Convergence properties of iterative decoders working at bit and symbol level

We compare the convergence thresholds of parallel and serially concatenated codes with interleaver that employ iterative decoding algorithms working at bit or symbol level. The analysis technique is an extension to that of the symbol-level decoder presented by S. ten Brink (see Electron. Lett., vol.35, no.10, p.806-8, 1999), and is valid for very large interleavers. The comparison shows that working at symbol level always yields a lower convergence threshold. In the serial concatenation, the advantage is quite significant