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DOCSIS 3.1: Coping With Design Gotchas Before Rollout

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LarryM99
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CEO
Upstream vs. downstream
LarryM99   9/27/2013 3:18:00 PM
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Jim, it looks like there is a significant increase in upstream bandwidth and a smaller increase in downstream. Am I reading that correctly? Does that indicate a departure from the assumption that most traffic goes to the cable customers instead of from them?

Bert22306
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CEO
This needs to be explained rather than just stated
Bert22306   9/27/2013 4:36:29 PM
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"The addition of OFDM modulation and other coding techniques results in a 50 percent increase in throughput versus QAM channels across an equivalent bandwidth."

Is this only because the guard bands between 6 MHz channels can be slightly narrower?

Use of OFDM is only really called for in environments with a lot of multipath interference. Are they having multipath problems in cable systems? Otherwise, a single carrier scheme like QAM should give measurably lower levels of threshold SNR, so I don't see this unqualified statement that OFDM is desirable in cable systems as a slam dunk. Sounds like another tradeoff that needs to be mentioned, at least in passing.

jackOfManyTrades
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Re: This needs to be explained rather than just stated
jackOfManyTrades   10/1/2013 3:23:50 AM
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I think the key phrase is "and other coding techniques". DOCSIS 3.1 uses an LDPC FEC, which makes it possible to use 4096-QAM. There's your 50% increase in capacity.

Bert22306
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CEO
Re: This needs to be explained rather than just stated
Bert22306   10/1/2013 3:31:58 PM
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Yes, that would make sense. In which case, though, the same would apply to using LDPC FEC with single-carrier QAM.

jackOfManyTrades
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Re: This needs to be explained rather than just stated
jackOfManyTrades   10/2/2013 3:10:11 AM
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I guess it would. I don't know why DOCSIS 3.1 uses OFDM. DVB-C2 does, too. It may be that it's six of one and half-a-dozen of the a other and as a lot of other systems use OFDM these days, it's easier to design a similar system. eg DVB-T2 (and T) uses OFDM; DVB-C is single carrier, but DVB-C2 is OFDM and very similar to DVB-T2. It makes designing a chip to do both easier and cheaper.

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