[Rmt] Comments about RaptorG I-D and the uploaded RMT presentation
Vincent Roca <vincent.roca@inrialpes.fr> Wed, 11 November 2009 05:12 UTC
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Date: Wed, 11 Nov 2009 06:12:59 +0100
From: Vincent Roca <vincent.roca@inrialpes.fr>
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Subject: [Rmt] Comments about RaptorG I-D and the uploaded RMT presentation
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Mike, The results presented in your slides are pretty impressive. Great work! However, with my colleagues, we have several comments WRT the slides uploaded on the IETF site: http://www.ietf.org/proceedings/09nov/slides/rmt-0.ppt Concerning your claim (slide 6): ------ RaptorG code properties * Linear time encoding and decoding * Comparable to Raptor specified in RFC 5053 ------ And that's all, no attempt to justify these claims! Our questions: 1) Of "linear complexity" does not mean it's fast. An algorithm can be of linear complexity while being an order of magnitude slower than another one which is in O(k^2). As k increases in the range of interest, it's even possible that the second algorithm be always significantly faster than the fist one. So I'd prefer to see encoding/decoding time evaluations, compared to a well known, widely available, codec. We've been using Luigi's Reed-Solomon to that purpose to compare with our LDPC codec, see for instance (slide 10 or the links provided in slide 11): http://www.ietf.org/proceedings/75/slides/fecframe-5/fecframe-5.htm 2) RFC 5053 specifies that decoding can rely on a Gaussian elimination technique, and explains how to do that. So the suggested decoding in RFC 5053 is *by no means of linear complexity*. draft-luby-rmt-bb-fec-raptorg-object-01 does exactly the same, so the suggested decoding in the I-D is *by no means of linear complexity*. Admittedly, other decoding techniques can be used and what is in the document is merely an example. However, at least with Raptor (I don't know for RaptorG), an ML (Maximum Likelihood) type of decoding remains required to achieve good erasure recovery capabilities (iterative decoding is not sufficient with Raptor codes). Can you clarify the kind of decoding algorithm used? Are you considering your "inactivation decoding" technique, as described in your Patent 6,856,263? Even in that case, it is not a purely iterative decoder, and complexity is not, strictly speaking, linear. In any case, it's a key point that must be clarified. 3) draft-luby-rmt-bb-fec-raptorg-object-01 considers source blocks of size up to K'_max=56404. The same I-D suggests to use Gaussian elimination for decoding. Is it realistic to suggest to use a Gaussian elimination for such large source blocks? Do you have practical recommendations? I imagine a decoder will use different types of decoding techniques, depending on the actual block size in use. Am I right? At least this is the way our LDPC codec works... 4) Does RaptorG introduce additional complexity WRT Raptor? The answer seems to be yes. From what I saw, RaptorG introduces operations over GF(2^8) for the creation of some of the Intermediate Symbols. It was not the case in Raptor where the generator matrix for intermediate symbols was fully binary. Having a matrix that is closer to a random matrix over GF(2^8) is probably the key point to achieve the decoding performances shown. However it makes math more complex. And it's the same kind of math as that of Reed-Solomon codes over GF(2^8). Can you clarify? What is your opinion, now you have developped a Raptor codec? Do you have measurements that compare both codecs (even if the codecs perhaps don't have the same maturity level)? 5) You're saying that sub-blocking has been improved. I don't see the fundamental differences but I probably missed something here (I didn't check thoroughly). And what do you mean by "improved derivation of parameters"? The above comments should not be considered negatively. We would just like to better understand some unclear points and perhaps have them clarified in the I-D and/or future presentations. That being said, *I do support this I-D to be WG Item*, of course. Regards, Vincent
- [Rmt] Comments about RaptorG I-D and the uploaded… Vincent Roca
- Re: [Rmt] Comments about RaptorG I-D and the uplo… Luby, Michael