[opendtv] Re: Charles Rhodes on unlicensed devices and white spaces
- From: Richard Hollandsworth <holl_ands@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Wed, 8 Aug 2007 17:22:14 -0700 (PDT)
Too much attention given to auto detect of "unoccupied" "White Spaces".
As if some "improved" sensitivity and/or algorithm will suddenly "fix"
everything.
[Why not simply enter zipcode to determine DTV assignments????]
Obviously, an indoor device with a negative gain antenna
(perhaps in ground floor appt...or suburban basement)
can't "detect" ALLLL occupied channels....
ATSC coverage from multiple directions is way too spotty....
But this little hiccup obfuscates the REAL problems.
DUE TO INTERFERENCE TO CABLE SYSTEMS, THERE ARE NO WHITE SPACES!!!!!
FCC OET tests determined that unlicensed devices would need
to be limited to 4 mW EIRP (vice desired 1 watt) based on
measurements at 2 meters separation....but this isn't "worst case".....
If a user fired up his brand new "White Space Device" on the other
side of a wall from his (or his neighbor's) DTV, the separation
could be only about a foot...reducing the allowable EIRP by 10+ dB.
[And many appt's don't have that 2-inch thick gypsum firewall.]
But the "EMI Susceptibility" test was conducted on only one
cable channel....if you've ever seen susceptibility test results,
you would know that some frequencies are more susceptible
than others....so there may be another 10+ dB reduction
whenever someone conducts tests across the entire VHF/UHF band....
But they only tested those (flat panel) DTV's that were "easy"
to transport to the test location....RPTV's may have less metal
shielding....and what about all of those people using USB Stick
Tuners connected to cable...where are their EMI test results???
Other than allocating these (unlicensed) devices to some other band,
or mandating that cable systems free up a "reserved" band (fat chance),
what other "options" are there that might be forthcoming
to somehow blow new life into this (hopefully) dying bird???
holl_ands
====================================
"Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx> wrote: Craig Birkmaier
wrote:
> A generic question here. Is a multi-carrier modulation system
> susceptible to the same interference issues, or does the use
> of multiple carriers help to mitigate the interference
> issues?
Same issues. Although I have read something written by Charles Rhodes
that stated, still without explanation though, that zero IF tuners work
better with multicarrier schemes than they do with single-carrier. I've
been waiting for the followup article on that.
There have been some excellent posts on this topic a few weeks ago. The
best and also cost effective solution today seems to be a tuned RF stage
followed by single conversion IF with active image cancelling circuit.
The tuned RF stage followed by dual conversion IF, which seemed to me to
cover all bases, just can't seem to match the cost of the single
conversion designs. And then there is the zero IF option, which I need
to find out more about.
>> One interesting stat he gives is that OTA signal strengths
>> under -68 dBm (what the FCC calls weak) occur in 84 percent of
>> DTT coverage area. (Note by area, not population.)
>
> Why should this be any different than it has been for NTSC?
> Obviiously the big sticks can't cover everything. This is the
> primary reason that there are thousands of TV translators
> around the country, trying to extend coverage to remote areas.
True. But even more obviously, SFNs can't cover everything. They improve
the immediate area over which they are deployed to the detriment of
outlying areas. The important point is that a -58 dBm signal is
considered close to the minimum margin for cable systems, yet 10 dB
*less* than that is very prevalent for OTA systems.
> There is also the issue of where the white spaces spectrum is
> in the highest demand. In general this will NOT be the remote
> areas, but rather, the urban areas where DTV signal strength
> should be relatively high. There is considerable interest in
> using the white spaces for rural broadband services, but this
> may be accommodated using properly designed services on
> channels that are adequately separated to prevent interference.
I think the auto-detection feature of these white space devices will be
their downfall. No matter where they are used, if they are deliberately
tuned for that location, they could be made to work.
The good news, though, is that these devices also interfere with cable
systems, and cable systems have no white spaces. So that should make
just about everyone concerned. Hopefully, this concern will prevent
white space devices from being prematurely marketed and then becoming a
royal pain in the behind for all of us.
Bert
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