[opendtv] Re: Spectrum Repacking Looms for TV Broadcasters
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
- Date: Mon, 5 Apr 2010 14:42:07 -0500
John Shutt wrote:
> Transmitters must be able to deliver peak power, and are sized
> accordingly. FCC planning factors, for both ATSC and for the submitted
> Sinclair petition, are based on average power.
John, there are two issues to be discussed here. I couldn't find the Sinclair
petition online (seems to have been deleted), so I can't quote the exact words,
but here's the deal as I understand it.
Yes, as you say, RF amps have to deal with power peaks in order not to clip too
severely. So if the peak to average ratio is greater than it was before, it
means you need a more powerful transmitter in order to transmit the same
average ERP. But this is only the broadcaster's problem. Meaning, as long as
Sinclair or anyone else was willing to install more powerful transmitters to
reach their allowed ERP, the FCC in principle should not care. Right?
But there's another problem. Interference is caused by PEAK power too, not just
average power. Which means, if the FCC is doing their channel allottments based
on average power, implicitly they are presupposing a given peak power limit
too. So you are going in with an understanding that peak to average ratio is X,
and then you substitue a modulation scheme where the peak to average ratio
greater than X, that table of allottments might have to go out the window.
Given that we have a market-based model in the US, and that we want to be able
to reuse frequencies as soon as possible between markets, it should not be
surprising that minimizing peak to average ratio is a good idea. If we instead
could have regional broadcasters, and good, practical SFNs, then this would be
less of a concern.
That is why, IIRC, Sinclair voiced that willingness to reduce power. Even if I
don't remember that correctly, the issue remains. Frequency reuse becomes
harder with higher peak to average ratios, and we need as much frequency reuse
as we can get. As a matter of fact, the FCC is now trying to change its rules,
to allow even tighter packing of channels, which only makes this issue that
much more important. Not to mention that VHF power limits are being revisited
as we speak, also contributing to this problem.
Bert
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