DORE, Renaud (7 rue Yves Mayeuc, Rennes, Rennes, F-35000, FR)
MOCQUARD, Olivier (12 rue Hoche, Rennes, Rennes, F-35000, FR)
GUILLOUARD, Samuel (10 rue de Champagne, Chantepie, Chantepie, F-35135, FR)
DORE, Renaud (7 rue Yves Mayeuc, Rennes, Rennes, F-35000, FR)
MOCQUARD, Olivier (12 rue Hoche, Rennes, Rennes, F-35000, FR)
| CLAIMS 1. Method for transmitting data, implemented in at least a transmitter (2, 1 11 to 1 13, 121 , 122) characterized in that it includes steps of: - constructing (92) OFDM symbols from first data and second data, a first set of subcarriers (61 ) carrying an information representative of said first data in each OFDM symbol, a second set of subcarriers (72) carrying an information representative of said second data in each OFDM symbol, said first and second sets of subcarriers being disjointed, said construction comprising: - a first coding of the first data by a first code of SISO type or MIMO type, the first code having a first dimension, - a second coding of the second data by a second code of MIMO type, the second code having a second dimension, said first dimension being strictly less than said second dimension, - an OFDM modulation of the first data which is coded by the first code and of the second data which is coded by the second code, and - a transmission (93) of the OFDM symbols intended for a plurality of receivers. 2. Method according to claim 1 , characterized in that said first coding has a dimension equal to 1. 3. Method according to claim 2, characterized in that said first coding corresponds to an identity coding. 4. Method according to claim 3, characterized in that said construction comprises a weighting of the first data, generating a plurality of symbols which are weighted from an incoming symbol and an OFDM modulation of each weighted symbol. 5. Method according to claim 1 , characterized in that said first coding has a dimension strictly higher than 1 and comprises a generation of a plurality of symbols which are coded from an incoming symbol and in that said construction comprises an OFDM modulation of each coded symbol. 6. Method according to any of claims 1 to 5, characterized in that it comprises 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 for receiving data, implemented in at least a receiver (5), characterized in that it comprises steps of: - receiving (101 ) OFDM symbols representative of first data and second data, - decoding (102) the OFDM symbols received, a first set of subcarriers (61 ) carrying an information representative of said first data in each OFDM symbol, a second set of subcarriers (72) carrying an information representative of said second data in each OFDM symbol, said first and second sets of subcarriers being disjointed, said decoding comprising an OFDM demodulation of the symbols received and supplying first decoded data and second decoded data: - said first data being coded by a first code of SISO type or MIMO type, the first code having a first dimension, - said second data being coded by a second code of MIMO type, the second code having a second dimension, said first dimension being strictly less than said second dimension. 9. Apparatus for transmitting data: characterized in that it comprises means: - for constructing (92) OFDM symbols from first data and second data, a first set of subcarriers (61 ) carrying an information representative of said first data in each OFDM symbol, a second set of subcarriers (72) carrying an information representative of said second data in each OFDM symbol, said first and second sets of subcarriers being disjointed, said construction comprising: - a first coding of the first data by a first code of SISO type or MIMO type, the first code having a first dimension, - a second coding of the second data by a second code of MIMO type, the second code having a second dimension, said first dimension being strictly less than said second dimension, - an OFDM modulation of the first data which is coded by the first code and the second data which is coded by the second code and a transmission (93) of the OFDM symbols intended for a plurality of receivers. |
1. Scope of the invention The invention relates to the domain of telecommunications and more precisely to the transmission of wireless data.
2. Prior art
According to the prior art, the transmission of data or the broadcast of services, television services for example, occurs through noisy channels that are subjected to interference (for example, fading corresponding to multiples paths related to echoes). In order to improve the reception, modulations for multi-subcarriers of OFDM (Orthogonal Frequency Division Multiplexing") type associated with SISO systems ("Single Input Single Output") implementing only one transmission antenna or with MIMO systems ("Multiple Input Multiple Output") implementing several transmission and reception antennas are relatively effective.
According to a technique known to the prior art and called SFN ("Single Frequency Network"), several SISO type transmitters transmit synchronously a same OFDM signal. In this way, a receiver receives a combination of the signals from several transmitters and decodes the combination obtained in this manner using the properties specific to the OFDM of cancellation of inter-symbol interference.
This technique has the disadvantage of monopolizing a large portion of the spectral resources in a wide geographical 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 purpose of the invention is to enable data transmission which is adapted to specific needs that do not require so many resources.
For this purpose, the invention proposes a data transmission method implemented in at least one transmitter. In order to improve the use of resources, the method comprises steps of:
- constructing OFDM symbols from first data and second data, a first set of subcarriers carrying information representative of the first data in each OFDM symbol; a second set of subcarriers carrying information representative of the second data in each OFDM symbol, the first and second sets of subcarriers being disjointed; the construction comprising:
- a first coding of the first data by a first code of SISO type or MIMO type, the first code having a first dimension;
- a second coding of the second data by a second code of MIMO type, the second code having a second dimension, the first dimension being strictly less than the second dimension;
- an OFDM modulation of the first data which is coded by the first code and of the second data which is coded by the second code and
- a transmission (93) of the OFDM symbols intended for a plurality of receivers.
Advantageously, the first coding has a dimension which is equal to 1.
According to a particular characteristic, the first coding corresponds to an identity coding, since the data are not modified by the coding (this may correspond to a simple link or a simple memory reading).
Advantageously, the construction comprises a weighting of the first data, generating a plurality of symbols which are weighted from an incoming symbol and an OFDM modulation of each weighted symbol.
According to another characteristic, the first coding has a dimension strictly higher than 1 and comprises a generation of a plurality of symbols which are coded from an incoming symbol and in that said construction comprises an OFDM modulation of each coded symbol. Advantageously, the method comprises steps of:
- receiving the first data from a first source; and
- receiving the second data from a second source different from said first source. According to a particular characteristic, the first data are associated with a first broadcast service and in that the second data are associated with a second broadcast service different from the first service. The invention also relates to a data reception method implemented in at least one receiver and characterized in that it comprises steps of:
- receiving (101 ) OFDM symbols representative of first data and second data,
- decoding (102) the OFDM symbols received, a first set of subcarriers (61 ) carrying information representative of the first data in each OFDM symbol; a second set of subcarriers (72) carrying information representative of the second data in each OFDM symbol, said first and second sets of subcarriers being disjointed; said decoding comprising an OFDM demodulation of the symbols received and supplying first decoded data and second decoded data: the first data being coded by a first code of SISO type or MIMO type, the first code having a first dimension; the second data being coded by a second code of MIMO type, the second code having a second dimension, the first dimension being strictly less than the second dimension. The invention also relates to a data transmission apparatus comprising means:
- for constructing (92) OFDM symbols from first data and second data, a first set of subcarriers (61 ) carrying information representative of said first data in each OFDM symbol; a second set of subcarriers (72) carrying information representative of said second data in each OFDM symbol, said first and second sets of subcarriers being disjointed; the construction comprising: - a first coding of the first data by a first code of SISO type or
MIMO type, the first code having a first dimension;
- a second coding of the second data by a second code of MIMO type, the second code having a second dimension, said first dimension being strictly less than said second dimension; - an OFDM modulation of the first data which is coded by the first code and the second data which is coded by the second code and
- a transmission (93) of the OFDM symbols intended for a plurality of receivers.
The invention also relates to a system comprising the transmission device and devices transmitting the first data synchronously.
The invention also relates to a receiver corresponding to the reception method.
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 wireless communications system implementing several transmitters and receivers, according to a particular embodiment of the invention,
- figure 2 presents a very diagrammatical block diagram of a transmitter of the system in figure 1 , according to the invention,
- figure 3 presents a very diagrammatical block diagram of transmitters of the system in figure 1 , - figures 4 and 5 diagrammatically illustrate a transmitter and a receiver of the system in figure 1 , respectively, according to the invention,
- figure 6 illustrates a broadcast spectrum used by a transmitter of the system in figure 1 ,
- figures 7 and 8 present the use of a broadcast spectrum by different antennas of a transmitter of the system in figure 1 , according to the invention,
- figure 9 illustrates a data transmission method according to a particular embodiment of the invention, implemented by transmitters of the system in figure 1 , and - figure 10 presents a data reception method according to a particular embodiment of the invention, implemented by receivers of the system in figure 1.
5. Detailed description of the invention The general principle of the invention is based on the use of
OFDM modulators by a transmitter for the transmission of two data sets. A first data set uses partial resources of a transmitter: first signals represent the data of the first set and are of SISO type or MIMO coded associated with a first code of a first dimension. A second data set fully uses the OFDM modulation resources of a transmitter: second signals represent the data of the second set and are of MIMO type associated with a second code of a second dimension. The first dimension is strictly less than the second dimension: if the second dimension is equal to Mx, the first dimension is for example equal to 1 if the first data set is transmitted in SISO mode and is greater than 1 and strictly less than Mx if the first data set is transmitted in MIMO mode. Figure 1 illustrates a broadcasting system 1 implementing several transmitters 101 to 106, 1 1 1 to 1 13, 121 and 122 according to a particular embodiment of the invention. The transmitters 101 to 106 have only one transmission antenna and are of SISO type. The transmitters 1 1 1 to 1 13 and
121 to 122 are of MIMO type and each have a MIMO encoder and several antennas transmitting a MIMO type signal. The system 1 also has receivers
(not illustrated), some of which are of MIMO type and are able to receive and decode the signals transmitted by the transmitters 1 11 to 1 13, 121 and/or
122 as well as the signals transmitted by the transmitters 101 to 106. The receivers are for example fixed receivers (including television decoders) or handheld devices, for example suitable for receiving and processing broadcast 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 itself geographic areas 1 1 and 12 which are different. Hence, 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 kilometers 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 punctual 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 disturbed 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 invention is not necessarily implemented in a system with several zones. Hence, according to a variant, the system 1 only has MIMO type transmitters 1 1 1 to 1 13. In this case, the synchronization is limited to these transmitters. Neither is the invention limited to a data broadcasting and also applies to the multicast or unicast transmission of data. In this case, the synchronization based on an external signal (transmitted by a server, for example) is advantageously not implemented. Figure 2 shows an architecture of a transmitter 2 suitable for transmitting two data sets (first and second sets) (associated or not with two different services, respectively) according to the invention (broadcasting, multicast or unicast mode) 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 comprises:
- an encoder 20 which receives a set of first data 200 associated, for example, with a first service and with a first broadcast area (for example the area 10),
- an encoder 21 which receives a set of second data 210 associated, for example, with a second service and with a second broadcast area (for example the area 1 1 ) different from the first broadcast area,
- Mx OFDM modulators 221 to 22Ntx which transmit OFDM symbols to at least one antenna respectively 231 to 23Mx; and - a synchronization module 24 which receives a synchronization signal 240 (for a transmitter used in broadcast mode) and which transmits a signal 241 which synchronizes the OFDM modulators 221 to 22Mx.
The synchronization signal 240 generates the signal 241 according to any one of the methods known to those skilled in the art. The signal 240 is for example a signal including a precise timestamp. The synchronization module 24 then generates the signal 241 by using a complementary means (for example by using a GPS (Global Positioning
System) signal). The synchronization module 24 can also generate the signal 241 from a signal carried on an Internet network. Advantageously, the OFDM modulators are synchronized 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. According to a multicast or unicast embodiment, the synchronization of the OFDM modulators is advantageously performed locally without using an external signal 240. 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 a 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 producing modulated symbols 204 (respectively 214).
The encoder 21 also comprises:
- a MIMO encoder 215 which encodes the modulated symbols 214 to form Mx streams of MIMO data blocks 2161 to 216Mx intended respectively for each one of the OFDMs 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. Q is for example equal to 1600 and is equal to the number m of subcarriers associated with the encoder 20.
The modulator 213 generates groups of Q' modulated symbols. 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 n of subcarriers associated with the encoder 21. n is for example equal to 100 with an encoder rate 215 worth 1 (with a full rank encoder of Blast type) and 2 antennas (Mx = 2). n is for example equal to 200 with an encoder rate 215 worth 0.5 (with an encoder of Alamouti type) and 2 antennas (Mx = 2). Advantageously, the total number of subcarriers is lower than the
FTT (Fast Fourier Transform) size used in the OFDM modulators. The number of subcarriers 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 more rate than the second service, the number of subcarriers assigned to the first service will be greater than those corresponding to the second service. The encoder 215 (and possibly the encoder 20) encode the modulated symbols at input with spatial multiplexing (for example based on a BLAST multiplexing of Bell laboratories (as described, for example, in the document written by G. J. Foschini, named "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" written 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" published in the IEEE Journal on selected area in communications in October 1998). At its output, the encoder 215 (respectively 20), the MIMO data blocks 2161 to 216Mx (respectively 2061 to 2060Mx' with Mx' being strictly less than Mx if the encoder 20 implements a MIMO coding) are each assigned to an OFDM modulator.
According to another variant, the encoder 20 is of SISO type (the associated code has a dimension equal to 1 ) and also comprises a weighting module in phase and in amplitude which uses a weighting vector. The weighting module receives the symbols generated by the modulator 203. For each symbol received, it produces Mx" symbols which are weighted phase- wise and amplitude-wise and feeds Mx" OFDM modulators 221 to Mx" and Mx" antennas 231 to 23Mx", so as to control the coverage area. Mx" can take any value less than or equal to Mx. According to different embodiments, the coverage area can thus be balanced (no direction being privileged) or, on the contrary, favour some directions (and/or penalize other directions). The weighting vector can be obtained using a simulation, a calculation and/or in situ measures (abstraction is made of the encoder 21 ) according to techniques known by those skilled in the art (for example, beam forming) and applied to a transmitter which does not use any encoder 21. According to another variant, the encoder 20 also comprises a MIMO encoder which codes the modulated symbols 204 forming Mx' streams of data blocks intended respectively for each one of the OFDM modulators 221 to 22Mx'. Hence, the dimension of the encoder 20 is equal to Mx 1 and the dimension of the encoder 21 is equal to Mx, Mx' being strictly less than Mx. The MIMO encoders of the encoders 20 and 21 can be of the same type or of a different type (for example, a full diversity scheme for the encoder 20 (including orthogonal space/time coding (or OSTBC for "orthogonal STBC"), for example an Alamouti coding) and a full rate scheme for the encoder 21 (including spatial multiplexing of BLAST coding type). Generally, the system comprises or does not comprise transmitters comprising only encoders which are similar to the encoders 20, this variant notably enables guarantying a quality of service and a rate which are different for the signals from the encoders 20 and 21 , and thus an adaptation to requirements. In a system 1 comprising transmitters 101 to 105 and transmitters 2 having a MIMO encoder 20, this variant enables a higher spectral efficiency for a first service associated with the encoder 20 and a higher rate for a second service associated with the encoder 21 to be obtained. Figure 3 shows an architecture of a transmitter 3 suitable for broadcasting data associated with a distinct 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 which allows the transmission of first data associated or not with 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);
- an OFDM modulator 221 which transmits OFDM symbols on a single antenna 231 ; and
- a synchronization module 24 which receives a synchronization signal 240 and which transmits a signal 241 that synchronizes the OFDM modulator 221. According to the embodiment described, the transmitter 3 comprises only one encoder associated with a transmission of a set of first data (for example, a first service) (encoder 20), which allows a dedicated implementation. According to a variant, the transmitter 3 has a structure similar to the transmitter 2, the part associated with the transmission of other data sets (for example, 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 the transmission of a second data set or a second service, an enable signal which validates the use of the encoder 20 and the corresponding subcarriers in the OFDM modulators or which acts directly at the level of OFDM modulators for authorizing or prohibiting the use of subcarriers associated with the second data set.
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 subcarriers F1 to Fm 610 to 61 m. The subcarriers F1 to Fm broadcast a signal which carries information representing data associated with a first data set (for example the first service S1 ) in each OFDM symbol.
Figure 7 illustrates the combination of the spectrum 61 and a spectrum 72 used by the antenna(s) (antenna 231 or antennas 231 to 23Mx 1 ) transmitting a signal from the encoders 20 and 21. The spectrum 72 includes n contiguous subcarriers F'1 to F'n 720 to 72n. The subcarriers F'1 to F'n transmitted by these antennas broadcast a signal which carries information representing data associated with the second data set (for example the second service S2) in each ODFM symbol.
Figure 8 illustrates the spectrum 72 used by the antenna(s) (antenna 232 to 23Mx if the encoder 20 uses only the antenna 231 or antennas 23(Mx 1 H-I ) to 23Mx if the encoder 20 uses the antennas 231 to
23Mx 1 ) which transmit a signal from the encoder 21 and which do not transmit any signal from the encoder 20.
Hence, at a time t, when the invention is implemented in a broadcast system, the transmitters 101 to 105, 1 1 1 to 113, 121 and 123 are synchronized with the signal 240 and broadcast in a synchronized way, on the same subcarriers of the spectrum 61 , information representing data associated with the first data set (or 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 subcarhers of the spectrum 72, information representing data associated with the second data set (for example second service S2), these data being modulated and coded in the same way; the transmitters 121 and 122 broadcast, on the same subcarhers of the spectrum 72, information representing data associated with a third data set (for example 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 data set. 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 third) data sets. 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 the receiver within the area 1 1 (respectively 12) to have a better reception of the data associated with the first data set, 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 kilometers of radius) and the area 1 1 (respectively 12) corresponds to an area inside buildings). Moreover, the use of spectral resources is optimized 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 subcarriers relative to the number of subcarriers available.
According to an advantageous embodiment of the invention, the first and/or second sets of subcarriers 61 and 72 are fixed. This notably enables the implementation to be facilitated.
According to another advantageous embodiment of the invention, the first and/or second sets of subcarriers 61 and 72 will 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 the 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 subcarriers 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 subcarriers used correspond in totality or in part to the subset 61. Hence, the transmitters 101 to 105 implement a selection of subcarriers 61 in a set of subcarriers available.
Advantageously, the subcarriers associated with at least one of the data sets (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 subcarriers respectively F1 to Fm and F'1 to F'n in the spectrum available including n+m subcarriers from F1 to F'n. Other embodiments are possible with contiguous (or consecutive) subcarriers in a spectrum including frequencies F"1 to F"m+n (F"1= F1 ; F"m= Fm ; F"m+1 = F'1 and F"m+n=F'n) for at least one of the data sets: for example, one embodiment where the spectrum associated with the first data set S1 uses m subcarriers from F"i to F"i+m and where the spectrum associated with the second (S2) or third (S3) data sets uses n subcarriers 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 data set S1 uses m subcarriers 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) data sets uses n subcarriers from PV to F"i+n.
According to another variant, the subcarriers 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 a bus 44 addresses and data, also transporting a clock signal: - a microprocessor 41 (or CPU),
- a non-volatile memory of ROM (Read Only Memory) type 42,
- a Random Access Memory (RAM) 43, - an interface 45 suitable for the reception of a first data set (for example service),
- an interface 46 suitable for the reception of a second data set different from the first set, - an interface 47 suitable for the transmission of data sets (for example service broadcast or multicast or unicast transmission) and performing notably the functions of the encoders 20 and 21 , the ODFM modulators 221 to 22Mx and the antennas 231 to 23Mx, - an interface 48 suitable for receiving the synchronization signal
240 and for synchronizing 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 subcarriers and data sets 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 data received and to be broadcast). The memory ROM 42 comprises in particular:
- a programme "prog" 420,
- parameters 421 of physical layers, and
- parameters 422 associated with the subcarriers. 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 the microprocessor 41 responsible for switching on the transmitter 4,
- incoming data 431 corresponding to the first data set S1 , - incoming data 432 corresponding to another data set S2,
- coded data 433 for the transmission of the data sets. Figure 5 illustrates schematically a hardware embodiment of a receiver 5 belonging to the system 1 and suitable for receiving and decoding the signals transmitted by the transmitters 2 (for example 1 1 to 1 13, 121 and 122). Advantageously, the receiver 5 is suitable for receiving the signals transmitted by the transmitters 101 to 105. Hence, the management of the system 1 is facilitated.
The receiver 5 comprises, connected to each other by a bus 54 of addresses and data, also transporting a clock signal:
- a microprocessor 51 (or CPU), - a non-volatile memory of ROM (Read Only Memory) type 52,
- a Random Access Memory (RAM) 53,
- a radio interface 55,
- an MMI interface 56 suitable for displaying information for a user and/or inputting data or parameters (for example the setting of subcarriers and data sets transmitted), and
- an interface 57 to a particular application suitable for processing the data of data set transmitted by the transmitters of the area to which the receiver 5 belongs and received by the receiver 5. 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 data sets received or decoded). The memory ROM 52 comprises in particular:
- a programme "prog" 520,
- parameters 521 of physical layers, and
- parameters 522 associated with the subcarriers of the data sets 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 realization, 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 the signals transmitted by the transmitters of the system 1. It notably includes a decoder associated with the encoders 20 and 21 of the transmitters. Advantageously, it includes Mx' OFDM demodulators if the corresponding transmitters include an encoder 20 implementing a code of dimension Mx 1 and/or Ntx OFDM demodulators corresponding to the encoder 21 of the transmitters. The demodulators used to decode the signals from the encoder 231 are reused for the decoding of the signals from the encoder 20 according to some embodiments and are not reused according to other embodiments. In particular, the receiver is suitable for receiving a broadcast signal which can include a part of the subcarhers available (for example subcarhers F1 to Fm corresponding to the first data set) or all the frequencies available (for example subcarriers F1 to Fm+n.) Advantageously, the receiver 5 decodes each OFDM symbol received for each set of subcarriers. 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 subcarriers 61 , implement also a selection of the set 61 to receive and decode the first associated data (for example by detecting subcarriers 61 used in the set of subcarriers available or by exchanging information between a transmitter or management device and the receiver). Figure 9 presents a data transmission method implemented in the transmitter 4 according to a particularly advantageous implementation of the invention.
During an initialization step 90, the different parameters of the transmitter 4 are updated. In particular, the parameters corresponding to the data sets to be transmitted and to the corresponding subcarhers are initialized in any manner (for example, following the reception of initialization messages transmitted by one or several servers or other element of the system 1 , or even, by commands from an user). Next, during a step 91 , the transmitter 4 generates or receives first data corresponding to a first data set S1 to be transmitted and second data corresponding to the second data set S2. According to some embodiments, the transmitter 4 is configured according to the area to which it belongs. Advantageously, it then generates or receives the data associated with the data sets (for example services) it broadcasts within its area. According to a variant, the transmitter 4 performs a step of filtering the data sets by processing for broadcasting only the data of sets 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 data set of a first source and of the data sets corresponding to a second service of a second source different from the 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 data sets comes from a same source. Hence, subcarriers and coverage areas are associated with each data set, irrespective of whether the sources of the first and second data sets are identical or different.
Then, during a step 92, the transmitter 4 realizes a step of OFDM symbols construction, each symbol carrying an information representing the first data and the second data. In particular, following a coding (including channel coding), a modulation (for example QAM or QPSK), a possible MIMO coding, the transmitter 4 associates with the data associated with the first (respectively second) service, a first (second) set of subcarriers F1 to Fn (respectively F'1 to F'm) which carries an information representing first (respectively) data in each OFDM symbol as illustrated with respect to the previous figures. The first and second sets of subcarriers are disjointed. Next, during a step 93, 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).
Figure 10 presents a reception method implemented in the receiver 5 suitable for receiving signals transmitted by the transmitter 2 according to a particularly advantageous implementation of the invention. If the receiver 5 receives signals transmitted by the transmitters 101 to 105, it is also suitable for decoding these signals.
During an initialization step 100, the different parameters of the receiver 5 are updated. In particular, the parameters corresponding to the data transmitted and to the corresponding subcarhers are initialized in any manner (for example, following the reception of initialization messages transmitted by one or several servers, transmitters or other element of the system 1 , or even, by commands from an user). Next, during a reception step 101 , the receiver 5 receives OFDM signals transmitted by the transmitter 2. The signals received include:
- corresponding first signals of SISO type or MIMO coded associated with a second code of a first dimension, the first signals coming from the encoder 20,
- second signals MIMO coded associated with a second code of a second dimension, the second signals coming from the encoder 21.
The first dimension Mx' is strictly less than the second dimension. A first set 61 of subcarhers carries the first signals as illustrated in figure 7. A second set 72 of subcarhers carries the second signals as illustrated in figures 7 and 8, the first and second sets 61 and 72 of subcarhers being disjointed.
Next, during a receiving step 102, the receiver 5 decodes the signals received during step 101. Advantageously, the receiver 5 decodes each OFDM symbol received for each set 61 and 72 of subcarhers. 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. Naturally, the invention is not limited to the embodiments previously described.
In particular, when the invention is applied to broadcasting, it 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 subcarriers. According to a variant, several services associated with a same area are grouped together and associated with a same set of subcarriers.
As mentioned above, the invention generally applies to the transmission of data sets and in particular to sets of two, three or more generally any number of data sets. Here, a data set corresponds in particular to a service (e.g. audiovisual), to data from a same source, to data from and to a same application and/or different types of transmission (for example, broadcast of a first data set and unicast or multicast transmission of a second data set). According to the invention, different types of transmission (e.g. unicast, multicast or broadcast) can advantageously be combined.
The spectrum of the sets of subcarriers assigned to data sets each 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 data set covers both areas 1 1 and 12 and data of a second (respectively third) data set covers the area 1 1 (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 subcarriers to the first service and a second (respectively third) set of subcarriers to the second (respectively third) service. The first, second and third sets do not intersect, which allows to avoid disturbances within the geographic areas which intersect. The choice of subcarriers 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 associated with different coverage areas, the local areas 1 1 and 12 are not totally covered by the area 10 and intersect only in part the area 10 (receivers can be both within the areas 10 and 1 1 , or only within one of the areas 10 and 11 ).