I NTRODUCTION TO ETSI DVB-T S TANDARD

In recent years, DTV (Digital TV) is widely adopted as the next-generation video broadcasting transmission technology. DTV can provide higher A/V quality and less transmission noise than conventional analog TV. The current developed DTV standards consist of DVB (Digital Video Broadcasting) in Europe, ATSC (Advanced Television Systems Committee) in U.S., ISDB (Integrated Services Digital Broadcasting) in Japan and DMB (Digital Multimedia Broadcasting) in China. The transmission modes of DTV include direct satellite broadcasting, cable and terrestrial broadcasting (over-the-air). In terrestrial broadcasting, particularly, video signal is transmitted against severer channel distortions such as multipath fading, co-channel interference and adjacent-channel interference. Since broadcasting transmission system is usually designed to operate within the UHF spectrum allocation for analogue transmissions, it has to provide sufficient protection against high levels of co/adjacent-channel interference emanating from existing PAL (Phase Alternative Line) / SECAM (SEquentiel Couleur Avec Memoire or sequential color with memory) services. Therefore, it is clearly that the terrestrial broadcasting has more challenges in research. The relative materials of terrestrial broadcasting systems are listed in Table 1.1

Standard DVB-T ATSC ISDB-T DMB-T

Modulation OFDM 8-VSB(Vestigial

Sideband)

OFDM OFDM

Area Europe U.S. Japan China

Table 1.1 Comparison of each DTV broadcasting standard

Terrestrial Digital Video Broadcasting (DVB-T) [1] has been subject to technical discussion for many years and undoubtedly been shown as a great success in delivering high quality and standard quality digital television by terrestrial means. DVB-T standard has been produced by European Telecommunication Standard Institute (ETSI) in Aug, 1997, and the second version is released in Jan, 2001. Although originating in Europe, it has been introduced in many countries around the world such as Taiwan. There are two other standards of different broadcasting modes, i.e., DVB-C (cable) and DVB-S (satellite) being specified by ETSI simultaneously. Both DVB-C and DVB-S are simpler system compared to DVB-T as listed in Table 1.2.

Broadcasting Mode Terrestrial (DVB-T) Satellite (DVB-S) Cable (DVB-C)

Modulation 2K / 8K-COFDM QPSK 16/32/64-QAM

Channel Bandwidth 5.71/6.66/7.61 MHz 26-54 MHz 2-7.86/7.96/7.92 MHz Table 1.2 Comparisons of DVB-T, DVB-S and DVB-C

DVB-S has high channel bandwidth (26-54 MHz) and uses QPSK modulation. DVB-C applies 16/32/64 QAM modulation without convolutional code because of lower channel noise and interferences. Nevertheless, in order to provide the high data rate required for video transmission and resist severe channel distortion in DVB-T, concatenated-coded Orthogonal Frequency Division Multiplexing (OFDM) has been adopted into DVB-T in particular.

OFDM is a very popular technology today due to its high data rate transmission capability with high bandwidth efficiency and its robustness to multipath distortion. It has been also chosen as the transmission technique of other communication systems such as ADSL, VDSL, XDSL, DAB and IEEE802.11a/g.

For coping with a multitude of propagation conditions encountered in the wireless

broadcasting channel, many parameters of OFDM for DVB-T can be dynamically changed according to channel conditions. The number of OFDM subcarriers can either be 2048 (2K) or 8192 (8K) so that the desired trade-off can be made between inter-symbol-interference (ISI) and Doppler spread. In the 2K mode, wide subcarrier spacing can reduce the distortion caused by Doppler frequency spread. In the 8K mode, long OFDM symbol duration can overcome large multipath fading. Other parameters like guard interval length, constellation mapping mode and coding rate of Viterbi can be also properly decided up to the broadcasting channel condition of the local area.

The transmission system is shown in Fig 1.1. It contains the blocks for source coding, outer coding and interleaving, inner coding and interleaving, mapping and modulation, frame adaptation and OFDM transmission.

Fig 1.1 Function block diagram of DVB-T system

In the case of two-level hierarchy, the functional block diagram of the system must be expanded to include the modules shown in dashed in. The splitter separates the incoming data stream into the high-priority and the low-priority stream. These two bitstreams are mapped onto the signal constellation by the mapper and therefore the modulator has a corresponding

number of inputs.

As in the baseline systems for satellite and cable, source coding of video and sound is based on the ISO-MPEG2 standards. After the MPEG2 Transport Multiplexer, the packet may optionally be split into two data streams of different priority in case of hierarchical modulation/channel coding. Subsequently, a Reed Solomon (RS) shortened code (204, 188 t = 8) and a convolutional bytewise interleaving with depth I = 12 is applied to the error protected packets (outer interleaving). As Fig 1.1, the outer interleaver is followed by the inner coder.

This coder is designed for a range of punctured convolutional codes (Viterbi), which allows code rates of 1/2, 2/3, 3/4, 5/6 and 7/8. If two-level hierarchical transmission is used, each of two parallel channel encoders has its own code rate. Afterward, the inner interleaver is block based bitwise interleaving.

The system uses OFDM transmission. All data carriers in one OFDM symbol are either QPSK, 16-QAM, 64-QAM, non-uniform-16-QAM or non-uniform-64-QAM. In addition to the transmitted data, an OFDM symbol contains scattered pilots, continual pilots and TPS (Transmission Parameter Signaling) pilots. These reference signals can be used for synchronization, channel estimation and transmission mode verification. The OFDM frame consists of 68 OFDM symbols and four frames constitute one super-frame. The frame structure involving distribution of scattered pilots is shown in Fig 1.2. Scattered pilots insert every 12 subcarriers and have an interval of 3 subcarriers in the next adjacent symbol.

Continual pilots locate at fixed indices of subcarrier, which contains 177 pilots in 8K mode and 45 in 2K mode. Both scattered pilots and continual pilots are transmitted at a boosted power level of 16/9 whereas the power level of other symbols is normalized to 1.

Fig 1.2 Frame structure

The TPS carriers are used for the purpose of signaling parameters related to the transmission scheme, i.e. to channel coding and modulation. The TPS is defined over 68 consecutive OFDM symbol and transmitted in parallel on 17 TPS carriers for the 2K mode and on 68 carriers for the 8K mode. Each OFDM symbol conveys one TPS bit which is differentially encoded in every TPS carriers. The TPS information contains frame number, constellation, hierarchy, code rate, guard interval, transmission mode and BCH error protection code. Unlike scattered and continual pilots, TPS pilots are transmitted at the normal power level of 1 with DBPSK modulation. The values of scattered pilots, continual pilots and TPS pilots are derived by PRBS sequence (X¹¹+X²+1). The guard interval may have four values, i.e. 1/4, 1/8, 1/16 and 1/32. Guard interval 1/4 would occupy 25% of the usable transmission capacity and hence only be used in case of SFN operation with long distances between transmitter sites. In the case of smaller transmitter distances (local SFN) or non-SFN operation the smaller values of guard interval can be selected. In conclusion, DVB-T system has good flexibility for various transmission conditions, so that it becomes a successful technology for video broadcasting.