Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B7/00—Radio transmission systems, i.e. using radiation field
  • H04B7/14—Relay systems
  • H04B7/15—Active relay systems
  • H04B7/185—Space-based or airborne stations; Stations for satellite systems
  • H04B7/1851—Systems using a satellite or space-based relay
  • H04B7/18513—Transmission in a satellite or space-based system
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B7/00—Radio transmission systems, i.e. using radiation field
  • H04B7/14—Relay systems
  • H04B7/15—Active relay systems
  • H04B7/204—Multiple access
  • H04B7/216—Code division or spread-spectrum multiple access [CDMA, SSMA]

Definitions

• the present invention relates to a method and a system for transmitting information (i.e. point-to-point link) or broadcasting (point-to-multipoint link) of information in digital form from a satellite to terrestrial receivers limiting reciprocal interference with a terrestrial radiocommunication system (designated by the term “terrestrial system” in the Telecommunications Regulations) occupying a frequency band overlapping with that of the satellite.
• a terrestrial radiocommunication system designated by the term "terrestrial system” in the Telecommunications Regulations
• the general term “terrestrial receivers” will designate fixed or mobile receivers located on or near the surface of the earth.
• the invention finds a particularly important, although not exclusive, application in the sharing of radioelectric resources between the downlinks of a satellite, in particular of a television satellite, and terrestrial systems such as broadcasting networks or digital television, radio links, communication networks with mobiles, for example according to the GSM or UMTS standard.
• Figure 1 shows schematically different links which can give rise to mutual interference in the event of coexistence in the same frequency band:
• the terrestrial network may include a base station T and stations or terminals TS.
• the base station is not just a transmitting station. It dialogues with TS receiving stations or terminals.
• Some of the stations T and TS can be in the transmission lobe L of the satellite S and some of the stations MS in an area of exchange between stations T and TS.
• the station MS receives not only a useful power coming from satellite S (arrow F1), but also a jamming power B1 coming from the stations of the terrestrial network.
• Network stations terrestrial placed in the satellite transmission lobe receive on their side a jamming power B2.
• the invention finds application whenever the terrestrial network is organized so that the exchanges in adjacent zones are carried out in different spectral sub-bands and of determined width, which one will qualify as narrow.
• the allocated radioelectric resource for example in the UHF channels between 470 and 860 MHz, is distributed according to a frequency plan between the coverage areas of the different transmitters and receivers.
• the allocated band is split between channels or sub-bands, with central frequencies spaced 8 MHz apart.
• a similar distribution is made in the case of telephony, with the difference that the links are then bidirectional and occupy two channels each allocated to a direction of communication.
• the obvious solution consists in allocating them separate frequency bands, for example 4GHz for the downlinks of a satellite, 620-790 MHz for terrestrial television and 900 or 1800 MHz for mobile telephony.
• This solution is less and less satisfactory, because it reduces the efficiency of management of the available frequency spectrum.
• It has also been proposed to reduce interference to terrestrial systems from satellite links by reducing the power surface density received at the surface of the earth from a satellite. However, this reduces the power received by the satellite terminals MS, which requires the use of directional antennas.
• Another solution usable in the case of fixed MS stations, consists in allocating to the links from the satellite a frequency band outside the band allocated to the terrestrial systems in the area where the MS station is located.
• the present invention aims to solve the problem of interference while allowing recovery of frequency ranges, in particular by making a new application of the already available spread spectrum techniques, including by direct sequence or by frequency hopping. More specifically, the invention starts from the observation that these techniques, by adapting them, make it possible to spread the power of the signal emitted by the satellite over a very wide spectral band, far greater than that of each of the terrestrial links, by taking advantage of, in the case of many terrestrial networks, and in particular in the case of GSM and television, the use different and narrow sub-bands in adjacent areas and possibly the presence of guard bands.
• the interference caused by the terrestrial system in the reception by the satellite terminal MS can be very reduced, without it being necessary for this to have a return channel providing a information allowing an adaptation of the spread of spectrum, according to the interference power received in each of the bands of the terrestrial system.
• Another consequence is to allow satellite downlink access to the bands of the terrestrial systems. Another consequence is to allow, with a simple adaptation, the use, for satellite links, of waveforms and in particular spreading techniques already widely used in the terrestrial field.
• the invention therefore proposes a method for transmitting or broadcasting information in digital form from a satellite to terrestrial receivers in the presence of a terrestrial network carrying out links each occupying a determined and narrow frequency sub-band of a extended band, the sub-bands being allocated to different terrestrial zones within which the connections are made, according to which: - the information is put in the form of digital symbols,
• the digital symbols are distributed over several carriers belonging to a group of carriers which are distributed throughout a channel covering at least four of said frequency sub-bands, by performing spectrum spreading. It is advantageous to use carriers which are all separate and / or to change the allocation of the carriers over time. It is also advantageous to distribute the spreading frequencies of the same program over all of one of the bands allocated to terrestrial communications in the region where transmission or broadcasting takes place from the satellite.
• the invention also provides a system for transmitting or broadcasting information programs in digital form on the downlink from a satellite to one or more terrestrial receivers, comprising: means for putting the information in the form of digital symbols, and means for distributing the digital symbols of each program over several carriers belonging to the whole of a group of carriers which are distributed in the whole of a band affected by a process of planning the radioelectric resource from the terrestrial network to the communication in disjoint sub-bands in a set of terrestrial zones having an overlap with the lobe of the satellite covering at least four of said frequency sub-bands, by performing spectrum spreading.
• the invention makes an application of the OFDM waveform which goes beyond the framework in which it has hitherto been proposed and which is to ensure diversity in time and frequency using the variable characteristics of the channel. propagation attenuation and multi-path to improve transmission.
• the invention takes advantage of the fact that the jammers taken into consideration are also subject to variations in the propagation channel.
• a transmitter carried by a satellite or transmitting from the earth to a satellite having a transparent payload for broadcasting towards the earth making it possible to implement the method defined above and comprising : means for putting the information in the form of digital symbols, and means for distributing the digital symbols of each program over several carriers belonging to the set of a group of carriers which are distributed in at least four sub-bands d '' a band affected by a radio resource planning process for communication in disjoint sub-bands in a set of zones terrestrial having an overlap with the lobe of the satellite, by carrying out a spread of spectrum.
• the invention also relates to a terrestrial reception terminal comprising means for carrying out the operations which are dual to those of the transmission or broadcasting method defined above.
• FIG. 1 is a diagram intended to show the reciprocal interference between terrestrial and satellite networks
• FIG. 3 shows an example of spread spectrum in the satellite link S-terrestrial station MS, as well as an appropriate width of spread band compared with the bands allocated to the different terrestrial links;
• FIGS. 4a and 4b respectively show an example of spectral occupancy of the first carriers available for a multi-carrier modulation called OFDM (Orthogonal Frequency Division Multiplex), usable for spreading spectrum in the event of application of the invention to satellite broadcasting in a band also allocated to terrestrial television, and frequency interleaving;
• OFDM Orthogonal Frequency Division Multiplex
• FIG. 5 is a simplified block diagram of a satellite transmitter and a land station receiver for implementing the invention
• FIG. 6 is a block diagram showing a possible implementation of the method according to the invention on transmission;
• - Figure 7 is a variant implementation of the modulator, in the event of simultaneous broadcasting of several programs;
• Figure 8, similar to Figure 7, shows another possible construction of the modulator;
• Figures 9 and 10 are partial diagrams of receivers;
• - Figure 1 1 is a block diagram showing an example of implementation using MSK or GMSK modulators at low speed (a few bkps);
• Figure 12 is a diagram showing the broadcast with a transparent payload satellite.
• FIG. 2 shows frequency sub-bands 10 in which the exchanges of a terrestrial network take place, and in particular the transmission sub-bands of terrestrial station T in the downlink lobe of a satellite S.
• a frequency band of this link generally of energy per unit of surface area lower on the terrestrial level, is indicated in 12.
• the emissions coming from station T in the reception band of the earth station MS of the satellite link lead to interference.
• the interference power added to other disturbances (thermal noise from the receiver, environment) reaches a value such that the ratio between the useful power and the interference power drops below a determined threshold, the downlink of the satellite is no longer usable.
• Interference can be particularly intense in the cells of a cellular telecommunications system if sub-bands are used which correspond to the frequency of the downlink of a satellite.
• the principle implemented by the invention to reduce the usual interference is illustrated in FIG. 3. It consists in spreading the frequency spectrum of the downlink of the satellite so as to distribute the power over a frequency band 14 much wider than each of the frequency sub-bands each allocated to a land station T.
• the interference caused by a land station T on a reception station MS is notably reduced.
• Spectrum spreading can be done by direct sequence or by frequency hoping.
• the application of these techniques to satellite downlinks, which are typically broadband, for example from 6 to 8 MHz, in the case of a digital television signal, is difficult. Indeed, spreading by frequency hopping or by direct sequence can only be used with an acceptable complexity of the systems for spreading bands of 1 to 4 Mcps (megachips per second).
• a digital television signal on a downlink should be spread over a band of at least 80 Mhz, which makes the existing conventional devices inapplicable.
• This type of waveform is advantageously associated with a time multiplexing, which makes it possible to take into account the variations in time of the propagation channel, thanks to a block interleaving of the bits to be transmitted. This avoids the risk of a large number of directly adjacent binary elements disappearing.
• Figure 4a shows only some of the first carriers available for such OFDM modulation.
• Each symbol to be transmitted can be distributed on 1705 elementary carriers, with a spacing of 4.4 kilohertz.
• the parameters can for example be as follows:
• the modulation for transmission will generally be a four-state phase modulation, called MDP4 or QPSK.
• MDP4 or QPSK phase modulation
• other types of modulation for example MDP8 or MAQ16 can be used.
• continuous phase modulation in particular the so-called MSK and GMSK modulations, have advantages with regard to the performance required of the satellite power amplifiers.
• a television program to be transmitted is formatted in 18 in the form of packets, of 188 bytes for example, in MPEG-2, MPEG-4 or DVB ASI format, then subjected to coding in 20.
• This coding is in two concatenated stages:
• Time interleaving 26 is then carried out, possibly merging with another stream 28; a time multiplexing by blocks is thus constituted which distributes the effects of the variations of the propagation channel over time.
• - OFDM or COFDM 30 modulation is then performed and ensures frequency multiplexing. It provides frequency interleaving and adds 32 carrier pilot or analysis symbols. These additional symbols allow synchronization of receivers.
• Each signal carrier is then subjected to a differential phase modulation 34 which gives rise to complex symbols.
• Guard intervals of duration Tg are introduced at 38.
• a frequency-time conversion 36 (generally inverse FFT) makes it possible to pass into the time domain.
• a digital / analog conversion precedes the frequency change 42 and the transmission 44.
• the usual OFDM signal is organized in super-frames of four frames of
• Each elementary carrier used for the downlink of a satellite is subjected to spreading by direct sequence, with a spacing between carriers sufficient to avoid their overlapping (consequently avoiding the use of two adjacent carriers in FIG. 4). This reduces the spectral power density, therefore the interference of terrestrial channels.
• This process also makes it possible to reduce the possibility of reception by unauthorized third parties. Indeed, the initialization of a pseudo-random spreading sequence can require an encryption key, the possession of which conditions the clear recovery of the transmitted data.
• each carrier can be spread with a sequence which is either specific to it or is the same for all the carriers of the same program (or of all the programs), which simplifies the implementation.
• This spreading mode is of the kind used in the DAB standard for example, but for the purpose of increasing interference immunity and not of reducing the interference effect on other transmissions.
• This method is the one that most reduces the power transmitted by the satellite by decreasing the spectral density of transmitted power.
• the spreading takes place by choosing, for a program, from all the available frequencies, P frequencies (corresponding to the P discrete carriers of the program).
• P frequencies corresponding to the P discrete carriers of the program.
• the selection will generally be made on a pseudo-random basis.
• the different elementary carriers of each of the different programs are chosen to be orthogonal, that is to say that the respective spectra of the different carriers do not overlap as in the standard OFDM mode.
• For a total band W, there are M solutions if each carrier occupies a band b, with M W / b.
• the diversity factor thus obtained locally avoids simultaneous interference of all the symbols.
• FIG. 4b gives an example of an elementary carrier frequency plan, with regular interleaving, in the case of ten interleaved programs.
• the interlacing scheme can be both fixed and random.
• each program is assigned a batch of 1705 determined frequencies.
• One particular solution consists in distributing the carrier frequencies used by a particular program regularly. However, to give equal treatment to all the programs, it is possible to exchange frequencies between programs by performing a pseudo-random draw of the frequencies allocated to each program in a global batch of frequencies.
• the assignment of the bits to carriers as a function of their position in the bit stream changes constantly over time. A time interleaving is thus added.
• An additional advantage of such an assignment compared to an invariable assignment is that it eliminates the risk of seeing bits corresponding to particularly important parameters (intra coding bits in MPEG television for example) transmitted permanently on frequencies subject to interference by narrowband jammers.
• FIG. 6 is a block diagram of the steps implemented on transmission in a particular embodiment.
• the data D admitted to the modulator has already been formatted, for example in accordance with the diagram in FIG. 5.
• This formatting can have very diverse natures, for example hierarchical multiplexing, MPEG-2 framing, block coding, convolutional coding, temporal interleaving, frequency interleaving, etc.
• the processed data is applied to a serial-parallel converter 46 which distributes the different binary elements on the different carriers which will compose the OFDM waveform.
• the modulator proper 48 modulates each of the carriers in phase and possibly in amplitude according to the binary elements of the data and the type of modulation chosen (MDP2, MDP4, MDAP8, MDAP16, ).
• the calculation of the phase and possibly of the amplitude is carried out by a control member 50, constituted by a processor which determines a time distribution and provides the modulation and spreading parameters by direct sequence in the case where this method is used. .
• the part of the modulator located downstream of the element 48 is conventional. It includes an element 36 performing an inverse Fourier transform and a digital-to-analog converter 40. This converter can also perform filtering. Finally, the frame 52 indicates a block grouping together the components performing the frequency change, the amplification, the broadband filtering and the emission.
• the constitution of the modulator can be that shown schematically in FIG. 7, where the elements corresponding to those already described are designated by the same reference number, assigned an index when the component is particular to a program.
• the same processor 50 can be used to control the central frequency of all the carriers and to ensure spreads.
• Each serial-parallel converter 46- ⁇ , ... 46n has practically the same constitution as in the previous case, since the input data of each program are processed separately.
• the modulator of FIG. 7 comprises a device inserted before the reverse Fourier transformation processors for reordering the carriers, so that each transformer only processes a subset of carriers which are contiguous and not disjoint.
• the presence of this device implies that the various programs to be transmitted are supported globally.
• a device 56 is inserted to perform an interleaving taking overall account of all the programs, before modulation.
• This arrangement makes it possible to have only subsets of contiguous carriers in baseband treated by the reverse Fourrier transformation processors 36. The load of the processors ensuring this transformation function is reduced. It then becomes necessary to assign to the frequency change module 52 a different frequency change function for each block constituting this device, whereas this was not the case for the blocks 52 1? ... 52 n of FIG. 7.
• the device 50 must moreover control the interleaving of the different symbols of the different programs on different carriers, according to a distribution diagram which can be fixed over time or which can vary over time.
• the receiver may have the general constitution shown in FIG. 9.
• the essential part of the demodulators is constituted by the means making it possible to ensure global deinterlacing by taking into account all the programs.
• the overall structure of the demodulator is practically not a function of the embodiment of the transmitter and accepts all the variants mentioned above. In fact, the demodulator operates over the entire wide spreading band and therefore must operate independently of the interleaving schemes.
• the radiofrequency signal RF originating from the satellite is converted into an electrical signal by the antenna 60.
• a module 62 provides amplification, filtering and frequency conversion to a lower frequency.
• Module 64 performs analog-to-digital conversion and baseband conversion. Moreover, he duplicates the signal to supply several fast Fourrier transformation processors 66. The use of several processors rather than one aims at reducing the processing capacities required of each of the processors. Their number is independent of that of the programs.
• a block 68 provides demodulation. This block includes several elementary demodulators, in order to limit their processing capacity. To operate, the demodulators must receive phase and amplitude information as well as control instructions.
• a module 70 ensures the synchronization in time and in frequency of the demodulators and provides them with direct spreading sequences, if necessary.
• the frequency deinterlacing block 72 ensures the rearrangement of the information according to the programs using a deinterlacing sequence which is either fixed and memorized once and for all, or variable in time, in which case it is provided by the device 70.
• This device receives itself DS signaling data originating from the transmitter and defining the interleaving path, the time distribution, the spreading sequences used, etc.
• This signaling data can be extracted by block 64.
• the signaling data DS can, in particular be transmitted on the pilot frequencies and the synchronization channels which are generally provided in the usual types of OFDM modulation.
• the demodulators 68 can also provide information on the state of the channel, the signal to noise ratio and the frequencies most affected by the interference, so as to send, on a return channel, indications making it possible to modify the frequencies.
• the frequency deinterlacing block 72 is placed before the demodulators, which makes it possible to integrate, in the demodulators, the functions of channel equalization and decoding before restitution of the binary information.
• the elements corresponding to those in FIG. 9 have the same reference number.
• FIG. 11 which gives an example of implementation, the elements corresponding to those of the preceding figures are also designated by the same number.
• the data and signaling D to be transmitted for each program can be subjected to a hierarchy at 60, for example by treating the two streams represented separately to further protect sensitive data (intra coding in digital television).
• FIG. 11 then shows the MPEG2 framing at 62, the block coding 64, the time interleaving 66, the convolutional coding 68 and the frequency interleaving 70 on each stream.
• the frequency distribution by OFDM formatting is then carried out globally by cartographic processors
• a block comprising a map memory and a group of low-speed MSK or GMSK 74 modulators (typically a few kbps).
• a first frequency change can then be made in digital form in order to distribute the carriers, which facilitates the implementation of the interference diversity.
• a second fixed conversion can take place, if necessary, and after conversion to 76.
• Each carrier is amplified by a low power amplifier 78 (of the order of 1 W) in solid state and operating in saturated mode (therefore having very good efficiency).
• Each amplifier amplifies only a very small part of the information (typically 0.01%).
• the MSK waveform supports this amplification mode well, and out-of-band emissions are limited. Finally, the signals are juxtaposed and transmitted by the antenna.
• This method is applicable with on-board demodulation and OFDM remodulation, or with digital signal processing without carrier demodulation (frequency conversion can be carried out at the time of digital processing).
• the payload of satellite S is transparent.
• the earth stations 80 transmit the programs on the uplink M after formatting (and synchronization of the spreading frequencies if necessary) to the satellite
• each of the TS stations transmits the signal in the form of a single carrier (for example in DVB-S format or in DVB-T standard).
• the satellite payload demodulates and decodes the received signals, then reformats them according to the process.
• the transmission stations 80 are then independent of the process.
• Another variant is applicable when using the DVB-T standard with remission. Since the method is independent of the DVB-T waveform, the variants not calling on direct spectrum spreading can be implemented without demodulation of the carriers but with only a reorganization of the carriers according to the laws programmed on board the satellite. These laws can be chosen at the command of a ground control station, for example.

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• 2001
  • 2001-09-14 FR FR0111921A patent/FR2829890B1/fr not_active Expired - Lifetime
• 2002
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  • 2002-09-11 WO PCT/FR2002/003087 patent/WO2003026163A1/fr active Application Filing
• 2004
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