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In [[computer science]], '''repeat-accumulate codes''' (RA codes) are a low complexity class of [[error-correcting code]]s. They were devised so that their [[ensemble weight distributions]] are easy to derive. RA codes were introduced by Divsalar ''et al.''
In an RA code, an [[information block]] of length <math>{N}</math> is repeated <math>{q}</math> times, scrambled by an [[interleaver]] of size <math>{qN}</math>, and then encoded by a rate 1 [[accumulator]]. The accumulator can be viewed as a truncated rate 1 [[recursive]] convolutional encoder with transfer function <math>{1/(1 + D)}</math>, but Divsalar ''et al.'' prefer to think of it as a block code whose input block <math>{(z_1, \ldots , z_n)}</math> and output block <math>{(x_1, \ldots , x_n)}</math> are related by the formula <math>{x_1 = z_1}</math> and <math>x_i = x_{i-1}+z_i</math> for <math>i > 1</math>. The encoding time for RA codes is linear and their rate is <math>1/q</math>. They are nonsystematic.
== References ==
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