[opendtv] Re: FCC Broadband Plan on TV spectrum
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
- Date: Wed, 17 Mar 2010 12:54:02 -0500
Al Limberg wrote:
> The FCC does not have competent engineering advice. SFNs with
> overlapping coverage areas simply will not work commercially
> for 8VSB DTV, despite some of the standards issuing from ATSC
> tending to indicate otherwise.
I'd go further, so as not to be misunderstood (as I'm sure this will be).
SFNs with overlapping coverage areas will also not work well with DVB-T,
because the guard intervals are not long enough in time unless, at best, tuned
to accept a big drop in spectral efficency. It takes something like DVB-T2,
with drastically lower symbol rates, to create long enough guard intervals for
such wide area SFNs. (Obviously, if dead zones in the area are acceptable, then
the problem is mitigated.)
SFNs with overlapping coverage areas are also not compatible with the US
market-based TV, which the FCC continues to promote.
And yet, SFNs with overlapping coverage areas are the only credible approach to
spectrum savings. Otherwise, the SFN's primary goal is ease of reception. And
conversely, a practical regional SFN, e.g. using DVB-T2, will probably have
towers spaced far enough apart that ease of reception will not be much
different from that of big sticks. As is also the case with the very low power
SFNs in Italy, for instance (same scenario, scaled down).
> Single-antenna receivers (particularly those already in the field)
> that move position cannot overcome the nulls caused by anti-phase
> signals from two transmission towers.
I think that in this case, COFDM helps only if the nulls are very narrow. Broad
nulls would kill that too, and both systems would benefit from diversity
antennas. I think that practical SFNs with DVB-T or with 8-VSB (assuming 5th
gen receivers or better) are those where all of the towers are relatively close
together, so that no tower would create an "echo" that is beyond the echo
tolerance of any of the other towers. Just as they have done in the few Euro
examples out there. Otherwise, you will get to the dense mesh that the Qualcomm
MediFLO system had to adopt, and even then, accept lots of interference zones
as they have to do.
Bert
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