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

  • From: "Allen Le Roy Limberg" <allimberg@xxxxxxxxxxxx>
  • To: <opendtv@xxxxxxxxxxxxx>
  • Date: Wed, 19 Jan 2011 19:27:17 -0500

Charles Rhodes concluded his recent article "Seeing Ghosts on a Single
Frequency Network" with the following comment:

"Let's hope the FCC doesn't mandate SFN topology for U.S. broadcasters."

Bert commented that a combination of matched filtering in the tuner,
tracking tuners, dual conversion IF stages, and good equalizers, can result
in very good performance.  He added that a more clever use of the rapid
symbol arrival times, to improve on dynamic echo equalization, the use of
dual receive antennas, and the better combined use of the Reed-Solomon and
Viterbi trellis codes, might further improve 8-VSB as currently
standardized.  IMO, these comments have validity.

However, the problem with SFNs for 8-VSB transmitters concerns more than
just the capabilities of 8-VSB receivers to receive from one individual
8-VSB transmitter.  The crux of the problem is how the 8-VSB receiver
combines the signals from a pair of 8-VSB transmitters using identical (or
complementary) modulation.  The SFN proponents originally contemplated
combining the signals at the antenna of a single 8-VSB receiver.  This can
result in as much as a 3 db improvement in SNR of the combined received
signals if they be in phase.  However, combining the signals at the antenna
of a single 8-VSB receiver is also apt to cause catastrophic loss of
reception should the received signals be in anti-phase.

If the 8-VSB signals from a pair of 8-VSB transmitters using identical
baseband modulating signal are to be constructively combined irrespective of
the siting of the 8-VSB receiver, the 8-VSB signals must be kept separate
from each other until the conversion to baseband where both signals have in
effect been de-rotated to a common phase.  The two baseband signals can then
be combined, either in a critically sampled digital regime or in an
over-sampled digital regime that is quasi-analog in nature.

The easiest way to maintain separation is for the pair of 8-VSB transmitters
using identical  baseband modulating signal to transmit over different RF
channels.  A pair of tuners convert respective ones of the RF signals each
to baseband to de-rotate both the identical baseband modulating signal to a
common phase.  Then the baseband signals can be constructively combined

Suppose the 8-VSB RF signals resulting from identical baseband modulating
signals are transmitted at alternate times from a pair of 8-VSB transmitters
that transmit with no more than 50% duty cycles using the same RF channel
allocation.  Then, the 8-VSB signals have been converted to baseband, they
can be differentially delayed and subsequently constructively combined.
Note that, overall, this is no more spectrally efficient than the
frequency-diversity transmissions using two RF channels.  Half as many RF
channels are used, but digital bandwidth is halved owing to the 50% duty
cycles.

The proponents of SFNs for 8-VSB signals seek to transmit respective 8-VSB
RF signals resulting from identical baseband modulating signals from a pair
of 8-VSB transmitters that transmit with 100% duty cycles using the same RF
channel allocation.  It appears that the proponents of SFNs for 8-VSB
signals now contemplate keeping the respective 8-VSB RF signals separate
from each other by directional antennas or by different antenna
polarization.

The practical problem with this is that such antenna systems are difficult
to graft onto handheld 8-VSB receivers.  The strain on one's wrist to cart
around a receiver with such antenna is something to consider.  Fifty years
from now old-timers will have arthritis in their wrists besides in their
thumbs.  Networks of 8-VSB transmitters with overlapping coverage areas
would better transmit at different carrier frequencies, rather than a single
carrier frequency.  Not only has most of the investment in transmission
antennas for frequency-diversity transmissions already been sunk and
amortized, handheld 8-VSB receivers can use single antennas of practical
size.

Al

 
 
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