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

  • From: "Allen Le Roy Limberg" <allimberg@xxxxxxxxxxxx>
  • To: <opendtv@xxxxxxxxxxxxx>
  • Date: Fri, 11 Feb 2011 00:48:44 -0500

I presumed a cooperating pair of stations, one in in each city that have
different 6 MHz RF channels that are not next to each other, so that
receivers do not have to deal with adjacent-channel interference problems
nor with the co-channel interference problems that are introduced by SFN.
If you allow receivers with two front-end tuners, the MFN currently in place
need not change.

If you want to use receivers with a single frequency-agile front-end tuner,
the transmitters must divide their transmission into two
time-division-multiplexed fields and transmit two different programs, one in
the odd fields and the other in the even fields.  E.g. the odd program
fields could be conveyed by the odd M/H Groups prescribed in A/153 and the
even program fields could be conveyed by the even M/H Groups prescribed in
A/153.

The point is that with an SFN, a receiver situated in a zone of overlapping
coverage has to deal with co-channel interference causing a deep fade in
received signal strength.  There is no energy for the receiver to detect;
the lost transmission has to recovered from redundant coding of less
affected portions of the frequency spectrum.  This costs spectral
efficiency.

With an MFN co-channel interference does not occur.  During the down
conversion to baseband, the two signals can be rotated to a common phase
orientation so they can be constructively, rather than randomly combined.
So, the signal combining process need not lose frequency spectrum.


Allen

----- Original Message ----- 
From: "John Shutt" <shuttj@xxxxxxxxx>
To: <opendtv@xxxxxxxxxxxxx>
Sent: Thursday, February 10, 2011 4:38 PM
Subject: [opendtv] Re: Seeing Ghosts on a Single Frequency Network


>
> ----- Original Message ----- 
> From: "Allen Le Roy Limberg" <allimberg@xxxxxxxxxxxx>
>
> > 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.
>
> Remember we're talking about a MFN here, so you're delivering the same
> multiplex using two separate 6 MHz carriers that nobody else can use in
your
> area.  Your station must be licensed to use 12 MHz of bandwidth.  You are
> not delivering 38.8 Mbps, you're delivering the same 19.4 Mbps twice using
> two separate 6 MHz channels.  And you cannot do so to portable or mobile
> devices.
>
> > 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.
>
> Again, we're talking about a MFN, not separate broadcasters in a single
> market.
>
> > 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.
>
> It only eats up the redundancy when in the area of overlap.  Which is why
I
> suggested using a wider bandwidth with more carriers, at a lower symbol
> rate, which restores SNR in the overlap area, and also improves SNR
outside
> of the overlap area.
>
> A MFN gains nothing when outside of the overlap area.  Your reception is
as
> fragile as ever, and cannot support mobile or portable use.  You have to
go
> to something like ATSC-MH, and then guess what?  You're right back to the
> lower bitrates of COFDM.
>
> > 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.
>
> Whether you're using two different 6 MHz channels in a MFN, or (for the
sake
> of argument) a single 12 MHz channel in an SFN, either way you're tying up
> 12 MHz of valuable spectrum.  And with the MFN you get zero improvement in
> reception capability when you're outside of the overlap zone.
>
> > As Bert observed, SFNs are a kludge.
>
> They're only a kludge when used with a single carrier system that is
> ill-equipped to handle them.  A kludge is ATSC-MH.
>
> John
>
>
>
>
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