[opendtv] Re: Spectrum is too valuable

  • From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
  • To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
  • Date: Tue, 10 Nov 2015 01:49:36 +0000

Ron Economos wrote:

DOCSIS 3.1 is based on LDPC FEC, so it uses a 16,200 bit (2025 byte)
frame (just like the short FEC frames in DVB-S2, DVB-T2 and DVB-C2).
The frame has 225 bytes of LDPC parity, 21 bytes of BCH parity and a
2 byte codeword header, leaving 1777 bytes of payload.

The minimum channel size is 24 MHz (of which 22 MHz contains active
carriers). Bandwidth can be increased in 2 MHz minimum sized chunks
up to 192 MHz (190 MHz of active carriers). The additional chunks
don't have to be contiguous.

We looked at DOCSIS 3.1 some time ago. The gains in spectral efficiency are
primarily thanks to the LDPC FEC and to the wider channel, which wider channel
eliminates the waste caused by the guardbands of previous 6 MHz channel DOCSIS
versions. The effect in b/s/Hz is noticeable but not gymongous, in a given
amount of spectrum. The use of higher modes of QAM are a necessity, given the
wider channel, rather than something surprising. So the rough estimates on
digital capacity which could be repurposed from broadcast channels would not be
markedly different, if you take into account the details of DOCSIS 3.1.

I did some comparitive numbers, long time ago, when 3.1 was initially being
(over)hyped.

Bert



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