Section 10.4.3 explains the DVB encoding system for the digital terrestrial system. At the receiving end, the signal has to be decoded. This section discusses the DVB specifications for the required channel decoding.
Clock
Figure 10.16 Hierarchical channel coding.
1 0 . 4 . 4 . 1 D e c o d i n g s y s t e m
In spite of the fact that the DVB specifications for the digital terrestrial sys-tem are in the final stage of becoming an ETSI standard, this syssys-tem still has to evolve from the laboratory environment to a practical implementa-tion. Tests have shown that portable reception in a car driving at 170 km/h is possible. This could make the system very interesting for all sorts of mobile digital broadband video services.
However, especially in the case of the 8k variant, the required sophis-ticated decoder technology is not yet being produced in mass production.
Depending on the actual costs of the decoder’s practical implementation, the requirements for the digital terrestrial system may still be subject to change.
1 0 . 4 . 4 . 2 R e c o v e r y o f R e f e r e n c e I n f o r m a t i o n
With the application of pilot cells, synchronization (frame, frequency, and time), channel estimation, and transmission mode identification are
Q
I
1011 1001
1010 1000
0010 0011
0000 0001
1101 0100
1111 1110
1100
0101 0111
0110
−2 2 4
−2
−4 2 4
−4
I Qk k=11 I Qk k=01
I Qk k=00 I Qk k=10
I Qk kare the two MSBs in each quadrant
Figure 10.17 Constellation diagram 16-QAM and alpha=2. (IkQkare the two MSBs in each quadrant.)
established. The TPS carriers contain specific information concerning the applied channel coding and type of modulation (see Table 10.6).
The reference information (pilots and TPS) can be recovered by means of a feedback circuit (PLL).
1 0 . 4 . 4 . 3 D e m o d u l a t o r a n d i n n e r d e - i n t e r l e a v e r
The demodulator and the inner de-interleaver operate in a reversed way compared to the interleaving and modulation process at the transmitting end. The (high- and low-priority) bit streams are de-interleaved in order to be demodulated. Next, the demodulated bit streams are multiplexed into a single bit stream again in order to be applied to the inner decoder.
The whole process is supported by the reference information.
1 0 . 5 S u m m a r y a n d c o n c l u s i o n s
DVB has provided specifications for a broad variety of digital transmission systems concerning communication via satellite, CATV, and terrestrial networks. These systems have been or are currently being standardized by ETSI. Table 10.7 provides an overview of the typical parameters of the DVB digital transmission systems.
Satellite communication suffers from power limitations. For this rea-son, QPSK is used as a modulation method. The advantage of satellite communication is the rich availability of bandwidth, which in the end allows a high bit rate. After receiving the satellite signals, further trans-mission via a SMATV network is enabled by the DVB SMATV system. This allows the use of QAM, but the available bandwidth is limited. Hence, a
Table 10.6 TPS Carrier Information
Constellation including the value of alpha (QAM modulation) Hierarchy information including inner code rate
Guard interval
Transmission mode (2k or 8k) Frame number in a super frame Synchronization word
lower bit rate is achieved. This last case more or less applies to the DVB cable system as well.
As a result of the different local characteristics, the DVB terrestrial system is the most complicated system of all. Multipath interference is countermeasured by means of the OFDM transmission technology. This multicarrier solution allows the application of QPSK as well as QAM. Due to the limited terrestrial frequency spectrum, this system has a lower maximum bit rate than the DVB DTH system.
MVDS, a terrestrial system that, for example, can be chosen as an alternative for transmission via CATV networks, is identical to the DVB DTH system. The only difference is the frequency band in which this
Table 10.7
Typical DVB Digital Transmission System Parameters
DVB System Modulation TERRESTRIAL2 OFDM (QPSK)
OFDM
MVDS QPSK 40.5-42.5 26.0–54.0 18.7– 68.0
MMDS3 16-QAM
1. Frequency band is chosen by CATV operator;
2. Nonhierarchical coding;
3. Frequency band has not yet been allocated to MMDS.
system is operated. This enables a compatible use of both systems. The MMDS system provides the user with the same functionality as the MVDS. The system design, however, is based on the DVB cable specifica-tions. This makes the DVB MMDS system compatible with the DVB cable system.
References
[1] Reimers, U., Digitale Fernsehtechnik, Datenkompression und Übertragung für DVB, Springer, April, 1995.
[2] ITU, Radio Regulations, 1990 edition, revised in 1994, Geneva, 1994.
[3] Ha, T. T., Digital Satellite Communications, New York: Macmillan Publishing Company, 1988, pp. 25–28.
[4] EBU/ETSI JTC, Digital broadcasting systems for television sound and data services;
Framing structure, channel coding and modulation for 11/12 GHz satellite services, ETS 300 421, December, 1994.
[5] EBU/ETSI JTC, Digital broadcasting systems for television sound and data services;
Satellite Master Antenna Television (SMATV) distribution systems, ETS 300 473, May, 1995, p. 8.
[6] EBU/ETSI JTC, Digital broadcasting systems for television sound and data services;
Satellite Master Antenna Television (SMATV) distribution systems, ETS 300 473, May, 1995.
[7] ISO/IEC DIS 13818-1, Coding of moving pictures and associated audio, June, 1994.
[8] Viterbi, A. J., Error Bounds for Convolutional Codes and an Asymptotically
Optimum Decoding Algorithm, IEEE Trans. On Information Theory IT-13, No. 2, 1967.
[9] EBU/ETSI JTC, Digital broadcasting systems for television sound and data services;
Framing structure, channel coding and modulation for cable systems, ETS 300 429, December, 1994.
[10] EBU/ETSI JTC, Digital broadcasting systems for television sound and data services;
Framing structure, channel coding and modulation for digital Terrestrial television (DVB-T), prETS 300 744, November, 1996.
[11] EBU/ETSI JTC, Digital broadcasting systems for television sound and data services;
Framing structure, channel coding and modulation for MVDS at 10 GHz and above, ETS 300 748, October, 1996.
[12] TNO FEL, Inventarisatie van MVDS systemen ten behoeve van beleidsvorming door HDTP, maart 1996, p. 35.
[13] EBU/ETSI JTC, Digital broadcasting systems for television sound and data services;
Framing structure, channel coding and modulation for MMDS systems below 10 GHz, prETS 300 749, 11 January, 1996.
[14] EBU/ETSI JTC, Digital broadcasting systems for television sound and data services;
Framing structure, channel coding and modulation for MMDS systems below 10 GHz, prETS 300 749, 11 January, 1996, p. 40.