[opendtv] Re: DTT tuner design

  • From: "Dale Kelly" <dalekelly@xxxxxxxxxxx>
  • To: <opendtv@xxxxxxxxxxxxx>
  • Date: Sat, 07 Jul 2007 18:57:21 -0700

> 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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