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.
Table 1: Indoor planning factors from Bill Meintel's IEEE presentation.
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 atdlung@transmitter.com
