PATENT ABSTRACT
An LDPC decoder has a determined number of processing units operating in parallel. Storage circuitry contains first words having a juxtaposition of a first type of message. The storage circuitry also contains second words having a juxtaposition of a second type of message. A message provision unit provides each processing unit with the messages. A message write unit may write words into the storage circuitry in a way that depends on the contents of the words. The message provision unit may provide data in a way that depends on the contents of the words.

PATENT DESCRIPTION
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an LDPC (Low Density Parity Check) decoder. 
   2. Discussion of the Related Art 
   LDPC decoding enables obtaining performance very close to the theoretical Shannon limit. LDPC decoders generally have better performance than turbo-code decoders and are currently used in broadcasting systems, for example, satellite digital television broadcasting systems. 
   The LDPC decoding consists, for a given word s containing k bits, of transmitting a word u containing n bits, among which n-k additional bits called parity bits which fulfill n-k parity equations. The matrix H having n-k lines and n columns verifying the following relation is called the code control matrix:
 
 H*   t   u =0  (1)
 
   The size of the matrices H generally used is significant since they may comprise more than 60,000 columns and 50,000 lines. Further, such matrices are practically empty and may comprise one “1” for approximately 10,000 “0”s. This is why they are called low-density matrices. 
   The number of “1”s in the line is called the check node degree of a line and the number of “1”s in the column is called the bit node degree. There are two types of LDPC codes: regular codes and irregular codes. Matrix H of a regular code has a constant number of “1”s per line and per column. The check node and bit node degrees are then constant. An irregular code has no constant check node degrees and/or bit node degrees and has a more random character. However, the best performances are obtained for irregular codes. The problem of an irregular matrix is that it is random. The decoding can then be difficult. European digital satellite television standard DVB-S2 advocates for the use of a matrix H which has a constant check node degree, and a bit node degree that can take three values. 
     FIG. 1  shows a graph corresponding to an equivalent representation of a matrix H of an LDPC code. Each circle  10  represents a bit node. As an example, six bit nodes BNi, with i ranging from 0 to 5, are shown. Each square  12  represents a check node. As an example, four check nodes CNj, with j ranging from 0 to 3, are shown. Branches  14  connect given bit nodes BNi to given check nodes CNj. The presence of a branch  14  between a bit node of index i and a check node of index j means that the equivalent matrix H has a “1” at line j and at column i. Note Ne the total number of branches  14  in the graph. Number Ne thus corresponds to the number of “1”s of matrix H. 
   The matrix H equivalent to the graph shown in  FIG. 1  is the following: 
   
     
       
         
           
             
               
                 H 
                 = 
                 
                   ( 
                   
                     
                       
                         110010 
                       
                     
                     
                       
                         101001 
                       
                     
                     
                       
                         010110 
                       
                     
                     
                       
                         001101 
                       
                     
                   
                   ) 
                 
               
             
             
               
                 ( 
                 2 
                 ) 
               
             
           
         
       
     
   
   Such a matrix H has a constant bit node degree equal to 2 and a constant check node degree equal to 3. 
   LDPC decoding uses log-likelihood ratios (LLR). For a transmitted information bit d to which corresponds a signal y received by the decoder after addition of the transmit channel noise, the LLR ratio of bit d with respect to signal y is defined as follows: 
   
     
       
         
           
             
               
                 
                   LLR 
                   ⁡ 
                   
                     ( 
                     y 
                     ) 
                   
                 
                 = 
                 
                   
                     LLR 
                     ⁡ 
                     
                       ( 
                       
                         d 
                         ❘ 
                         y 
                       
                       ) 
                     
                   
                   = 
                   
                     ln 
                     ⁢ 
                     
                       
                         P 
                         ⁡ 
                         
                           ( 
                           
                             ⅆ 
                             
                               = 
                               
                                 0 
                                 ❘ 
                                 y 
                               
                             
                           
                           ) 
                         
                       
                       
                         P 
                         ⁡ 
                         
                           ( 
                           
                             ⅆ 
                             
                               = 
                               
                                 1 
                                 ❘ 
                                 y 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
         
       
     
   
   where P(d=0|y) is the probability for transmitted bit d to be equal to 0 according to the received value y and P(d=1|y) is the probability for transmitted bit d to be equal to 1 according to the received value y. Such probabilities especially depend on the features of the transmit channel. The larger LLR(y), the greater the probability for transmitted bit d to be equal to “0”. As an example, it may be decided, for a received signal y, that if LLR(y) is negative, then transmitted bit d is a “1” and that, otherwise, transmitted bit d is a “0”. 
   LDPC decoding is based on a so-called message passing algorithm. Such an algorithm consists of associating with each bit node a received signal to which corresponds an initial LLR ratio, and of iteratively exchanging messages between bit nodes  10  and check nodes  12 , along branches  14  of the graph corresponding to matrix H. The exchanged messages are log likelihood ratios and enable determining, after several iterations, for each received signal a corrected LLR ratio based on which the bit which has been transmitted is finally determined. 
   An iteration of such an algorithm mainly comprises two steps: 
   a “bit node determination” step which consists, for each bit node, of gathering the messages transmitted to the bit node by the check nodes to which the bit node is connected and of sending a new message to each check node to which the bit node is connected; and 
   a “check node determination” step which is the symmetrical operation and which consists, for each check node, of gathering the messages transmitted to the check node by the bit nodes to which the check node is connected and of sending a new message to each bit node to which the check node is connected. 
   It can actually be considered that each branch  14  corresponds to a memory element from which a message is read before each bit or check node determination step and into which a message is written after each bit or check node determination step. 
   LDPC decoding algorithm will now be more specifically described for a graph having dbn bit nodes BNi, i ranging from 0 to dbn-1, and dcn check nodes CNj, j ranging from 0 to dcn-1. For the first iteration, the bit node determination step is carried out by using, for each bit node BNi, value LLR(y i ) where y i  is the received signal associated with bit node BNi. Values LLR(y i ) are obtained from the characteristics of the transmit channel. Message v i→j , called a bit node message, transmitted by bit node BNi to check node CNj, is initialized as follows:
 
 v   i→j   =LLR(y   i )  (4)
 
   The next check node determination step consists, for each check node CN j , of transmitting a message w j→i , called a check node message, to each bit node BNi connected to check node CNj. Each message w j→i  is determined from the parity equation associated with check node CNj. For this purpose, the messages v k→j  provided by the bit nodes BNk connected to check node CNj are simply used, except for bit node BNi to which message w j→i  is transmitted, and message w j→i  is obtained by verifying the parity equation. Let S 1 j be the subset of [0, dbn-1)] containing the indexes k such that bit nodes BNk are connected to check node CNj. The expression of w j→i  is given by the following relation: 
   
     
       
         
           
             
               
                 
                   w 
                   
                     j 
                     -&gt; 
                     i 
                   
                 
                 = 
                 
                   ln 
                   ( 
                   
                     
                       1 
                       + 
                       
                         
                           ∏ 
                           
                             
                               k 
                               ∈ 
                               S1j 
                             
                             , 
                             
                               k 
                               ≠ 
                               i 
                             
                           
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           tanh 
                           ⁡ 
                           
                             ( 
                             
                               
                                 v 
                                 
                                   k 
                                   -&gt; 
                                   j 
                                 
                               
                               / 
                               2 
                             
                             ) 
                           
                         
                       
                     
                     
                       1 
                       - 
                       
                         
                           ∏ 
                           
                             
                               k 
                               ∈ 
                               S1j 
                             
                             , 
                             
                               k 
                               ≠ 
                               i 
                             
                           
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           tanh 
                           ⁡ 
                           
                             ( 
                             
                               
                                 v 
                                 
                                   k 
                                   -&gt; 
                                   j 
                                 
                               
                               / 
                               2 
                             
                             ) 
                           
                         
                       
                     
                   
                   ) 
                 
               
             
             
               
                 ( 
                 5 
                 ) 
               
             
           
         
       
     
   
   For the next iterations, the bit node determination step is carried out, for each bit node BNi, based on messages w j→i  provided by check nodes CNj connected to bit node BNi. For each check node CNj connected to bit node BNi, a message v i→j  corresponding to the sum of initial value LLR(y i ) and of all the messages received from the check nodes connected to bit node BNi, except for the message transmitted by node CNj, is provided. Let S 2 i be the subset of [0, dcn-1] containing indexes k such that check nodes CNk are connected to bit node BNi. The expression of v i→j  thus is the following:
 
 v   i→j   =LLR ( y   i )+Σ kεS2i,k≠j   w   k→i   (6)
 
   Once a determined number of iterations has been performed, a value Li corresponding to a corrected LLR ratio is determined for each bit node BNi. Li is determined by the following equation: 
   
     
       
         
           
             
               
                 
                   L 
                   i 
                 
                 = 
                 
                   
                     LLR 
                     ⁡ 
                     
                       ( 
                       
                         y 
                         i 
                       
                       ) 
                     
                   
                   + 
                   
                     
                       ∑ 
                       
                         k 
                         ∈ 
                         S2i 
                       
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       w 
                       
                         k 
                         -&gt; 
                         i 
                       
                     
                   
                 
               
             
             
               
                 ( 
                 7 
                 ) 
               
             
           
         
       
     
   
   It is then decided, for each bit node BNi, based on value L i , which is the value of the bit associated with the received signal y i . For example, if Li is negative or zero, it may be decided that the bit is equal to 1 and if Li is strictly positive, it may be decided that the bit is equal to 0. 
     FIG. 2  schematically shows an example of the forming of an LDPC decoder  20  implementing the previously-described algorithm. 
   Decoder  20  comprises a processing unit  22  comprising NB separate elementary processing units  24  (Mod j , j ranging from 1 to NB) capable of performing in parallel calculation operations to provide the bit and check node messages. 
   Decoder  20  comprises a RAM  26  in which are stored the bit node messages provided by units  24  after a bit node determination step and check node messages provided by units  24  after a check node determination step. Memory  26  comprises Ne/NB lines. At each line of memory  26  is stored a word containing a number of bits equal to the product of number NB of units  24  by the number of bits that can be processed by a unit  24 . Each word corresponds to the juxtaposition of NB bit node messages or of NB check node messages. 
   Decoder  20  comprises an input memory  28  in which are stored initial values LLR(y i ), i ranging from 0 to dbn-1. Decoder  20  also comprises an output memory  30  in which are stored values Li, i ranging from 0 to dbn-1. 
   Processing unit  22  is connected to a multiplexer  32 , controlled by a control signal S 1  which, according to the value of control signal S 1 , provides processing unit  22  with a word from input memory  28 , a word directly read from memory  26 , or a word provided by a rearrangement unit  34  and which corresponds to a word of memory  26  in which the order of the messages forming the word has been modified. More specifically, when a word is transmitted to processing unit  22 , the message at the first position in the word is transmitted to the first elementary processing unit Mod 1 , etc., and the message at the NB th  position in the word is transmitted to the NB th  elementary processing unit Mod NB . 
   Processing unit  22  is capable of successively providing words corresponding to the juxtaposition of NB bit node messages or of NB check node messages. More specifically, when a word is transmitted to processing unit  22 , the message at the first position in the word is provided by first elementary processing unit Mod 1 , etc., and the message at the NB th  position in the word is provided by the NB th  elementary processing unit Mod NB . The words provided by processing unit  22  may be transmitted to output memory  30  or to a rearrangement unit  38  capable of providing a new word from a received word and corresponding to the received word in which the order of the NB bit node messages or of the NB check node messages is modified. 
   Rearrangement unit  38  drives a first input of a multiplexer  40 , controlled by a signal S 3 . Processing unit  22  drives a second input of multiplexer  40 . According to the value of S 3 , multiplexer  40  provides memory  26  with the word provided by rearrangement unit  38  or by processing unit  22 . 
   Decoder  20  comprises an address provision unit  42 , for example, comprising a ROM, at each line of which are stored an address of memory  26  and rearrangement data. Rearrangement units  34  or  38  are capable of modifying the order of the messages in a word based on rearrangement data. The rearrangement data may correspond to a number of shift positions. By successively reading the lines of address provision unit  42 , a series of addresses in memory  26  is obtained in an order different from the regular order of the reading from the lines of memory  26 . 
   Upon operation of decoder  20 , for a bit node determination step or for a check node determination step, an elementary unit  24  is brought to use bit or check node messages which are located at different locations in memory  26 . 
   Generally, the messages are distributed in memory  26  so that, in a check node determination step, the words can be read successively according to the regular order of the reading from memory  26 , the bit node message placed at the first position of each word of memory  26  being used by first elementary unit MOD 1 , etc., and the message placed at the NB th  position being used by the NB th  elementary unit MOD NB . After a check node determination step, each check node message is stored at the same position as the bit node message previously associated with the same branch. 
   In the next bit node determination step, the rearrangement unit reads the words from memory  26  in the read order provided by address provision unit  42 . For each read word, rearrangement unit  34  provides a new word corresponding to the read word in which the message order is modified based on the rearrangement datum of address provision unit  42 . The messages are, for example, shifted to the left by a number of positions equal to the number of shift positions stored in address provision unit  42  for the read address. The message at the first position of the new word is then provided to first elementary unit Mod 1 , etc. and the message at the NB th  position of the new word is provided to the NB th  elementary processing unit Mod NB . 
   From a word transmitted by processing unit  22 , rearrangement unit  38  provides a new word that it stores in memory  26  according to an order provided by address provision unit  42  so that each bit node message is stored at the same location as the previous check node message associated with the same branch. For example, each new word corresponds to the received word in which the bit node messages are shifted to the right by a number of shift positions stored in address provision unit  42 . 
   Memory  26  is generally of large size since Ne can be greater than 200,000. As an example, a processing unit  22  may contain several hundreds of elementary processing units  24  operating in parallel. If each message is coded over some ten bits, each word manipulated by rearrangement units  34 ,  38  comprises several thousands of bits. A rearrangement circuit capable of performing circular shift operations on such words can take up several square millimeters when it is formed on an integrated circuit according to a conventional semiconductor technology. This corresponds to a non-negligible portion of the total decoder surface area. 
   SUMMARY OF THE INVENTION 
   The present invention aims at obtaining an LDPC decoder formed on an integrated circuit having a reduced surface area. 
   The present invention also aims at obtaining an LDPC decoder formed on an integrated circuit having a simple structure. 
   To achieve these and other objects, the present invention provides an LDPC decoder comprising a determined number of processing units operating in parallel, each processing unit being capable of successively receiving messages of a first type and of successively providing messages of a second type based on a first processing of the messages of the first type and of receiving messages of the second type and of providing messages of the first type based on a second processing of the messages of the second type, the second processing being different from the first processing; a storage means capable of containing first words and second words, each first word containing a juxtaposition of messages of the first type in a number equal to said determined number, and each second word containing a juxtaposition of messages of the second type in a number equal to said determined number; a message provision unit capable of reading first words or second words from the storage means; of providing each processing unit, for each first read word, with a message of the first type at a position in the first word which depends on the processing unit; and of providing each processing unit, for each second read word, with a message of the second type at a position in the second word which depends on the processing unit; and a message write unit capable of writing, into the storage means, first words, each first word containing messages of the first type, each provided by a different processing unit, each message of the first type being at a position in the first word which depends on the associated processing unit; and of writing, into the storage means, second words, each second word containing messages of the second type, each provided by a different processing unit, each message of the second type being at a position in the second word which depends on the associated processing unit, the message provision unit further being capable of providing said message of the first type at a position in the first word which depends on the first word and said message of the second type at a position in the second word which depends on the second word, or the message write unit being further capable of writing each message of the first type at a position in the first word which depends on the first word and each message of the second type at a position in the second word which depends on the second word. 
   According to an embodiment of the present invention, the message provision unit and the message write means are simultaneously capable of respectively performing read and write operations from and into the storage means. 
   According to an embodiment of the present invention, the storage means comprises locations arranged according to a determined order, each location being capable of containing a first word or a second word, and the message provision unit is capable of successively reading the first words from the storage means according to the determined order, and of successively reading the second words from the storage means according to an order different from the determined order, or the message write unit is capable of successively writing the first words into the storage means according to the determined order, and of successively writing the second words into the storage means according to an order different from the determined order. 
   According to an embodiment of the present invention, the processing units are arranged according to a determined order and the message provision unit is capable of providing said message of the first type at a position in the first word which depends on the first word, the message write unit being capable of writing into the storage means a first word formed of messages of the first type arranged according to said determined order, and of writing into the storage means a second word formed of messages of the second type arranged according to said determined order. 
   According to an embodiment of the present invention, the processing units are arranged according to a determined order, and the message write unit is capable of writing each message of the first type at a position in the first word which depends on the first word and each message of the second type at a position in the second word which depends on the second word, the message provision unit being capable of reading from the storage means a first word and of providing each processing unit with a message of the first type contained in the first read word, the messages of the first type being arranged in the first word according to said determined order, and of reading from the storage means a second word and of providing each processing unit with a message of the second type contained in the second read word, the messages of the second type being arranged in the second word according to said determined order. 
   According to an embodiment of the present invention, the storage means comprises locations each associated with an address, the decoder comprising a means for providing a second address of the storage means from a first address of the storage means. 
   The present invention also provides an iterative LDPC decoding method, comprising a determined number of processing units operating in parallel, and in which an iteration comprises the steps of: 
   a) reading first words from a storage means, each first word containing a juxtaposition of messages of a first type in a number equal to the determined number; 
   b) providing each processing unit, for each first read word, with a message of the first type at a position in the first word which depends on the processing unit; 
   c) having each processing unit successively provide messages of a second type based on a first processing of the messages of the first type; 
   d) storing in the storage means second words, each second word corresponding to a juxtaposition of messages of the second type in a number equal to the determined number and each provided by a different processing unit, each message of the second type being at a position in the second word which depends on the associated processing unit; 
   e) reading the second words from the storage means; 
   f) providing each processing unit, for each second read word, with a message of the second type at a position in the second word which depends on the processing unit; 
   g) having each processing unit successively provide new messages of the first type based on a second processing of the messages of the second type, the second processing being different from the first processing; 
   h) storing in the storage means new first words, each new first word corresponding to a juxtaposition of messages of the first type in a number equal to a determined number and each provided by a different processing unit, each message of the first type being at a position in the new first word which depends on the associated processing unit. Further, at step b), the position of the message of the first type in the first word further depends on the first word and, at step f), the position of the message of the second type in the second word further depends on the second word or, at step d), each message of the second type is at a position in the second word which depends on the second word and, at step h), each message of the first type is at a position in the new first word which depends on the new first word. 
   According to an embodiment of the present invention, the first words are read from the storage means according to a first order of reading from the storage means, the second words being read from the storage means according to a second order of reading from the storage means different from the first order. 
   According to an embodiment of the present invention, the first words are stored in the storage means according to the first read order, the second words being stored in the storage means according to the second read order. 
   According to an embodiment of the present invention, the storage means comprises a memory capable of containing at the same locations alternately first words or second words, for a first word and a second word stored at a same location in the storage means, and for a same processing unit, at steps b) and f), the position in the second word of the message of the second type provided to said processing unit being equal to the difference, modulo the determined number, between the determined number and the position in the first word of the message of the first type provided to said processing unit. 
   The foregoing and other objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1 , previously described, shows a graph illustrating an example of an LDPC decoding; 
       FIG. 2 , previously described, schematically shows an example of the forming of a conventional LDPC decoder; 
       FIG. 3  shows an example of embodiment of an LDPC decoder according to the present invention; 
       FIG. 4  shows an example of the forming of an element of the circuit of  FIG. 3 ; and 
       FIG. 5  shows another example of the forming of the element of  FIG. 4 . 
   

   DETAILED DESCRIPTION 
   For clarity, the same elements have been designated with the same reference numerals in the different drawings. 
   The present invention comprises eliminating one of rearrangement units  34 ,  38 . A check node message “exchanged” between check node NCj and bit node NBi, obtained at a check node determination step, will then not be written at the same “location” as the bit node message “exchanged” between the same bit node NBi and the same check node NCj obtained at a bit node determination step. A rearrangement of the messages of each read word must then be performed upon reading of the words from memory  26  both in a bit node determination step and a check node determination step or a rearrangement of the messages of each word resulting from processing unit  22  must then be performed on reading of the words from memory  26  both in a bit node determination step and a check node determination step. 
     FIG. 3  shows an example of embodiment of an LDPC  45  according to the present invention. Conversely to the decoder shown in  FIG. 2 , the present decoder  45  comprises a single rearrangement unit  46  capable of reading words from memory  26 , possibly according to an order different from the regular read order, and of performing a rearrangement of the messages forming the word. Rearrangement unit  46  is controlled by a shift control unit  47  which indicates based on the rearrangement data provided by address provision unit  42  how the messages of the read word are to be rearranged. 
   A multiplexer  48 , controlled by a signal S 4 , is connected at a first input to rearrangement unit  46  and at a second input to input memory  28 . According to the value of S 4 , multiplexer  48  provides processing unit  22  successively with words stored in input memory  28  or words provided by rearrangement unit  46 . Processing unit  22  drives output memory  30  and memory  26 . The words provided by processing unit  22  are transmitted to output memory  30  or to memory  26 . 
   A comparison between the operation of decoder  20  shown in  FIG. 2  and of decoder  45  according to the present invention will now be described for the following matrix H: 
   
     
       
         
           
             
               
                 H 
                 = 
                 
                   ( 
                   
                     
                       
                         1 
                       
                       
                         0 
                       
                       
                         1 
                       
                       
                         0 
                       
                       
                         1 
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         1 
                       
                       
                         1 
                       
                       
                         1 
                       
                       
                         0 
                       
                     
                     
                       
                         0 
                       
                       
                         0 
                       
                       
                         1 
                       
                       
                         1 
                       
                       
                         1 
                       
                       
                         0 
                       
                       
                         1 
                       
                       
                         1 
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         1 
                       
                     
                     
                       
                         1 
                       
                       
                         1 
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         1 
                       
                       
                         1 
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         1 
                       
                       
                         1 
                       
                     
                     
                       
                         1 
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         1 
                       
                       
                         1 
                       
                       
                         0 
                       
                       
                         1 
                       
                       
                         1 
                       
                       
                         1 
                       
                       
                         0 
                       
                       
                         0 
                       
                     
                     
                       
                         0 
                       
                       
                         1 
                       
                       
                         1 
                       
                       
                         1 
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         1 
                       
                       
                         0 
                       
                       
                         1 
                       
                       
                         0 
                       
                       
                         1 
                       
                     
                     
                       
                         0 
                       
                       
                         1 
                       
                       
                         0 
                       
                       
                         1 
                       
                       
                         0 
                       
                       
                         1 
                       
                       
                         1 
                       
                       
                         0 
                       
                       
                         1 
                       
                       
                         0 
                       
                       
                         1 
                       
                       
                         0 
                       
                     
                   
                   ) 
                 
               
             
             
               
                 ( 
                 8 
                 ) 
               
             
           
         
       
     
   
   Number Ne of “1”s of matrix H is equal to 36. Matrix H is associated with a graph with 12 bit nodes and with 6 check bit nodes. A (bit or check node) message exchanged between a bit node and a check node is called ek, with k ranging from 0 to 35. The numbering of messages ek is performed along the order of the lines of matrix H and is illustrated by the following matrix H′: 
   
     
       
         
           
             
               
                 
                   H 
                   ′ 
                 
                 = 
                 
                   ( 
                   
                       
                   
                   ⁢ 
                   
                     
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           0 
                         
                       
                       
                         0 
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                       
                       
                         0 
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           3 
                         
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           4 
                         
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           5 
                         
                       
                       
                         0 
                       
                     
                     
                       
                         0 
                       
                       
                         0 
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           6 
                         
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           7 
                         
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           8 
                         
                       
                       
                         0 
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           9 
                         
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           10 
                         
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           11 
                         
                       
                     
                     
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           12 
                         
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           13 
                         
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           14 
                         
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           15 
                         
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           16 
                         
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           17 
                         
                       
                     
                     
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           18 
                         
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           19 
                         
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           20 
                         
                       
                       
                         0 
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           21 
                         
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           22 
                         
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           23 
                         
                       
                       
                         0 
                       
                       
                         0 
                       
                     
                     
                       
                         0 
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           24 
                         
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           25 
                         
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           26 
                         
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         0 
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           27 
                         
                       
                       
                         0 
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           28 
                         
                       
                       
                         0 
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           29 
                         
                       
                     
                     
                       
                         0 
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           30 
                         
                       
                       
                         0 
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           31 
                         
                       
                       
                         0 
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           32 
                         
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           33 
                         
                       
                       
                         0 
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           34 
                         
                       
                       
                         0 
                       
                       
                         
                           e 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           35 
                         
                       
                       
                         0 
                       
                     
                   
                   ⁢ 
                   
                       
                   
                   ) 
                 
               
             
             
               
                 ( 
                 9 
                 ) 
               
             
           
         
       
     
   
   As an example, it is considered that number NB of elementary processing units  24  is equal to 3. For decoder  20  shown in  FIG. 2 , messages ek, k ranging from 0 to 35, may be stored in memory  26  according to an arrangement corresponding to the following memory MEM: 
   
     
       
         
           
             
               
                 MEM 
                 = 
                 
                   
                     [ 
                     
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             0 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             13 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             25 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             12 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             24 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             14 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             26 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             3 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             15 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             27 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             4 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             16 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             29 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             5 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             17 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             28 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             6 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             18 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             30 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             7 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             19 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             32 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             8 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             20 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             31 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             9 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             21 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             34 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             10 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             22 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             33 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             11 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             23 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             35 
                           
                         
                       
                     
                     ] 
                   
                   ⁢ 
                   
                     
                       
                         0 
                       
                     
                     
                       
                         1 
                       
                     
                     
                       
                         2 
                       
                     
                     
                       
                         3 
                       
                     
                     
                       
                         4 
                       
                     
                     
                       
                         5 
                       
                     
                     
                       
                         6 
                       
                     
                     
                       
                         7 
                       
                     
                     
                       
                         8 
                       
                     
                     
                       
                         9 
                       
                     
                     
                       
                         10 
                       
                     
                     
                       
                         11 
                       
                     
                   
                 
               
             
             
               
                 ( 
                 10 
                 ) 
               
             
           
         
       
     
   
   where the last column corresponds to the lines of memory  26 . 
   Inversions should be noted between messages e 12  and e 13 , e 24  and e 25 , e 28  and e 29 , e 31  and e 32 , and e 33  and e 34 . Such inversions avoids messages intended to be used by the same elementary processing unit  24  to be in the same word. For example, messages e 0  and e 12  used in first elementary processing unit Mod 1  in a bit node determination step are contained in two different words. 
   In a check node determination step, the words of memory  26  are read in the regular order of the reading from memory  26 . For each read word, the message at the first position is used by first elementary unit Mod 1 , the message at the second position is used by second elementary unit Mod 2 , and the message at the third position is used by third elementary unit Mod 3 . As an example, first elementary unit Mod 1  thus successively receives bit node messages e 0  to e 5 , from which it determines check node messages e 0  to e 5 , respectively stored at the same place as previous messages e 0  to e 5 . The distribution of messages ei, i ranging from 0 to 35, in memory  26  thus corresponds to matrix MEM of relation (10). 
   In a bit node determination step, rearrangement units  34 ,  38  use the information in address provision unit  42  to read and write the right messages at the right places. 
   As an example, address provision unit  42  comprises, at each line, a line number of memory  26  and a number of shift positions. In the present example, address provision unit  42  is formed of a memory having a composition corresponding to the following matrix ROM: 
   
     
       
         
           
             
               
                 ROM 
                 = 
                 
                   [ 
                   
                     
                       
                         0 
                       
                       
                         0 
                       
                     
                     
                       
                         1 
                       
                       
                         1 
                       
                     
                     
                       
                         6 
                       
                       
                         1 
                       
                     
                     
                       
                         7 
                       
                       
                         0 
                       
                     
                     
                       
                         2 
                       
                       
                         2 
                       
                     
                     
                       
                         8 
                       
                       
                         2 
                       
                     
                     
                       
                         9 
                       
                       
                         0 
                       
                     
                     
                       
                         3 
                       
                       
                         1 
                       
                     
                     
                       
                         10 
                       
                       
                         2 
                       
                     
                     
                       
                         4 
                       
                       
                         0 
                       
                     
                     
                       
                         11 
                       
                       
                         1 
                       
                     
                     
                       
                         5 
                       
                       
                         2 
                       
                     
                   
                   ] 
                 
               
             
             
               
                 ( 
                 11 
                 ) 
               
             
           
         
       
     
   
   where the first column corresponds to line numbers of memory  26  and the second column corresponds to a number of shift positions. 
   In a bit node determination step, rearrangement unit  34  reads the words from memory  26  in the order indicated by address provision unit  42 . As an example, the determination of the bit nodes of index  0 ,  1 , and  2  is obtained from lines  0 ,  1 ,  2  of address provision unit  42  which respectively send back to lines  0 ,  1 , and  6  of memory  26 . Rearrangement unit  34  thus reads the word stored at line  0  of memory  26  and performs a leftward shift of the messages of the read word by a number 0 of shift positions and provides message e 0 , at the first position of the modified word, to first elementary processing unit Mod 1 , message e 13 , at the second position, to second elementary processing unit Mod 2 , and message e 25 , at the third position, to third elementary processing unit Mod 3 . Rearrangement unit  34  then reads the word stored at line  1  of memory  26  and performs a leftward shift of the messages of the read word by 1 shift position. Finally, rearrangement unit  34  reads the word stored at line  6  of memory  26  and performs a leftward shift of the messages in the read word by 1 shift position. 
   At the end of the determination of bit nodes n°  0 ,  1  and  2 , rearrangement unit  38  successively receives the next three words (e 0 , e 13 , e 25 ), (e 12 , e 24 , e 1 ), (e 18 , e 30 , e 6 ). To properly write word (e 18 , e 30 , e 6 ), rearrangement unit  38  uses the rearrangement data stored at line n°  2  of address provision unit  42  which indicates a number of shift positions of 1. Rearrangement unit  38  then provides a new word (e 6 , e 18 , e 30 ) corresponding to the initial word circularly shifted to the right by one shift position. Line n°  2  of address provision unit  42  sends back to line n°  6  of memory  26 . Rearrangement unit  38  thus writes the new word (e 6 , e 18 , e 30 ) at line n°  6  of memory  26 . The distribution of messages ei, i ranging from 0 to 35, in memory  26  thus corresponds again to matrix MEM of relation (10). 
   At the first iteration, processing unit  22  reads initial values ui, i ranging from 0 to 11, stored in memory  28  and corresponding to initial values LLR(y i ). Rearrangement unit  38  initializes memory  26  according to the following matrix MEM: 
   
     
       
         
           
             
               
                 MEM 
                 = 
                 
                   
                     [ 
                     
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             0 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             0 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             4 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             5 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             3 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             8 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             6 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             7 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             9 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             10 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             11 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             10 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             11 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             9 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             0 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             3 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             4 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             5 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             4 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             5 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             3 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             6 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             7 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             8 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             7 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             8 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             6 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             11 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             9 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             10 
                           
                         
                       
                     
                     ] 
                   
                   ⁢ 
                   
                     
                       
                         0 
                       
                     
                     
                       
                         1 
                       
                     
                     
                       
                         2 
                       
                     
                     
                       
                         3 
                       
                     
                     
                       
                         4 
                       
                     
                     
                       
                         5 
                       
                     
                     
                       
                         6 
                       
                     
                     
                       
                         7 
                       
                     
                     
                       
                         8 
                       
                     
                     
                       
                         9 
                       
                     
                     
                       
                         10 
                       
                     
                     
                       
                         11 
                       
                     
                   
                 
               
             
             
               
                 ( 
                 12 
                 ) 
               
             
           
         
       
     
   
   In the present invention, data ui are written into memory  26  by using the order of the lines of memory  26  provided by address provision unit  42  but without the application of a shifting, an arrangement of data ui in memory  26  is thus obtained, according to the following matrix MEM: 
   
     
       
         
           
             
               
                 MEM 
                 = 
                 
                   
                     [ 
                     
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             0 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             0 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             3 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             4 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             5 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             6 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             7 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             8 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             9 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             10 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             11 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             9 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             10 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             11 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             0 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             3 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             4 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             5 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             3 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             4 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             5 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             6 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             7 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             8 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             6 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             7 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             8 
                           
                         
                       
                       
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             9 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             10 
                           
                         
                         
                           
                             u 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             11 
                           
                         
                       
                     
                     ] 
                   
                   ⁢ 
                   
                     
                       
                         0 
                       
                     
                     
                       
                         1 
                       
                     
                     
                       
                         2 
                       
                     
                     
                       
                         3 
                       
                     
                     
                       
                         4 
                       
                     
                     
                       
                         5 
                       
                     
                     
                       
                         6 
                       
                     
                     
                       
                         7 
                       
                     
                     
                       
                         8 
                       
                     
                     
                       
                         9 
                       
                     
                     
                       
                         10 
                       
                     
                     
                       
                         11 
                       
                     
                   
                 
               
             
             
               
                 ( 
                 13 
                 ) 
               
             
           
         
       
     
   
   In the following check node determination step, rearrangement unit  46  successively reads, line after line, the words from memory  26  and performs, for a word read from memory  26  at a determined line number, a circular rightward shift, by the shift position number indicated at the same line number of address provision unit  42 . The new obtained messages provided by processing unit  22  are then written into memory  26  in the normal order of the lines of memory  26  with no shift. An arrangement of the messages in memory  26  corresponding to the following matrix MEM is thus obtained: 
   
     
       
         
           
             
               
                 MEM 
                 = 
                 
                   
                     [ 
                     
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             0 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             13 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             25 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             12 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             24 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             14 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             26 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             3 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             15 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             27 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             4 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             16 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             29 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             5 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             17 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             28 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             6 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             18 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             30 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             7 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             19 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             32 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             8 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             20 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             31 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             9 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             21 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             34 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             10 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             22 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             33 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             11 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             23 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             35 
                           
                         
                       
                     
                     ] 
                   
                   ⁢ 
                   
                     
                       
                         0 
                       
                     
                     
                       
                         1 
                       
                     
                     
                       
                         2 
                       
                     
                     
                       
                         3 
                       
                     
                     
                       
                         4 
                       
                     
                     
                       
                         5 
                       
                     
                     
                       
                         6 
                       
                     
                     
                       
                         7 
                       
                     
                     
                       
                         8 
                       
                     
                     
                       
                         9 
                       
                     
                     
                       
                         10 
                       
                     
                     
                       
                         11 
                       
                     
                   
                 
               
             
             
               
                 ( 
                 14 
                 ) 
               
             
           
         
       
     
   
   In the next bit node determination step, rearrangement unit  46  provides messages to processing unit  22  by reading the words from memory  26  in the order indicated by address provision unit  42  and by performing, on each read word, a leftward shift by the number of shift positions indicated by address provision unit  42 . As an example, rearrangement unit  46  successively provides the three words (e 0 , e 13 , e 25 ), (e 12 , e 24 , e 1 ), and (e 18 , e 30 , e 6 ) respectively from lines  0 ,  1  and  2  of address provision unit  42 . 
   The new obtained words provided by processing unit  22  are then written into memory  26  by following the order indicated by address provision unit  42  but with no shift. As an example, processing unit  22  successively provides three new words (e 0 , e 13 , e 25 ), (e 12 , e 24 , e 1 ), and (e 18 , e 30 , e 6 ) which are respectively written at lines n°  0 ,  1 , and  6  of memory  26 . 
   A message distribution is then obtained in memory  26 , which corresponds to the following matrix MEM: 
   
     
       
         
           
             
               
                 MEM 
                 = 
                 
                   
                     [ 
                     
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             0 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             13 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             25 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             12 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             24 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             26 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             14 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             15 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             27 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             3 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             4 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             16 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             29 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             28 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             5 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             17 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             18 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             30 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             6 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             7 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             19 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             32 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             31 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             8 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             20 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             9 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             21 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             34 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             33 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             10 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             22 
                           
                         
                       
                       
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             23 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             35 
                           
                         
                         
                           
                             e 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             11 
                           
                         
                       
                     
                     ] 
                   
                   ⁢ 
                   
                     
                       
                         0 
                       
                     
                     
                       
                         1 
                       
                     
                     
                       
                         2 
                       
                     
                     
                       
                         3 
                       
                     
                     
                       
                         4 
                       
                     
                     
                       
                         5 
                       
                     
                     
                       
                         6 
                       
                     
                     
                       
                         7 
                       
                     
                     
                       
                         8 
                       
                     
                     
                       
                         9 
                       
                     
                     
                       
                         10 
                       
                     
                     
                       
                         11 
                       
                     
                   
                 
               
             
             
               
                 ( 
                 15 
                 ) 
               
             
           
         
       
     
   
   The message arrangement in memory  26  is thus different after a check node determination step and after a bit node determination step. 
   As described previously in relation with the first iteration, at the next check node determination step, rearrangement unit  46  successively reads, line after line, the words from memory  26  and performs, for a word read from memory  26  at a determined line number, a circular shift to the right, by the shift position number indicated at the same line number of address provision unit  42 . Thus, rearrangement unit  46  successively reads words (e 0 , e 13 , e 25 ), (e 12 , e 24 , e 1 ), etc. until (e 23 , e 35 , e 11 ) and provides processing unit  22  successively with words (e 0 , e 13 , e 25 ), (e 1 , e 12 , e 24 ), etc. until (e 11 , e 23 , e 35 ), that is, by performing a circular shift to the right of the messages by a number of positions equal to the number of shift positions 0, 1, etc. to 2 respectively stored at lines n°  0 ,  1 , etc. to  11  of address provision unit  42 . 
   The shift direction (rightwards or leftwards) is obtained from the data stored in memory  42 . Thus, when a leftward shift operation has to be performed, the numbers of shift positions contained in memory  42  are directly used. When a rightwards shift operation is to be performed, an equivalent leftward shift value corresponding to the difference, modulo NB, between NB and the number of shift positions provided by memory  42  is determined. In the present example, a circular shift by 2 positions to the right is equivalent to a circular shift by 1 position to the left. This enables using a rearrangement unit  46  only performing leftward shift operations. 
     FIG. 4  shows a first example of the forming of shift control unit  47  and of address provision unit  42 . Address provision unit  42  comprises a memory  51 , for example, a ROM, capable of receiving a signal L 1  representative of an initial line number of memory  26  and of providing a signal L 2  representative of a new line number of memory  26 , a signal D_NB representative of a number of shift positions (to the left) to be used in a bit node determination step, and a signal D_NC representative of a number of a shift positions (to the right) to be used in a check node determination step. Address provision unit  42  comprises a multiplexer  52 , controlled by a binary control signal BN 0 _CN 1 , receiving signal L 1  at a first input and signal L 2  at a second input and providing a signal Address to memory  26 . As an example, when signal BN 0 _CN 1  is high, which indicates that a check node determination step is going on, signal Address is equal to L 1 . The lines of memory  26  are then successively read. When signal BN 0 _CN 1  is at a low state, which indicates that a bit node determination step is going on, signal Address is equal to L 2 . The lines of memory  26  are thus read in the order provided by address provision unit  42 . Shift control unit  47  comprises a multiplexer  54 , controlled by signal BN 0 _CN 1 , receiving signal D_NB on a first input and signal D_NC on a second input and providing a signal Shift to rearrangement unit  46 . As an example, when signal BN 0 _CN 1  is high, which indicates that a check node determination step is going on, signal Shift is equal to D_NC and when signal BN 0 _CN 1  is low, which indicates that a bit node determination step is going on, signal Shift equal to D_NB. 
     FIG. 5  shows a second example of embodiment of shift control unit  47  and of address provision unit  42 . Address provision unit  42  is formed of first and second separate memories  56 ,  58 . First memory  56  is capable of receiving signal L 1  and of providing signal L 2 . Address provision unit  42  comprises a multiplexer  60 , controlled by binary control signal BN 0 _CN 1 , receiving signal L 1  on a first input and signal L 2  on a second input and providing signal Address to memory  26  and to second memory  58 . Second memory  58  is capable of receiving signal Address and of providing the number of shift positions D_NB adapted to the implementation of a bit node determination step. Unit  47  comprises a unit  61  capable of determining the shift number D_NC to be used in a check node determination step based on D_NB. Shift control unit  47  comprises a multiplexer  62 , controlled by signal BN 0 _CN 1 , receiving signal D_NB on a first input and signal D_NC on a second input and providing a signal Shift to rearrangement unit  46 . As an example, when signal BN 0 _CN 1  is high, which indicates that a check node determination step is going on, signal Shift is equal to D_NC and when signal BN 0 _CN 1  is low, which indicates that a bit node determination step is going on, signal Shift is equal to D_NB. 
   Decoder  45  according to the present invention comprises a single rearrangement unit  46  as compared to a conventional decoder  20  which comprises two. The surface necessary to the forming of shift control unit  47  is negligible. The surface of a rearrangement unit is thus practically completely freed. Further, rearrangement unit  46  of decoder  45  according to the present invention may have a structure identical to that of a conventional rearrangement unit  34 ,  38 . 
   Further, rearrangement unit  46  is only used, in the present example of embodiment, in read operations from memory  26 . Decoder  45  according to the present invention thus enables simultaneously performing read and write operations in memory  26  and thus enables optimizing the duration of an iteration of the decoding algorithm. 
   According to an alternative of the present invention, the decoder comprises a single rearrangement unit  46  provided between processing unit  22  and memory  26 , downstream of processing unit  22  according to the data flow direction. Rearrangement unit  46  is then capable of writing a word into memory  26  based on a word provided by processing unit  22 , by modifying the position of the messages in the word provided by processing unit  22 . In a bit node determination step or a check node determination step, the message at the first position of each read word is provided to first elementary processing unit  24 , the message at the second position of each read word is provided to second elementary processing unit  24 , etc. and the message at the last position of each read word is provided to the last elementary processing unit  24 . Rearrangement unit  46  then modifies the message position in each word provided by processing unit  22  so that all messages to be provided to a same elementary processing unit  24  at the next determination step are at the same position in each word. A different distribution of the bit node messages and of the check node messages is then obtained in memory  26 , as in the decoder previously described as an example, respectively after a bit node determination step or a check node determination step. Since such a rearrangement unit is only used in write operations into memory  26 , read and write operations can thus be simultaneously performed, which enables optimizing the duration of an iteration of the decoding algorithm. 
   According to another variation of the present invention, memory  26  is replaced with first and second memories, the bit node messages being stored in the first memory and the check node messages being stored in the second memory. With such a structure, in a bit node determination step, the check node messages are read from the second memory while the new bit node messages obtained from the read check node messages are written into the first memory and, in a check node determination step, the bit node messages are read from the first memory while the new check node messages obtained from the read bit node messages are written into the second memory. 
   Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the rearrangement unit may perform an operation other than a shift. Block permutations may be used. 
   Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.