Distributed transmission system

In North American digital terrestrial television broadcasting, a distributed transmission system (DTS or DTx) is a form of single-frequency network in which a single broadcast signal is fed via microwave, landline, or communications satellite to multiple synchronised terrestrial radio transmitter sites. The signal is then simultaneously broadcast on the same frequency in different overlapping portions of the same coverage area, effectively combining many small transmitters to generate a broadcast area rivalling that of one large transmitter or to fill gaps in coverage due to terrain or localised obstacles.

History

While the idea of a single-frequency network of multiple transmitters broadcasting the same programming on the same channel from multiple transmitter sites is not a new concept, the ATSC digital television standard in use in North America was not designed for this mode of operation and was poorly adapted to these applications. The restrictive timing requirements and poor multipath interference handling of early ATSC implementations would have precluded multiple synchronous transmitters on the same frequency at the time of the first wide-scale commercial ATSC deployment in 1998; these restrictions eased somewhat as receiver design advanced in subsequent years. By 2004, technology existed to provide digital television receivers with the means to detect static (not mobile or changing) multipath interference (subject to certain timing constraints) and compensate for its effects on the digital signal.

Tests have been run by various individual broadcasters or broadcast groups, including the Metropolitan Television Alliance (MTVA, a consortium of New York city television stations).[1] A series of initial tests involving four distributed transmission sites and over 100 test measurement sites in NYC and New Jersey were completed in June 2008, along with smaller-scale tests in New York in 2007. The New York market is uniquely problematic for multipath reception due to the large number of man-made obstacles which prevent adequate digital coverage of the entire city from the main broadcast facilities atop the Empire State Building.

Technical issues

To the receiver, a signal from a single-frequency network appears as a single broadcast with strong multipath interference; in the worst case, it is detected as a main signal and a reflection both of equal strength as signals arrive from multiple transmitters to the same intermediate location at slightly different times.

The ATSC standard used for digital television in North America, unlike the DVB-T standard in Europe and other nations, uses 8VSB instead of OFDM—a modulation which allowed a station to transmit at lower peak power levels, but which historically has been far inferior in handling multipath reflections and RF interference.

The first widespread commercial deployment of US ATSC digital television began in 1998, with the first early adopters being stations in the largest markets (including New York city, served by transmitters atop the World Trade Center). Digital receivers of this era, while expensive, were poorly equipped to deal with reflected signals—a severe drawback in urbanised environments. Later generations of receiver design significantly mitigated these limitations; by 2004 technology existed to build receivers capable of detecting and compensating for static multipath interference conditions where a single echo was 10 dB weaker (within a 30 microsecond time difference) or the same strength (the worst case, but within a 12 microsecond range).[2]

If the transmitters could be kept at sufficiently precise synchronisation and sufficiently close geographical spacing to operate within these limits, a single-frequency network using the new receiver design would be possible even with the existing North American ATSC digital broadcast standards.[3]

Tests by Pennsylvania State University public educational WPSX-TV (now WPSU-TV) were initially made in 2003[4] WPSU was in analog a VHF 3 station which serves State College, Pennsylvania from a distant transmitter which must also cover Johnstown and Altoona. As a digital station, WSPU had used a large UHF 15 transmitter at the location of the original low-VHF broadcast tower, leading to localised problems with terrain shielding which interfered with UHF reception in State College itself. Relocation of the main transmitter would have interfered with the station's ability to serve the other two communities. Addition of a small (50 kW) synchronised digital TV transmitter in State College, on the same frequency as the main UHF 15 signal, proved a means to improve reception; further improvements would be possible by adding small co-channel 50 kW transmitters in each community to be served.

ATSC released standards on September 25, 2004 as guidance on the design of multiple transmitters, single frequency networks and multiple frequency networks.[5] The new 2004 standards included:

  • A/110A, "Synchronization Standard for Distributed Transmission, Revision A"
  • A/111, "Design of Synchronized Multiple Transmitter Networks" [6]

Technical issues addressed included that of synchronization between transmitters (GPS was used to supply a 1 Hz and a 10 MHz reference frequency, as well as timing information) and precise control of transmitted frequencies (to within 1 Hz). Identification for each individual transmitter needed to be embedded in the signal for troubleshooting purposes, yet the main data stream on every synchronised transmitter must be identical; this is done by adding a second, low bit rate spread spectrum signal 27–30 dB weaker than the main signal. As this "watermark" identifier is buried under the stronger main signal, multiple repetitions of this same identifier could be received and summed in order to provide a readable version of the watermark to broadcast technicians. A standard receiver, meanwhile, would see the same signal from all transmitters by design.

The generation of non-MPEG data carried as part of the transport layer (such as the position of transmitted frame sync, or the initial state of trellis encoding devices) would also have to be matched exactly between every synchronized transmitter. Even though this data is discarded after the received signal is demodulated, any mismatch could create interference between the various co-channel signals. An extra “operations and maintenance” distributed transmission packet (OMP, packet identifier PID:0x1FFA) would need to be added to the ATSC data at the studio and used to control various parameters needed for configuration and synchronization of the individual transmitters.

The location, directional pattern and power levels for each of the transmitters would also have to be very carefully chosen, as the ATSC system is subject to very strict limits on the maximum time difference between arrival of multiple versions of the same signal at the receiver. In problem reception areas, significant improvements could be obtained but careful design would be required to operate multiple co-channel transmitters without destructive interference.[7]

Further tests run by Telemundo owned-and-operated station WNJU-TV, Ion TV and broadcast tower owner Richland Towers using one main New Jersey transmitter and a Times Square fill-in DTS secondary transmitter[8] in 2007 indicated that, of fifteen test sites for reception of the station in New York city, 40% would obtain a substantial improvement in signal by the addition of a second transmitter to the existing station,[9] while all but one would receive at least the same signal quality as was observed without a distributed transmission system.[10] New York's Metropolitan Television Alliance was to run similar tests, but on a larger scale, in 2007 and 2008.[11]

Regulatory issues

While the US Federal Communications Commission has supported DTS in principle since 2004, an FCC call for public comment at the end of 2005 garnered a wide spectrum of responses in early 2006, ranging from strong support by groups such as the National Association of Broadcasters[12] to widespread opposition by groups who advocate the free use of "white spaces" (unused broadcast frequencies)[13] for non-broadcast purposes[14] such as wireless data.[15]

The FCC granted six-month special technical authority to WTVE Reading, Pennsylvania in December 2006, allowing it to operate a distributed transmission system on an experimental basis but did not authorise the systems on any permanent, licensed basis at that time.[16]

An FCC-sponsored test market exercise in Wilmington, North Carolina shut down all analogue full-power commercial broadcasts at noon on September 8, 2008. While a large number of the resulting calls from viewers were straightforward questions about installation of antennas and converters, or the need to scan for channels before being able to watch digital television, hundreds more were about a more intractable problem. Viewers of longtime full-power low-VHF broadcasters like WECT (NBC 6 Wilmington), a signal which in its analogue form reached to the edge of Myrtle Beach, could no longer receive the station - even with the converter and proper antenna installation. The move to UHF 44 and a different transmitter site had substantially reduced WECT's coverage area[17] and, for many who for many years were on the fringes of the analogue NBC 6 signal, WECT was no more.[18]

On November 7, 2008 the FCC issued an order approving the use of distributed transmission systems by terrestrial DTV broadcasters, subject to various restrictions.[19] This allows broadcasters to apply for DTS facilities to cover the area once covered by analogue TV, while not expanding coverage beyond the existing analogue coverage area. It also prohibits a broadcaster "cherry picking" a coverage area in such a way as to cover urban areas while leaving rural viewers with no signal.

This waiver has come too late to allow the newly proposed DTS facilities to be constructed and operational before the federally mandated 2009 analogue shutoff.[20]

The Consumer Electronics Association and CTIA proposed in December 2009 to force all stations to use this method, so that the companies they represent could use the remaining space in the TV band for mobile broadband. Unlike the digital television transition in the United States, they do not propose that stations be forced to pay for it however, much like the 2 GHz broadcast auxiliary service was forced to move by the FCC, but only after the beneficiary (Sprint Nextel) compensated broadcasters for the regulatory taking.[21]

Individual broadcasters

In Puerto Rico, Spanish language independent WSTE 7 "Super Siete" currently operates multiple analogue transmitters on the same frequency to cover various portions of the same island; this system has shown limitations due to interference between the transmitters if all are operational simultaneously. Use of a properly synchronised digital DTS could help to reduce this interference.

In Pennsylvania, independent WTVE is licensed to serve Reading even though its primary audience is in Philadelphia. A distributed transmission system now allows it to tailor its coverage area to improve coverage in areas where its signal is currently marginal.

In Virginia public television WVPT/WVPY operate a combined total of five additional on-channel synchronised transmitters to fill areas blocked by mountains from two main VHF/UHF transmitters; a set of US$100,000 synchronised digital transmitters can replace service from the same number of conventional analogue broadcast translators and also enable overnight datacasting of instructional materials to the area's 188 schools.[22][23]

In New Mexico, Telemundo affiliate KTDO proposes DTS as a means of pairing a low-power DTV facility currently operating in its community of license (Las Cruces) with a second facility atop a mountain overlooking El Paso, Texas in order to reach a wider audience.[24]

In Missouri, FOX affiliate KRBK operates a DTS as a way to service the Springfield, Missouri market from 5 transmission points based around the Springfield DMA. This system went on air in late 2011, and is still being revised today.

In Alaska, Anchorage MyTV affiliate KYES-TV operates with limited resources and equipment, covering a large and sparsely populated area with many small broadcast translator stations. While broadcast signal synchronization is not an issue (as the overlap between signals falls entirely into unpopulated areas), the ability to re-use multiple small transmitters may allow the station to avoid the cost of building one large, expensive main transmitter for its digital signal.

See also

References

  1. ^ NTIA: NYC 9/11 Digital Television Transition Project
  2. ^ Wu, Y.; Xianbin Wang; Citta, R.; Ledoux, B.; Lafleche, S.; Caron, B. (2004). "An ATSC DTV receiver with improved robustness to multipath and distributed transmission environments". IEEE Transactions on Broadcasting. doi:10.1109/TBC.2004.823843.
  3. ^ Design procedures and field test results of a distributed-translator network, and a case study for an application of distributed-transmission; SALEHIAN K., WU Y., CARON B.; Communications Research Centre (CRC) Ottawa, IEEE transactions on broadcasting, 2006, ISSN 0018-9316 IETBAC.
  4. ^ WPSX-TV set to begin experimental DTX transmission, May 15, 2003 12:00 PM
  5. ^ ATSC approves new recommended practice A/111: Design of Synchronized Multiple Transmitter Networks, September 25, 2004 Archived December 15, 2008, at the Wayback Machine
  6. ^ ATSC distributed transmission, Broadcast Engineering, February 2, 2007
  7. ^ The ATSC Distributed Transmission System and Applications to Translator Service Archived March 26, 2009, at the Wayback Machine, David L. Hershberger, Axcera LLC
  8. ^ SFN: Are Many Transmitters Better Than One?, Merrill Weiss, TV Newsday, Sep 13 2007
  9. ^ Richland Towers/Telemundo/ION report on DTS
  10. ^ SFN TV Broadcasting in The United States?, Randy Hoffner, TV Technology, October 3, 2007
  11. ^ MTVA Gets Approval for NYC Distributed Transmission System, TV Technology, May 25, 2007
  12. ^ National Association of Broadcasters submission to FCC, 2006, supporting DTS
  13. ^ Opposition to DTV DTS, various organisations, as filed with FCC in 2006
  14. ^ CommonCause.org objections to DTS, FCC 2006 filing Archived November 28, 2008, at the Wayback Machine
  15. ^ New America: Opposition to DTV DTS, 2006
  16. ^ Transmission boost: The FCC permits a distributed transmission system, Harry C. Martin, Broadcast Engineering, Feb 1, 2007 Archived December 15, 2008, at the Wayback Machine
  17. ^ Confronting the Cliff Effect, Paige Albiniak, TV Broadcast, December 26, 2008
  18. ^ FCC OKs digital workaround for DTV signal range problems, Matthew Lasar, ArsTechnica, November 11, 2008
  19. ^ FCC order on distributed transmission, November 2008
  20. ^ Home Theater News: FCC Green-Lights DTV Range Fix, Mark Fleischmann, November 17, 2008
  21. ^ "Archived copy". Archived from the original on 2010-01-13. Retrieved 2009-12-29.CS1 maint: Archived copy as title (link)
  22. ^ WVPY application for experimental DTS transmitter at Luray VA
  23. ^ http://www.wvpt.net/pi/WVPY2%20Stamped.pdf
  24. ^ http://fjallfoss.fcc.gov/cgi-bin/ws.exe/prod/cdbs/forms/prod/prefill_and_display.pl?Application_id=1232496&Service=TV&Form_id=387&Facility_id=36916
Broadcast relay station

A broadcast relay station, also known as a satellite station, relay transmitter, broadcast translator (U.S.), re-broadcaster (Canada), repeater (two-way radio) or complementary station (Mexico), is a broadcast transmitter which repeats (or transponds) the signal of a radio or television station to an area not covered by the originating station. It expands the broadcast range of a television or radio station beyond the primary signal's original coverage or improves service in the original coverage area. The stations may be (but are not usually) used to create a single-frequency network. They may also be used by an FM or AM radio station to establish a presence on the other band.

Relay stations are most commonly established and operated by the same organisations responsible for the originating stations they repeat. However, depending on technical and regulatory restrictions, relays may also be set up by unrelated organisations - such as community groups in areas that would otherwise not be served.

City of license

In American, Canadian and Philippine broadcasting, a city of license or community of license is the community that a radio station or television station is officially licensed to serve by that country's broadcast regulator.

In North American broadcast law, the concept of community of license dates to the early days of AM radio broadcasting. The requirement that a broadcasting station operate a main studio within a prescribed distance of the community which the station is licensed to serve appears in U.S. law as early as 1939.Various specific obligations have been applied to broadcasters by governments to fulfill public policy objectives of broadcast localism, both in radio and later also in television, based on the legislative presumption that a broadcaster fills a similar role to that held by community newspaper publishers.

KYES-TV

KYES-TV, virtual channel 5 (VHF digital channel 7), is a MyNetworkTV-affiliated television station licensed to Anchorage, Alaska, United States. The station is owned by Gray Television, as part of a duopoly with NBC affiliate KTUU-TV (channel 2). The two stations share studios on East 40th Avenue in Anchorage; KYES-TV's transmitter is located in Knik, Alaska. On cable, the station is available on GCI channel 5. It is also carried on DirecTV and Dish Network in the Anchorage television market.

List of North American broadcast station classes

This is a list of broadcast station classes applicable in much of North America under international agreements between the United States, Canada and Mexico. Effective radiated power (ERP) and height above average terrain (HAAT) are listed unless otherwise noted.

All radio and television stations within 320 kilometers (about 200 miles) of the US-Canada or US-Mexico border must get approval by both the domestic and foreign agency. These agencies are Industry Canada/Canadian Radio-television and Telecommunications Commission (CRTC) in Canada, the Federal Communications Commission (FCC) in the US, and the Federal Telecommunications Institute (IFT) in Mexico.

Metropolitan Television Alliance

The Metropolitan Television Alliance, LLC (MTVA) is a group organized in the wake of the loss of the transmission facilities atop the World Trade Center in 2001. Its mission is to identify, design and build a facility suitable for the long-term requirements of its member stations to meet their over-the-air digital broadcast requirements. This could include designing facilities for the Freedom Tower in Lower Manhattan, assessing alternative sites and technologies and dealing with local, state and federal authorities on relevant issues.The group, which includes stations WABC-TV 7, WCBS-TV 2, WFUT–TV 68, WNBC–TV 4, WNET–TV 13, WNJU–TV 47, WNYE-TV 25, WNYW–TV 5, WPIX–TV 11, WPXN-TV 31, WWOR-TV 9 and WXTV–TV 41, signed a memorandum of understanding in 2003 with the developer, Larry A. Silverstein, to install antennas atop the Freedom Tower. Broadcasters have used the Empire State Building (and, to a lesser degree, 4 Times Square) since the September 11 attacks. In 2006, control of the project was transferred to the Port Authority of New York and New Jersey, with which further discussions have been ongoing.

The group received a grant from the NTIA to study distributed transmission system (DTS) in New York City. Multiple tests were run from various sites in the New York and Newark region in 2006 and 2007 by MTVA and individual member stations, with the use of distributed transmission on a permanent, non-experimental basis ultimately approved for US stations by the Federal Communications Commission on November 7, 2008.

In 2008, Saul Shapiro was appointed President.

Single-frequency network

A single-frequency network or SFN is a broadcast network where several transmitters simultaneously send the same signal over the same frequency channel.

Analog AM and FM radio broadcast networks as well as digital broadcast networks can operate in this manner. SFNs are not generally compatible with analog television transmission, since the SFN results in ghosting due to echoes of the same signal.

A simplified form of SFN can be achieved by a low power co-channel repeater, booster or broadcast translator, which is utilized as gap filler transmitter.

The aim of SFNs is efficient utilization of the radio spectrum, allowing a higher number of radio and TV programs in comparison to traditional multi-frequency network (MFN) transmission. An SFN may also increase the coverage area and decrease the outage probability in comparison to an MFN, since the total received signal strength may increase to positions midway between the transmitters.

SFN schemes are somewhat analogous to what in non-broadcast wireless communication, for example cellular networks and wireless computer networks, is called transmitter macrodiversity, CDMA soft handoff and Dynamic Single Frequency Networks (DSFN).

SFN transmission can be considered as a severe form of multipath propagation. The radio receiver receives several echoes of the same signal, and the constructive or destructive interference among these echoes (also known as self-interference) may result in fading. This is problematic especially in wideband communication and high-data rate digital communications, since the fading in that case is frequency-selective (as opposed to flat fading), and since the time spreading of the echoes may result in intersymbol interference (ISI). Fading and ISI can be avoided by means of diversity schemes and equalization filters.

UHF television broadcasting

UHF television broadcasting is the use of ultra high frequency (UHF) radio for over-the-air transmission of television signals. UHF frequencies are used for both analog and digital television broadcasts. UHF channels are typically given higher channel numbers, like the US arrangement with VHF channels 2 to 13, and UHF channels numbered 14 to 83.

UHF broadcasting became possible due to the introduction of new high-frequency vacuum tubes developed by Philips immediately prior to the opening of World War II. These were used in experimental television receivers in the UK in the 1930s, and became widely used during the war as radar receivers. Surplus tubes flooded the market in the post-war era. At the same time, the development of color television was taking its first steps, initially based on incompatible transmission systems. The US FCC set aside a block of the then-unused and now-practical UHF frequencies for color television use. The introduction of the backward compatible NTSC standard led to these channels being released for any television use in 1952.

Early receivers were generally less efficient at UHF band reception, and the signals are also subject to more environmental interference. Additionally, the signals are less susceptible to diffraction effects, which can improve reception at long range. UHF generally had less clear signals, and for some markets, became the home of smaller broadcasters who were not willing to bid on the more coveted VHF allocations. These issues are greatly reduced with digital television, and today most over-the-air broadcasts take place on UHF, while VHF channels are being retired. To avoid the appearance of disappearing channels, digital broadcast systems have a virtual channel concept, allowing stations to keep their original VHF channel number while actually broadcasting on a UHF frequency.

Over time a number of former television channels in the upper UHF band have been re-designated for other uses. Channel 37 was never used in the US and some other countries in order to prevent interference with radio astronomy. In 1983, the US FCC removed channels 70 through 83 and reassigned them to Land Mobile Radio System. In 2009, with the move to digital television complete in the US, channels 52 through 69 were reallocated as the 700 MHz band for cellular telephone service. In 2011, Channel 51 was removed to prevent interference with the 700 MHz band. The US UHF channel map now includes channels 14 through 36 and 38 through 50.

WSTE-DT

WSTE-DT, virtual and VHF digital channel 7, branded on air as Teleisla, is an independent television station serving San Juan, Puerto Rico that is licensed to Ponce. The station is owned by the Univision Local Media subsidiary of Univision Communications, as part of a duopoly with Caguas-licensed Univision owned-and-operated station WLII-DT (channel 11); it is also sister to radio stations WKAQ (580 AM) and WKAQ-FM (104.7). These stations share studios on Calle Carazo in Guaynabo. WSTE maintains a network of five transmitter sites, located at Cerro Maravilla in Ponce, at Cerro La Marquesa in Aguas Buenas, at Cerro Canta Gallo in Aguada, on Highway 22 in Arecibo, and at the Monte del Estado in Mayagüez.

WTVE

WTVE, virtual channel 51 (UHF digital channel 50), is a Sonlife-affiliated television station serving Philadelphia, Pennsylvania, United States that is licensed to Willow Grove. Owned by NRJ TV LLC, it is a sister station to Trenton, New Jersey-licensed Class A station WPHY-CD (channel 25). WTVE's studios are located on North 11th Street in Reading, and its transmitter is located in the Chestnut Hill section of Philadelphia.

WVPT

WVPT, virtual channel 51 (VHF digital channel 11), is a Public Broadcasting Service (PBS) member television station licensed to Staunton, Virginia, United States, serving Harrisonburg, the Shenandoah Valley of Virginia and West Virginia. The station is owned by the Richmond-based Commonwealth Public Broadcasting Corporation. WVPT's studios are located in Harrisonburg near the campus of James Madison University, and its transmitter is located in central Augusta County, Virginia.

WVPT operates a second station, WVPY, licensed to New Market, Virginia. WVPY was formerly a full-time satellite which served Winchester and the upper Shenandoah Valley. Through a channel-sharing agreement, it now broadcasts from WVPT's transmitter as a satellite of Richmond's WCVW, using virtual channel 51.2.

WYCI

WYCI is a primary Heroes & Icons and secondary MyNetworkTV affiliated television station licensed to Saranac Lake, New York, United States serving Upstate New York's North Country. The station currently brands as YCN, an initialism for "Yankee Communications Network". It previously broadcast a digital signal on virtual and ultra high frequency (UHF) channel 40 from a transmitter on the WNBZ-FM tower north of the village along the Essex and Franklin county lines. The station can also be seen on Spectrum channel 18 and Comcast channel 80. Owned by Cross Hill Communications, LLC, WYCI has studios on Pine Haven Shores Road in Shelburne, Vermont.

In July 2018, the station went silent to meet a contractual deadline in agreement with T-Mobile. The station will resume broadcasting on channel 34 from transmitter sites on Terry Mountain and Mount Pisgah once an application to construct a distributed transmission system passes coordination with Canada, gets approved by the Federal Communications Commission (FCC) and the facilities are built.

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