Doug Lung / RF Technology
Reviewing the IEEE Broadcast Symposium, Part II
This month I’ll wrap up my coverage of the IEEE Broadcast Technology Symposium. These are short summaries of much larger presentations. Let me know if you would like me to cover any of the topics in more detail.
ALTERNATIVES TO OET-65
Anyone responsible for an FCC licensed transmitter should be familiar with the FCC Office of Engineering and Technology Bulletin 65. “OET-65” provides a set of tools for evaluating RF exposure from radiators ranging from AM towers to microwave dishes. At the symposium, Martin Doczkat from the FCC Office of Engineering and Technology presented RF Exposure: Simulation & Compliance, in which he reviewed antenna RF field theory and compared computational methods, including OET-65, for predicting RF exposure.
Due to the height and location of broadcast antennas, most broadcast engineers use OET-65 formulas to calculate far-field RF power density. OET-65 offers formulas for the reactive near-field and radiating near-field, but other methods may yield more accurate results. For example Doczkat noted the FM Model “May be grossly inaccurate in the near-field” due to the null fill, phase offset, and feed power of the antenna and environmental effects such as reflections. The other methods he described, including Method of Moments (MoM) and Finite-Element Method, are familiar to antenna engineers. One of his slides shows a possible extension to OET-65 using MoM for localized exposure. It is usable for distances up to 40 cm for emitters between 300 MHz and 6 GHz.
Doczkat cautioned that multi-use sites may be more challenging to accurately model due to approximations of sources and the environment. He noted that correctly performed measurements take precedence over computation, adding “Licensees/permittees must consider all RF sources, and must coordinate to reduce power or cease operations as necessary to ensure compliance.”
NEXT GEN TV
While watching TV on the big screen remains popular, more people are watching video on their smartphones and tablets. The 19.39 Mbps available with from the current ATSC 8VSB standard currently isn’t likely to be enough for UHDTV. Work is underway in this country on ATSC 3.0 and internationally with the Future of Broadcast TV (FoBTV), of which NAB and ATSC are members, to develop the next generation broadcast (NGB) TV platform.
Mark Richer from ATSC outlined the FoBTV vision and provided his thoughts on requirements for the NGB system. These include an IP transport layer, interactivity and addressability and a focus on broadcasting to mobile devices, although ultra-high definition wasn’t ignored. Regarding transmission systems, Richer listed efficiency (number of bits/Hertz) and flexibility/scalability as key points. We can’t assume fixed bandwidth channels such as 6 MHz; they should be dynamically configurable from 0.25 to 25 MHz.
In his presentation “Progress in ATSC 3.0: The Next Generation Broadcast Television System,” Jim Kutzner, senior director of Advanced Technology at PBS, outlined the methodology ATSC and FoBTV are using to develop NGB-TV. The ATSC TG3 subcommittee is first looking to outline the use cases for ATSC 3.0 and develop scenarios around them. Kutzner pointed out that a highly granular, “bottom up” approach stimulates expansion thinking (ATSC 3.0 won’t be limited by backwards compatibility to the existing broadcast system), while providing a perspective that includes business and regulatory impacts. Use cases are used to create scenarios that define requirements. Scenarios include a harmonized world standard, new services to provide an immersive viewing experience, “smart” interaction and personalization. Some high level system target attributes include high spectral efficiency, improved reliability and robustness, flexible system configuration, and support for multiple terminal characteristics (mobile/fixed). Future work will consider deployment scenarios.
The demand for increased quality and quantity of services will not diminish, so the application, transport and physical layers of a future system need to be decoupled to allow for changes. The system must have the ability to evolve to future standards.
Kutzner cautioned that the ATSC 3.0 puzzle is more than technology; it also includes business and regulation. In my opinion, those could be more challenging than the technology side!
Luke Fay from Sony Electronics, Skip Pizzi from NAB, and Jerry Whitaker from ATSC also contributed to the presentation paper.
ATSC 3.0 and the FCC incentive auction of UHF TV spectrum were the two most common topics of discussion, at least among engineers involved with TV broadcasting in the United States. At this point, we don’t know exactly how the FCC will comply with the law that requires them to use “all reasonable efforts” to “protect the coverage area and population served” by TV stations that do not give up their spectrum, according to the FCC’s current Notice of Proposed Rulemaking. Reply comments to the NPRM are not due until March 12 so it will likely be after the NAB Show before we find out what the FCC has decided on.
Robert Weller, FCC Office of Engineering and Technology Technical Analysis Branch Chief, provided a summary of the Incentive Auctions. He noted that not all terms in the “all reasonable efforts” clause are defined. The FCC has tentatively defined coverage area as “terrain-limited coverage,” effectively preserving the noise-limited protected contour but with areas affected by terrain assumed to be identical within the same band. Multiple options were presented for dealing with interference after repacking.
The desired outcome of the repacking is optimized placement of stations, minimized spectrum holes, minimized spectrum interference and maximized utilization of spectrum. He acknowledged that during the DTV transition, the process generated antenna patterns that couldn’t be built. To avoid this problem in the repacking, the FCC is considering maintaining antenna patterns and scaling them.
During Q&A, Bill Meintel, of the San Francisco based RF consulting firm, Meintel, Sgrignoli and Wallace, asked about the dipole factor, to which Weller responded that it was still being considered and he welcomed comments backed up with a technical analysis. As you may recall, the FCC dipole factor correction requires a higher signal level for coverage at UHF channels above 37 compared with those below Channel 37. If dipole factor is used in calculating protected coverage, a 1,000 kW station on Channel 50 moving to Channel 26 could have to reduce its power by 2 dB, to 631 kW to maintain the service contour.
In these columns I’ve provided just a sample of the papers at this year’s Symposium. The entire 2012 Symposium is available on-line at http://vabs2012.infoneedle.com/. Next year the IEEE Broadcast Technology Symposium will move to San Diego. The IEEE BTS web site at http://bts.ieee.org/ has preliminary details.
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