Across the world, free-to-air (FTA) broadcast television is rapidly shifting from analog to digital format. Digital television (DTV) deployment—combined with rapid growth in the personal media market—is creating opportunities for many broadcasters to deliver television content not only to large screen displays, but also to mobile and portable devices.
This article will provide an overview of major DTV standards that are being deployed worldwide, with a specific focus on those in use in Europe, North America, and China The article will also provide a more comprehensive examination of the China Digital Television Terrestrial Broadcasting (DTTB) System Standard, also known as GB20600-2006, which is unique among all the digital broadcast television standards for its capability to deliver high-definition television (HDTV) to mobile devices moving at speeds of greater than 200 km/hour.
Digital Television Architecture
A top-level view of DTV architecture, with the main building blocks connected from end-to-end, is shown in Figure 1. The UHF/VHF antenna grabs the digital television broadcast signal from the air. The front-end is comprised of a tuner and demodulator, which includes complex error correction. In any given DTV, the number and the type of tuners and demodulators is dependent on how many and what types of signals are being received by the TV; for instance, whether it is receiving and demodulating HDTV or analog TV. The front-end separates the digital audio/video signal from the radio frequency (RF) and converts it into an intermediate frequency so it can be digitally processed. The demodulator also performs complex error correction on the RF signal. This demodulation produces a transport stream (TS) containing the compressed and encoded audio, video, and program data, which is then transferred to the back-end section of the television circuit.
The back-end is comprised of two major blocks: audio and video decode and scaler-de-interlacing-post processing. It performs any necessary conditional access function and decodes the TS, which then is passed on to the post-processing circuitry before being displayed on the DTV screen.
There are separate standards that govern the technology for both the front-end and back-end building blocks. The front-end block specification is usually referred to as an “air-interface standard,” signifying the specification of the method by which the digital information of the programming is packed, modulated, and sent over the air-waves using the RF carrier.
The DTV standards examined in this article refer to the front-end air-interface standards.
DTV Standards Comparison
The transition from analog to digital television is taking place at different rates around the world. Table 1 outlines the timeframes and identifies which DTV standards are being used, as well as other unique characteristics of those standards.
Regardless of the geographical region and the specific type of the standard itself, the motivation behind the specification of a digital TV standard is the same: to enhance the consumer’s television viewing experience and to create new business models for the next-generation digital era. However, as is apparent in Table 1, the United States, Europe, Japan, South Korea, and China each have adopted a different terrestrial DTV standard.
A common feature to all the standards is the MPEG-2 compression scheme, which is used to encode the audio-video content into a TS. Then, depending on the region and the standard, this TS stream is packed into frame, modulated, and transmitted over airwaves using the scheme described in the terrestrial DTV standard specification.
In the U.S., the Advanced Television Systems Committee (ATSC) standard uses 8-vestigial side-band (VSB) signaling. A modulation scheme called coded orthogonal frequency division multiplexing (COFDM) is employed in Europe’s digital video broadcast (DVB) standard. Japan utilizes the concept of integrated services digital broadcasting (ISDB), in which the structure of segmented OFDM is used for terrestrial broadcasting (ISDB-T). In South Korea, the terrestrial digital multimedia broadcasting (T-DMB) standard also employs a variation of the OFDM. China’s GB20600-2006 employs a unique and innovative variation on the OFDM, called time-domain synchronous OFDM (TDS-OFDM).
What is different about the TDS-OFDM used in China? For starters, channel estimation is done via a pseudo-noise (PN) sequence rather than a set of pilot tones, which is how it is done in COFDM. The innovation behind TDS-OFDM is that the PN sequence is in the time-domain, which dramatically accelerates the channel estimation process and enables high-speed channel estimation. For COFDM, a certain minimum number of symbols are required to complete the channel estimation fully. However in TDS-OFDM, only one symbol is required to achieve the same thing. The 8-VSB technology is even more complicated. The preamble sequence, which is used to estimate the channel, requires 0.5 ms in TDS-OFDM. But in the 8-VSB technology of the U.S. ATSC standard, it requires 24 ms, resulting in an even longer time for the channel estimation to be completed.
From a digital communication viewpoint, the OFDM signals (which are the basis for the COFDM signaling) using cyclic prefix (CP) and zero-padding prefix are equivalent, as shown in Figure 2 and Figure 3. The primary variation between these two schemes lies only in the use of different prefix signals.
The insight that drove the TDS-OFDM performance is that the superposition of a PN to the prefix will not destroy the orthogonal properties because its effects can be removed. Moreover, the addition of a PN signal brings about a number of benefits, such as fast synchronization, accurate channel estimation, and high spectral efficiency, which are vital for mobility.
In the development of GB20600-2006, researchers considered other DTV standards—including the European DVB and U.S. ATSC standards. Chinese researchers then adopted the best features of each and improved upon them further. Table 2 compares performance of the three standards, including the handheld version of the DVB standard (DVB-H).
Among these various DTV transmission standards, ATSC only supports one modulation and one data rate, while Europe’s DVB-H supports multiple modulation and data rates. For example, to compare China’s GB20600-2006 standard with DVB-H, let’s look at some key parameters in Table 2. DVB-H and GB20600-2006 have the same modulation and FEC rate, and almost the same guard-interval. The performance can be compared using data rate, required carrier-to-noise (C/N), and the maximum Doppler frequency.
In terms of data rate, GB20600-2006 is about 10 percent higher than that of DVB-H. It also means that GB20600-2006 has about 10 percent better spectrum efficiency. Based on simulation, the performance of GB20600-2006 is 2 dB better than DVB-H for the required C/N in Gaussian channel. This is mainly due to the low-density parity-check code (LDPC). For mobile performance in the typical-urban six path (TU6) channel, GB20600-2006 can tolerate 30 Hz higher Doppler frequency. For ATSC versus GB20600-2006, ATSC also achieves good spectrum efficiency, but it does not support any mobility.
Mobile Optimization: Key Features of China’s DTV Standard
While the terrestrial DTV standards are designed to provide HDTV services with fixed antenna reception, most are not well-suited to meet the stringent low-power requirements of mobile and portable devices. As a result, broadcasters continue to face challenges in determining which technology and broadcast mechanism can be used to reach the fast growing market of mobile and portable users.
The one exception is China’s GB20600-2006 standard, which was purposely built to deliver a consistent, high-quality DTV viewing experience whether viewers are sitting in their living room watching television or on a high-speed train watching shows on their cell phones. The technology can broadcast audio and video at transmission rates of greater than 24 Mbps to consumer devices. Because the mobile reception capability is inherently built into the standard, these consumer devices now have a mobile TV feature that works not only when stationary, but even while traveling at speeds greater than 200 km per hour.
Mobile TV receivers require vastly more complicated signal processing algorithms at their core because they have to estimate the channel sufficiently and frequently at high speeds. It is in this processing that GB20600-2006’s TDS-OFDM technology outperforms both the U.S. ATSC’s 8-VSB technology and Europe’s DVB standard.
The block diagram of a China DTV transmitter is shown in Figure 4. The transmitter includes randomizer, forward error correction (FEC) encoder, mapping and interleaving, system information generation, multiplexing, frame data processing, frame header generation, framing, base-band processing, and up-conversion.
The input TS is randomized and then sent to the FEC encoder block. The FEC encoder creates concatenated code of Bose and Ray-Chaudhuri (BCH) and LDPC code. The outer code is the BCH (752,762) block code, while the inner code is the LDPC code. There are three different codes with similar structure, which are LDPC (7493, 3048), LDPC (7493, 4572,) and LDPC (7493, 6912). The encoded bit stream is mapped to the constellation and interleaved by a convolutional time-domain interleaver and a frequency-domain block interleaver. The time-domain interleaver has two modes: M=240 and M=720. The frequency-domain interleaver applies only for multi-carrier. The modulation schemes supported by the standard are 4QAM-NR, 4QAM, 16QAM, 32QAM, and 64QAM.
The modulation scheme, LDPC rate, and time interleaver mode are referred to as system information (SI). SI is encoded using a 32-bit Walsh code. The multiplexing block combines 3744 data symbols and 36 information symbols into one frame with 3780 symbols. These 3780 symbols are processed by the frame body processing block. In the case of multi-carrier mode (C=3780), these 3780 symbols are converted to the orthogonal frequency division multiplexing (OFDM) signal using a 3780-point inverse discrete fourier transform (IDFT). In the case of the single-carrier mode (C=1), the 3780-point IDFT is by-passed. The 3780 symbols form one frame body.
The key parameters of China DTV standard are summarized in Table 3. It is clear from the review of the air-interface specification that the GB20600-2006 standard accommodates significantly higher Doppler rates and faster channel estimation, which directly translate into higher mobile and vehicular speeds.
While other countries around the world developed their digital TV standards a number of years ago, the GB20600-2006 standard is a much more recent development. Standard development began in 2000 at China’s DTV Technology Research Center.
While working closely with the officials on China’s standards board, the research team studied the digital television standards of various countries around the world, including Europe’s DVB-T and ATSC in United States. They adopted and improved on the best features of each. Motivated by a self-imposed deadline to deliver free-to-air HDTV to consumers for the 2008 Beijing Olympics, China’s standard focus from the beginning was primarily on the user experience.
The core technology that empowers the GB20600-2006 standard is Time Domain Synchronous – Orthogonal Frequency Division Multiplexing (TDS-OFDM), created by Legend Silicon Corp. and developed jointly with Tsinghua University in Beijing. The GB20600-2006 standard—with TDS-OFDM at its core—is inherently more robust than other DTV standards. While it is being deployed at a rapid rate for fixed applications, it can cost effectively be used to adapt broadcast technology for more interactive and specialized programs and services for mobile audiences.