[opendtv] 2008 IEEE Broadcast Symposium in Review
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Thu, 11 Dec 2008 12:14:41 -0500
VERY interesting column.
Note the results of the Brazilian tests, note which propagation model seemed
the most accurate in predicting coverage, note the comparisons of single tower
vs different SFN configurations, note how amazingly long the echo lags was that
were measured in the San Francisco tests, sometimes preventing M/H reception,
and note what Doug Lung says about DTT stations moving back to VHF.
Bert
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http://www.tvtechnology.com/article/70966
2008 IEEE Broadcast Symposium in Review
by Doug Lung, 12.04.2008
This month, I'll discuss some of the papers presented at the 58th Annual IEEE
Broadcast Symposium that focused on TV broadcast RF topics and the U.S. DTV
transition.
COVERAGE MEASUREMENT AND PREDICTION
The paper "Brazilian DTTB Coverage Performance Evaluation" by Gunnar Bedicks
Jr.; Fujio Yamada, Edson Horta, Cristiano Akamine and Francisco Sukys describes
the results of DTV signal measurements in the Sao Paulo metropolitan area. Sao
Paulo was selected for study because of its rugged terrain and obstructions
from high buildings. Where possible, measurements were done in 24 radial
sectors (15 degrees wide) at 10 distances ranging from 1 km to 57 km from the
transmitting antenna. DTV signal quality was compared with that of an adjacent
channel analog signal from the same site.
The DTV effective radiated power (ERP)-based on information in the paper-was
212 kW at an antenna height of 170 meters. The analog transmitter peak output
power was 60 kW, four times the average output power of the DTV transmitter.
The measured results were compared with the field strengths calculated using
the Okumura-Hata model and the ITU 1546 curve.
A graph showed the measured field strength in the sectors at different
distances from the transmitter. There was a variation in measured signal
strength across the sectors, as expected given the terrain and obstructions.
Overall, the Okumura-Hata model provided the best prediction. ITU 1546 usually
underpredicted field strength and, as expected, free space calculations almost
always overpredicted field strength.
The measurements indicate indoor reception should be feasible up to 20 km (12.5
miles) from the transmitter site but beyond that an outdoor antenna is needed.
Areas with failed DTV reception 30 km (18.6 miles) from the transmitter site
were terrain obstructed. Perfect quality DTV reception was possible in areas
where analog picture quality was poor. Bedicks said that six or seven gap
fillers could be used to fill in the coverage "dark spots" in the city.
Two papers focused on mobile video using the ATSC M/H balloted candidate
standard.
This year's symposium had two presentation tracks on Thursday, making it
impossible to catch all the presentations. Copies of the papers are available
in the symposium proceedings. I've summarized the key points from one of them
based on the paper in the proceedings.
The "Statistical Analysis of Digital Television Planning for the ISDTV System"
by Késia C. Santos, Erik F. Silva and Marcelo S. Alencar compares a single
stick transmission facility with three variations of a single frequency network
using multiple lower power transmitters. The study used a 3D ray tracing method
to calculate field strength and power data in a 2.4 km by 3.4 km urban area.
Three antennas at 60 meters with a power of 5 kW each resulted in an outage
probability of 5.5 percent (94.5 percent coverage). Five antennas at 45m, with
a power of 3 kW each, provided an outage probability of 5.8 percent. Using a
single tower at 100 meters with 5 kW provided only 81.2 percent coverage. It
was necessary to increase the transmitter power to 30 kW to reduce the outage
probability to 5.3 percent (94.7 percent coverage).
Mobile TV was a popular topic inside and outside the meeting rooms. Two papers
focused on mobile video using the ATSC M/H balloted candidate standard. Dennis
Wallace a partner at the firm, Meintel, Sgrignoli and Wallace, described mobile
and handheld field measurements. Mark Aitken, director of advanced technology
at Sinclair and Brett Jenkins, director of technology strategy at Ion Media
Networks covered mobile video business opportunities, the ATSC A/153
standardization activity and ATSC M/H technology from the presentation and
management layers down to RF system performance. Aitken's part of the
presentation, which covered everything except the RF system performance, is in
the proceedings. This month, I'll focus on Jenkins' part of the presentation.
Jenkins' presentation is not in the proceedings. If there are minor errors in
the numbers, blame them on my notes. He focused on the results of the
Independent Determination of Viability (IDOV) conducted by the Open Mobile
Video Coalition's Technical Advisory Group (OMVC OTAG) and provided more detail
on the performance of the Samsung/Rohde & Schwarz A-VSB technology and the
LG/Harris MPH technology in San Francisco and Las Vegas. This information had
not been released publicly, as far as I know, until his presentation. There
were no surprises-overall MPH performed better. What was interesting was a
discussion of the failure of both systems at a few measurement locations in San
Francisco. The data collected during the IDOV showed sufficient signal
strength, but reception wasn't possible. OTAG coordinated a second round of
measurements with LG to try to determine the cause of the failures.
More detailed measurements showed the problem was due to strong echoes outside
the equalizer range of the receiver. Jenkins said a -6.4 dB reflection at 86.6
ms was observed at one of the locations. Not only is this echo outside the
range of ATSC equalizers, it is outside the guard interval for COFDM (Coded
Orthogonal Frequency Division Multiplex). He presented maps showing the time
delay for reflections from the two towers of the Golden Gate Bridge. They
closely matched the echoes observed at the Broadmoor test site. In the
Embarcadero part of San Francisco, a -7 dB reflection was seen at 101.8 ms.
These and other ghosts in the 80-100 ms range were likely due to reflections
from the San Raphael and Oakland Bay bridges.
While locations with echoes like these are rare, providing robust mobile video
service in these isolated areas will be a challenge. Equalizer performance is
important. A directional antenna improved reception. These failures occurred in
very few locations, so another solution might be to install low-power boosters
to provide a strong enough signal to overcome the echoes.
CATASTROPHE?
Bill Meintel's presentation, titled "The U.S. DTV Transition-Will February 18,
2009 be a Catastrophe? What are the Problems and How to Fix Them" certainly
attracted a lot attention (see "FCC Lacks Focus on Transition," Nov. 5, 2008).
The paper isn't in the proceedings, so blame me for any errors.
Meintel presented a list of warning signs of problems on Feb. 18. I've
paraphrased some of his key points:
700 stations are changing channels in February;
There is no requirement that DTV coverage replicate analog coverage;
Lack of confidence in converter box receiver performance;
Lack of confidence in non-over-the-air providers to change out analog receivers;
Government has failed to understand the transition and has lost sight of the
goal-continuity of service, and
Problems in the economy may create more off-air viewers, but some broadcasters
might not survive.
Meintel sees the "cliff effect" as the biggest problem. DTV reception doesn't
fail like analog reception. Viewers with poor analog reception will not have
DTV reception. The problem affects distant viewers and ones closer in using
indoor antennas.
Building penetration loss, height loss, loss of antenna gain and multipath all
need to be considered when determining the field strength required for indoor
reception. Meintel presented his suggested indoor planning factors, see Table 1.
I didn't catch Meintel's explanation of how these numbers were derived, but I
know the firm Meintel, Sgrignoli and Wallace has done ATSC field-testing in
many cities checking indoor and outdoor reception. Dennis Wallace has also
evaluated indoor antenna performance. One thing you will notice-in all but the
worst case, higher field strength is required for high band VHF reception than
UHF reception. The main reason for this is that indoor antennas have lower gain
at VHF than at UHF.
Meintel examined the impact these planning factors would have on people living
within 20 miles of the transmitter site where indoor antennas are likely to be
used. For all scenarios, around twice as many people are expected to have
problems with high VHF reception as with UHF reception. If this analysis is
correct, it will be a major issue for UHF DTV stations moving back to their
high band VHF analog channels.
Meintel outlined ways to solve the problem. Consumer education is at the core
of many of them. He urged government to grant maximization applications
promptly, with restrictions as needed, and promptly notify applicants of
technical deficiencies in applications. Regarding TV spectrum, he emphasized no
rulemakings should go forward unless they have a direct positive impact on
improving free off-air service and contribute to a successful transition.
In addition to consumer education efforts and enlisting help from retailers and
local civic groups, stations need to make sure cable and satellite providers
will be ready on Feb. 18. It is also important to determine areas where you
anticipate reception problems. This may be due to a sidemount antenna or
directional pattern. Keep track of DTV service complaints to define areas that
need more work. Long-term solutions will involve improved transmission
facilities, translators, distributed transmission systems, better receivers and
better receive antennas, including smart antennas.
This is only a small sample of the presentations at the 2008 IEEE Broadcast
Symposium. For more information, visit www.ieee.org/bts.
Even though time constraints may make it difficult to respond to all e-mail, I
read all comments. Your question could provide the seed for a future RF
Technology column.
E-mail me at Dlung@xxxxxxxxxxxxxxx
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