We’ve been hearing a lot about single-frequency networking lately, especially in connection with the rollout of ATSC 3.0. However, the technology has been around for a long, long time, with the first implementation taking place in the mid-1920s, and involving operation of radio stations fairly close to each other and sharing a common frequency.

This “synchronized broadcasting” technology involved the locking of transmitter carrier frequencies together (via a wired connection) to avoid unpleasant beat notes generated by the transmitter oscillators as they drifted slightly in frequency. (It helped if the two stations were broadcasting a common show [network-supplied, for instance] and weren’t geographically too far apart so that the slight delay due to audio delivery via telephone line wasn’t sufficient to cause an audible echo or “flanging” effect on consumer radio sets located midway between the common-frequency signals.)

During the 1950s and beyond, this sort of “on-channel” operation extended to television with the implementation of unlicensed “boosters” that retransmitted an off-air television station to small communities, or perhaps an individual, located in a valley area below the reach of the main TV signal. (These devices were typically located on mountain ridges with a receiving antenna directed at the desired station, followed by an RF amplifier feeding a transmitting antenna directed down into the valley without TV service.) These “boosters” could — and sometimes did — cause interference to television reception and other communications, and as such were a source of aggravation to the FCC. However, they did prove that on-channel operation of two or more transmitters was possible, if there was sufficient shielding from natural terrain to prevent multiple signals from reaching receivers.

[Read: Dallas ATSC 3.0 SFN Buildout Nears Completion]

Somewhat later, in the evolution of broadcast television, legal operation of such signal boosters or “gap-fillers” was permitted — but with care given to prevent interference to consumers in a station’s main service area through the use of terrain shielding and/or highly directional antennas.

Fast forward to the advent of digital television, and SFN technology took a giant leap forward due to the nature of the transmitted signal and the ability of receivers to reject unwanted signals (analogous to “ghosts” in analog television produced by reflections that create multiple signal paths). No longer did SFN implementers have to rely strictly on blocking of unwanted signals by terrain. According to ONE Media’s Fred Baumgartner, the very flat terrain in the Dallas/Ft. Worth area was one of the factors in locating the initial ATSC 3.0 SFN trial in that market, as it provides a “worst case” scenario in terms of terrain shielding.

Television industry consultant S. Merrill Weiss recognized early on, and engineered the world’s first ATSC 1.0 SFN in the State College, Penn., area to provide better coverage for the city’s WPSX-TV (now WPSU-TV). The use of DTV SFN technology has since spread to a number of other areas.

“There were 29 [SFN] applications filed before the repack,” said Weiss. “Of those 19 were approved before the freeze, and I believe all were built.”

Even though the technology has proven successful with ATSC 1.0, ATSC 3.0 should make television SFNs easier to implement.

“ATSC 1.0 is dependent on the design of the adaptive equalizer which can be quite different in its performance from receiver to receiver,” said Weiss. “ATSC 3.0 is dependent on the modulation that’s used, and all receivers will respond to it identically. And on top of that having multiple carriers (inherent in the 3.0 OFDM [orthogonal frequency division multiplex) signal] makes things a lot easier.”

Yiyan Wu, a scientist at Canada’s Communications Research Centre who has been heavily involved in the development of ATSC 3.0, added another factor in the signal’s potential versatility “ATSC 3.0’s OFDM modulation has a longer guard interval [than 1.0’s] and this provides more protection,” said Wu. “ATSC 1.0 uses variable equalization, and the longest echo that it can handle is about 64 microseconds. ATSC 3.0 can handle 150 microseconds. Longer echo means that your power can push out further. The longer the guard interval, the less transmission power you need.”

[Want more information like this? Subscribe to our newsletter and get it delivered right to your inbox.]

This 800-year-old former Ghent monastery was the site of the 10th annual BMSB conference.

GHENT, BELGIUM—A former 13th century Dominican monastery in this medieval city recently served as the backdrop for the dissemination of information that was anything but historical or “old world.” The event was the 10th annual Broadband Multimedia Systems and Broadcasting conference, with video experts from around the world convening here to present and exchange ideas on the latest developments in television.

The June 17-19 symposium drew some 150 television scientists and engineers and featured presentations covering everything from the embodiment of MIMO technology in UHD broadcasting to a proposal for enhancing viewing experiences through the resurrection of “smell-o-vision.” However, a relatively new transmission technology—layered division multiplex or “LDM,” which will be incorporated in to the U.S. ATSC 3.0 television standard—appeared to garner a great deal of interest among conference participants. It was the subject of at least 10 papers and was referenced directly or indirectly in a number of other presentations.

LDM SEEN AS POWERFUL TOOL IN NEXT-GEN TV

LDM seems destined to play a big role in future television systems as it addresses content delivery to a variety of viewing devices, according to Richard Chernock, BMSB session chair, chief science officer at Triveni Digital and member of the ATSC 3.0 standards development committee.

Richard Chernock

“Broadcasters are interested in the different broadcast delivery models provided by ATSC 3.0, which range from robust transmission to moving or indoor receivers up to very efficient delivery of UHDTV to rooftop antennas,” said Chernock. “One of the new tools available is layer division multiplexing, which allows simultaneous transmission of both modes through the use of power multiplexing. LDM (in addition to TDM [time domain multiplexing]) will provide broadcasters the capability to... pursue the mixture of business models that they desire.”

LDM, in simplest terms, lets broadcasters pack more digital information within their allotted channel slots. The technology allows stacking of physical layer data streams with varying power levels, modulation schemes and data coding, thus offering a great deal of flexibility and robustness in delivering video to fixed and mobile receive sites. Such architecture ensures that all available transmission bandwidth is fully utilized and can provide a boost of three to six dB of signal-to-noise improvement—something that can make the difference between a great picture or no reception at all for television receiving devices situated in challenging receive site locations.

HOW IT ALL STARTED
Yiyan Wu, another BMSB session chair and also principle research scientist at the Communications Research Centre Canada is credited with being the originator of the LDM concept and was asked how his idea came about.

“About five years ago I was driving my 13-year-old daughter Cindy to a school activity and there was a brand-new double-decker bus in front of me,” said Wu. “I said ‘I hate those double-deckers; they totally block my view. I cannot even see the traffic lights. I do not know what they are good for.’”

Wu said that his daughter responded that such vehicles had the same footprint as a regular bus, but could carry twice the passenger load—something she had heard mentioned a few days before in a radio broadcast. 

“That made me start thinking,” said Wu. “If we want to increase the capacity [in connection with DTV transmission], we [could] pile up the ‘bits’ vertically in multiple layers.”

NOT JUST FOR ATSC 3.0

Yiyan Wu When asked about the applicability of LDM technology in connection with advanced digital television transmission systems other than ATSC 3.0, Wu responded:

“A good technology speaks for itself…the NHK Sciences and Technology Research Laboratories (NHK-STRL) of Japan and the National Engineering Research Center on Digital TV (NERC-DTV) of China are strong supporters of the LDM technologies. They are major standard-setting organizations in their countries.”

Wu also cited research work being done at the Mackenzie University of Brazil on applying LDM to the ISDB-T digital television systems adopted in Brazil and Japan.

“I would say that LDM has a good chance to make the next generation DTV standards in other parts of the world,” Wu said.

As mentioned by Wu, other research organizations have been quick to pick up on the LDM concept. One of these was Spain’s University of the Basque Country, which was well represented at the BMSB event.

One of the conference participants was Pablo Angueira, an associate professor in the department of electronics and telecommunications at the University. He spoke about the school’s involvement with LDM technology.

“The first [iteration] was called cloud transmission; however, this was not exactly the concept of LDM. But based on that, a cooperative project was initiated between CRC Canada, ETRI [Electronics and Telecommunications Research Institute] in Korea, and our university. We developed the final LDM concept that will be part of the ATSC 3.0 standard.”

LDM ON DISPLAY
Angueira and his associates brought a working demonstration model of LDM technology to the conference that operated throughout the three-day event. He explained that this “breadboard” version was really the initial realization of the new transmission technology.

“The first version of this development was shown one year ago, so it’s fairly new,” said Angueira. “In fact the software-defined radio setup being demonstrated here was the first LDM prototype. ETRI later developed its own full-power real-time transmitter and receiver.

Spain’s University of the Basque Country provided this working model of an LDM transmission system.

“We have two power modules for transmitting and receiving, but they are only performing the IF part of the transmission system. The baseband part is all software—software defined radio. We are transmitting two layers, with the upper layer the robust one with QPSK and the lower layer is a non-uniform carrying a non-uniform constellation—64 QAM. These are the constellations now accepted in ATSC 2.0. The upper one carries 4.1 mbps and the lower one carries 19 mbps, so in total we’re running 23 or 24 mbps. The use case for this is sending content on the upper layer for mobile receivers and portable receivers. The lower layer would be for fixed reception with a rooftop antenna, with results similar to what you would expect from ATSC 1.0 or 2.0.

Angueira stated that equipment shown would be going to Shanghai in October for testing there in connection with the ATSC 3.0 standards process.

Television scientists and engineers from more than 25 nations participated in the Ghent IEEE BMSB conference, which serves as a forum for the presentation and exchange of ideas in the fields of multimedia media broadcasting, telecommunications, data networking technologies and consumer electronics. The 2016 conference will be held June 1-3 in Nara, Japan.