Efficient use of spectrum resources is a necessity for modern wireless communications systems. Regulators are not interested in authorizing, and service providers are not interested in launching, new services that would use spectrum inefficiently, given the unquenchable thirst for bandwidth and resource scarcity in the current spectrum-constrained world. As Will Rogers might have said about spectrum: they aren’t making any more of that stuff. For those involved in developing the ATSC 3.0 digital television transmission system, spectrum efficiency has been a primary guiding principle.
One of the spectrum efficient techniques documented in the baseline physical layer of the draft ATSC 3.0 candidate standard (currently being balloted) is called Layer Division Multiplexing (LDM). LDM allows multiplexing two signals in a single RF TV channel enabling simultaneous delivery of two independent broadcast services by using spectrum overlay and signal cancellation techniques. Layered Division Multiplexing is the result of a cooperative project between the Communications Research Centre (CRC) in Canada, the Electronics and Telecommunication Research Institute (ETRI) in Korea and University of the Basque Country in Spain.
I’m from Missouri—you’ve got to show me
LDM has been demonstrated in simulation and in prototype form a number of times over the past months at the NAB Show and other venues. Operation in the real world is another story and field test verification studies are just beginning to emerge. At IBC2015 held in Amsterdam in mid September, Heung Mook Kim, Director of the Terrestrial Broadcasting Research section of ETRI, presented results of LDM field tests conducted earlier this year in Jeju Island Korea.
Figure 1 Heung Mook Kim from ETRI in Korea describes field test results of Layer Division Multiplexing techniques at IBC 2015.
Dr. Kim first described the basic concept of LDM as shown in the figure below. In the ATSC 3.0 physical layer draft standard, two layers are initially identified, the core (or upper) layer and the enhanced (or lower) layer.
Figure 2 The basic Layer Division Multiplexing Concept
In the ETRI field experiments, a 16K FFT was used: the upper (robust) layer employed QPSK modulation with 4/15 coding for a 2.7 Mbps mobile HD stream while the lower (enhanced) layer used 64-NUC (non-uniform constellation) modulation with 10/15 coding for a 20.5 Mbps UHDTV fixed service stream. The threshold carrier-to-noise ratios for AWGN were -0.3 dB and 18.5 dB for the upper layer and lower layer respectively.
The transmission facility constructed for the tests represents a low power/low tower case, with the transmit antenna at a height of 21 meters from the ground and transmission power of 520 W ERP on UHF channel 50. In the set of initial reported field tests, there were 21 measurement sites for fixed reception, 16 points for indoor reception and 5 routes for mobile reception.
All very well, Orville, but will it fly?
For relative comparisons during the fixed and indoor portions of the field tests, ATSC 1.0 transmissions at the same power level were also included. For fixed outdoor reception sites, the success rate for the three modes are shown below.
Figure 3 Success rate of DTV and LDM modes for fixed outdoor reception
For indoor reception, the rate for successful reception was 19% for DTV, 6% for the LDM lower layer and 69% for the LDM upper layer. Similarly, mobile reception for the LDM signal was generally highly reliable for field strengths above the threshold level.
Videos showing received picture quality and spectrum analyzer displays during the mobile field tests can be found here:
For more information, the developers of LDM have developed a website (www.ldm-tech.com) which includes lots of additional details about the technology. Also Heung Mook Kim at ETRI can be contacted for additional information about the Jeju Island field tests.