[opendtv] Re: Diversity USB DVB-T stick
- From: "Albert Manfredi" <bert22306@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Sun, 25 Feb 2007 16:59:37 -0500
Richard Hollandsworth wrote:
With the two included short whip antennas, the "optimum
spacing" may be as far apart as the short cords permit....
but it's gonna depend on where those sweet spots are....
1/2 lambda is all it takes for used in mobile devices, where you have to
keep things small.
Drat!!! Can only dream of Diversity Antenna for ATSC....
must be "too hard".....
Too hard why? Because there are interests out ther that don't want good OTA
equipment to be available in the US? Or too hard because of technical
reasons?
The A-VSB work included a diveristy antenna. When used in the turbo code
mode, that combination came within less than 2 dB of the Shannon limit. Not
half bad.
And why can't diversity antennas be used for any 8-VSB receiver? The antenna
combining technique mentioned in this thread, maximum ratio combining (MRC),
is the most elaborate of the choices, not the only choice. You would need a
separate demod for each antenna if you use MRC, because you need to know the
channel characteristics of each channel. (Always true, by the way, not just
for ATSC.) But a simple threshold technque could be implemented with a
single receiver. Potentially, as a separate outside box, as Tom was asking.
By the way, as far as I know, these automatic diversity antenna schemes were
originally developed for DS-CDMA, a technique in which the symbol arrival
period is expected to be much longer than symbol duration. Short blips
spaced far apart. It was intended for use with rake filters. So, if it these
technqiues were adaptable to something totally different, like COFDM, I
don't see why one should assume they aren't adaptable to other modulations
too.
Bert
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