[opendtv] Re: DTT tuner design

  • From: "Dale Kelly" <dalekelly@xxxxxxxxxxx>
  • To: <opendtv@xxxxxxxxxxxxx>
  • Date: Sun, 08 Jul 2007 12:56:58 -0700

That's as I saw them when inspecting those sites. Twelve foot high
efficiency dishes per tower and using both space and frequency diversity, a
very major investment per site. Some sites did use very large horn type
antennas across the Sierras and there were likely others. I don't know the
selection criteria but I assume they are a higher performance antenna.
  -----Original Message-----
  From: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx]On
Behalf Of johnwillkie
  Sent: Saturday, July 07, 2007 11:15 PM
  To: opendtv@xxxxxxxxxxxxx
  Subject: [opendtv] Re: DTT tuner design


  And, having spent many, many hours looking at cc and ofs microwave station
records (original, master, in paper), in many instances, space diversity
required three 12" tx and rx antennae on each tower, with two and sometimes
three frequencies per tx point, and multiple routing across the country
because in some locations, sometimes NOTHING worked for short periods.



  And, I should point out, they usually had very generous noise and RF
budgets.



  John Willkie




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  De: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx] En
nombre de Dale Kelly
  Enviado el: Saturday, July 07, 2007 6:57 PM
  Para: opendtv@xxxxxxxxxxxxx
  Asunto: [opendtv] Re: DTT tuner design



  > In microwave communications, a similar effect occurs with antenna space
diversity....



  and.... add  frequency diversity and it's almost bullet proof - though
very expensive. The old AT&T system (before fiber) was the the model for
reliability.

    -----Original Message-----
    From: opendtv-bounce@xxxxxxxxxxxxx
[mailto:opendtv-bounce@xxxxxxxxxxxxx]On Behalf Of Richard Hollandsworth
    Sent: Saturday, July 07, 2007 1:54 PM
    To: opendtv@xxxxxxxxxxxxx
    Subject: [opendtv] Re: DTT tuner design

    Yes, multipath can (sometimes) be a GOOD THING.

    When performing lab tests with the Brazil Ensembles, et. al., the C/N is
established
    for one and only one path.  Additional paths are then turned on one at a
time to
    ensure that they all have the desired C/N (SAME if equal strength case).

    With uncorrelated noise on each path, total C/N actually improves when
the signals
    are in-phase....which is the job of the adaptive correlator.

    This explains why the reported C/N values under simulated multipath
conditions
    are less than for just a single path.

    In HF communications, adaptive equalizers coherently combine different
    propagation paths (e.g. F1 and F2) so that when the signal fades away
    on one path, the other path is usually still usable.

    In microwave communications, a similar effect occurs with antenna space
diversity....

    holl_ands

    ======================================
    Albert Manfredi <bert22306@xxxxxxxxxxx> wrote:

    Dale Kelly wrote:

    >I misspoke, either modulation system requires increased C/N
    >to handle multipath.

    I this strictly true? I don't think so. And I believe that's why the
Chinese
    standard adopted a training sequence in the GI (same one they use in
their
    single carrier mode), rather than using the end of the previous symbol,
as
    standard COFDM does. And that's also my bet for DVB-T2. (Just
speculation.)

    The ideal equalizer should not require a higher C/N to twist multipath
    distortion back into shape.

    Here is an example. I'm assuming here that the numbers provided are
C/(N+I),
    and that when this value is less than 15 dB, what you're actually seeing
is
    a (C+I)/N that is close to 15 dB.

    (I also believe that eventually we'll be seeing (C+I)/N better than 15
dB,
    when the combined trellis and R-S FEC schemes are more cleverly
exploited.)

    These are C/N numbers that apply to the Samsung Gemini chip.

    No multipath 15.2 dB
    Brazil A 15.0
    Brazil B 17.0
    Brazil C 13.5
    Brazil D 13.8
    Brazil E 19.9

    Compare this to the CSA values for 64-QAM and 3/4 FEC (same spectral
    efficiency if GI is 1/16).

    No multipath is 18 dB, Ricean is 18.6 dB, and their Rayleigh, which is
less
    strenuous than Brazil E, is 21.7 dB. (Brazil E has 3 dB echoes, vs two.)

    So basically, the planning factors used by the CSA do assume a deficit
of ~3
    dB or more in C/N margin, compared with a well executed equalizer demod.
And
    that's reflected in the field strength minima. If good demods are used,
the
    FCC and CSA planning factors should provide very close to the same
coverage,
    is my thinking.

    Bert

    _________________________________________________________________
    http://liveearth.msn.com



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