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Sometime the definition of what constitutes an 'iteration' is blurred. Technically an interation is the combination of two component decode operations (MAP/Log-MAP/SOVA etc), one using the non-interleaved parity bits, and one using the interleaved ones (i.e. 6 iterations involves 12 component decodes). Sometimes people use the term iteration to just mean number of component decodes. It is not uncommon for a turbo decoder to feature an 'early termination' option, where you compare the hard output of the previous N iterations (where N is generally between 2 to 3). If the outputs are the same (i.e. they have converged), you can generally stop decoding with very little effect on BER. In high SNR channels this is good as the decoder generally quickly converges to the right answer (i.e. more iterations don't tell you anything new). Typically its also good in low SNR channels for the opposite reason (the decoder quickly converges to the wrong answer so more iteration doesn't help either). In mid-SNR channels the decoder usually run to its maximum number of iteration anyway. The maximum number of iterations required for a given application is generally derived from the expected SNR of the channel and the a required bit or frame error rate target for the link.
--[[User:Miterdale|miterdale]] 12:51, 15 January 2006 (UTC)
== Error Floor ==
In this article, no distinction is made between Turbo codes and Turbo product codes. Being no expert in the field, I'm not sure which is discussed here. I am, however, aware of one important limitation of Turbo codes: they exhibit an error floor at high SNRs. Turbo Product codes (from my understanding) exhibit no such error floor, and LDPC codes definitely do not. The orphaned article [[Error floor]] mentions that Turbo codes have an error floor (and, incorrectly, that LDPCs do), but other than that I find no mention of error floors anywhere in Wikipedia. I'm not knowledgeable enough to fix that, but if anyone else is looking for a good project... [[User:129.128.210.68|129.128.210.68]] 17:23, 17 April 2007 (UTC)
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