Look back to the definition of the outer code and linear inner codes, this definition of the Justesen code makes sense because the codeword of the outer code is a vector with N elements, and we have N linear inner codes to apply for those N elements.
22.
This paper analyzed repeat-accumulate ( RA ) codes which are the serial concatenation of an inner two-state recursive convolutional code ( also called an'accumulator'or parity-check code ) with a simple repeat code as the outer code, with both codes linked by an interleaver.
23.
The main idea is that if the inner block length is selected to be logarithmic in the size of the outer code then the decoding algorithm for the inner code may run in exponential time of the inner block length, and we can thus use an exponential-time but optimal maximum likelihood decoder ( MLD ) for the inner code.
24.
Thus, for an outer symbol " y " "'i " to be incorrect after inner decoding at least " d " / 2 inner symbols must have been in error, and for the outer code to fail this must have happened for at least " D " / 2 outer symbols.
