.But the weak radio performance of these terminals requires the provision ofcompensation obtained with the superior radio performance of the satellite.Thedifference between the orbits higher than the \"Van Allen belts\" and those at loweraltitude is in space attenuation especially since the satellite is close to earth.For communication, two transmission architectures are possible, depending onthe available frequency bands.The first uses L band for only mobile-satellite links, theother S and L bands. Moreover, these architectures can be changed according to thecountry, since several transmission standards can be used: for example use of TDMAaccess techniques, use of CDMA type techniques, etc.The communication system of the invention is compatible with MSG (mobile specialgroup) standards.It is also adaptable to CDMA transmission techniques.The systemuses only one frequency band for terminal-satellite links.The invention therefore aims to propose a particular transmission sequence,since the satellite and terminal alike can receive and transmit simultaneously.Thistherefore requires the setting up of a transmit-receive frame and arrangement ofspecific synchronisation functions, in time for a TDMA system, and in time and codefor a CDMA system.A CCIR report (document number US IWP 8/14-52; 1st August 1990) entitled\"Technical characteristics of a personal communication mobile satellite system\"describes a communications system by low orbit satellites with multi-beamantennas;each having 37 conical beams.Such a system has the big disadvantage ofproducing a large number of beams, each one forming a small trace on the earth.Inaddition, on account of even the mobility of the users and the passage of the.i satellites, beam changes can occur with timo.Tho latter are generally accompanied bya change of resources (\"hand-over\").Their high number during a conversationconstitutes a phenomenon detrimental to the quality of the connection and listeningcomfort.Such a system is disadvantaged by a long transmit-receive frame.An object of th present invention is to get around these disadvantages byproviding a communication system which improves the capacity of satellites in a verysignificant way, by using a short transmit-receive frame.According to the invention there is provided a communication system betweensatellites in low orbit and terminals, characterised in that the beams of each satellitecoverage area are switched on in a spatial scan referred to as \"beam-hopping\inthat for each satellite and for each terminal, transmission and reception aresynchronised and separated over time, and in that time-division duplexing is used.In one example the communication antenna system of each satellite providesan isoflux coverage comprised of several beams which can, to advantage, be ellipticaland elongated in the orbital direction of the satellite.In order that the invention may be readily carried into effect, an embodimentthereof will now be described in relation to the accompanying drawings, in which:Figure 1 depicts a coverage area by a satellite communications system of theprior art;Figure 2 illustrates the satellite communications system according to theinvention;Figures 3 and 4 illustrate two types of transmit-receive frames related to thesystem represented in Figure 2;Figure 5 illustrates a particular representation of the invention's system.The system of satellites or \"constellation\" considered is a worldwide coveragesystem which supplements the existing terrestial communications systems.In thissystem a terminal, either mobile or non-mobile,transmitter-receiver or receiver only, is identified, localised and linked by a satellite ofthe constellation through a connection station in the public stationary networkproviding access for it to the entire services of the public telephone network or thefuture ISDN.The radio facilities allocated to all of the terminals are divided into satellite4cells, each corresponding to a zone of small radius in comparison with the satellitecoverage area and of large radius in comparison with a terrestial cell of a mobilenetwork of the \"Mobile Special Group\" (MSG) type.This coll is linked to a connectionstation, and the terminals belonging geographically to this zone are linked to thestation.In such a satellite communication system there are two classes of possible loworbits:polar orbits: whose plane passes over the poles (or quasi- polar to take account ofthe case of heliosynchronous orbits; that is to say, whose plane remains fixed inspaco).These orbits have, in theory, the property of guaranteeing a continuous andcomplete coverage of the earth.inclined orbits; whose plane makes a given angle, in practice less than degrees, with the equatorial plane.The continuous coverage is then made up of twobands of parallel boundary lines at the equator and symmetrical with respect to thelatter.Each type of orbit has points of intersection with the orbit planes.In the case ofpolar orbits, the intersection zone of these is adjacent to the poles.In the case ofinclined orbits, this zone is adjacent to the equator.Furthermore, the service area of asatellite is defined by a geometric condition:lt is the set of points on the earth fromthe satellite is seen with an elevation (the angle which the user-satellitedirection makes with the plane tangent to the earth at the place of the terminal)greater than a predetermined value (the practical value is between 10 degrees and15 degrees).The two types of orbit have the same property: the service area of each satelliteoverlap each other at times or at different places:in the case of polar orbits, it is while approaching the poles that the service areasof each satellite gradually overlap each other;in the case of inclined orbits, the description of the phenomenon of overlap ismore complex, but in certain areas it can achieve 100%.There are even satelliteconstellations which provide a quadruple overlap in certain areas.Such a property is an advantage, because it establishes communications withat least two satellites in most cases.The design of the system according to the*where invention takes account of multiple coverages so as to avoid interference betweencoverage areas. The access mode to satellites in low orbit also takes account of theinterference problem.In a satellite communications system of the prior art, such as that previouslydefined, a coverage area is achieved on earth with several beams 10 as shown inFigure 1 ;the useful coverage obtained being the area 11 .Such a coverage has severaldisadvantages:ln the path from satellite to terminal, it has several areas in which theintensity of inteference is very high.These areas, for TDMA (time division multipleaccess) and CDMA (code division multiple access) alike, are of consequence, that isto say that they play a very large part in the size, weight and cost of the satellite inquestion.If the orbits are more inclined, the interference can lead to breaks incommunication for several tens of seconds.In addition, such a coverage with finebeams requires hand-overs fairly frequently.In a system where the number of beamsis high, this leads to a hand- over every minute for example.The processing loadinduced at ground level is then for from negligible.Moreover, in CDMA (where the \"near-far effect\" can be significant) as much asin TDMA/FDMA or in FDMA (frequency division multiple access), where the harmfuleffect of large amplitude carrier waves on those which are of lesser power is known,it is desirable to have an antenna gain transmitting at ground level at a receivedpower (per unit area) as uniform as possible.In the same way as terrestial systems of radio-communicat- ions with mobiles, thesatellite system of the invention, such as that shown in Figure 2, is a cellular systemfor which the largest dimension cells are constituted by the ground trace of the variouscoverage beams 12 of each multi-beam satellite 13. From a radio standpoint, a cell ischaracterised by all the resources (frequencies, time intervals, codes), from which theterminal happens to take an element at the time a connection is made.The systemaccordii., to the invention has N beams 12.In the example illustrated in Figure 2 which shows the ground coverage 14 ofa satellite 13 in the constellation, a group of several spots 12 selected sequentiallyamong the N spots of the satellite are successively switched on.This operation isachieved during transmission as well as reception which naturally leads to atransmission frame.Generally speaking the coverage is comprised of a set of N spots:.20 (which can be circular, elliptical or any shape) simultaneously switched on accordingto the \"beam- hopping\" principle.Transmission and reception in various spots are made accord ing to the extensive,carefully designed satellite frame pattern,In effect in the case of a multi-beam system there are two means of freeing itfrom the interference between adjacent beams connected to the same satellite:each of the beams is continuously switched on and the limiting of interferencebetween beams is possible by a scheme of frequency reutilisation.All of the availableband is then not used in a beam.all of the available band is used in the beam.The technique which frees it frominterference between adjacent beams is the spatial scan or beam-hopping.The beamsare simultaneously ilium inating sufficiently long spatially so that their mutualinterference levels are acceptable.Concerning the interference level between beams of different satellites: whentwo or more satellites cut across or approach each other their traces at ground levelmore or less partially overlap.These occurrences are encountered relatively frequentlyin a multi-beam system.On the other hand further interference can come fromtrans-horizon propagation phenomena.The random interference in this case can beminimised only by the beam- hopping technique.Among each of the existing access modes, several modes consist of a judiciouscompromise between performance (number of channels) and the complexity (andtherefore the cost) of the demodulator.It is a question of modes using either signalseparation in frequency (FDMA), or separation in time (TDMA) or a separation in code(CDMA) or hybrid modes: a CDMA-TDMA combination for example.The most advantageous access modes are those which are compatible with themodes used by the ground cellular networks. There are three:The frequency division multiple access (FDMA) mode uses frequencyduplexing.lt requires four frequency bands to establish connections between theterminal and the satellite, two frequency bands to establish connections between thesatellite and the fixed stations of the ground networks (communication links).A capacity slightly less than 40 carrier waves per MHz and by fine beam (of thehalf-output MSG type, or of the speech signal coded at 4800 bits/second) can be20 achieved.rhe time division multiple access (TDMA) mode is distinctive in that it increasesthe output in a way such that a given user has access to the satellite only during ashort period which has been pre-assigned to him.In the system according to theinvention several carrier waves per frequency band are used so that the output is nottoo high.The selected output is that one of the ground cellular network which thesatellite system complements. For example in Europe the output of the MSG network(ETSI European standard) is preferably selected, in the United States that of theDAMPS network (USA digital standard).ln this t'ype of access beam-hopping is used,the frequency band used by each carrier wave being higher than the Dopplereffect..However beam-hopping imposes a synchronisation between transmission andreception, for the satellite and terminal alike.Depending on the frequency bandswhich are avai'able for the mobile, several solut ions are then possible:In the classic case where two frequency bands are available for the mobileterminal-satellite paths, it is possible to simplify the structure of the terminal by using atransmission and a reception always separated in time (this technique is known as\"time duplexing\" and is written \"TDD\").The principle of reserved access in that case istherefore as follows:to establish a connection a frequency resource (choice of carrier wave) isallocated;then, within this resource, transmission times are defined.Synchronisationmust be guaranteed by the terminal and the connection station.It is effected first of allin a specific channel then in the transmission channel, when the change oftransmission time is made incrementally.In the case of a single band of frequencies for mobile terminal-satelliteconnections the operation in TDD for the mobile terminals and the satellite ismandatory, which results in a useful capacity of the particularly straightforwardsatellite.The synchronisation of the mobile terminal transmission is done by using firstall a specific channel then by using a closed loop procedure, which increases ordecreases transmission time.In this case, as in the previous one, a centalisedmanagement of frequency resources limits interference.In addition, it does not ruleout rapid hand-overs having to be made.But the system of the invention is requiredto supplement a ground network which already has these functions.The management.20 S. 8is based on the following principle: from a certain altitude of the satellite, thegeographic zones where interference is possible are limited.ln these zones alonethere is a sharing of spectral resources.In each of the others a terminal can haveaccess to the whole spectrum.Nevertheless, it is possible to find a remedy forinterference without resorting to a completely centralised management of the systemby using a slow beam-hopping, so that if there is interference it exists only for a brieftime.The capacity which is then possible to achieve is slightly less than 35 carrierwaves per MHz.(This capacity appears lower than that mentioned for FDMA, but thefact must be taken into account that there is only a single frequency band: this type ofsystem has in fac twice the capacity in practice).The main advantage of this type ofsystem is that allows the use of a very simple satellite pay load.The code division multiple access (CDMA) mode, otherwise known as\"spectrum expansion\provides a decentralised solution to the interferenceproblems.The use of a spectrum spacing in effect superimposes several carrier waves,coming from one or several satellites.This mode can be used either with a FDD-typeaccess (transmission and reception having different frequency bands), or with aTDD-type access.In the case of two frequency bands for mobile terminal- satellite links the twoaccess types, FDD or TDD, are possible. A TDD-type solution decreases theinterference rate coming from multiple coverages in the case of inclined orbits.lneffect, when there is coverage superposition, there is a degradation in capacitylocally, which can be compensated for by a power monitoring device.Such a device isused essentially over the satellite- terminal path.It assures each user of a minimumquality of connection.ln effect in situations of multiple coverage, certain users arepenalised by too high an intensity of interference.On the other hand if the interferenceis not too high, it is possible to increase the satellite power intended for these users:the increase in total resultant power is minimal in principle.But it has an impact onthe link quality of other users who then see their intensity of interference increase.Theuse of a power monitoring device therefore has limits which must not be exceeded.As in the case of TDMA, it is possible to operate with a single frequencyband.In that case the access process is the TDD- type.However spectrum expansion.o20 9poses several specific problems. The demodulation of the spectrally-expanded signalssupposes that the receiver is capable of regaining the time reference which has beenused for transmission.Two methods are then usable: either reconstruct the timereference from the received signals, but the use of long codes, made necessary by thenumber of users present simultaneously in the system, results in this technique beingvery complex to use; or retain the time reference in memory, then from an estimationof the variations which it can have between reception of two packets, reconstitute it atthe time of reception.The main problem of TDD access is the initial acquisition of transmissionsynchronisation.The synchronisation is effected first of all in open loop then in closedloop by the station which controls the network.To begin with, a terminal acquires thesignalling channel of the network.Then, if it has to transmit, it sends a first messagewhose reception defines the time interval which it is advisable to apply so as to beperfectly synchronised.The synchronisation in loop being made and the terminalsynchronised, the tracking and checking of the synchro- nisation is obtained by themeasurement of the synchronisation made in the ground station providing theinterface with the switched telephone network.There is however a particularly simplecase of TDD operation where the process does not need to be applied, and where thesignal reception alone by the terminal is sufficient to provide synchronisationinformation.It is the case where only the terminal is operating in TDD.In this case, theterminal has a bi-frequency, but alternating, operation.As soon as it receives a signalcoming from the satellite, it transmits.On the satellite end, with the differences indistance assisting, it is not possible to receive every signal coming from the terminalssimultaneously in the same beam.The satellite transmission is framed with beamhopping, that is to say that it is done alternately by half the number of beams, eachtransmitting beam being separated from another by a non-transmitting beam.In thenext time interval, it is the reverse.However reception of the satellite can only becontinuous on account of the temporal scattering due to the spread of distances.Codeaccess is sensitive to amplitude dispersion in the various carrier waves of the samefrequency.Codes, which are perfectly orthogonal (for example Walsh-Hadamardcodes) and perfectly synchronised, are used so that the effect is not perceptible.But thesynchronisation is never perfect with satellites in low orbit, even though it can be very,20 'for good.The shape of the coverage area obtained by satellite antennas is thus veryimportant if it is wished to avoid what is known as the \"near-far problem\".ln otherterms the large amplitude carrier waves interfere a lot more than the small amplitudecarrier waves.In a correctly functioning system, all the carrier waves are thereforebrought as close as possible.In the system of the invention there are two possibilities for synchronisationwhich lead to two different frame structures. But first of all it is advisable to identify theconstraints.The registered constellation of satellites assures a coverage by at least twosatellites at every point on earth, thus it is necessary to control interference betweensatellites.The most bothersome interference being that coming from satellites at lowelevation, when a satellite at high elevation is used;it is necessary to have coverage by fine beams, in order to establish aconnection outcome compatible with the constraints due to the use of portableterminals. The minimum number of coverage areas is six.Thus the interference of onebeam on another has to be controlled which leads, in TDMA, to the use ofbeam-hopping. Out of six beams, two or three will be switched on simultaneouslywhile leaving one or two beams spaced between the active beams.In the system of the invention only a single frequency band is available: it istherefore necessary to operate alternately. The alternate operation must be possiblea mobile situated adjacent to the subsatellite point and for a mobile situated at thecoverage edge.The previous constraint envisages two frame structures: a) that is to saytransmissions E from the satellite in each of the beams are temporally connected justat the times of reception R. Then the length of the frame is determined by the timedivision duplexing operation of the mobile furthest from the satellite. Supposing thatthe transmission and reception times in each beam are of the same length, theconstraint is expressed by the following inequality:Te 2 Dmax/cF =2 n Ten is the number of transmission times,Te the length of each of these times,20 Dmax is the maximum distance (at the minimum elevation),F the length of the frame.Such a \"short\" frame is shown in Figure 3.It is composed of a succession oftransmit-receive pairs, to form a frame of length Tf.For a given transmission thecorresponding reception is made a frame afterwards.The non-interference conditions of the transmit-receive packets are interpretedgeometrically by the separation of the T/R bands drawn in solid lines in the altituderange included between Dmin and Dmax.In an example particularly advantageous to the system of the invention shownin Figure 4 a group of 2 spots selected from among 6 spots of the satellite areswitched on.The operation is achieved just as well in transmission as reception.In thissystem the geometric shape of the beams 12 has been modified: from circular theyhave become elongated e'liptical.Thus the communication time without hand-over hasbeen appreciably increased;the large axis of the ellipse being arranged parallel tothe direction 15 of the orbital direction.ln this way, as long as the user remains insight of the satellite he is illuminated in a continuous way by the same beamthroughout the connection.The adoption of a limited number of elongated ellipticalbeams thus provides undoubted added value at the system level: the complexity ofseveral sub-systems is reduced (pay load, antenna) and the management of certainfunctions happens to simplified at the global level (management of communicationsand resources). It thus results in a greater flexibility and a greater availability of thecommunications system with the mobile terminals.Advantageously the system of the invention includes satellite constellationswhich belong to the group of constell- ations well-known and listed under the name of\"Walker symmetrical constellations\". (See on this subject the article by J.G.Walkerentitled \"Continuous whole earth coverage by circular orbit satellites\" published in\"Satellite systems per mobile communications and surveillance\"; IEEE conferencepublication 1973). These constellations are symmetrical due to the regular allocation of satellitesin the same orbit, as well as the distribution of satellites in the same orbit and thedistribution and the matching inclination of the orbital planes in space. They havebeen selected because they minimise the number of satellites for a given coverage,12and because they are particularly effective for coverage of a latitude band.A Walker constellation is charaderised by five parameters:Altitude, in this case 1389 km (for life reasons).Inclination.The parameter triplet T/P/F T is the total number of satellites,P is the number of orbital planes,F is the phasing parameter which indicates the relative position of the satellitesin a following orbital plane.To optimise the coverage of the inhabited areas, that is to say includedbetween the equator and 65 degrees of latitude (North und South), a Walkerconstellation (1389 km, 52 degrees, 48/8/1) is needed. The constellation has theadvantage of allowing an optimum coverage of the area, notably from the point ofview of elevation, but it has the drawback, as with all constellations with a largenumber of satellites, of requiring at least two years to set up. The systm in questiontherefore uses two constellations which can be set up consecutively.The adopted constellations are the followingWalker (1389 km, 47 degrees, 24/08/3), which accurately covers Continental USA(CONUS) and southern Europe (typically as far as the latitude of Lille), but which hassignificant holes in coverage below 30 degrees in latitude.Walker (1389 km, 55 degrees, 24/08/3), which accurately covers the rest of theworld and optimises the coverage, notably the elevation, in the countries with latitudesbetween 10 degrees and 60 degrees.Thus at 10 degrees elevation and 1389 km altitude this leads to Te 23. ms that is to say 5 frames MSG.if one chooses n=2, then the minimum frame length is 4 Te, that is to say F 92.3 ms.if one chooses n=3, then the value Te which is suitable for GSM frame is 3. Thisleads to F 83. 07 ms.if one chooses n=6, then the value of Te is one MSG frame, and F 55. 38 ms.the periods are long and it must be noted that as n increases, the capacity of the banddecreases.:which 13b) The other possibility is of alternating transmission and reception over each ofthe beam groups operating simultaneously. The alternate operation leads to twoconstraints, one from the terminals close to the zenith, the other from those which areat the coverage edge. The two disparities which govern these constraints are:for the terminals with the satellite at the zenith:Te Tr 2 Dmin/cfor the terminals at the coverage limit:m (Te Tr) FF 2 Dmax/c where:Te is the illumination period of a transmission beam,Tr is the illumination period of a reception beam,Dmin is the minimum distance (satellite altitude),Dmax is the distance between the satelliie and the mobile at the edge of the area atminimum elevation,m is the number of transmission-reception pairs comprising the frame,f is the frame period.Such a \"long\" frame is represented in Figure 4. In a frame period alltransmissions appear, then the corresponding receptions. The non-interferencecondition is interpreted geometrically by the separation of the T/R bands up to adistance Dmax.In the example shown in Figure the first constraint gives, by supposing Te Tr Te 4. 63 ms, which allows thechoice of:Te 1 MSG frame 4. 615 ms;the second constraint gives:at 10 degrees elevation F 23. 07 ms or F 5. 01 MSG frames, fromm 3 and F 27. 69 msat 20 degrees elevation F 18. 39 ms or F 3. 98 MSG frames, from which m=2 and F 18. 46 ms.In a worthwhile design based on the system shown in Figure 5 asynchronisation method has been selected with a short time frame, F 18. 46 mssince once the constellation is in place in most of the regions of the globe, the:.20 *is 14elevation exceeds 20 degrees. During the start-up phase, a double frame periodcan be selected so as to be able to operate at zero elevation. As the two frames canoperate simultaneously, in the regions at low latitude, which are also regions of lowtraffic, the frame period can be 36. 92 ms. The even terminals and odd terminalsare then differentiated, each operating with a single space frame. The doubleframe system maximises the capacity of the satellites.The one problem which can arise is the transmission of a parity from aterminal at low elevation during reception from a terminal of opposite parity situatedadjacent to the latter. But there are two remedies to the situation in which acalculation of probability indicates the weak frequency. The first is a matter for theallocation of resources by the same station. In effect, the two terminals are inprinciple dependent on the same connection station, since they are close to eachother. The resource allocation algorithm can arrange it so that the even and oddterminals do not have access to the same channels, which in theory resolves theproblem for the vast majority of operating situations. The second is to bring about ahand-over from detection of persistent interference.The structure of the selected frame, seen from the satellite, thus comprises twotransmission-reception intervals, the trans- mission length being equal to that ofreception. The length of a transmission interval is equal to that of a MSG frame, thatto say 4. 615 ms. The lengtn of the base frame is four MSG frames, but anoperating mode with a double period is possible, to take account of an operatingmode at elevation lower than 20 degrees. The structure of the MSG frame istherefore vital, since constrained by the altitude of the satellites. It must be notedhowever that the organisation (temporal arrangement) of the frames cannot beretained like it is, at least in the alternating mode of operation. However,functionally it is possible to have the same channel types, even if the numbering ischanged.The frame rate is retained, just as the modulation and the length of thepackets. The communication channels are kept identical, Therefore with anadditional time-delay, of a few milli-seconds, almost the same procedures can beapplicable.It is agreed that this invention has been described and represented only byway of preferential example and that its constituent components will be able to bereplaced by equivalent components without, for all that, going beyond the scope ofthe invention.a *e a.ee.polar .20 The claims defining the invention are as follows:1. A communications system between satellites in low orbit and terminals,wherein beams of each satellite coverage area are switched on in a spatial scanreferred to as \"beam- hopping\in that for each satellite and for each terminaltrans-mission and reception are synchronised and separated over time; and in that atime-division duplexing is used.2. A communications system according to claim 1,wherein each satellite'scommunication antenna system provides an isoflux coverage composed of severalbeams.3. A communications system according to claim 2,wherein the beams areelliptical beams elongated in the orbital direction of the satellite 4. A communications system according to any one of claims 1 to 3, whereinmode of access to the satellites is a TDMA mode.A communications system according to any one of claims 1 to 3, whereinmode of access to the satellites is a CDMA mode.6. A communications system according to claim 1, wherein the satellites haveorbits.7. A communications system according to claim 1, wherein the satellites haveinclined orbits.8. A communications system according to claim 7, wherein said system uses twoWalker-type satellite constellations.9. A communication system substantially as herein described with reference toFigures 2 5 of the accompanying drawings.DATED THIS EIGHTEENTH DAY OF JUNE 1996ALCATEL N, V.ABSTRACTThis invention relates to a communications system between satellites in loworbit and terminals, in which the beams (12) of each coverage area (14) are switchedon in a spatial scan referred to as \"beam- hoppi ng\". For each satellite and terminal,transmission and reception are separated over l-me and synchronised, a time-divisionduplexing being used.FIGURE 2.0 p so:*.,Sp#us.p %seepso.1 FIG. 111 FIG. 2131312/C C C CS. C CC. C C C C.C C C C CC SCCC. CC *C CCC CC** SC C. p 5 SCCFIG. 3TtDminD maxH U U4147 9jJC CCC Ca.C C a a a a a aa a a a C a C C C a Cbe. S aa a eC a(Ln~Ln-91