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John R. Smith IBM

Standards Digital Terrestrial Television Broadcasting in China Weiqiang Liang, Wenjun Zhang, Dazhi He, Yunfeng Guan, Yao Wang, and Jun Sun Shanghai Jiao Tong University

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nterest in digital television technologies has been growing for many years, leading to three main digital TV terrestrial transmission standards around the world.1 The eight-level Vestigial SideBand (8-VSB) modulation system, developed by the Advanced Television System Committee in the US,2 paved the way for other standards. The second one, Digital Video Broadcasting-Terrestrial, uses traditional Coded Orthogonal Frequency Division Multiplexing (COFDM) technology and was widely adopted in Europe, Australia, Singapore, and other areas.3 The third, the Band Segmented Transmission (BST)-OFDM system, is the basis for Japan’s Terrestrial Integrated Service Digital Broadcasting Standard (ISDB-T).4 A working group in China, home to the world’s largest television market, recently developed a fourth standard that meets that country’s unique needs. In China, the proportion of television viewers using terrestrial reception is pretty high. Other characteristics of China’s TV audience made it necessary to develop a new digital terrestrial television broadcasting standard that provides high-quality multimedia service as well.

History and roadmap To offer a brief context in which this technology developed, it’s important to note that China began developing its own digital television ter-

Editor’s Note Television audiences around the world want to enjoy high-quality digital programming. China, which has the world’s largest TV market, recently established a digital terrestrial television broadcasting standard. DTTB will not only allow improved digital broadcast delivery to rural and urban areas, but it will also support mobile TV, making possible television reception on buses, taxis, and even high-speed magnetic levitation mass-transit trains. DTTB will be used for HDTV broadcast during the 2008 Beijing Olympics, and China’s large-scale transition to the DTTB standard will follow. —John R. Smith

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restrial broadcast standard in 1994. In 1995, the central government founded the HDTV Technical Executive Experts Group (TEEG), whose members came from several domestic universities and research institutes. After three years’ effort, the group developed the first HDTV prototype, which included a high-definition encoder and decoder, as well as a multiplexer, modulator, demodulator, and demultiplexer. It covered all the key parts from transmitter to receiver and was successfully applied for the live broadcast of the 50th Anniversary National Day program in 1999. In 2001, China called for proposals for a digital terrestrial television broadcasting standard. The five submitted proposals included ❚ Advanced Digital Television BroadcastingTerrestrial (ADTB-T), a single-carrier approach from HDTV TEEG, and ❚ Digital Multimedia Broadcasting-Terrestrial (DMB-T), a multicarrier approach from Tsinghua University. In 2003, the Academy of Broadcasting Science proposed another multicarrier approach: Terrestrial Interactive Multiservice Infrastructure (TiMi). After lab tests, field trials, and intellectual property analyses, in 2004 the Chinese Academy of Engineering led a united working group to merge these three leading proposals. Finally, in August 2006, the working group unveiled a milestone for the Chinese digital television industry by issuing the merged standard, “Frame Structure, Channel Coding and Modulation for a Digital Television Terrestrial Broadcasting System.”5 The standard, which contains both single- and multicarrier options and supports various multiprogram standard and highdefinition (SDTV/HDTV) broadcasting services, will be mandatory after 1 August 2007.

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Figure 1. Roadmap of China’s digital terrestrial television broadcasting (DTTB) standard development.

1994 Fundamental R&D

1999 DTV live broadcasting

2004 Terrestrial test

2008 HDTV Olympics

2006 Standard issued

2015 Analog switched off

2010 Large-scale transition

Frame header Input Randomizer

FEC

Interleaver and mapping MUX

Frame body processor

MJX

Postprocessor

UP converter

System information

This standard will lead to commercial use through three steps (see Figure 1). First, Beijing, Shanghai, Tianjin, Shenyang, Qingdao, and Qinhuangdao will host commercial trials of technologies that use the standard and, during the Olympic Games in 2008, stations will use it to provide HDTV services. Second, localities will carry out large-scale transitions from analog to digital broadcast services from 2010 to 2015. Finally, in about 2015, existing analog TV broadcasting capabilities will be switched off permanently.

Brief description of the Chinese standard Figure 2 shows a block diagram of the Chinese DTTB transmission system.5-7 The signal is transmitted in frames. In this section we’ll describe the frame structures, frame body processing, system information, and forward error correction.

Figure 2. Block diagram of the DTTB transmission system.

Frame body processing As for the FB processing block, parameter C (denoting the number of carriers) offers two options. The C1 mode indicates a single-carrier modulation scheme; C3780 mode indicates multicarrier modulation with 3,780 subcarriers. In both options, the input data is low-density paritycheck (LDPC) coded with rates of 0.4, 0.6, or 0.8. Constellation mapping schemes for each mode Table 1. Descriptions of the frame structures from the DTTB standard. Frame Structure 1 2 3

Frame Header 420 symbols 55.6 μs 595 symbols 78.7 μs 945 symbols 125 μs

July–September 2007

Frame structure Each frame contains a frame header (FH) and frame body (FB). The frame body contains system information and coded data. Both FH and FB have the same symbol rate of 7.56 mega symbols per second (Msps). Table 1 shows the definitions of these three frame structures, as described in section 4.6 of the DTTB standard. The FHs use pseudonoise sequences in three different lengths: 420, 595, and 945. The three

FH types are correspondingly abbreviated as PN420, PN595, and PN945. PN420 and PN945 are made up of complete m-sequences of length 255 and length 511, with their cyclical extensions as preambles and postambles, while PN595 is made up of the first 595 symbols of an msequence of length 1,023.

Frame Body 3,780 symbols 500 μs 3,780 symbols 500 μs 3,780 symbols 500 μs

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Table 2. Low-density parity check (LDPC) performance.

Block Length Code Rate (in Bits) 0.4 7,488 0.6 7,488 0.8 7,488

Information Bits 3,008 4,512 6,016

Bit Energy to Noise Density Ratio (Eb/N0) 2.1 dB 2.3 dB 3.3 dB

Performance away from the Shannon Limit 2.3 dB 1.6 dB 1.2 dB

include 64-quadrature amplitude modulation (QAM), 32-QAM, 16-QAM, 4-QAM, and 4-QAMNR, which uses the Nordstrom-Robinson code.8 It should be noted that in C3780 modes, each FB also employs a block interleaver. System information Each FB contains 36 (Binary Phase Shift Keying) BPSK-mapped system information symbols, the first four of which indicate the C1 or C3780 mode. The last 32 symbols are spread-spectrumprotected Walsh codes that show necessary system information including constellation mapping, LDPC code rates, and interleaving modes. Forward error correction The standard’s forward error correction (FEC) includes LDPC code at three different rates: LDPC (7,493, 3,048), LDPC (7,493, 4,572), and LDPC (7,493, 6,096). Bose, Ray-Chaudhuri and Hocquenghem (BCH) (762, 752) coding is concatenated outside the LDPC. BCH (762, 752) can only correct one-bit errors. Without an interleaver between the BCH and LDPC code, BCH can’t contribute much for error correction. Nonetheless, a frame always includes integral Transmission Stream packets. A convolutional symbol interleaver with two different depths is used with the LDPC coding. One is M  240 and the other is M  720. They both have 52 interleaving branches.

IEEE MultiMedia

Square-root raised cosine filter To limit the bandwidth of the transmitted signal to 8 MHz, the standard uses as a shaping filter a square-root raised cosine (SRRC) filter. Additionally, in C1 mode, dual pilots whose power is 16 decibels (dB) lower than the total average power are optionally inserted onto the transmitted data at a ±0.5 symbol rate. Argument The option of multicarrier modulation in the standard has no virtual subcarriers and pilots. The

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FH modulated by a single carrier replaces Cyclic Prefix in this approach. Thus, the only difference between single- and multicarrier modes is the Inverse Fast Fourier Transform of 3,780 points, which could be regarded as a data filter for FB. Each mode has two types of channel estimation and compensation methods. The all-time domain processing approach, which employs a time domain, is code enhanced and has a datadirected adaptive decision feedback equalizer with LMS algorithms.9 The hybrid time and frequency domain processing approach implements a channel estimator within the time domain, using the known FH and compensating for the FB in the frequency domain.10

Novelty of this standard Technical innovations for the DTTB standard include the following: ❚ Using time-domain PN sequences as frame headers.6 These can serve as training sequences of equalizers and also as guard intervals to mitigate intersymbol interference. Choosing the known PN sequences as frame headers also provides higher-spectrum efficiency, because PN can also be used for the channel estimation. This takes the place of inserting both continuous and scattered pilot methods, as the traditional COFDM approach does.7 Furthermore, it achieves faster channel acquisition because it can be done directly in the time domain. ❚ Using LDPC code as part of the FEC. Until now, performance of LDPC has been regarded as the best in coding theory. It can provide superior error correction capabilities for better sensitivity, resulting in larger coverage at the same radiative power. Table 2 shows the LDPC’s performance. ❚ Including spread-spectrum technology that protects system information.6 As we previously noted, DTTB contains many options such as singlecarrier modulation, multicarrier modulation, three frame header options, three FEC coding rates, five constellation mappings, two interleave depths, and fixed or rotating PNs in frame headers. Different combinations result in hundreds of operation modes. This spreadspectrum approach protects system information and makes it possible for the receiver to identify the correct working mode in all kinds of severely distorted channels.

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Figure 3. Digital terrestrial fixed reception in rural areas of Chongming Island, Shanghai.

Possible future applications

Figure 4. First HDTV digital terrestrial trial in May 2006.

terrestrial reference project that has dramatically improved coverage for its viewers. Previously, the inhabitants in Chongming could only receive five unstable analog TV stations. Now, the digital terrestrial network, using single-carrier modulation, delivers 18 SDTV programs in two 8-MHz channels. The low-cost system upgrade and reliable signal coverage provides a viable broadcasting solution. More than 50 cities in 15 provinces across China are currently establishing digital terrestrial systems. China is also encouraging its larger cities to begin delivering free HDTV terrestrial broadcast content, which should trigger growth not only for the digital terrestrial market but for the entire HDTV industry, including high-definition flatpanel displays, chipsets, transmitters, software, and content production. The Shanghai Media Group and Shanghai Jiao Tong University successfully launched the first HDTV digital terrestrial trial in May 2006. Now, with the approach of the 2008 Beijing Olympics, the city of Beijing as well as other big cities such as Shanghai and Shenzhen are actively preparing for HDTV live broadcasting.

Field trials China’s government is also working on improving people’s viewing experience by launching a project called Village to Village TV Coverage. As an example, Chongming Island, a county in Shanghai, is participating in a digital

Mobile options Perhaps the biggest difference between analog and digital terrestrial television is mobile reception. China’s DTTB standard enables mobile reception, which allows viewers to enjoy TV and other multimedia content in buses, taxis, and

July–September 2007

Introducing digital television terrestrial broadcasting in China has not only enhanced existing services but has paved the way for new applications. With the DTTB standard, large-scale fixed reception both in HDTV and of multiple SDTV programs is now possible. New services include mobile, portable, and high-speed applications. In China today (see Figures 3 and 4), fixedreception broadcasting represents the main television viewing option, and it enjoys enormous market potential. Of the 349 million families in China, 67 percent—or 235 million—watch TV by terrestrial reception. These viewers are scattered throughout city areas and the countryside. Often there are large distances between the cable suppliers and the viewers, so it’s too expensive for cable TV networks to reach viewers who reside far from city centers. Most of these viewers can receive between two and five analog (mainly public access) TV stations. The analog channels for these stations are sometimes unstable and noisy, but aside from that, these audiences are also eager for more content. With high bandwidth and compression efficiency, China’s DTTB standard can transmit 20–26 megabits per second (Mbps) per 8-MHz TV channel. This will greatly improve the effective coverage in the vast middle and western areas of China, as well as the more rural areas. These areas will be able to receive between 40 and 50 stable and clear digital TV signals. Thanks to the standard’s strong reception in both indoor and outdoor environments, its resistance to cochannel and adjacent-channel interference, and its large-scale single-frequency network coverage, viewers can use outdoor or indoor antennas and receivers with digital terrestrial tuners to watch their favorite programs.

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Figure 5. Mobile digital terrestrial multimedia services in Shanghai.

430 km/h

every year would benefit from the potential for live TV on trains. China’s railway operators hope to provide live sports broadcasting during the 2008 Olympics. The single-carrier technology in the DTTB standard can provide mobile reception at speeds of up to 430 kilometers per hour on a magnetic elevated train.6

Conclusion

IEEE MultiMedia

Figure 6. Ultra highspeed mobile reception, available at 430 kilometers per hour.

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private automobiles (see Figures 5 and 6). The 20 million vehicles in China could comprise a large part of the potential mobile TV market. Because buildings in China’s big cities tend to be tall and tightly compacted, the shadows from high rises and terrain shielding can cause receiving difficulties. Using efficient equalization technology, the DTTB standard supports stable mobile reception under either single-tower or singlefrequency network environments. As a typical example of a successful mobile digital terrestrial network, in mid-2004 a five-tower digital terrestrial single-frequency network was put into commercial operation in Shanghai, providing mobile TV in cabs, electronic bus stations, and multimedia donation boxes in the downtown area. The new standard also makes high-speed mobile reception possible. The more than four billion passengers who ride the railways in China

China’s DTTB standard, ratified in August 2006, indeed marked a milestone in the Chinese digital television industry. Fixed digital television reception will be a primary market for DTTB, providing SDTV delivery in rural areas and live HDTV in cities. Mobile reception capabilities promise to satisfy a huge demand in the transportation industry. Field trials to date have demonstrated DTTB’s good performance, providing a solid foundation for building on these applications. We’re very willing to collaborate with people from academic, engineering, and industrial groups to improve the DTTB standard in the future. MM

Acknowledgment The work described in this article was partially supported by Natural Science Foundation of China (NSFC) under grant 60332030.

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References 1. Y. Wu et al., “Overview of Digital Television Development Worldwide,” Proc. IEEE, IEEE Press, 2006, pp. 8-21. 2. ATSC Digital Television Standard, Advanced Television Systems Committee, document A/53, 16 Sept. 1995; http://www.atsc.org. 3. European Telecommunication Standards Institute, “Framing Structure, Channel Coding and Modulation for Digital Terrestrial Television,” ETSI, document 300 744, version 1.4.1, 2001; http://www.dvb.org. 4. Association of Radio Industries and Businesses (ARIB), “Terrestrial Integrated Services Digital Broadcasting (ISDB-T)—Specifications of Channel Coding, Framing Structure, and Modulation,” ARIB, 28 Sept. 1998; http://www.arib.or.jp/english/. 5. Standardization Administration of the People’s Republic of China, Frame Structure, Channel Coding and Modulation for a Digital Television Terrestrial Broadcasting System, [in Chinese], Chinese National Standard GB 20600, 2006; http://www.sac.gov.cn. 6. W. Zhang et al., “An Introduction of the Chinese

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9. 10.

DTTB Standard and Analysis of the PN595 Working Modes,” IEEE Trans. Broadcasting, vol. 53, no. 1, 2007, pp. 8-13. J. Song et al., “Technical Review on Chinese Digital Terrestrial Television Broadcasting Standard and Measurements on Some Working Modes,” IEEE Trans. Broadcasting, vol. 53, no. 1, 2007, pp. 1-7. A. Vardy, “The Nordstrom-Robinson Code: Representation over GF (4) and Efficient Decoding,” IEEE Trans. Information Theory, vol. 40, no. 5,1994, pp. 1686-1693. S. Haykin, Adaptive Filter Theory, 4th ed., Publishing House of Electronics Industry, 2003. B. Song et al., “On Channel Estimation and Equalization in TDS-OFDM Based Terrestrial HDTV Broadcasting System,” IEEE Trans. Consumer Electronics, vol. 51, no. 3, 2005, pp. 790-797.

Readers may contact Yunfeng Guan at yfguan69@ sjtu.edu.cn. Contact Standards editor John R. Smith at jsmith@ us.ibm.com.

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