Single Frequency Networks (SFNs) are considered the optimal network configuration to maximize the spectrum efficiency and to minimize the co- channel interference problems in the advanced broadcasting planning. As a matter of fact, they have been widely used in the European countries since the dawn of the first Digital Terrestrial Television (DTT) standard, namely DVB-T.
Their main advantage is that, providing that all the transmitters are time and frequency synchronized, the same content can be delivered over the whole network occupying a single RF channel. Nevertheless, the local/regional content delivery is still one of the major drawbacks for SFNs.
In this paper, Layered Division Multiplexing (LDM) is proposed as the definitive technique that will allow the seamless delivery of local contents or targeted advertisements over SFNs.
LDM is a spectrum efficient non-orthogonal multiplexing technology that has been adopted in the ATSC 3.0 Physical Layer Standard as Baseline Technology, which consists on the superposition of two or more data streams of different power.
In this scenario, LDM upper layer can be used to deliver TDM-ed mobile-HD and 4k-UHD services, whereas the LDM lower layer with a negative SNR threshold (dB) can reliably provide seamless local coverage/service for each SFN transmitter without coverage gaps.
Terrestrial broadcast services delivery is based on Single Frequency Networks (SFNs) in most European countries, and most likely, it will be the future of the North American broadcasting networks.
No matter the implemented terrestrial standard (DVB-T, DVB-T2, ATSC 3.0), the SFNs will enable the re-use of the same spectral resource over the entire service area provided that all transmitters are synchronized in frequency and time.
Layered Division Multiplexing (LDM) - is a non-orthogonal multiplexing (NOM) scheme that has recently been adopted in the ATSC 3.0 Next Generation Digital TV Standard .
In the literature, it has been already proved both theoretically and practically that LDM is more spectrum efficient than the classical Time Division Multiplexing (TDM) or Frequency Division Multiplexing (FDM) approaches.
The main enabling technologies for this NOM technique are the strong Forward Error Correction (FEC) codes, the successive signal cancellation, and adjustable power assignations for multiplexing different data streams using different layers. None of these techniques requires major changes in the current receiver structures.
LDM FOR SFN SEAMLESS LOCAL COVERAGE
SFN networks are designed to increase the spectrum efficiency and reduce the co-channel interference, so the overall service quality is improved. Nevertheless, they are not the best approach for delivering local content in the regional areas or serving targeted individual markets.
What is more, none of the existing solutions have completely addressed the problem of the the local content delivery within SFN networks. The first, and simplest solutions, were based on the classical TDM/FDM schemes .
The main problem of these approaches is that in order to allocate resources to the local service, the global configuration resources should be decreased in a linear basis, and as a consequence, the coverage and spectrum efficiency shrinks accordingly.
As a result, this family of solutions is not very attractive for such a competitive market, where the frequency is a very scarce resource.
In recent years, the use of hierarchical modulation was also proposed for the local content delivery over SFN networks, being the global content transmitted on the high priority layer of the hierarchical modulation, and the local content modulated in the low priority layer .
Even if it can show a better performance than the previous approach, it requires a substantial complexity increase in order to be implemented in the receiver part.
Finally, the distributed MIMO-SFN centralized architecture has also been presented as a solution for providing both global and local contents within an SFN .
In this case, the main problem for the operators is the required investments in the infrastructure and the consequent increased complexity for the iterative cancellation stage at the receiver site.
In conclusion, up to now there are no proposals that fully address all the challenges related with the seamless local service content over SFN networks.
In this work, LDM is proposed as the alternative to provide local and global contents within the same SFN network, maximizing the spectrum efficiency and minimizing the required infrastructure upgrades. In addition, it can guarantee the seamless reception of the local service, and therefore, the coverage gaps will be eliminated.
The upper layer of the LDM system can be used to broadcast TDM combined mobile HD, 4k UHD HDR, or multiple enhanced 1080p HD services to the entire regional service area, whereas the LDM lower layer can be used to deliver localized services from each SFN transmitter. The main advantage of LDM is that the regional and global services are totally decorrelated.
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