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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.

Bert22306
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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   10/1/2013 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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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.

LarryM99
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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?



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As data rates begin to move beyond 25 Gbps channels, new problems arise. Getting to 50 Gbps channels might not be possible with the traditional NRZ (2-level) signaling. PAM4 lets data rates double with only a small increase in channel bandwidth by sending two bits per symbol. But, it brings new measurement and analysis problems. Signal integrity sage Ransom Stephens will explain how PAM4 differs from NRZ and what to expect in design, measurement, and signal analysis.

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