Abstract:
The invention relates to a digital demodulator whose architecture is adapted to multicarrier modulations (radio wave transmissions), but which remains suitable for use for monocarrier modulations (cable and satellite transmissions). With multicarrier modulations, the demodulator must carry out certain functions at a frequency of the order of sampling frequency and other functions at a frequency of the order of the symbol frequency. The invention comprises a separation of the architecture into three modules: a first module which carries out programs which are repeated with a first frequency, a second module capable of using programs which are repeated with a second frequency, and an interface module between the first and the second module. An advantage is that the memory size necessary for storing instructions for the first module is reduced. An application is for DVB standard transmission of digital TV programs.

Description:
FIELD OF THE INVENTION 
     The invention relates to a transmission system comprising at least a transmitter and a receiver, said receiver comprising a digital data demodulator. The invention also relates to a receiver and a demodulator designed for use in such a system. 
     The invention has important applications in the field of digital modulations. 
     BACKGROUND OF THE INVENTION 
     The standardization project DVB-T (Digital Video Broadcasting-Terrestrial) as defined by the ETSI and relating to the distribution of digital TV programs by radio links describes an example of such a transmission system. In this project in particular multicarrier modulations which are used which renders it possible to use the characteristics of the transmission channel to the best advantage. 
     The use of multicarrier modulations leads to a particular problem. The multicarrier transmission technique consists in a frequency multiplexing of N carriers which are modulated by points of a constellation (for example, points of a QAM constellation). Each transmitted symbol (called FDM symbol, short for Frequency Division Multiplexing) thus corresponds to a block of N points, each point of the block modulating one of the N carriers. In the case of multicarrier transmissions, the sampling frequency of the transmitted signal is accordingly much higher than the frequency of the FDM symbols. At the demodulator level, certain demodulation functions are carried out at a frequency of the order of the sampling frequency of the received data, whereas other functions are carried out at a frequency of the order of the symbol frequency. 
     On the other hand, it is desirable to use a static communication model for managing the data exchanges inside such a demodulator. The use of a static communication model comprises the realization of given functions at given moments by executing programs which are repeated at one and the same frequency, so that data can be provided in a regular rhythm. This type of communication model is advantageous because it enables to guarantee that all the data are correctly transmitted and that accordingly the functions are correctly executed. 
     If the architecture of a demodulator for multicarrier modulations is a static architecture, the choice of the common repetition frequency must necessarily be the lowest frequency from among the frequencies which can be used, i.e. a frequency which is of the order of the symbol frequency. This means that a very large number of instructions must be stored in a memory. In particular, the instructions relating to the treatment of the various carriers will have to be stored as many times as there are carriers. This solution is extremely costly from a memory point of view. 
     SUMMARY OF THE INVENTION 
     The invention has for its object to provide a demodulator which provides a solution to this problem. 
     This object is achieved with a transmission system, a receiver, and a demodulator as described in the opening paragraphs which are characterized in that said demodulator comprises: 
     a first module designed for carrying out first demodulation functions in accordance with at least one first program which is repeated with a first frequency, 
     a second module capable of carrying out second demodulation functions in accordance with at least one second program which is repeated with a second frequency, and 
     an interface module for exchanging data between said modules. 
     The invention thus provides a separation of the demodulator architecture into two parts which are in communication through an interface. In the case of multicarrier modulations, one of these parts (the first module) is designed for carrying out functions which must be performed at a frequency of the order of the sampling frequency. The other part (the second module) is designed for carrying out functions which must be performed at a frequency of the order of the symbol frequency. The operating programs of the first module have a repetition frequency of the order of the sampling frequency. Consequently, the instructions relating to the processing of the various carriers are only stored in a memory a minimum number of times. The necessary memory length for the storage of said programs is accordingly reduced to a minimum. 
     The invention furthermore provides the advantage that an architecture is created which can be used for transmission systems which use monocarrier modulations, for example for transmission systems by cable or by satellite. In this type of system, the sampling frequency and the symbol frequency are of the same order of magnitude such that the problem described above does not arise. In that case, only one of the modules is used (the first module). The operating programs of this module have a repetition frequency of the order of the symbol frequency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood and further details will become apparent from the following description with reference to the annexed drawings, given by way of non-limitative example, in which: 
     FIG. 1 shows an example of a transmission system according to the invention, 
     FIG. 2 shows the structure of an FDM symbol, 
     FIG. 3 is a diagram of the architecture of a digital demodulator according to the invention, 
     FIG. 4 is an explanatory diagram of the management of the internal communications in a demodulator as shown in FIG. 3, 
     FIG. 5 is a diagram of an embodiment of an interface module of a demodulator according to the invention, 
     FIG. 6 is a diagram of an OFDM demodulator according to the invention, and 
     FIG. 7 is a diagram of a demodulator according to the invention for monocarrier modulations used in cable or satellite broadcasting systems for digital programs in accordance with the definitions of the standardization projects DVB. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows an example of a digital transmission system according to the invention, showing a transmission between a transmitter  1  and a receiver  2  via a transmission medium  3 . The transmitter  1  comprises a data source  11 , a source coder  12 , a channel coder  13 , and a digital modulator  14 . The receiver  2  comprises a digital demodulator  21 , a channel decoder  22 , and a source decoder  23 . The transmission medium  3  may be of various kinds, for example, it may be a cable network, a satellite channel, or a radio wave channel. The modulation used is chosen as a function of the transmission medium, taking into account to an optimum degree the characteristics of the transmission medium. In particular, monocarrier modulations will be used for cable and satellite transmissions, and multicarrier modulations for radio wave transmissions, because the technique of multicarrier modulation offers a good protection against the selectivity of the radio wave channels, the multipath propagation, and interferences between radio wave channels. 
     In the DVB-T standardization project as defined by the ETSI, the multicarrier modulations used are OFDM (Orthogonal Frequency Division Multiplexing) modulations. The OFDM technique consists in a frequency multiplexing of N orthogonal carriers which are modulated by points of a constellation. (for example, points of a QAM constellation). Each transmitted symbol (referred to as OFDM symbol) thus corresponds to a block of N points, each point of the block modulating one of the N orthogonal carriers. 
     FIG. 2 shows the structure of an example of an OFDM symbol as defined in the DVB-T standardization project. Each symbol is composed of a guard interval Tg followed by a useful part Tu. The guard interval Tg serves to eliminate interferences between the symbols. The useful part comprises N=8192 samples. These 8192 samples correspond to 6817 useful carriers. Some of these 6817 useful carriers transport data and other ones transport control information. The nature of a carrier (useful carrier, data carrier or control information carrier) is determined by its position in the OFDM symbol. The control information carriers are used essentially for the synchronization and channel determination. Certain processes to be carried out on the carriers are thus carried out at a frequency of the order of the symbol frequency. By contrast, the processes to be carried out on the useful data carriers all take place at a frequency of the order of the sampling frequency. 
     FIG. 3 shows a diagram of the basic architecture of a digital demodulator  21  according to the invention. The demodulator  21  comprises a plurality of calculation units PU i  (i=1, . . . , K) controlled by programs PGM i,j  (j=1, . . . , L i ) which are repeated with a first frequency. These calculation units PU i  communicate with one another via an interconnection network INT. 
     The digital demodulator described here is a programmable demodulator which can be programmed so as to be used for different types of modulation. This implies in particular that the symbol frequency is not necessary known in advance. The architecture used being a static architecture, periodic communication moments (t 0 , t 1 , . . . ) are provided for transferring the data (D 0 , D 1 , . . . ) from one calculation unit to another (see FIG.  4 ), independently of the value of the symbol frequency. It may accordingly happen that no data is available when a communication must take place (for example, at moment t 2 ). A validity indicator Iv is accordingly associated with each communication interval for informing the calculation unit which is expecting a data whether or not there will be a data in this communication time interval. At moment t 2 , for example, the validity indicator has a zero value because no data is available. 
     The calculation units PU i  (i=1, . . . , K−1) form a first module  30 . They carry out first demodulation functions in accordance with PGM i,j  programs which are repeated with a first frequency. The calculation unit PU K  comprises a second module  32  and an interface module  34 . The second module  32  is capable of carrying out second demodulation functions in accordance with one or several programs  36  which are repeated with a second frequency. 
     An embodiment of an interface module  34  is shown in detail in FIG.  5 . It comprises selection means  40  for selecting data to be transmitted, a FIFO memory  42  for storing the data to be transmitted to the second module  32 , a FIFO memory  44  for storing results provided by the second module, and transmission means  46  for transmitting the results stored in the memory  44  to the first module  30 . This interface module  34  is controlled by PGM K,j  programs, in particular by a writing program PGM K,1  and by a reading program PGM K,2 . 
     The data received by the calculation unit PU K  are either symbols containing N carriers or any data whatsoever which are to be transmitted to the second module  32 . The selection means  40  are used when the data received is a symbol so as to select in this symbol the carriers to be transmitted to the second module  32  (only the carriers for control information only must be transmitted). The selection means  40  are controlled by the writing program PGM K,1 . They comprise a counter  50  and a table  52 . The counter  50  numbers the carriers contained in the symbol in their order of appearance. The table  52  contains for each carrier number a transfer identifier It which indicates whether the corresponding carrier must or must not be transmitted to the second module  32 , i.e. whether it should be copied into the FIFO memory  42 . 
     The data which are not symbols and which are to be transmitted from the first module  30  to the second module  32  are copied directly into the FIFO memory  42 . 
     The FIFO memory  42  contains on the one hand the data to be transmitted to the second module  30  and on the other hand a function identifier If for each data which indicates the source function and/or the destination function of the data. This function identifier If renders it possible for the second module  32  to know where the corresponding data is to be stored in the memory in view of its subsequent processing. 
     The second module  32  carries out one or several functions which lead to results. These results are stored in the FIFO memory  44  with a function identifier If which indicates the source function and/or the destination function of the result. This function identifier If renders it possible to control, in conjunction with the transmission means  46 , the communication time slot at which a result must be transmitted to the first module  30 . 
     The transmission means  46  are controlled by the reading program PGM K,2 . The program PGM K,2  comprises instructions which indicate the type of communication which must take place at the communication time slot. Each type of communication corresponds to the transmission of a result of a certain type. The type of a result is indicated by the function identifier If which is associated therewith. The transmission means  46  are formed by a table of correspondences which indicates the correspondence between a type of communication and the type or types of results to be transmitted each time. For example, there may be two different types of communications C 1  and C 2 , a communication of type C 1  corresponding to the transmission of a result whose function identifier If is equal to Z 1  or Z 2 , and a communication of type C 2  corresponding to the transmission of a result whose function identifier is equal to Z 3 . The reading program PGM K,2  consults the table of correspondences for each communication time slot so as to determine whether the function identifier for the result at the output of the FIFO memory  44  corresponds to the type of the communication. If this is the case, the result at the output of the FIFO memory is transferred to the first module with a validity identifier Iv equal to one. The reading program then moves to the next data. If it is not the case, the output result is not transferred (which means that it remains in the FIFO memory), and the validity indicator Iv associated with the current communication time slot is set for zero. This mechanism ensures that the results are transmitted at correct communication time slots with respect to the needs of the first module  30 . 
     FIG. 6 shows an example of an OFDM demodulator according to the invention. The calculation units of the first module  30  here carry out the demodulation functions at a frequency of the order of the sampling frequency. They are controlled by PGM i,j  programs which are repeated with a frequency of the order of the sampling frequency. These calculation units each comprise essentially a calculation unit PU 1  which transposes the received signal into the baseband, a calculation unit PU 2  which carries out the treatments for the purpose of synchronization, a calculation unit PU 3  which carries out essentially an inverse Fourier transform operation for retrieving symbols, a calculation unit PU 4  for channel correction, a unit PU 5  which is a delay memory for storing a symbol during the channel correction operation, and a decoding unit PU 6 . The interface module  34  and the second module  32  are integrated into a calculation unit PU 7 . The second module  32  carries out the demodulation functions which take place at a frequency of the order of the symbol frequency. It is controlled by programs  36  which are repeated with a frequency of the order of the symbol frequency. The exchanges between the first module  30  and the second module  32  take place, for example, in the following manner: 
     the unit PU 3  transmits symbols to the unit PU 7 , 
     the unit PU 7  transmits results relating to the synchronization to the unit PU 2 , 
     the unit PU 7  transmits results relating to the channel correction to the unit PU 4 . 
     FIG. 7 shows an example of a demodulator according to the invention for monocarrier modulations. This demodulator comprises a first module  30 , a second module  32 , and an interface module  34 . The first module  30  essentially comprises a calculation unit PU 10  which transposes the received signal into the baseband, a calculation unit PU 20  which carries out functions for the purpose of synchronization, a calculation unit PU 30  which carries out filtering operations for retrieving the symbols, and a decoding unit PU 40 . The interface module  34  and the second module  32  are integrated into a calculation unit PU 7 . The calculation unit PU 7  is not used. All demodulation functions are carried out by the first module  30 . The programs which control the operation of the calculation units PU 10 , PU 20 , PU 30 , and PU 40  are repeated with a frequency of the order of the symbol frequency. 
     A programmable digital demodulator has been described which can be used for various types of modulations. The invention, however, is not limited to this example. It is in particular applicable to an OFDM demodulator which operates at a given symbol frequency and in which the communications (in particular the communication time slots at which the data must be transmitted) can be managed in a simpler manner than in the example described. It is clear that in that case the interface module may be simplified compared with the one described above. 
     Similarly, if all the data transmitted from the first module  30  to the second module  32  originate from the same calculation unit, for example if the only data transmitted are symbols provided by the unit which performs the Fourier transform calculations, it will be of no use to store a function indicator in the fifo memory  42  which indicates the source and/or destination function of the data.