s are scrambled and encoded again, but now only the even-positioned coded bits are stored. The idea at the base of the decoder of turbo codes is that the signal arriving at the receiver is composed of two words encoded with the same code where the symbols are in a differ- ent order. It is intuitive, even if it is not SO easy to demonstrate, that the quantity of information the decoder has on the sent word is much greater than what it would have if only one code were used. Telecommunication Networks The input word is organized like a matrix Output of the line-row interleaver Output of the helical interleaver Even-odd interleaver working Encoded word without interleaver Encoded word with row-column interleaver Final code word after encoding and interleaving FIGURE 5.69 Scheme of the operation of a few possible interleaver algorithms. In this situation, the generic decoder of a turbo code is a sort of soft decoder having at the input the row samples of the received data and constituted by two different decoders, one for each component code. The decoders exchange information about the received word and at the end, if no errors happen, both produce the same output symbol stream. Naturally, if the two decoders produce in a certain step two different outputs, they detect it by exchanging data, and interpret it as the presence of errors starting a joined procedure to determine the nature of the error and to correct it. All this procedure is much more complicated than the decoding algorithm of BCH codes, but nevertheless can be carried out fast using dedicated logic, like a very fast DSP or FPGA or more probably an ASIC. There is a strong research effort to study properties and implementation of turbo code decoders and the interested reader can start from the following bibliographies [102,103]. 5.8.5.4 ITU-T OTN Standard and Advanced FEC The use of a FEC implies that besides the signal to be transmitted, a redundancy is sent to be used by the decoder to correct decision errors or to perform optimum soft decision. Technolo