Talk:Low-density parity-check code: Difference between revisions

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For a person who has not studied information theory, the introduction chapter says absolutely nothing. It's some state of the art code, rrright. For what/Where/Why is it used? I think the introduction should answer these questions first. Then it's okay to jump into the technics. Besides, what's Forney's factor graph notation anyway? Maybe an article about it should be created, or at least [[Forney]] linked somewhere. (Elsewhere than to [[Forney, Texas]]). --[[User:ZeroOne|ZeroOne]] 19:52, 12 Oct 2004 (UTC)

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I think some of the details need to be better written here. The variable 'k' is not even defined. Also, the sentence "or there must be an even number of odd values" is confusing. Instead of writing that, should write something straight-forward, such as - the "Exclusive OR" of all the bits linked by the lines will be logical 0. Also, as if by 'magic', the matrices "P" and "I" just pop out of nowhere, and nobody has any idea what they are. [[User:KorgBoy|KorgBoy]] ([[User talk:KorgBoy|talk]]) 08:10, 25 May 2017 (UTC)

I think H should be reduced to -P^T | I_{k} and not (n-k). Because the number of rows of H is actually k. It works for this example since n-k = k here, but for general n and k, it will not work. <!-- Template:Unsigned --><small class="autosigned">—&nbsp;Preceding [[Wikipedia:Signatures|unsigned]] comment added by [[User:Swaprava|Swaprava]] ([[User talk:Swaprava#top|talk]] • [[Special:Contributions/Swaprava|contribs]]) 17:53, 5 January 2020 (UTC)</small> <!--Autosigned by SineBot-->

Similarly, G will be starting with I_{n-k} and not k. LDPC codes encode n-k bits into n bits, right? <!-- Template:Unsigned --><small class="autosigned">—&nbsp;Preceding [[Wikipedia:Signatures|unsigned]] comment added by [[User:Swaprava|Swaprava]] ([[User talk:Swaprava#top|talk]] • [[Special:Contributions/Swaprava|contribs]]) 17:56, 5 January 2020 (UTC)</small> <!--Autosigned by SineBot-->

== Forgotten? ==

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:[R. G. Gallager. Low-Density Parity-Check Codes. MIT Press, 1963]

:[[User:Markovisch|Markovisch]] ([[User talk:Markovisch#top|talk]] • [[Special:Contributions/Markovisch|contribs]]) 01:02, 27 March 2017 (UTC)

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== Decoding ==

The decoding section directly jumps to technicalities and state-of-the-art improvements while never really introducing belief propagation and bit flipping algorithms which I believe are not that hard to understand.

The part about seeing an LDPC as independent SPC codes and using SOVA or BCJR is confusing. I believe it tries to draw an analogy between turbo codes or convolutional codes and LDPC codes. It currently has no reference and the only references I could find on applying those algorithms to LDPC were in really specific constructions with structured codes or concatenations.

The part about lookup table has no reference either. I have no doubt that a bit flipping algorithm can run on a low power CPU and that some computations can be avoided by using a lookup table (for the check nodes computations for example). But I am not sure that this is what the author meant and if this is what was meant I don't see how it is relevant to detail the lookup table and the usage of the FIFO.

[[User:Bpcov|Bpcov]] ([[User talk:Bpcov|talk]]) 19:44, 28 July 2018 (UTC)