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

  • From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
  • To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
  • Date: Wed, 9 Feb 2011 15:14:33 -0600

John Shutt wrote:

> 8 MHz UHF channels are the norm in Europe, and 6-7-8 MHz adaptive
> receivers have existed since the mid 2000s, mainly as DVB-T USB
> "thrumbdrive" receivers.   (Google "DVB-T USB Receiver".) My
> suggestion of a 12 MHz wide channel was more to compare two
> different approaches that each tied up 12 MHz of spectrum than it
> was an actual proposal for a working system.

I got that part of your proposal. But the point you ignore is, SFNs are a 
kludge.

What does that mean? It means that in order to create a workable SFN with a 
given channel width, you are going to eat into that channel capacity. Instead, 
if you use an MFN, you wouldn't have to degrade the channel spectral efficiency.

It's true that multicarrier schemes can *better* survive destructive 
interference between SFN towers, as Al has said. But *better* is the key word. 
All of the subcarriers you clobber in the process can only be survived by 
making the scheme spectrally inefficient. If you can survive half the carriers 
being clobbered, it means simply that you have had to cut in half the spectral 
efficiency of the channel.

Not an issue with MFNs. And you don't need to worry about time-aligning the 
symbols, either.

I think of SFNs a bit like 5 cylinder engines. A bit of an engineering tour de 
force, to keep engineers interested in trying out new stuff, but what's the 
point? Fours and sixes are easier to balance. You don't need to spend R&D funds 
inventing super fancy new engine mounts to **mask the inevitable problems**.

> You neglected to comment on the other speculated benefits of my
> approach using a "wideband" COFDM receiver when it is well within
> the city grade contour of a single transmitter, far away from the
> overlap area, compared to your dual frequency approach.

What is the advantage here, compared with two 6 MHz channels?

Bert
 
 
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