WO2010000655A1

WO2010000655A1 - Method and system for broadcasting data

Info

Publication number: WO2010000655A1
Application number: PCT/EP2009/057900
Authority: WO
Inventors: Samuel Guillouard; Renaud Dore; Olivier Mocquard
Original assignee: Thomson Licensing
Priority date: 2008-06-30
Filing date: 2009-06-24
Publication date: 2010-01-07
Also published as: FR2933254A1

Abstract

The invention relates to a data broadcast, implemented in at least a first transmitter (111 to 113,121,122). In order to optimize resources, the broadcast comprises steps consisting in: constructing OFDM symbols, each symbol comprising first data and second data associated respectively with first (10) and second (11, 12) broadcast areas (10), the first area covering at least part of the second area, a first set of sub-carriers carrying information representative of the first data in each OFDM symbol, a second set of sub-carriers carrying information representative of the second data in each OFDM symbol, the first and second sets of sub-carriers being disjointed, and broadcasting of OFDM symbols intended for a plurality of receivers.

Description

Method and system for broadcasting data.

1. Scope of the invention

The invention relates to the telecommunications field and more specifically to data broadcasting in a network comprising several transmitters.

2. Prior art

According to the prior art, the broadcasting of services, for example television, is performed by transmitters having a limited range. In order to improve the broadcasting coverage, several transmitters are planned covering a large territory. The modulations for multiple sub-carriers of OFDM (Orthogonal Frequency Division Multiplexing) type are relatively efficient for data broadcasting through a channel subject to disturbances (for example, channel with multiple paths creating echoes).

According to a technique known in the prior art and called SFN (Single Frequency Network), several transmitters transmit synchronously a single OFDM signal. In this way, a receiver receives a combination of signals from several transmitters and decodes the combination obtained in this manner using the properties specific to the OFDM for the cancellation of inter-symbol interference.

This technique has the disadvantage of monopolizing a big portion of the spectral resources in a large geographic area for the broadcasting of a single service.

3. Summary of the invention

The purpose of the invention is to overcome the disadvantages of the prior art.

More particularly, the object of the invention is to enable a broadcasting of services which is suitable for specific needs for smaller geographic areas.

For this purpose, the invention proposes a method for broadcasting data, implemented in at least a first transmitter. In order to improve the broadcasting according to the geographic areas to be covered, the method comprises the steps of:

- constructing OFDM symbols, each symbol including first data and second data, the first data being associated with a first broadcast area; the second data being associated with a second broadcast area which differs from the first broadcast area, the first broadcast area covering at least one part of the second broadcast area; a first set of sub-carriers which carry information representative of the first data in each OFDM symbol; a second set of sub-carriers which carry information representative of the second data in each OFDM symbol, the first and second sets of sub-carriers being disjointed, and

- broadcasting of OFDM symbols intended for a plurality of receivers.

In this way, in the first broadcast area, the first data is broadcast through symbols using the first set of sub-carriers. In the second broadcast area, the first and second data is broadcast through symbols using the first and second sets of sub-carriers. The first and second broadcast areas intersect in part or the second (respectively first) area encompasses the first (respectively second) area. Hence, a receiver present in the second area correctly receives the first and second data. Data transmissions can thus be defined specific to the second area without neglecting the transmission coverage of the first in the second area. If a receiver is located within the second area and can receive a signal associated with the first data transmitted in the first area outside of the second area, the reception of the first transmitted data is improved. Advantageously, the broadcast is time and frequency synchronized in said first and second broadcast areas.

According to a particular characteristic, the first set or the second set includes a plurality of contiguous sub-carriers.

According to advantageous characteristics, the first set of sub- carriers is fixed or variable.

According to a particular characteristic, the broadcasting method comprises the steps of:

- receiving the first data from a first source; and

- receiving the second data from a second source different from the first source. According to an advantageous characteristic, the first data is associated with a first broadcasting service and the second data is associated with a second broadcasting service different from the first service.

Advantageously, the broadcasting is of MIMO (Multiple Input Multiple Output) type.

The invention relates also to a data broadcasting system remarkable in that it includes:

- a first set of transmitters belonging to a first broadcast area,

- a second set of transmitters belonging to a second broadcast area, the first broadcast area covering at least one part of the second broadcast area, the first and second broadcast areas being different, each transmitter of the first set is suitable for broadcasting first OFDM symbols including first data, the first data being associated with the first broadcast area, a first set of sub-carriers carries an information representative of the first data in each first OFDM symbol, each transmitter of the second set is suitable for broadcasting second OFDM symbols including the first data and second data, the second data being associated with the second broadcast area, the first set of sub-carriers carries an information representative of first data in each second OFDM symbol; and a second set of sub-carriers carries an information representative of the second data in each second OFDM symbol, the first and second sets of sub- carriers being disjointed.

4. List of figures

The invention will be better understood, and other specific features and advantages will emerge upon reading the following description, the description making reference to the annexed drawings wherein:

- figure 1 illustrates a broadcasting system implementing several transmitters, according to a particular embodiment of the invention;

- figure 2 presents a diagrammatical block diagram of a transmitter of the system in figure 1 , according to the invention; - figure 3 presents a diagrammatical block diagram of the system in figure 1 , - figures 4 and 5 diagrammatically respectively illustrate a transmitter and a receiver of the system in figure 1 , according to the invention;

- figures 6 to 8 present the use of a broadcasting spectrum by different transmitters of the system in figure 1 ; and - figure 9 illustrates a method for broadcasting data according to a particular embodiment of the invention, implemented by transmitters of the system in figure 1.

5. Detailed description of the invention Figure 1 illustrates a broadcasting system 1 implementing several transmitters 102 to 106, 1 1 1 to 1 13, 121 and 122 according to a particular embodiment of the invention. The system 1 also includes receivers (not illustrated in figure 1 ). The receivers are for example fixed receivers (including television decoders) or handheld devices, for example suitable for receiving and processing broadcasting services (for example recording and/or displaying of video data). The system 1 includes a global server 100 connected to each one of the transmitters 101 to 106, 1 1 1 to 1 13, 121 and 122 by links 106. The links 106 are, for example, wire links (for example, using an IP (Internet Protocol) type network), wireless or satellite links. The global server 100 transmits data corresponding to a first service (including a video broadcasting service) to the set of transmitters to which it is connected through the links 106.

The system 1 covers a geographic area 10 including geographic areas 1 1 and 12 which are different. So, each one of the transmitters 101 to 106, 1 1 1 to 1 13, 121 and 122 covering the area 10 broadcasts the data corresponding to the first service to the set of receivers present within the area 10. The transmitters 101 to 106 are, for example, high power transmitters each covering a large area (for example several kilometres of radius (for example 3, 10 or 50 km)) included within the area 10. The system 1 includes a local server 1 10 connected to the transmitters 1 1 1 to 1 13 which are present within the area 1 1. The local server 1 10 transmits data corresponding to a second service (including a video broadcasting service) to the set of transmitters to which it is connected through any links (for example wire, wireless or satellite links). The broadcasting of transmitters 1 1 1 to 1 13 covers this area 1 1 , in other words, the transmitters 1 1 1 to 1 13 broadcast the data corresponding to the second service to the set of receivers present within the area 1 1. The system 1 includes a local server 120 connected to the transmitters 121 and 122 which are present within the area 12. The local server 120 transmits data corresponding to a third service to the set of transmitters to which it is connected through any links. The broadcasting of transmitters 121 and 122 covers this area 12, in other words, the transmitters 121 and 122 broadcast the data corresponding to the third service to the set of receivers present within the area 12.

The servers 1 10 and 120, according to the embodiment described above, are advantageously managed locally and physically separated from the global server 100, which facilitates the implementation of networks specific to the areas 1 1 and 12 (including their installation to cover occasional or time limited services).

According to a variant, the servers 1 10 and 120 are integrated in the server 100. Advantageously, the server 100 transmits the services to each one of the transmitters according to their coverage area 10, 1 1 or 12.

This variant has the advantage of a centralized management and of limiting the number of servers.

According to the embodiment described, the areas 1 1 and 12 inside the largest area 10 are disjointed. Hence, the reception of the services is facilitated in each one of the areas 1 1 and 12.

According to a variant, the areas 1 1 and 12 inside the largest area 10 are different and intersect in part. The reception of the second and third services is then perturbed in the intersecting part. However, the receivers present within the area 1 1 (respectively 12) and not present within the area 12 (respectively 1 1 ) correctly receive the first and second (respectively third) services.

The figure 2 shows an architecture of a transmitter 2 suitable for broadcasting data associated with two different services according to the invention to receivers. The transmitters 1 1 1 to 1 13, 121 and 122 have, for example, the architecture of the transmitter 2. The transmitter 2 includes:

- an encoder 20 which receives data 200 associated with a first service and with a first broadcast area (for example the area 10), - an encoder 21 which receives data 210 associated with a second service and with a second broadcast area (for example the area 1 1 ), different from the first broadcast area,

- Mx modulators OFDM 221 to 22Ntx which transmit OFDM symbols to at least one antenna respectively 231 to 23Mx; and

- a synchronisation module 24 which receives a synchronisation signal 240 and which transmits a signal 241 which synchronises the OFDM modulators 221 to 22Mx.

The synchronisation signal 204 generates the signal 241 according to any method known by those skilled in the art. The signal 240 is for example a signal including a precise time stamp. The synchronisation module 24 then generates the signal 241 by using a complementary means

(for example by using a GPS (Global Positioning System) signal). The synchronisation module 24 can also generate the signal 241 from a signal carried on an Internet network. Advantageously, the OFDM modulators are synchronised with a precision in the order of 1 μs and more generally with a precision which allows guard intervals of the OFDM symbols to absorb, at the receivers level, the possible shifting between the transmitters and the transmission delays linked to the paths taken by the broadcast signals. The encoder 20 (respectively 21 ) comprises:

- a channel encoder 201 (respectively 21 1 ) which receives the data 200 (respectively 210), protecting them with a channel coding (for example convolutional code or in block) against the errors linked to a noisy transmission and shaping them to form blocks of coded data 202 (respectively 212),

- a modulator 203 (respectively 213) (for example of QAM (Quadrature Amplitude Modulation) or PSK (Phase Shift Keying) type) which receives the coded data 202 (respectively 212) and produces the modulated symbols 204 (respectively 214); and

- a MIMO encoder 205 (respectively 215) which encodes the modulated symbols 204 (respectively 214) to form Mx streams of MIMO data blocks 2061 to 206Mx (respectively 2161 to 216Mx) intended respectively for each one of the OFDM 221 to 22Mx. Here, the antennas 231 to 23Mx include the radio or RF (Radio Frequency) part, and in particular frequency rotations, amplifications and filtering.

The modulator 203 generates groups of Q modulated symbols S1 to SQ. Q is for example equal to 1600 and is the product of the code rate used by the MIMO encoder 205 by the number Mx of transmitting antennas 231 to 23Mx and by the number m of sub-carriers associated with the encoder 20. m is for example equal to 800 with an encoder rate 205 worth 1 (with a full rank encoder of BLAST type) and two antennas ((Mx = 2). m is for example equal to 1600 with an encoder rate 205 worth 0.5 (with an encoder of Alamouti type) and 2 antennas (Mx = 2).

The modulator 213 generates groups of Q' modulated symbols S1 to SQ'. Q' is for example equal to 200 and is the product of the code rate used by the MIMO encoder 215 by the number Mx of transmitting antennas 231 to 23Mx and by the number m of sub-carriers associated with the encoder 21. n is for example equal to 100 with an encoder rate 215 worth 1 and 2 antennas (Mx = 2). n is for example equal to 200 with an encoder rate 205 worth 0.5 and 2 antennas (Mx = 2).

Advantageously, the total number of sub-carriers is lower than the FTT (Fast Fourier Transform) size used in the OFDM modulators. The numbers of sub-carriers m and n used respectively by the first and second services can be equal or different. Advantageously, they are adapted to the rate necessary: if the first one requires a higher rate than the second service, the number of sub-carriers assigned to the first service will be greater than those corresponding to the second service.

The encoders 205 and 215 encode the modulated symbols at input with a spatial multiplexing (for example based on a Bell laboratories BLAST multiplexing (as described, for example, in the document by G. J. Foschini, entitled "Layered Space-Time Architecture for Wireless Communication in a Fading Environment When Using Multiple Antennas" and edited in the Bell Labs Technical Journal, Vol. 1 , No. 2, Autumn 1996, pp 41 -59) or a STBC/SFBC code (Space Time Block Code/ Space Frequency Block Code). The STBC/SFBC code is, for example, an Alamouti or Golden orthogonal code (for example). A Golden code is described in the document "The Golden Code: A 2 x 2 Full-Rate Space-Time Code with Non- Vanishing Determinants," written by J. -C. Belfiore, G. Rekaya, E. Viterbo (and published in IEEE Transactions on Information Theory, vol. 51 , No. 4, pp. 1432-1436, April 2005.). According to a variant, the STBC code is described in "Space-Time block codes from orthogonal designs" by V.Tarokh, H. Jafarkhani, and R. A. Calderbank (and published in IEEE Transactions on Information Theory, vol. 45, pp. 1456-1467, in July 1999). An Alamouti orthogonal code is described in the document "A simple transmit diversity technique for wireless Communications". At its output, the encoder 205 (respectively 215) and the MIMO data blocks 2061 to 206Mx (respectively 2061 to 2060Mx) are assigned to an OFDM modulator.

According to a variant, the transmitters do not operate in MIMO mode. In particular, the transmitter 2 does not include a MIMO coding and includes only one OFDM modulator associated with an antenna; the modulators 203 and 213 then directly supply the OFDM modulator.

The figure 3 shows an architecture of a transmitter 3 suitable for broadcasting data associated with a separate service to receivers. The transmitters 101 to 105 have, for example, the architecture of the transmitter 3.

The transmitter 3 comprises the elements of the transmitter 2 allowing the transmission of a first service. These elements are similar to the corresponding elements of the transmitter 2 and have the same references. The transmitter 3 comprises:

- the encoder 20 which receives data 200 associated with the first service and the first broadcast area (for example the area 10);

- Mx OFDM modulators 221 to 22Mx which transmit OFDM symbols to at least one antenna respectively 231 to 23Mx; and

According to the embodiment described, the transmitter 3 includes one encoder associated with a service (encoder 20), which allows a dedicated implementation.

According to a variant corresponding to a particular embodiment of the invention, the transmitter 3 has a structure similar to the transmitter 2, the part associated with other services (encoder 21 ) being: - hard invalidated, this part cannot be used but the manufacturing can be facilitated, or - can be validated on demand; for example, following the starting of a second service, an enable signal which validates the use of the encoder 20 and the corresponding sub-carriers in the OFDM modulators or which acts directly at the level of OFDM modulators for authorising or prohibiting the use of sub-carriers associated with the second service. The figure 6 illustrates a spectrum 61 used by the transmitters 101 to 105 at a given time t. The spectrum 61 includes m contiguous sub- carriers F1 to Fm 610 to 61 m. The sub-carriers F1 to Fm broadcast a signal which carries an information representative of data associated with the first service S1 in each OFDM symbol.

The figure 7 illustrates the combination of the spectrum 61 and a spectrum 72 used by one of the transmitters 1 1 1 to 1 13 at the time t. The spectrum 72 includes n contiguous sub-carriers F'1 to F'n 720 to 72n. The sub-carriers F'1 to F'n transmitted by one of the transmitters 1 1 1 to 1 13 broadcast a signal which carries an information representative of data associated with a second service S-? in each OFDM symbol.

The figure 8 illustrates the combination of the spectra 61 and 72 used by one of the transmitters 121 and 122 at the time t. The sub-carriers F'1 to F'n transmitted by one of the transmitters 121 and 122 broadcast a signal which carries an information representative of data associated with the third service S3 in each OFDM symbol.

Hence, at a time t, the transmitters 101 to 105, 1 1 1 to 1 13, 121 and 123 are time and frequency synchronised with the signal 240 and broadcast in a synchronised way, on the same sub-carriers of the spectrum

61 , an information representing data associated with the first service S1, these data being modulated and coded in the same way. At the same time t, the transmitters 101 to 105 do not use the spectrum 72; the transmitters 1 1 1 to 1 13 broadcast, on the same sub-carriers of the spectrum 72, an information representing data associated with the second service S2, these data being modulated and encoded in the same way; the transmitters 121 and 122 broadcast on the same sub-carriers of the spectrum 72, an information representing data associated with the third service S3, these data being modulated and coded in the same way. Hence, a receiver located within the area 10 receives the signal broadcast by at least one of the transmitters within the area 10 and decodes the signal corresponding to the spectrum 61 in order to obtain the data associated with the first service. A receiver located within the area 1 1 (respectively 12) receives the signal broadcast by at least one of the transmitters within the area 1 1 (respectively 12) and possibly a signal broadcast by a transmitter within the area 10 out of the area 1 1 (respectively 12); it decodes the signal corresponding to the spectra 61 and 72 in order to obtain the data associated with the first and the second (respectively the third) services. The fact that the transmitters within the area 1 1 (respectively 12) broadcast the data associated with the first and second (respectively third) services, allows receivers within the area 1 1 (respectively 12) to have a better reception of the data associated with the first service, which is particularly advantageous when the reception of the signals transmitted by the transmitters out of the area 1 1 (respectively 12) is difficult, even impossible (case for example, where the area 10 is very large (several kilometres in radius) and the area 1 1 (respectively 12) corresponds to an area inside buildings). Moreover, the use of spectral resources is optimised within areas where all the frequency bands are not used for the first service covering the largest area. This also allows the first service to use a reduced number of sub-carriers relative to the number of sub-carriers available.

According to an advantageous embodiment of the invention, the first and/or second sets of sub-carriers 61 and 72 are fixed. This notably enables to facilitation of the implementation.

According to another advantageous embodiment of the invention, the first and/or second sets of sub-carriers 61 and 72 vary; they vary temporally, for example, every 10 or 100 OFDM symbols which allows a compromise between a statistical gain linked to the changes of frequency (the more frequently the changes are, the better the gain is) and a reliability of the canal estimate (the channel can be better estimated if the changes are less frequent). Hence, if one or several frequencies are noisy or poorly received, the corresponding sub-carriers are affected. By varying the spectra 61 and 72, this allows a diversity of frequencies for the receivers and the effects of disturbances on a given service are more limited. According to this embodiment, the transmitters of the system 1 transmit OFDM symbols whose sub-carriers that are used correspond in totality or in part to the subset 61. Hence, the transmitters 101 to 105 implement a selection of sub- carriers 61 in a set of available sub-carriers.

Advantageously, the sub-carriers associated with at least one of the services (first, second or third) are contiguous, which allows receivers to have a more precise estimate of channel response (for example through frequency interpolations). Hence, according to the figures 6, 7 and 8, the spectra 61 and 72 use consecutive sub-carriers respectively F1 to Fm and F'1 to F'n in the available spectrum including n+m sub-carriers from F1 to F'n. Other embodiments are possible with contiguous sub-carriers (or consecutive) in a spectrum including frequencies F"1 to F"m+n (F"1= FV, F"m= Fm; F"m+1 = F'1 and F"m+n=F'n) for at least one of the services, for example, one embodiment where the spectrum associated with the first service S1 uses m sub-carriers from F"i to F"i+m and where the spectrum associated with the second (S2) and third (S3) services uses n sub-carriers from F"1 to F"i- 1 and from F"i+m+1 to F"m+n. According to another embodiment, the spectrum associated with the first service S1 uses m sub- carriers from F"1 to F"i- 1 and from F"i+n+1 to F"m+n and where the spectrum associated with the second (S2) and third (S3) service uses n sub- carriers from PV to PV+n.

According to another variant, the sub-carriers used by the services are not consecutive, which can allow in some cases a greater diversity of frequencies and a better resistance to channel disturbances to be obtained. The figure 4 illustrates schematically a hardware embodiment of a transmitter 4 corresponding for example to the transmitter 2.

The transmitter 4 comprises, connected to each other by an address and data bus 44, also transporting a clock signal:

- a microprocessor 41 (or CPU);

- a non-volatile memory or ROM (Read Only Memory) type 42; - a Random Access Memory or RAM 43;

- an interface 45 suitable for the reception of a first service;

- an interface 46 suitable for the reception of a service different from the first service;

- an interface 47 suitable for the broadcasting of services and notably realizing the functions of the encoders 20 and 21 , the

OFDM modulators 221 to 22Mx and the antennas 231 to 23Mx;

- an interface 48 suitable for receiving the synchronisation signal 240 and for synchronising the interface 47; and/or - an MMI (Man Machine Interface) interface 49 or to a specific application suitable for displaying information for a user and/or inputting data or parameters (for example the setting of sub- carriers and services to be transmitted).

It is noted that the word "register" used in the description of memories 42 and 43 designates in each of the memories mentioned, a memory zone of low capacity (some binary data) as well as a memory zone of large capacity (enabling a whole programme to be stored or all or part of the data representing services received and to be broadcast). The memory ROM 42 comprises in particular:

- a program "prog" 420; - parameters 421 of physical layers; and

- parameters 422 associated with the sub-carriers.

The algorithms implementing the steps of the method specific to the invention and described below are stored in the ROM 42 memory associated with the transmitter 4 implementing these steps. When powered up, the microprocessor 41 loads and runs the instructions of these algorithms.

The random access memory 43 notably comprises:

- in a register 430, the operating programme of microprocessor 41 responsible for switching of transmitter 4, - incoming data 431 corresponding to the first service S1,

- incoming data 432 corresponding to another service S2,

- coded data 433 for broadcasting services.

The figure 5 illustrates schematically a hardware embodiment of a receiver 5 belonging to the system 1 and suitable for receiving and decoding the signal transmitted by the transmitters of the areas 10, 1 1 and 12.

The transmitter 5 comprises, connected to each other by an address and data bus 54, also transporting a clock signal:

- a microprocessor 51 (or CPU);

- a non-volatile memory of the ROM (Read Only Memory) type 52;

- a Random Access Memory or RAM 53,

- a radio interface 55,

- a MMI interface 56 suitable for displaying information for a user and/or inputting data or parameters (for example the setting of sub-carriers and services to be transmitted), and

- an interface 57 to a particular application suitable for processing the service data transmitted by the transmitters of the area to which the receiver 4 belongs and received by the receiver 4.

It is noted that the word "register" used in the description of memories 52 and 53 designates in each of the memories mentioned, a memory zone of low capacity as well as a memory zone of large capacity

(enabling a whole programme to be stored or all or part of the data representing services received and decoded).

The memory ROM 52 comprises in particular:

- a program "prog" 520, - parameters 521 of physical layers, and

- parameters 522 associated with the sub-carriers of the services received.

The algorithms implementing the steps of the method specific to the invention and described below are stored in the ROM 52 memory associated with the receiver 5 implementing these steps. When powered up, the microprocessor 51 loads and runs the instructions of these algorithms.

The random access memory 53 notably comprises:

- in a register 530, the operating programme of the microprocessor 51 responsible for switching on the receiver 4, - incoming data 531 corresponding to the data received and decoded by the receiver 55,

- decoded data 532 shaped to be transmitted to the interface to the application 57.

Other structures of the transmitter 4 and/or of the receiver 5 than those described with respect to the figures 4 and 5 are compatible with the invention. In particular, according to variants, the transmitters and/or the receivers compatible with the invention are implemented according to a purely hardware realisation, for example in the form of a dedicated component (for example in an ASIC (Application Specific Integrated Circuit) or FPGA (Field-Programmable Gate Array) or VLSI (Very Large Scale

Integration)) or of several electronic components embedded in an apparatus or even in a form of a mix of hardware elements and software elements.

The radio interface 55 is suitable for the reception of signals broadcast by the transmitters of the system 1. It notably includes a MIMO decoder associated with the transmitters if these last are of MIMO type and a

SISO (Single Input Single Output) decoder associated with the transmitters if these last are of SISO type (or not MIMO). Advantageously, it includes an OFDM demodulator corresponding to the OFDM modulation used. In particular, the receiver is suitable for receiving a broadcast signal which can include a part of the sub-carriers available (for example sub-carriers F1 to Fm corresponding to the first service) or all the frequencies available (for example sub-carriers F1 to Fm+n.) Advantageously, the receiver 5 decodes each OFDM symbol received for each set of sub-carriers. According to a particular embodiment, the receiver decodes separately the first and second signal sequentially in time by using common or parallel hardware resources by using resources dedicated to each service. The receivers corresponding to the transmitters of the system 1 , which, according to a variant, implement also a selection of sub-carriers 61 , implement also a selection of the set 61 to receive and decode the first associated data (for example by detecting sub-carriers 61 used in the set of sub-carriers available or by exchanging information between a transmitter or management device and the receiver).

The figure 9 presents a broadcasting method implemented in the transmitter 4 according to a particularly advantageous implementation of the invention.

During an initialisation step 90, the different parameters of the transmitter 4 are updated. In particular, the parameters corresponding to the services to be broadcast and to the corresponding sub-carriers are initialised in any manner (for example, following the reception of initialisation messages transmitted by one or several servers or other element of the system 1 , or even, by commands from an user). Next, during a stage 91 , the transmitter 4 generates or receives first data corresponding to the first service S1 to be transmitted and second data corresponding to the second service S2. The transmitter 4 is configured according to the area to which it belongs. Advantageously, it generates or receives the data associated with the services it broadcasts within its area. According to a variant, the transmitter 4 realizes a step of services filtering by processing for broadcasting only the services data corresponding to the area to which it belongs, the other data not being broadcast. Advantageously, the step 90 includes a reception of the data corresponding to a first service of a first source and of the data corresponding to a second service of a second source different from said first source. So, although the data come from different sources, it is advantageously transmitted with an optimization of the hardware and spectral resources. According to a variant, the data corresponding to the first and second services comes from a same source. Hence, sub-carriers and coverage areas are associated with each service, no matter whether the service sources are the same or different.

Then, during a step 92, the transmitter 4 realizes a step of OFDM symbols construction, each symbol carrying an item of information representing the first data and the second data. In particular, following a possible coding (including channel coding), modulation (for example QAM or QPSK), possible MIMO coding, the transmitter 4 associates with the data associated with the first (respectively second) service and with a first (respectively second) broadcast area, a first (second) set of sub-carriers F1 to Fn (respectively F'1 to F'm) which carries an item of information representing the first (respectively second) data in each OFDM symbol as illustrated with respect to the previous figures. The first and second sets of sub-carriers are disjointed. Next, during a step 93 of OFDM symbols broadcasting intended for a plurality of receivers, the transmitter 4 broadcasts the OFDM symbols corresponding to the services associated with the area to which it belongs. The step 91 is then repeated. According to some embodiments, the steps 91 to 93 are successive. According to other advantageous embodiments, the steps 91 to 93 are realized simultaneously and in parallel (data being received at the same time than OFDM symbols, corresponding to other data previously received, are constructed and than OFDM symbols are broadcast).

Naturally, the invention is not limited to the embodiments previously described.

In particular, the invention is not limited to the broadcast of two services but can be extended to more than two services (for example three, four, ten or more). Within a given geographic area, for each service, a transmitter belonging to this area constructs OFDM symbols by associating with each service a set of dedicated sub-carriers. According to a variant, several services associated with a same area are grouped together and associated with a same set of sub-carriers.

The spectrum of the sets of sub-carriers assigned to services each one associated with a specific area is advantageously dissociated when specific areas intersect: for example, if the areas 1 1 and 12 intersect and if data of a first service covers both areas 1 1 and 12 and data of a second (respectively third) service covers the area 11 (respectively 12) without covering the area 12 (respectively 1 1 ), advantageously, the transmitters of the area 1 1 (respectively 12) assign a first set of sub-carriers to the first service and a second (respectively third) set of sub-carriers to the second (respectively third) service. The first, second and third sets do not intersect, which enables disturbances to be avoided within the geographic areas which intersect. The choice of sub-carriers can, for example, be realized according to messages transmitted by a transmitter, by a receiver (through, for example, a return channel), a user or any other type of means.

According to a variant embodiment, the local areas 1 1 and 12 are not totally covered by the area 10 and intersect only in part of the area 10 (receivers can be both be within the areas 10 and 1 1 , or only within one of the areas 10 and 1 1 ).

Claims

1. Method for broadcasting data, implemented in at least a first transmitter (2, 1 1 1 to 1 13, 121 , 122) characterized in that it includes steps of: - constructing (92) OFDM symbols, each symbol including first data and second data, said first data being associated with a first broadcast area (10); said second data being associated with a second broadcast area (1 1 , 12) which differs from said first broadcast area, said first broadcast area covering at least one part of said second broadcast area; a first set of sub-carriers (61 ) which carry an information representative of said first data in each OFDM symbol; a second set of sub-carriers (72) which carry an information representative of said second data in each OFDM symbol, said first and second sets of sub-carriers being disjointed; and

- broadcasting (93) of the OFDM symbols intended for a plurality of receivers.

2. Method according to the claim 1 , characterized in that the broadcast is time and frequency synchronised in said first and second broadcast areas.

3. Method according to any of claims 1 and 2, characterized in that said first set or said second set includes a plurality of contiguous sub-carriers.

4. Method according to any of claims 1 to 3, characterized in that said first set of sub-carriers is fixed.

5. Method according to any of claims 1 to 3, characterized in that said first set of sub-carriers is variable.

6. Method according to any of claims 1 to 5, characterized in that it comprises the steps of:

- receiving said first data from a first source; and - receiving said second data from a second source different from said first source.

7. Method according to any of claims 1 to 6, characterized in that said first data are associated with a first broadcast service and in that said second data are associated with a second broadcast service different from said first service.

8. Method according to any of claims 1 to 7, characterized in that the broadcast is of MIMO type.

9. System for broadcasting data, characterized in that it comprises: - a first set of transmitters belonging to a first broadcast area

(10);

- a second set of transmitters belonging to a second broadcast area (1 1 , 12), said first broadcast area covering at least one part of said second broadcast area, said first and second broadcast areas being different; each transmitter of said first set being suitable for broadcasting first OFDM symbols including first data, said first data being associated with the first broadcast area, a first set of sub-carriers (61 ) carrying an information representative of said first data in each first OFDM symbol, each transmitter of said second set being suitable for broadcasting second OFDM symbols including said first data and second data, said second data being associated with the second broadcast area, the first set of sub-carriers carrying an information representative of said first data in each second OFDM symbol, and a second set of sub-carriers (72) carrying an information representative of said second data in each second OFDM symbol, said first and second sets of sub-carriers being disjointed.