DTV (Digital TV) is popularly used as the next-generation video broadcasting transmission technology in recent years. DTV provides much higher A/V quality and less transmission noise than conventional analog TV. Nowadays, the 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.
DVB-T standard, one kind of the most popular standards, has been produced by European Telecommunication Standard Institute (ETSI) in Aug, 1997. It has been applied in many countries in the world. In Taiwan, DVB-T standard is also applied as the broadcasting standard. 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 (COFDM) has been adopted into DVB-T in particular. COFDM 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 resisting all kinds of propagation conditions encountered in the wireless broadcasting channel, many parameters of COFDM for DVB-T can be dynamically changed according to channel conditions. The number of COFDM 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 2K mode, wider subcarrier spacing can significantly reduce the distortion caused by Doppler frequency spread. In 8K mode, longer OFDM symbol duration can overcome larger 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.
Like guard interval, longer ones have more powerful capability in severe multipath channel than shorter ones. QPSK can resist more noise distortion than 16-QAM and 64-QAM. Less coding rate can detect out more incorrect bit, it is suitable in the severe error channel.
Although the DVB-T reception can also be applied in mobile environment, the ability of reception for handheld terminals is still not good enough because of its high operation power.
Therefore, Digital Video Broadcasting-Handheld (DVB-H) was also proposed based on the DVB-T technology to provide broadcast services for handheld devices such as PDAs or mobile phones [6].
DVB-H, the specification for bringing broadcast services to battery-powered handheld
receivers, was formally adopted as an ETSI standard in November 2004. This is the offcial DVB-H website maintained by the DVB Project Office. The DVB-H technology is a spin-off of the DVB-T standard. It is large extent compatible to DVB-T but takes into account the specific properties of the addressed terminals- small, lightweight, portable, battery-powered devices in mobile environment. Unlike the DVB-T transport stream adopted from the MPEG2 standard, the DVB-H system is IP (Internet Protocol)-based, therefore the outer DVB-H interface is the IP interface. The IP data are embedded into the transport stream by means of the MPE (Multi Protocol Encapsulation) frame, an adaptation protocol defined in the DVB Data Broadcasting Specification [4]. One MPE frame contains one or more IP datagrams and has a maximum number of 1024 rows and a constant number of 255 columns. The transmission system for DVB-T/H standard 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 COFDM transmission. The additional features of DVB-H are also shown in Fig 1.1.
Fig 1.1 Functional block diagram of the additional features
As we can see the DVB-H codec is additional composed of the MPE, MPE-FEC, and time-slicing. Time-slicing architecture will be introduced in section 1.3. For mobile channels reception and long delay spread conditions, an enhanced error protection scheme on the link layer is needed. This scheme is called MPE-FEC and employs powerful channel coding and time interleaving. The MPE-FEC scheme consists of an RS code in conjunction with an extensive block interleaving. The RS (255, 191, 64) code is utilized to perform MPE-FEC error protection. Besides, a virtual block interleaving effect is also performed by reading from and writing to the MPE frame in column direction whereas coding is applied in row direction.
The parameters for 8MHz channel bandwidth in DVB-H standard are listed in Table 1-1.
Table 1-1 Parameters for 8MHz channel in DVB-H standard
Parameter 8k mode 4k mode 2k mode
Number of subcarriers K 6817 3409 1705
Value of carrier number Kmin 0 0 0
Value of carrier number Kmax 6816 3408 1704
FFT size N 8192 4096 2048
Symbol duration TU 896μs 448μs 224μs
Subcarrier spacing 1/TU 1.116KHz 2.232KHz 4.464KHz
Spacing between Kmin and Kmax 7.61MHz 7.61MHz 7.61MHz
Guard interval Ng/N 1/4,1/8,1/16,1/32 1/4,1/8,1/16,1/32 1/4,1/8,1/16,1/32
Table 1-2, Table 1-3, and Table 1-4 shows the different parameters for 7MHz, 6MHz, and 5MHz channel in DVB-H standard. The DVB-H also supports 5MHz transmission channel bandwidth in addition.
Table 1-2 Parameters for 7MHz channel in DVB-H standard
Parameter 8k mode 4k mode 2k mode
Symbol duration TU 1024μs 512μs 256μs
Subcarrier spacing 1/TU 0.977KHz 1.953KHz 3.906KHz
Spacing between Kmin and Kmax 6.66MHz 6.66MHz 6.66MHz
Table 1-3 Parameters for 6MHz channel in DVB-H standard
Parameter 8k mode 4k mode 2k mode
Symbol duration TU 1194.67μs 597.33μs 298.67μs
Subcarrier spacing 1/TU 0.837KHz 1.674KHz 3.348KHz
Spacing between Kmin and Kmax 5.71MHz 5.71MHz 5.71MHz
Table 1-4 Parameters for 5MHz channel in DVB-H standard
Parameter 8k mode 4k mode 2k mode
Symbol duration TU 1433.60μs 716.80μs 358.40μs
Subcarrier spacing 1/TU 0.697KHz 1.395KHz 2.790KHz
Spacing between Kmin and Kmax 4.75MHz 4.75MHz 4.75MHz
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 mapping and therefore the modulator has a corresponding.
This system uses COFDM transmission. All data carriers in one COFDM symbol are mapped either as QPSK, 16-QAM, 64-QAM, non-uniform-16-QAM or non-uniform-64-QAM. In
addition to the transmitted data, a COFDM 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 COFDM frame consists of 68 COFDM 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.
Fig 1.2 frame structure
The carrier indices of scatter pilots are shown as
min min max
{ 3 ( mod 4) 12 | integer, 0, [ ; ] }
SP= k=K + × l + p p p≥ k∈ K K (1-1) The corresponding modulation of scattered pilots is expressed as
m,l,k k
m,l,k
Re{c } = 4 / 3 2 (1/2 - w ) Im{c } = 0
× (1-2)
where wk means Pseudo Random Binary Sequence (PRBS), and cm,l,k means k-th subcarrier in l-th symbol in m-th frame. PRBS sequence (X11+X2+1) determines the values of scattered pilots, continual pilots and TPS pilots. The PRBS generator is shown as in Fig 1.3.
Fig 1.3 Generation of PRBS sequence
Continual pilots locate at fixed indices of subcarrier, which contains 177 pilots in 8K mode, 89 pilots in 4K mode, and 45 pilots in 2K mode. The corresponding modulation is expressed as
m,l,k k
m,l,k
Re{c } = 4 / 3 2 (1/2 - w ) Im{c } = 0
× (1-3)
The subcarrier indices of continual pilots in 8K mode are shown in Table 1-5. The carrier indices of pilots in 2K mode are 0 to 1704 in Table 1-5, and that in 4K mode are 0 to 3408.
Table 1-5 carrier indices for continual pilot carriers for 8K mode
0 48 54 87 141 156 192 201 255 279 282 333 432 450 483 525 531 618 636 714 759 765 780 804 873 888 918 939 942 969 984 1050 1101 1107 1110 1137 1140 1146 1206 1269 1323 1377 1491 1683 1704 1752 1758 1791 1845 1860 1896 1905 1959 1983 1986 2037 2136 2154 2187 2229 2235 2322 2340 2418 2463 2469 2484 2508 2577 2592 2622 2643 2646 2673 2688 2754 2805 2811 2814 2841 2844 2850 2910 2973 3027 3081 3195 3387 3408 3456 3462 3495 3549 3564 3600 3609 3663 3687 3690 3741 3840 3858 3891 3933 3939 4026 4044 4122 4167 4173 4188 4212 4281 4296 4326 4347 4350 4377 4392 4458 4509 4515 4518 4545 4548 4554 4614 4677 4731 4785 4899 5091 5112 5160 5166 5199 5253 5268 5304 5313 5367 5391 5394 5445 5544 5562 5595 5637 5643 5730 5748 5826 5871 5877 5892 5916 5985 6000 6030 6051 6054 6081 6096 6162 6213 6219 6222 6249 6252 6258 6318 6381 6435 6489 6603 6795 6816
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.
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. There is 17-bit word as synchronization word in one frame. Unlike scattered and continual pilots, TPS pilots are transmitted at the normal power level of 1 with DBPSK modulation. The modulation is expressed as
m,l,k m,l-1,k m,l,k
m,l,k m,l-1,k m,l,k
if sl = 0, then Re{c } = Re{c }; Im{c } = 0;
if sl = 1, then Re{c } = -Re{c }; Im{c } = 0. (1-4) The absolute modulation of the TPS carriers in the first symbol in a frame is derived from the reference sequence wk as follows:
m,l,k k
m,l,k
Re{c } = 2 (1/2 - w )
Im{c } = 0 (1-5) The carrier indices for TPS carriers in 8k mode are listed in Table 1-6, and that is in 2k mode and 4k mode are from 0 to 1687 and from 0 to 3048. It concludes 17, 34 and 68 TPS carriers in 2K, 4K and 8K mode respectively.
Table 1-6 Carrier indices for TPS carriers for 8K mode
34 50 209 346 413 569 595 688 790 901 1073 1219 1262 1286 1469 1594 1687 1738 1754 1913 2050 2117 2273 2299 2392 2494 2605 2777 2923 2966 2990 3173 3298 3391 3442 3458 3617 3754 3821 3977 4003 4096 4198 4309 4481 4627 4670 4694 4877 5002 5095 5146 5162 5321 5458 5525 5681 5707 5800 5902 6013 6185 6331 6374 6398 6581 6706 6799
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/H system has good flexibility for various transmission conditions, so that it becomes a successful technology for video broadcasting.