How To Move More Bits Through TV Channels

Recognizing that demands on spectrum are only going to increase, two spectrum efficiency experts at IBC 2014 — one Spanish and one German — discussed new technologies for transmitting more bits through any given sliver of TV spectrum.

Pablo Angueira, a professor at the University of the Basque Country in Spain, presented work on layer division multiplexing, LDM, a hierarchical spectrum re-use technique based on cloud transmission that could be used for the physical layer in the next-generation ATSC 3.0 broadcast standard.

And Frank Herrmann, project leader, at Panasonic in Germany, talked about multiple input-multiple output, MIMO, antenna technology that promises to boost the data-carrying capacity of a channel by leveraging multiplexing and antenna diversity.

While each is separate and distinct, they are not mutually exclusive, says Rich Chernock, chief science officer of Triveni Digital and chairman of TG3, the technology group working on ATSC 3.0. “I think they are sort of orthogonal” — independent of one another and could be used together to put more data through a channel, Chernock said.

Angueira is working with the Communications Research Centre in Canada and the Electronics and Telecommunications Research Institute, ETRI, in South Korea.

His LDM technology draws on an article that appeared two years ago in the IEEE Transactions of Broadcasting, “Cloud Transmission: A New Spectrum-Reuse Friendly Digital Terrestrial Broadcasting Transmission System,”

LDM uses “spectrum overlay technology to transmit multiple data streams in one RF channel,” Angueira said during his IBC presentation. Each layer has a different robustness and data capacity to support the reception requirements of different types of services, such as mobile and fixed.

Angueira compared LDM to a double-decker bus, saying that layering allows more data to be carried in the same UHF channel just as the stacked bus makes it possible to carry more passengers in the same footprint as a single-deck bus. Multiple layers are possible to provide for future growth with backwards compatibility, he said.

The upper layer is intended for low signal-to-noise mobile services, while the lower layer is used for high signal-to-noise, high data rate services, such as delivering HDTV to fixed receivers in the home, he said.

Key to LDM is how robust — impervious to co-channel interference — the receiver is. The authors of the IEEE cloud transmission article say it is best to have a “very low” signal-to-noise ratio of “-2 to -3dB.”

However, this level of robustness comes with a cost: relatively low data throughput -in the area of 2.4 to 3Mb/s, the paper says.

While this would seem to run contrary to the goal of maximizing data throughput, the authors say increasing the amount of data broadcast is easily achieved by transmitting two LDM signals from the same tower and antenna.

During his presentation, Angueira said “a strong error correction code and error mitigation system” for Cloud Txn (transmission) and “a good cancellation scheme” are important LDM components.

Signal cancellation would come into play, for instance, when a home TV receives the robust upper layer, cancels it from the received signal and begins decoding the lower layer signal for its own use, said Angueira.

He added that a field test has been conducted in Bilbaq, Spain, using a DVB-T2 software-defined radio, and that an LDM receiver supporting an ATSC 3.0 layer is in process.

Herrmann’s presentation on MIMO attacked the problem of increasing data throughput from a different angle. “Multiple antennas exploit the spatial domain,” he said, and “increase data rate without bandwidth expansion.”

Using a 2×2 MIMO setup yields an 80% gain in channel capacity, Herrmann said, noting that such gains through diversity rely on using transmission paths that are independent from each other.

He added that the maximum data rate possible in a MIMO system is proportional to the smaller of the number of transmit and receive antennas.

“DVB-NGH [Next Generation Handheld] is the first broadcast system to apply pure MIMO,” he said, adding that there is the possibility to exploit MIMO as part of eSFN [single frequency networks] under discussion in ATSC.  

Chernock said he was impressed by LDM and MIMO and that the ATSC 3.0 group working on the physical layer has both under consideration.

At a fundamental level, ATSC 3.0 is being designed so that every layer, including the physical layer, is extensible, which means that even if MIMO and LDM aren’t part of the initial standard they can be added later, he said.

Chernock also pointed out that the presentations were good examples of the international participation he has worked to promote in the development of ATSC 3.0.

According to Chernock, the greater the input from researchers, broadcasters, standards bodies and vendors from around the world, the better the chance that ATSC 3.0 may be able to serve as a framework for an international broadcasting standard that can stand the test of time.

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