Repeat-accumulate code

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In computer science, repeat-accumulate codes (RA codes) are a low complexity class of error-correcting codes. 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 ${\displaystyle {N}}$ is repeated ${\displaystyle {q}}$ times, scrambled by an interleaver of size ${\displaystyle {qN}}$, and then encoded by a rate 1 accumulator. The accumulator can be viewed as a truncated rate 1 recursive convolutional encoder with transfer function ${\displaystyle {1/(1+D)}}$, but Divsalar et al. prefer to think of it as a block code whose input block ${\displaystyle {(z_{1},\ldots ,z_{n})}}$ and output block ${\displaystyle {(x_{1},\ldots ,x_{n})}}$ are related by the formula ${\displaystyle {x_{1}=z_{1}}}$ and ${\displaystyle x_{i}=x_{i-1}+z_{i}}$ for ${\displaystyle i>1}$. The encoding time for RA codes is linear and their rate is ${\displaystyle 1/q}$. They are nonsystematic.