AU645905B2 - Low orbit communications satellite system for mobile terminals - Google Patents

Description

64590 Regulation 3.2

AUSTRALIA

Paten ts Act 1990 ten C. 0 C C

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ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT "LOW ORBIT COMMUNICATIONS SATELLI.TE SYSTEM FOR MOBILE TERM INALS" The following Statemoent is a F1.ll (leSerip6ion Of tis invontion, including thc bost mdchod of performing it known tV us:- This invention relates to a low orbit communications satellite system designed for mobile terminals.

Until now, satellite orbits or elliptical orbits strongly inclined, both sharing the property of being located, on average, above zones in space with high particle concentration, called "van Allen belts". Recently, lower orbits have been considered.

Their altitude being between 800 and 2000km. One of the characteristics of communications satellite systems using such orbits is their possibility to communicate with a large number of mobile terminaiss, portable terminals for instance. But the poor radio-electrical performance of these terminals requires to provide for compensation which is obtained from higher satellite radio-electrical performances.

The difference between orbits at an altitude higher than the "van Allen belt" and those at a lower altitude is space attenuation, the closer the satellite is to the earth, the weaker the attenuation. Nevertheless, it is possible to compensate space attenuation by selecting the satellite antenna size accordingly. Nevertheless, this compensation is limited. For communications with mobile terminalss, these limits are exceeded. Low orbit satellite antennae provide lower gains; due to the geometry of communications satellite systems using these orbits, these antennae must compensate path variations much higher than those occurring with higher orbits.

A CCIR report (Document No. US IWP 8/14-52, 1 August 1990) titled 'Technical characteristics of a personal communication mobile satellite system" describes a low orbit communications satellite system with multibeam antennae; each comprising 37 conical beams. A major drawback of this system is the large number of beams, each of them forming a small trace on earth, Furthermore, due to users' mobility and satellites flyby, beam changeover may occur as time goes by. These are generally accompanied by a hand-over. Their significant number during a conversation is detrimental to the quality of the connection and to easy listening.

An object of the present invention is to eliminate these drawbacks by describing a communications system which can dramatically improve satellite capacity.

According to the invention there is provided a low orbit communications satellite system for mobile terminalss, wherein the communications antenna system of each satellite provides isoflux coverage formed by several elliptical beams elongated in the flyby direction of the satellite.

In an advantageous implementation, the beams in each coverage are illuminated according to spatial scanning referred to as beam hopping; transmission and reception being synchronised for each satellite and for each mobile terminals.

In another advantageous implemcntation, a time-division duplexing is used; transmission and reception being separated in time and synchronised for each satellite and for each mobile terminals.

Such a system offers the advantage of increasing the capacity of a satellite weighing approximately 500 kg from 60 channels (geostationary) to over 5000 channels in low orbit. Moreover, this communications system has a simple configuration.

It operates with polar orbits and inclined orbits. With regard to inclined orbits, it limits interferences irrespective of the type of access used. Finally, it improves the operation of antennae for each satellite.

The satellites used in the system according to the invention offer the advantage of being capable of carrying a payload including: an antenna linking a fixed station for connection to the earth network; a duplexing circuit separating transmission from reception, as well as •polarisation directions;

S.,

S 15 a set of circuits for the reception of signals issued from earth network connection stations; a set of filters and frequency translation circuits for translation to the satellite link frequency band and to the mobile terminals; this set also controls the gain in the .transmission chain to the mobile terminaiss; a multiport amplifier; a set of communication antennae directed to the mobile terminalss; a set of receivers and mixers for translation of the mobile/satellite link frequency to the satellite/earth network connection station frequency; a frequency and/or time division multiplexing circuit; a power amplifier circuit for the satellite/connection station link, which is also fitted with an output filter per polarisation direction.

The characteristics and advantages of the invention are highlighted in the following description given as a non-limiting example, to be read in conjunction with the attached figures, where: Figure 1 illustrates the coverage in a communications satellite sys:em according to the known technology.

Figure 2 illustrates the communications satellite system in accordance with the invention.

Figures 3 to 6 are diagrams illustrating the operation of the co'nmunications satellite system in accordance with the invention.

Figure 7 illustrates the payload of the satellite in the communications system in accordance with the invention.

Figure 8 illustrates the performances of the satellite antenna in the communications system in accordance with the invention.

In a communications satellite system, there are two possible classes of low orbit: Polar Orbits. Their plane passes via the poles. (Or quasi-polar to take into account synsynchronous orbits, ie. their plane remains fixed in space). Normally, these orbits have the property of guaranteeing permanent and global earth coverage.

Inclined Orbits. Their plane is at an angle, in practice lower than 600, with the plane of the equator. The permanent coverage is then formed of two boundary bands parallel and symmetrical to the equator.

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o Each type of orbit has some points intersecting the orbit planes. With polar orbits, the intersection zone is near the poles. With inclined orbits, this zone is near the equator. Furthermore, a satellite service zone is defined by a geometrical condition: the set of places on earth from where the satellite is seen with an elevation (angle defined by the user/satellite direction and the plane tangent to the earth at the location of the mobile terminals) greater than a preset value (the practical value being between 100 and 150).

S Both types of orbits have the same property: the service zones of each satellite overlap at different instants or locations: With polar orbits, it is towards the poles that the service zones of each satellite overlap a little.

With inclined satellites, the description of the overlap phenomenon is more complex, but in some zones it may reach 100%. There arc even constellations of satellites which, in certain zones, ensure quadruple coverage.

Such a property is an advantage since it allows to set up communications with at least two satellites in most cases. The design of the system in accordance with the invention takes into account multiple coverage to prevent interferences between coverag interference is characterised by the superimposition of several signals, one of them is the "desired" signals, the others are interfering and may impair or prevent good reception of the desired signal. The method of access to these low orbit satellites also takes into account the interference problem.

In a communications satellite system from the known technology, such as previously defined, on earth a coverage zone is formed from several beams (10) as shown in Figure 1; the useful coverage obtained being zone Such coverage has several drawbacks: in the path satellite/mobile, it contains several zones where interferences are very high. Whether operating in TDMA (Time Division Multiple access) or in DCMA (Code Division Multiple Access), these zones dictate the dimensions, ie. they greatly contribute to the size, weight and cost of the satellite considered. Furthermore, if the orbits are inclined, these interferences may lead to the link being cut off for several tens of sections. Moreover, such a coverage with fine beams requires relatively frequent hand-overs. In a system with a significant number of beams, these lead to a hand-over every minute for instance, The processing load then required on the ground is far from negligible.

Furthermore, whether operating in CDMA where the near-far effect may be significant, or in TDMA/FDMA or in FDMA (Frequency Division Multiple Access), where the detrimental effect the high amplitude carriers have on lesser amplitude I carriers is known, it is desirable to have an antenna gain leading to the ground at a power received (per unit of surface) as uniform as possible.

As for the mobile earth communications systems, the satellite system in accordance with the invention, as shown in Figure 2, is a cellular system in which larger cells are defined by the ground track of different coverage beams (12) from each multibeam satellite On a radioclctrical point of view, a cell is characterised by a set of resources (frequencies, time slots, codes) from which the mobile terminals extracts an element during call set up.

The system in accordance with the invention is a system with 6 beams for instance, instead of 37 as is the case of the system described in the previously mentioned CCIR report. The number of hand-overs is therefore automatically divided by the ratio of the beam number, ie. by a factor of 6 approximately. To ensure identical global coverage each of the beams (12) must therefore ccver a greater earth surface, which results in the definition of a single and small size antenna (16).

Moreover, in this system the geometrical shape of the beams (12) has been modified: from circular they have become elongated elliptical as as shown in Figure 3. Communication time without hand-over has thus been significantly increased; the large axis of the ellipse having been placed parallel to the satellite flyby direction Therefore, as long as the user remains visible to the satellite, he is continuously illuminated by the same beam all along the communication.

Selecting a reduced number of elongated elliptical beams is therefore a true bonus at system level: it minimises the complexity of several sub-systems (payload, antenna) and the management of certain functions is thus simplified overall (communications and resources management). Therefore, this leads to a greater flexibility and availability of the mobile communications system.

Moreover, the system in accordance with the invention may be used in conjunction with time division multiplexing and beam-hopping techniques: the N-spots in the earth coverage are sucessively and sequentially illuminated to form groups of P-spots among the N-spots.

In the example shown in Figures 2 and 3, we successively illuminate a group of 2 spots (18) among the 6 satellite spots, and sequentially according to 6/2=3 time pitch. This operation is performed during transmission as well as during reception, which naturally leads to a transmission frame including a total of 6 time slots (3 for transmission to the mobile and 3 for reception by the satellite) as shown in Figure 6.

Indeed, with multibeam systems there are two ways of eliminating interferences between neighbouring beams connected to the same satellite: each of the beams is permanently illuminated and it is possible to limit interfercnce between beams thanks to a frequency re-utilisation diagram, Then, all the available band is not used in one beam.

all the available band is used in one beam. The technique used to eliminate interference between neighbouring beams is spatial scanning or beam-hopping. The beams being sufficiently fart apart in space to provide acceptable mutual interference levels are simultaneously illuminated, With regard to limiting interference between beams of different satellites: when the paths of two or several satellites cross or get closer, their ground tracks overlap more or less in part. These events occur quite frequently in a multibcam system.

S: Furthermore, additional interference may result from transhorizon propagation phenornena. In this case, this random interference can only be limited by the beam hopping technique. This spatial scanning technique combined with the use of elongated elliptical spots is therefore the best solution for this type of interference. Furthermore, the elongated shape of the beams (12) helps reduce the overlapping surface of the tracks from the various satellites, as shown in Figure 3. Moreover, this solution has the advantage of being compatible with the various useable transmission diagrams and leads to a significant simplification of the anicnna.

Among the existing access modes, several modes make up a judicious compromise between demodulator performance (number of channels) and complexity (hence cost). They consist of modes using either frequency division of signals (FDMA), or time division (TDMA) or code division (CDMA), or hybrid modes: a combination of CDMA-TDMA for instance.

The most advantageous access modes are those which are compatible with the modes used by the cellular earth networks. There are three of these modes.

Frequency division multiple access (FDMA). This mode uses frequency duplexing. It requires four frequency bands to set up a connection: two frequency bands to set up the connection between the mobile terminals and the satellite and the fixed stations in ,he earth network (connection links).

A capacity slightly lower than 40 carriers per NMHz and per fine beam (half rate GSM type, or coded speech signal at 4800 bit/second) can be achieved.

S Time division multiple access (TDMA). This mode has the peculiarity of increasing the rate such that a given user can only access the satellite during a short preallocated period. In the system in accordance with the invention, we use several carriers per frequency band such that the rate is noi too high. The rate selected is that of the ground cellular network which is complemented by the satellite system.

For instance, in Europe we preferably choose the rate of the GSM network (ETSI European standard), while in the United States we choose that of the DAMPS network (USA digital standard).

With this type of access, the frequency band used by each carrier being greater than the Doppler effect, we use beam hopping. Nevertheless, beam hopping implies synchronisation between transmission and reception, with regard to the satellite as well as the mobile. Depending on the frequency bands available to the mobile, scveral solutions are then possible: In a typical case whereby two frequency bands arc available for mobile terminals-satellite routes, it is possible to simplify the terminal structure by using a transmission and a reception always separate in time (this technique is called time duplexing and abbreviated as TDD). The principle of the then selected access is as follows: to set up a call, a frequcntial resource is allocated (selection of a carrier frequency), then within this resource is allocated (selection of a carrier frequency), then within this resource transmission moments are defined. Synchronisation must be guaranteed by the terminal and the connection station. It first occurs in a specific channel, then in the transmission channel, where the change of transmission moment occurs by increment.

When dealing with a single frequency band for mobile terminals-satellite connections, TDD operation is compulsory for mobile terminalss and satellite. This leads to a particularly simple satellite payload. Transmission synchronisation of the mobile terminals occurs using first a specific channel, then using a closed loop procedure which either increments or decrements the transmission moment.

In this case, as in the previous one, centralised frequency management of the resources is used to limit interferences. Moreover, it is not excluded that quick hand-overs may have to be carried out. But the system in accordance with the invention is designed to complement an earth network which is already fitted with these functionalities. This management is based on the following principle: once the satellite is at a certain altitude, the geographical regions where interference is likely to be encountered are limited. In these regions alone there is sharing of spectral resources. In all others, a mobile terminals can access the entire spectrum. Nevertheless, it is possible to find a remedy for the interference without resorting to an entirely centralised management of the system by using a slow frequepcy jump; such that if interference occurs, it will only be for a short time.

The capacity which is then possible to reach is slightly smaller than 35 carriers per MHz. (This capacity seems lower than that mentioned with the FDMA. But the fact that there is only one frequency band must be taken into account: in fact this :type of system has a capacity nearly double), The main advantage of this type of system is that it makes it possible to use a very simple satellite payload.

Code Division Multiple Access (CDMA). This mode, also called "spectrum spreading" offers a decentralised solution to interference problems. Indeed, spectrum spreading allows for superimposition of several carriers coming from one or several satellites. This mode can be used cither with an FDD type access (transmission and reception having different frequency bands), or a TDD type access. Figures 4 and diagrammatically illustrate the amplitude/frequency curves with two possibilities for TDD access; depending on whether the access occurs with time division or code division of the carriers. Indeed, nothing prevents the use of a TDMA or a CDMA access.

In the case of two frequency bands for mobile terminals-satellite connections, both types of access are possible, FDD or TDD. A TDD type solution makes it possible to reduce the rate of interferences coming from multiple coverage in the case of inclined orbits. Indeed, when coverages are superimposed, there is local downgrading of the capacity, which may be compensated by a power control device. Such a device is mainly useful on the satellite-mobile terminals route. It makes it possible to guarantee each user a minimum quality in the communication, Indeed, when dealing with multiple coverage, some users are penalised by two powerful an interference. Conversely, if this interference is not too significant, it is possible to increase the satellite power aimed at these users: the resulting total power increase is generally minimum. But it has an impact on the quality of the connections of the other users who then see their interference power increase. Therefore, the use of a power control device has its limits which must not be exceeded.

As with TDMA, it is possible to operate with a single frequency band. In this case, the method of access is of the TDD type. Nevertheless, spectrum spreading poses a few specific problems. Demodulation of the signals spectrally spread assumes that the receiver is capable of finding the time reference which was used for transmission. Two methods can then be used: either restore the time reference from the signals received: but the use of long codes, required due to the number of users simultaneously present on the system, makes this technique very complex to use; or I store the time reference in memory then, from estimates of the variations that may 15 have occurred between reception of two packets, restore it during reception.

The main problem with TDD access is the initial acquisition of transmission synchronisation. This synchronisation is first carried out in an open loop then in a closed loop using the station controlling the network. First, a mobile terminals acquires the network signalling channel, Then, if it needs to transmit, it sends an initial message, the reception of which helps define the time shift to be applied for perfect synchronisation. This looped synchronisation being carried out and the mobile being synchronised, sync tracking and control is obtained by measurement of this synchronisation made by the earth station interfacing with the switched telephone networks. Nevertheless, there is a particular simple case of TDD operation whereby S 25 this method need not be applied, and where only signal reception by the mobile terminals is sufficient to supply the sync information, this is the case where only the mobile terminals operates in TDD.

In this case, the mobile terminals operates in dual frequency but in half duplex.

As soon as it receives a signal from the satellite, it transmits. On the satellite side, due to the differences in distance, it is not possible to receive all signals from the terminals in the same beam simultaneously.

Satellite transmission is framed with beam hopping, ic. it alternately occurs by half the number of beams, each transmitting beam being separated from the other by a non-transmitting beam. At the next time pitch, the reverse occurs. Nevertheless, satellite reception can only be permanent due to time dispersion resulting from distance dispersion. Code access is sensitive to amplitude dispersion of the various carriers on the same frequency. Perfectly orthogonal codes (Walsh-Hadamard codes for instance) and perfectly synchronised are used to neutralise this effect. Nevertheless, even though synchronisation may be very good, it can never be perfect with low orbit satellites. The shape of the coverage obtained with the satellite antennae is therefore very important if near-far problems are to be avoided. In other words, high amplitude carriers create a lot more interferences than low amplitude carriers. In a correctly operating system, all carriers are brought to the closest possible level.

Conversely, when only a single frequency band is available, it is necessary to operate in half duplex. This mode of operation has already been described. Figure 6, showing the operating principle of single frequency TDD for a distance from the satellite in relation with time for a transmission period (TI) and a reception period describes the mode of operation of the system when the latter is associated with beam hopping; in this instance, the coverage being achieved by six elliptic beams two beams (18) being simultaneously switched on. The frame has been designed, without it being a specific constraint, such that the maximum duration of the path is the one corresponding to the transmission of one beam. Nevertheless, it can be demonstrated that this duration is limited (to approximately twice the duration of the maximum path) if the frame is to be used in an optimum way.

In the system in accordance with the invention, the payload can be designed very advantageously, as shown in Figure 7 block diaram. This figure indicates two possible payloads, single or dual frequency, for mobile terminals-satellite links.

This payload includes: an antenna (20) linking a fixed station for connection to the earth network. In most cases, it will be a horn. Nevertheless, another type may be used if need be.

a duplexing circuit It separates transmission from reception, as well as polarisation directions.

a set (22) of circuits for the reception of signals issued from earth network connection stations.

a set (23) of filters and frequency translation circuits for translation to the satellite link frequency band and to the mobile terminalss. This set also controls the gain in the transmission chain to the mobile tcrminalss. When beam hopping is required, swi'ching occurs within this set. It may also contain the ultra stable time base required for the operation of the system. This time base controls the supply of the power amplifiers, which, when the satellite operates in TDD, are switched on then off at appropriate times to save energy.

a multiport amplifier It typically includes N-inputs, a Butler type divider matrix or other type, P-amplifiers, a combiner matrix inverse to the input matrix.

a set (25) of communication antennae. These antennae are in a plane network, with direct radiation to the mobile terminalss. Before each inlet, a filter and a circulator are placed such that the antenna can be used for transmission and reception when it has to operate in the same frequency band. Otherwise, either the antenna is double, or its radiating elements are dual frequency.

a set (26) of receivers and mixers for translation of the mobile/satellite link frequency to the satellite/earth network connection station frequency.

a frequency and/or time division multiplexing circuit (27) depending on the case considered. All signals are combined therein before being transmitted in one polarisation direction.

a power amplifier circuit (28) for the satellite/connection station link, which is also fitted with an output filter per polarisation direction.

The curves ,'hown in Figure 8 illustrate the antenna performance viewed from the satellite for a coverage as shown in Figure these curves being isoflux with a common reference which is the earth centre; the gain (in dB) shown 5.0; being reduced by the attenuation due to the distance. Only three beams are shown in this figure, the other three (not shown) can be obtained by symmetry in relation to a vertical axis of symmetry.

It is of course understood that the present invention was described and illustrated using a preferred example only and that its components may be replaced with equivalent components without, in any way, departing from the context of the invention.

Claims (11)

1. A low orbit communications satellite system for mobile terminalss, wherein the communications antenna system of each satellite provides isoflux coverage formed by several elliptical beams elongated in the flyby direction of the satellite.

2. A communication system as claimed in claim 1, wherein the beams in each coverage are illuminated according to spatial scanning referred to as beam hopping; and in that, transmission and reception are synchronised for each satellite and for each mobile terminals.

3. A communication system as claimed in claim 1 or 2, wherein time-division du- plexing is used; and wherein transmission and reception are separated in time and synchronised for each satellite and for each mobile terminals.

4. A communication system as claimed in any one of claims I to 3, wherein trans- mission and reception use different frequency bands.

5. A communication system as claimed in any one of the preceding claims, wherein 15 access to the satellites occurs in FDMA mode.

6. A communication system as claimed in any one of claims 1 to 4, wherein access to the satellites occurs in TDMA mode.

7. A communication system as claimed in any one of claims 1 to 4, wherein access to the satellites occurs in CDMA mode.

8. A communication system as claimed in claim 1, wherein the satellites have a polar orbit.

9. A communication system as claimed in claim 1, wherein the satellites have an inclined orbit.

10. A communication system as claimed in claim 1, wherein the satellites have a payload including: an antenna linking a fixed station for connection to the earth network. a duplexing circuit which separates transmission from reception, as well as polarisation directions. a set of circuits for the reception of signals issued from earth network con- nection stations. a set of filters and frequency translation circuits for translation to the satellite link frequency band and to the mobile terminalss, this set also controlling the gain in the transmission chain to the mobile terminalss. a multiport amplifier, a set of antennae towards the mobile terminalss. 13 -a set of rcccivers and mixers for translation of thc mobilo/satellite link fre- quency to the satellite/earth network connection station frequency. a frequency and/or time division multiplexing circuit. a. power amplificr circuit for the satcllitc/connicction station link, which is also fitted with an output filter per polarisation direction.

11. A. low orbit communication satellite system, substantially as herein described with reference to Figures 1-8 of the accompanying drawings. DATED THIS SE2VENTH- DAY OF MAY 1992 ALCATEL N. ABSTRACT The present invention relates to a low orbit communications satellite system de- signed for mobile terminals; the com-munications antennia system.- of each satellite provides isoflux coverage formed by several elliptical beams elongated in the flyby direction of the satellite. Applicable to the space telecommunications field. Figure 2. 0* 00*0 a S #Se 0 0.