WO2008006886A1

WO2008006886A1 - Time-dependent method of synchronizing of a radio communication system

Info

Publication number: WO2008006886A1
Application number: PCT/EP2007/057205
Authority: WO
Inventors: Patrick Bruas
Original assignee: Thales
Priority date: 2006-07-13
Filing date: 2007-07-12
Publication date: 2008-01-17
Also published as: EP2050208A1; FR2903828A1; FR2903828B1

Abstract

The invention relates to a time-dependent method of synchronizing an orthogonal frequency evasion waveform. The method comprises for each station a step of acquiring and keeping the time, the time thus determined being a locally estimated time at each station, and a step of acquiring and keeping the Transit Time between the station and a satellite, the transit time being estimated locally. The moments of transmission by said station are determined as a function of the locally estimated time and the locally estimated transit time. A further object of the invention is a satellite radio communications system by decentralized transit time management. In particular, the invention applies to satellite communications wherein the waveform implements spread spectrum techniques and frequency hopping in general, and more particularly to transparent geostationary satellite communications.

Description

TIME SYNCHRONIZATION METHOD OF A RADIO COMMUNICATION SYSTEM

The invention relates to a method for temporally synchronizing an orthogonal frequency evasion waveform. The invention further relates to a satellite radio system with decentralized transit time management. In particular, the invention applies to satellite communications whose waveform implements spread spectrum and frequency hopping techniques in general, and more particularly to transparent geostationary satellite communications.

A satellite system comprises on the one hand one or more artificial satellites orbiting the Earth and on the other hand a set of ground station or earth station adapted to communicate with the satellites of said system. The ground stations comprise all the equipment necessary to establish a satellite link and in particular one or more modems.

A satellite communication system must provide for each user a network capacity (that is, a portion of the bandwidth and the satellite power that is used to establish one or more communication channels) adapted to the needs and the constraints of each user with respect to system-wide and environment-related constraints. This includes canceling or minimizing network interference. In addition, a satellite communication system must provide adequate protection against electromagnetic interference as needed. A satellite communication system must still have high reliability and / or availability, especially for users. Also a satellite communication system must avoid as much as possible to particularize a station vis-à-vis the waveform, the vulnerability of this station thus jeopardizing all communications network. In conjunction with these operational constraints, the design of a satellite communication system must meet economic constraints, and consequently optimize the simplicity of implementation and as far as possible allow at least partial re-use. of known waveforms. Among the other constraints, it is still possible to mention the security constraints.

In a conventional commercial communication satellite system, ground station modems are not protected, even against a low level of interference or unplanned interference. In this case, even a low level of interference can cause a break in service and compromise the level of availability of the system. On the other hand, the payload of the satellite has optimum performances, in particular in terms of quality factor (G / T) and operational gain, in a context of use without interference.

In a protected communications satellite system, ie a system whose ground stations include protected modems and whose satellites include protected repeaters, the links are protected against any jammer up to a certain level of maximum interference. This type of system uses for this both a waveform protected against electromagnetic disturbances and repeaters protected against electromagnetic interference. The minimum repeater protection is provided by a hard limiter device on board the repeater which closes any jammer too powerful and protects the power amplifier on board. As for the protected waveform, it may in particular be a spread spectrum waveform whose codes are orthogonal. In return, the orthogonality requires a very precise synchronization of all the carriers arriving in the same band of spreading at the level of the satellite, which implies the implementation of procedures of acquisition and maintenance of synchronization between the various stations on the ground which are complex and centralized compared to a time master. In addition, the time master station is the vulnerable link in the network because its unavailability causes the communication network to stop. The secure payload of the satellite is also an expensive investment. In addition, the protection of the payload is at the expense of its performance.

The purpose of the invention is in particular to overcome the aforementioned drawbacks. For this purpose, the subject of the invention is in particular a time synchronization method for stations using an orthogonal frequency evasion waveform for satellite communications, characterized in that it comprises for each station:

• a step of acquiring and maintaining the time; A step of acquiring and maintaining the transit time between the station and a satellite, the transit time being estimated locally and autonomously; the times of transmission by said station being determined according to the time and transit time estimated locally

In one embodiment, the waveform comprises stages whose transmission instants and the instantaneous frequency band are determined by a pre-established coding law, the moment of transmission of a bearing by a station being determined. according to the pre-established coding law, the estimated time and the estimated transit time locally.

The transit time acquisition and maintenance step can be a local estimate at any instant of the transit time depending on the ephemeris of the satellite, the geographical position of the ground station and a common reference network hour. The times of emission by said station can then correspond substantially to the common reference network time to which is subtracted the estimated Transit Time.

The invention also relates to a satellite radio system with decentralized transit time management. It comprises at least one radiocommunication satellite, a ground segment comprising a set of ground stations. The ground segment implements a protected waveform with orthogonal frequency evasion. Each ground station implements transit time management procedures between said ground station and the satellite and a necessary time management procedure. observing the orthogonality criterion of said waveform, the transit time of each station being determined locally and autonomously at each station.

In one embodiment, each ground station implements a step of acquiring and maintaining the time, the time thus determined being a time estimated locally at each ground station and a step of acquisition and maintenance Transit time between the ground station and the satellite. The transit time is estimated locally. The times of emission by said ground station are determined according to the locally estimated time and the estimated transit time locally. The waveform is for example used for links via a satellite between ground stations exchanging traffic. The waveform may include steps whose transmission instants and the instantaneous frequency band are determined by a pre-established encoding law, said waveform being orthogonal. The moment of emission of a landing by a ground station is then determined according to the pre-established law of coding, the time estimated locally and the transit time estimated locally.

The advantages of the invention include that it significantly improves the service availability of the system. In addition, the invention makes it possible to reduce the time of entry into the network of ground stations and to reduce the bandwidth requirement of network management information. The invention also makes it possible to obtain radio silence without losing the orthogonality of the waveform.

Other features and advantages of the invention will become apparent from the description which follows, given with reference to the appended drawings which represent:

FIGS. 1a and 1b, a satellite system protected against electromagnetic disturbances according to the state of the art, as well as an example of a frequency evasion spreading code and the associated time-frequency law implemented by said protected system; • Figure 2, by a block diagram, a satellite radio system with decentralized transit time management according to the invention;

FIG. 3, by a block diagram, a time synchronization method of an orthogonal frequency evasion waveform according to the invention.

FIGS. 1a and 1b illustrate a satellite system protected against electromagnetic disturbances according to the state of the art, as well as an example of a frequency evasion spreading code and the associated time-frequency law implemented by said system protected. The satellite system according to the state of the art comprises a satellite 15. The satellite 15 may be a geostationary satellite comprising one or more transparent repeaters. These transparent repeaters can also be protected against electromagnetic disturbances. The satellite system according to the state of the art still comprises ground stations 10, 20. The ground stations 10, 20 comprise in particular electromagnetic transmission and reception means and modems for generating and decoding forms of the signal. Protected waves against electromagnetic disturbances. It can especially be spread spectrum waveforms, such as the waveform designated by the acronym OFHMA for Orthogonal Frequency Hopping Multiple Access. The ground stations 10, 20 can establish communication links between them via the satellite 15 to exchange traffic. FIG. 1a shows a diagram showing a frequency-evading spreading code used by the waveform used to protect the links between the ground stations 10, 20. The diagram comprises an axis of abscissae 1 representing the time while the ordinate axis 3 represents the frequency. In the diagram as shown, there are three traffic carriers. Thus, are represented three different codes, also called bearings, 2a, 2b and 2c corresponding to three different traffic carriers. For example, emissions from a first ground station 10, 20 may use the codes or landing 2a, emissions from a second ground station 10, 20 may use codes or 2b and 2c. Each level 2a, 2b, 2c occupies at a given moment a band of a distinct frequency included in the set of an allocated frequency band called BE spreading band. At a given moment, there is no recovery of the frequency band used by the different pallies 2a, 2b, 2c. The bearings 2a, 2b, 2c have a fixed duration depending on the characteristics desired for the waveform. At the end of this period, and after a duration TP called bearing hole, new levels 2a, 2b, 2c meeting the same criteria as those described above are assigned. However, each level 2a, 2b, 2c does not necessarily occupy the same instantaneous frequency band. A pre-established encoding law is used to determine the instantaneous frequency band Bl occupied at each instant by the bearings 2a, 2b, 2c. Thus, each ground station 10, 20 establishing a link via the satellite thus codes the signal comprising the traffic to be transmitted according to such a predetermined coding law. Each ground station 10, 20 for which the traffic is intended must decode the signal received according to such a predetermined code-coding law. Therefore, since the coding law depends not only on the frequency but also the time it is absolutely necessary that all the ground stations 10, 20 sharing the same pre-established coding law be synchronized. The orthogonality criterion of the waveform is a temporal accuracy criterion of the levels 2a, 2b, 2c. Also, the set of bearings 2a, 2b, 2c must be received by all the stations on the ground sharing the same preset coding law in a synchronous manner with a precision Δh less than the duration TP of the bearing holes. Also, if it is desired to enhance the capacity of the waveform with respect to electromagnetic disturbances, it is desirable to increase the number of steps 2a, 2b, 2c per second, and reduce the duration TP of the holes of bearings. However, the shorter the TP duration of the bearing holes, the more difficult the orthogonality criterion to fulfill. Orthogonality requires very precise synchronization of all the carriers arriving in the same BE spreading frequency band at the satellite 15. This problem is solved in the satellite communication systems of the state of the satellite. art, by the implementation of procedures for acquiring and maintaining synchronization between the different ground stations 10, 20. Thus a long loop supported by links 11, 12 slaves the time systems present in each ground station called slave 10 to a centralized reference time system in a so-called master ground station. As a result, although this system achieves very high resistance to electromagnetic interference, radio time synchronization procedures and network entry procedures are complex or time-consuming because of centralized waveform of master-slave type. In addition, this solution consumes space resources.

FIG. 2 illustrates a satellite radio system with decentralized transit time management according to the invention. The system according to the invention comprises a constellation of radiocommunication satellites 30. In the description and in FIG. 2, only one satellite is represented. However, the invention applies to any satellite system having as many satellites as necessary. The satellite 30 does not necessarily include a transparent repeater protected against electromagnetic interference. In addition, the satellite 30 may be an ordinary geostationary radiocommunication satellite, operated for example by a telecommunication operator. The ground segment comprises a ground station assembly 31. The ground segment uses a protected waveform with an orthogonal frequency evasion. Frequency hopping speed can optionally be reduced with respect to the hopping rate of a protected system according to the state of the art. Unlike state-of-the-art protected systems, the time and synchronization management procedures necessary to comply with the orthogonality criterion are not centralized on a master ground station 20. According to the invention, each ground station 31 implements autonomously and therefore decentralized TT transit time management procedures between said ground station 31 and the satellite 30. As for the time management procedure, it is either decentralized or centralized .

In one embodiment, the time management is decentralized.

This means that it is the responsibility of each ground station 31 to acquire and maintain its synchronization of time and transit time. The time can then be obtained either locally, for example by means of a GPS / GNSS device or an atomic clock, or by radio at the initiative of the ground station 31 automatically and transparently for the operator who interrogates one of the stations of the network and obtains the time by radio by a bilateral procedure which compensates for the propagation time.

In another embodiment, time management is centralized. This means that it is the responsibility of each ground station 31 to acquire and maintain its transit time synchronization but it is the responsibility of a master clock station 31 to guarantee the time synchronization of all the stations at the same time. ground 31 of the ground segment. The time is broadcast by a master synchronously or asynchronously in the form of a permanent or periodic beacon. The time master must compensate for his transit time to emit his beacon. The stations on the ground 31 copy the time of the station to the master hourly ground, tainted with two biases: the estimation error of transit time amount and the estimation error of downstream transit time. Regardless of the time management used, the transit time is calculated locally, for example by virtue of the three input parameters of an orbitography calculation algorithm known to those skilled in the art:

The ephemeris of the satellite 30, initial data which can be refreshed every month for example; The geographical position of the ground station 31 in a given geodesic landmark;

• Universal Time UTC, which can be deducted from the time of the station.

FIG. 3 illustrates a time synchronization method of an orthogonal frequency evasion waveform according to the invention. The method according to the invention comprises, for each station on the ground 31, a step 40 for acquiring and maintaining the time and a step 41 for acquiring and maintaining the TT Transit Time between a ground station 31 and a satellite These steps are implemented continuously within each ground station 31 in order to permanently update said parameters. Step 40 of the procedure for acquiring and maintaining time can be set by the means illustrated in FIG. 2. At the end of step 40, the ground station 31 has an estimated time local to any time, however, suffering from some maximum uncertainty +/- ΔT compared to the reference time network (which is defined at the satellite level). At the end of step 41, the ground station 31 has the locally estimated Transit Time between said station 31 and the satellite 30. The locally estimated transit time at all times suffers from a certain maximum uncertainty +/- Δτ compared to the real instantaneous transit time. The value of these two maximum uncertainties ΔT and Δτ is dimensioning to calculate the temporal dispersion tolerance +/- σ of the arrival times of the bearings 2a, 2b, 2c uplinks of the network on the satellite. Indeed, this dispersion value 2σ must be increased by the duration of the bearing hole TP so as to cancel the self interference of the protected waveform. The state of the art gives a certain ratio between the duration of the stage and the duration of the dead time at the beginning of the stage (which is the sum of several elementary times: frequency change time, downward and upward shaping, possibly ramp-up time of the amplifier). Consequently, the maximum allowed jump speed will be a function of the maximum temporal synchronization uncertainties (time and transit time). From the local estimation of the transit time and the time, and according to the pre-established law of coding of the waveform, each station can then transmit the local level 2a, 2b, 2c at the instant planned. In one embodiment, the step 41 for acquiring and maintaining the Transit Time TT is a local estimate at any instant of the transit time knowing the ephemeris of the satellite 30, the geographical position of the ground station 31 and one hour Href reference common network. This local estimate makes it possible to transmit the local level 2a, 2b, 2c at the instant Href-TT so that all the levels 2a, 2b, 2c arrive synchronously on the satellite 30 at the time

Href +/- σ with a compatible tolerance σjnax of the bearing hole TP of the waveform. The tolerance σjnax is a function of the following 5 inaccuracies: i. local uncertainty about Href's knowledge; ii. local uncertainty about geographical position; iii. inaccuracy of the satellite ephemerides used to calculate the TT transit time; iv. bias introduced by the transit time calculation method TT which corresponds to a modeling of the trajectory of the orbit (it is often a Keplerian ellipse, whereas the true plane of the trajectory moves under the effect of the non-centripetal terrestrial gravitational field due to the flattened shape of the geoid); v. variation of the TT transit time since the last refresh time of the calculation up to the instant of use of the transit time estimate TT to transmit.

The first item i) of uncertainty depends on the type of time management, centralized or decentralized. For example, for decentralized time management based on GPS receiver or equivalent, an accuracy of +/- 1 μs can be achieved. If we want to get rid of GPS, we return to a time distribution by radio whose accuracy can vary from 1 microsecond to 10 microseconds depending on the procedure used. The second item ii) of uncertainty is between 1 and 10 km, ie an uncertainty of 3.3 μs to 33 μs at the edge of the zone. For a fixed platform, the geographical accuracy of 1 km is easily reached, thanks to any method of localization. The third item iii) of uncertainty is between 500 m and 4 km, ie an uncertainty of 1.6 μs at 13.2 μs. Indeed, the satellite ephemeris can predict the position of the satellite 30 at any time and with a Delta_distance uncertainty of a few km typically. This depends on the inclination of the satellite, the orbital parameter refresh period, the initial ephemeris bias, the presence or absence of a satellite station keeping maneuver by the SCS (Satellite Control Station) and the celestial mechanics calculation algorithm used to derive the satellite-earth station distance, all else being known (geographical position of the station in a given geodetic coordinate system, current UTC time). The fifth position v) of uncertainty slides at a rate of 1 ns / s / degree of inclination / degree of latitude in the center of the "eight" of the satellite 30. Indeed the maximum variation of the sinusoid of TT of period 24 hours is 81 ns / s for an inclination of 3 ° and for a maximum latitude of 81 ° is the limit of optical visibility in the meridian of the satellite. If one refreshes the TT transit time calculation which serves to enslave the time of emission Hemi, then the drift is worth to the maximum, in limit of overall cover and for 3 ° of inclination, 2 μs. The orthogonality constraint requires that twice the sum of the above five positions does not exceed the duration of the bearing hole of the waveform. In one embodiment, the temporal synchronization method of a waveform according to the invention is used for the synchronization of a waveform that meets the ETSI DVB-RCS standard (according to the English expression). Digital Video Broadcast - Return Channel System). The DVB-RCS standard defines a return path. This return path can then be supported by an orthogonal frequency evasion waveform, in order to share the rising capacity.

Claims

A method of time synchronization of stations using an orthogonal frequency evasion waveform for satellite communications, characterized in that it comprises for each station (31):

A step (40) for acquiring and maintaining the time; A step (41) for acquiring and maintaining the Transit Time (TT) between the station (31) and a satellite (30), the transit time (TT) being estimated locally and autonomously; the transmission times by said station (31) being determined according to the time and transit time (TT) estimated locally.

2. Method according to claim 1 characterized in that the waveform comprises bearings (2a, 2b, 2c) whose transmission instants and the instantaneous frequency band (Bl) are determined by a predetermined law of coding, the moment of emission of a bearing by a station (31) being determined according to the pre-established law of coding, the estimated time and the transit time (TT) estimated locally.

3. Method according to any one of claims 1 to 2 characterized in that the step (41) for acquiring and maintaining the Transit Time (TT) is a local estimate at any time of the transit time according ephemeris the satellite (30), the geographical position of the station (31) and a common reference hour network (Href).

4. A satellite radio system with decentralized transit time management characterized in that it comprises at least one radiocommunication satellite (30), a ground segment comprising a set of ground stations (31), the ground segment implementing an orthogonal frequency evasion protected waveform, each ground station (31) implementing transit time management (TT) procedures between said ground station (31) and the satellite (30) and a time management procedure, necessary to comply with the orthogonality criterion of said waveform, the transit time of each station being determined locally and autonomously at each station

5. System according to claim 5 characterized in that each ground station (31) implements a step (40) for acquiring and maintaining the time, the time thus determined being a time estimated locally at each station at (31) and a step (41) for acquiring and maintaining the Transit Time (TT) between the station (31) and the satellite (30), the transit time (TT) being estimated locally, the instants of emission by said ground station (31) being determined according to the locally estimated time and the locally estimated transit time (TT).

6. System according to claim 6 characterized in that said waveform is used for links via a satellite (30) between ground stations (31) exchanging traffic, said waveform comprising bearings (2a, 2b, 2c) whose transmission instants and the instantaneous frequency band (Bl) are determined by a pre-established encoding law, said waveform being orthogonal, the instant of emission of a bearing by a station at sol (31) being determined according to the pre-established encoding law, locally estimated time and locally estimated transit time (TT).

7. System according to any one of claims 6 to 7 characterized in that the step (41) for acquiring and maintaining the Transit Time (TT) is a local estimate at any time of the transit time according ephemeris the satellite (30), the geographical position of the ground station (31) and a common reference hour network (Href).

8. System according to claim 8 characterized in that the transmission instants by said ground station (31) substantially correspond to the time common reference network (Href) to which is subtracted the estimated Transit Time (TT).