Efficacy of Physical Layers of Digital Signals

The free-to-air (FTA) terrestrial TV sector still forms the largest single broadcast platform worldwide. For an efficient and viable platform, the network operators need a high channel capacity and flexible configuration of transmission parameters for various services for fixed or portable devices. The use of multiple physical layer pipes in the European DVB system (DVB-T2), layered division multiplexing in the Advanced Television Systems Committee (ATSC3.0), and segmented layers in Japanese ISDB-T, make it possible to carry multiple services in a single channel. In the multiple physical layer pipes (m-PLP) the physical layer is divided into separate logical signals or pipes. Each PLP carries one logical data stream. Each PLP can have different modulation and different error protection parameters. The PLP can be with static bit rate (CBR) or dynamic bitrates (VBR). For optimum utilization of available channel capacity, the standard allows for the dynamic allocation of data to the individual PLPs during operation to accommodate for varying data rate requirements.

The commercial requirement for service-specific robustness together with the need for different stream types is met by the concept of fully transparent physical-layer pipes, which enable the transport of data independent of its structure, with freely selectable, PLP-specific physical parameters. For DVB-T2 if we use two PLPs with PLP1 configured with 256 QAM, G.I 1/128, FFT 32 k, the data capacity may be 33.36 Mbps, which may deliver about 15 SDTV channels and C/N requirement for 10 m directional antenna will be 18 dB. The second PLP2 configured with QPSK, CR ½, FFT 8 k, the data capacity can be about 1 Mbps,
which may relay about three mobile SDTV channels and require C/N of 1.3 dB. DVB-T2 group has also defined a method called FEF (future extension frame) which can be implemented over an existing DVB-T2 network.

Layered division multiplex (LDM) concept used in ATSC3.0 facilitates transmission of multiple data streams in one RF channel with different robustness and data capacity for different services and reception environments. The upper layer (UL) needs to be ultra-robust. The lower layer (LL) power is injected from 3 to 6 dB below the UL signal. Signal cancellation is used to retrieve the robust UL signal first, cancel it from the received signal, and then start the decoding of the lower layer signal. Here a strong error correction code and error mitigation system for the UL can achieve a negative SNR value, closer to the Shannon limit, and save power.

ISDB-T (Integrated Services Digital Broadcasting – Terrestrial) using segmented OFDM transmission system enables fixed/mobile/portable reception service in the same channel. One-seg service, its unique portable service of ISDB-T, uses one segment of 6 MHz. One seg receiver is easily mounted into the mobile-phone, portable PDA, USB tuner etc., so it enables the broadcast mobile service using layer A with QPSK for LDTV, audio, data etc. and fixed reception using Layer B with 64 QAM for HDTV or multi-SDTV with data.

Both the allocated capacity and the robustness can be adjusted to the content/service providers’ particular needs, depending on the type of receiver and the usage environment to be addressed. The PLP architecture is designed to be flexible so that arbitrary adjustments to robustness and capacity can be easily done. Thus, the optimum utilization of DTT for its capacity versus signal robustness (coverage area) can be achieved by employing different PLP for specific services employing different protection level, different coverage level, multiple receiver platform, and local content insertion. The service guide toolbox enables the end-to-end platform to offer advanced and interactive services. The services range from enriched EPG and service information, interactive services, PVR, connected TV, push VOD to convergence services with IPTV systems such as catch-up services. The efficacy of physical layers in DTT is a boon for broadcasters and viewers as well.