[opendtv] Re: Seeing Ghosts on a Single Frequency Network

  • From: "Allen Le Roy Limberg" <allimberg@xxxxxxxxxxxx>
  • To: <opendtv@xxxxxxxxxxxxx>
  • Date: Thu, 10 Feb 2011 15:51:46 -0500

John Shutt wrote:

> I suppose it depends on your definition of spectral efficiency.  If I tie
up 8 MHz to deliver 19.4 Mbps, and you tie up 12 MHz to deliver 19.4 Mbps,
which one is more spectrally efficient?  Sure, you're only receiving 6 MHz
at time, but you've got that other 6 MHz cooking off in the distance, unable
to be used by anyone else.  From the FCC's point of view, you're using 12
MHz.  My question was if you were going to tie up 12 MHz of spectrum,  is a
MFN the best way use it?  I say no, because of your last question below.

The other 6 MHz is sending a different program to somebody else, which other
program has the same content as the one you are receiving. 12 MHz is tied up
to deliver 38.8 Mbps.

If four 19.4 Mbps programs are being simultaneously transmitted by
single-carrier-modulation signals, 24 MHz is tied up.  This 24 MHz  can only
transmit three 19.4 Mbps programs using the COFDM.  To my mind this means
COFDM is only 3/4 as spectrally efficient as single-carrier-modulation
signals.

That is not to say that there are no returns to be obtained from the
redundancy in COFDM.
The point is that SFNs eat up this redundancy, making it unavailable for
improving SNR.

I am not condemning COFDM.  I am simply pointing out that SNRs waste
frequency spectrum to no advantage to COFDM nor to single-carrier-modulation
signals, owing to the co-channel interference that is inevitable with the
use of SFNs.  I believe this is the reason that Charlie Rhodes hoped that
the FCC would never adopt this SFN topology in the US.  If COFDM is adopted,
MFNs are better used with it than SFNs would be, because the properties of
COFDM are not squandered overcoming co-channel interference from the network
itself.

As Bert observed, SFNs are a kludge.

Al

 
 
----------------------------------------------------------------------
You can UNSUBSCRIBE from the OpenDTV list in two ways:

- Using the UNSUBSCRIBE command in your user configuration settings at 
FreeLists.org 

- By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word 
unsubscribe in the subject line.

Other related posts:

  1. Home
  2. opendtv
  3. Archive
  4. 02-2011