The Bendov paper supports my understanding that single-frequency
networks (SFNs) are not well adapted for use with 8-VSB or other
single-carrier transmissions. Oded views the problem primarily from
the standpoint of providing satisfactory transmission of
electromagnetic field variations to certain reception sites.
Experienced receiver designers view the problem posed by SFNs as
primarily one of satisfactory reception being impaired by having to
combine signals received from two transmitters before the signals can
be processed to insure that they should combine constructively. Some
experienced receiver designers have been skeptical since SFNs were
first proposed for 8-VSB that practical receivers for it could be
designed. This doubt was felt especially if the receivers were to be
hand-held, so simple reception antennas had to be used.
Frequency-diversity transmissions from multiple single-carrier
transmitters are superior to simultaneous SFN transmissions. This is
because the respective signals from transmitters transmitting in
different RF channels need not be combined until each of the signals
is de-rotated during its individual down conversion to baseband. The
de-rotated baseband signals should always constructively combine.
Simultaneous SFN transmissions are usually combined before down
conversion, which combination may be constructive, somewhat
constructive or destructive in nature.
If multiple-carrier-signal transmissions are used in an SFN, the
situation can be better since usually some components of the plural
transmissions will combine constructively before down conversion. The
chance of the two multiple-carrier signals constructively combining is
generally improved if the two signals experience different multipath
conditions and if those different multipath conditions are dynamic in
nature. If there is redundant coding of the modulation of the
multiple-carrier-signal transmissions, the loss of the components that
combine destructively before down conversion can be overcome. This is
why SFNs are possible using COFDM.
The Sinclair submission cites the Bendov paper for the proposition
that 8-VSB transmissions do not work well with SFNs, which is
quite true for all single-carrier-signal transmissions. This fact is
used to support the premise that COFDM multiple-carrier-signal
transmissions are superior to single-carrier-signal transmissions
because they work in SFNs, as proven by experience in Europe. This
provides a subtle indictment against frequency-diversity
transmissions, which are supported by transmitter networks using
current transmitters, as opposed to SFN transmissions, which would be
supported by transmitter networks using new or reconstructed transmitters.
What the Sinclair submission ignored is that multiple-carrier-signal
transmissions, as well as single-carrier-signal transmissions, can be
better received from transmitters that broadcast over different RF
channels, rather than the same RF channel. This is because the
simultaneous multiple-carrier-signal transmissions can be processed
independently by a receiver until baseband responses to those
transmissions can be combined with that combination assuredly being
constructive in nature. Then coding of the multiple-carrier-signal
transmissions does not have to overcome the cancellations of
anti-phase components caused by simultaneous multiple-carrier-signal
transmissions being combined willy-nilly before down-conversion from
RF to baseband.
What the Sinclair submission could have made clear to the FCC is that
investing in SFNs for broadcast DTV is not a particularly good idea,
even if the transmitters in the SFN make simultaneous
multiple-carrier-signal transmissions. Frequency-diverse simultaneous
multiple-carrier-signal transmissions are more practical than
simultaneous multiple-carrier-signal transmissions over SFNs, insofar
as receivers capable of top-notch reception are concerned. The time
and money investments in transmitters for frequency-diverse
transmissions have been substantially sunk already.
When the SFN red herring is discounted, the controversy between
single-carrier-signal transmissions and multiple-carrier-signal
transmissions reverts to pretty much what it was before considering
the networking of transmitters for them.
Al
----- Original Message -----
*From:* Ron Economos <mailto:w6rz@xxxxxxxxxxx>
*To:* opendtv@xxxxxxxxxxxxx <mailto:opendtv@xxxxxxxxxxxxx>
*Sent:* Saturday, January 29, 2011 10:47 PM
*Subject:* [opendtv] Re: Seeing Ghosts on a Single Frequency Network
http://fjallfoss.fcc.gov/ecfs/document/view?id=7021027035
Ron
On 1/23/2011 4:30 AM, Mark Aitken wrote:
I can assure you that I share in your understanding and intend to
change the nature of the debate.
