[opendtv] Interference tests for use of TV white spaces

  • From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
  • To: <opendtv@xxxxxxxxxxxxx>
  • Date: Wed, 1 Aug 2007 16:36:46 -0400

Brand new reports from the the FCC on use of white spaces. This one is already 
available to the public. This describes co-channel interference between a white 
space device using a free OTA channel and a TV connected to a cable system, 
tuned to that same frequency band.

Transmission power of the signal arriving through the cable was -58.8 dBm, 
toward the minimum of what's acceptable for cable systems.

The transmission power of the DVB-H device using the white space that caused 
interference at 2 meters was 6.3 dBm, or 4 mW.

The transmission power that caused  interference at 10 meters was 15.3 dBm, or 
34 mW.

Note: the proposed rules are to permit these white space devices to operate up 
to 1 W output power. So that appears to be a bit excessive.

As expected, it would appear that apartment or townhouse dwellers are most at 
risk in this. Intervening walls (of standard materials in US construction) did 
not seem to change the situation appreciably.

What this report doesn't address is how the white space device determined that 
the space was available, and the effect of a transmission on OTA reception if 
the transmitting device mistakenly assumed the space to be "white." If the 
space was not really available, I would expect a TV connected to an outdoor 
antenna to be somewhat more vulnerable, because OTA signal are often less than 
-58 dBm. A TV connected to rabbit ears would of course be at considerably 
higher risk.

Bert

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http://hraunfoss.fcc.gov/edocs_public/attachmatch/DOC-275668A1.pdf

EXECUTIVE SUMMARY

This report presents the results of in situ measurements of the interference 
susceptibility of three digital television (DTV) receivers to direct-pickup of 
emissions within the channel width of a digital cable TV signal to which the 
receivers were tuned. The tests were intended to identify the susceptibility of 
cable TV reception by a digital cable TV receiver connected directly to the 
cable TV system (without the use of a set-top box) to interference from devices 
that might operate within the TV broadcast spectrum on locally unused broadcast 
channels (TV white spaces) that overlap the frequencies of channels used by the 
cable TV system. The Commission is currently considering rules that would 
permit the use of such white-space devices.

The three DTV receivers were Digital Cable-Ready (DCR) models that were on the 
market in 2005. The digital cable TV signal used in these tests was a 
256-quadrature amplitude modulation (256-QAM) signal adjusted to a signal level 
near the minimum level specified by the "Digital Cable Network Interface 
Standard"* for the "input terminals of the first device located on the 
subscriber’s premises." Tests were performed on EIA cable TV channel 70, which 
overlaps the spectrum of broadcast UHF TV channel 19.

The interfering signal was a 4.8-MHz wide orthogonal frequency-division 
multiplexing (OFDM) signal operating in the spectrum occupied by the selected 
cable TV channel. This signal was radiated from an antenna having near 0 dBi 
gain.

The tests were performed with the interfering signal source separated from the 
DTV receiver by distances of 2 meters or 10 meters and, in most cases, by one 
of two residential walls: an exterior wall of a singlefamily house or a wall 
separating two townhouse units. The interfering source was operated at two 
different heights at most locations and at two different polarizations. Most 
measurements were performed with the interferer located behind the TV receiver, 
but some measurements were also performed with interference from the front of 
the TV and a few measurements were performed at other aspect angles. A total of 
108 measurements were made.

The tests show that an OFDM source operating at an effective isotropic radiated 
power (EIRP) as low as 6.3 dBm can cause interference to cable DTV reception at 
a distance of 2 meters and that an EIRP as low as 15.3 dBm can cause 
interference at a distance of 10 meters. † The TVs exhibited less 
susceptibility to interference from the front than to interference from the 
rear. For interference from the rear, the median interference threshold EIRPs 
across the three tested receivers and all antenna heights, polarizations, and 
lateral-offset positions were 16.9 dBm and 24.2 dBm for 2 and 10 meter 
distances, respectively. Median thresholds for interference from the front were 
21.2 dBm and >25.1 dBm at 2 and 10 meters, respectively.

(We note that interference from a device in a neighboring residential unit is 
unlikely to occur with the front aspect at a distance of only 2 meters.)

Due to the limited scope of these tests (three TV sets, one cable-TV channel, 
and two primary test sites), the results are not intended to constitute a 
complete basis for defining criteria necessary to protect cable TV viewers from 
interference by devices operating in the TV white spaces. Nevertheless, the 
tests provide an empirical demonstration of the potential for such interference 
at relatively low power levels, and, as such, a useful input to the decision 
process.
 
 
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