[opendtv] Re: SFN considerations (was Doug is Missing the Point)
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Fri, 17 Feb 2006 19:53:56 -0500
Neil Pickford wrote:
> Lets keep it simple so the 8-VSB encumbered can understand.
>
> The reason SFN concept works in DVB-T COFDM is that we
> have given up some data capacity to extend each Transmitted
> symbol time with a symbol guard interval.
>
> The receiver still works with the nominal symbol time but
> adjusts a symbol sampling window so it incorporates the
> majority of the significant echoes (pre or post) received
> for each symbol. This minimises the Inter Symbol
> Interference normally caused by multipath. This is why
> DVB-T COFDM can handle passive/active (SFN) multipath so
> well.
To implement SFNs, what you need is multipath tolerance. Guard intervals
are merely one system's technique for doing this.
Furthemore, to implement the simpler unsycnhronized variey of SFNs, you
need not just lagging echo tolerance but also the more difficult leading
echo tolerance.
What you describe above is the way COFDM combats multipath. But 8-VSB
can also be used in SFNs, as long as receivers are designed to combat
multipath.
Al was describing how 8-VSB receivers are designed to combat multipath.
With 8-VSB, this is done by means of an equalizer in the receiver. To
better combat pre-echo, the 8-VSB solution is to add a matched FIR
filter upstream of the equalizer, so as to make the job easier for the
equalizer when pre-echo exists. Because the equalizer only has to
process what looks like lagging echo. (That seems to be the best
technique as of now, at any rate.)
And also important, SFNs solve problems in certain specific situations,
but are not practical in many of the scenarios that marketing hype
suggests. They aren't as trivial to implement as marketers would have us
believe, for any system. So in fact, whether COFDM or 8-VSB is used, a
practical national network has to rely on a system of translators, and
SFNs may be usable within smaller areas, such as urban centers. Large
scale SFNs, while theoretically possible with either COFDM or 8-VSB
through synchronization of transmitters, are expensive and certainly
problematic, and therefore not contemplated in either system.
So the bottom line is, what we have here is only a difference in degree,
at most. It's true that as of now, COFDM does permit dialing back of
spectral efficiency to make SFNs somewhat more practical than 8-VSB, but
still within the confines of urban areas. As of now, with respect to
SFNs, 8-VSB is similar to COFDM with a GI of between 1/16 and 1/32.
Something like that.
Bert
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