Digital Video Broadcasting-Terrestrial (DVB-T) has been subjected to technical discussion for many years and undoubtedly been shown as a great success in delivering high quality digital television by terrestrial means [3]. DVB-T standard has been produced by European Telecommunication Standard Institute (ETSI) in Aug, 1997. It has been applied in many countries around the world such as Taiwan. Although the DVB-T reception can 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 [7]. The detailed concepts of DVB-T and DVB-H will be illustrated later.
The transmission system of the DVB-T standard is shown in Fig. 1.1. It contains the blocks for source coding, outer coding and interleaving, inner coding and interleaving, mapping, OFDM modulation, and frame adaptation, respectively. In the case of two-level hierarchy, the functional block diagram of the system must be expended to include the modules shown in dashed line. As we can see the source coding of audio and video signals is based on ISO-MPEG2 standard. After the MPEG2 transport multiplexer, a Reed-Solomon (RS) shortened code (204,188, t=8) and a convolutional byte-wise interleaving with depth I=12 shall be applied to generate error protected packets. As Fig. 1.1 shows, the outer
interleaver is followed by the inner coder. This coder is designed for a range of punctured convolutional codes, 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 inner codes has its own code rate.
Afterward, the inner interleaver is block based bit-wise interleaving. The constellation mapping for OFDM subcarriers operates with various modes after the inner interleaver. The constellation modes are QPSK, 16-QAM, 64-QAM, non-uniform 16-QAM, and non-uniform 64-QAM, respectively. The transmission channel bandwidth is 6MHz, 7MHz, and 8MHz, respectively.
Fig. 1.1 Functional Block diagram of DVB-T system
The DVB-T system uses OFDM technique with various transmission parameters. The parameters for 8MHz channel bandwidth in DVB-T standard are listed in Table 1-1. Two modulation modes are defined: a 2k mode and an 8k mode. The 2k mode is suitable for short distance transmission and high speed mobile reception because of its short symbol duration and wide subcarrier spacing. On the contrary, the 8k mode is suitable for long distance transmission and deep multipath spread. Other parameters such as code arte, constellation mode, and guard interval length can also be decided properly according to the broadcasting channel condition of the local area.
Table 1-1 Parameters for 8MHz channel in DVB-T standard
Parameter 8k mode 2k mode
Number of subcarriers K 6817 1705
Value of carrier number Kmin 0 0
Value of carrier number Kmax 6816 1704
FFT size N 8192 2048
Symbol duration TU 896µs 224µs
Subcarrier spacing 1/TU 1.116KHz 4.464KHz
Spacing between Kmin and Kmax 7.61MHz 7.61MHz
Guard interval Ng/N 1/4,1/8,1/16,1/32 1/4,1/8,1/16,1/32 An OFDM frame consists of 68 OFDM symbols and four frames constitute a super-frame. In addition to the transmitted data, an OFDM symbol contains several kinds of reference signals for synchronization and channel estimation such as scattered pilots, continual pilots, and TPS (Transmission Parameter Signaling) pilots. Scattered pilots are inserted every 12 subcarriers and have an interval of three subcarriers in the next adjacent symbol. Continual pilots locate at fixed subcarrier index which contain 177 for 8k mode and 45 for 2k mode, respectively. Both scattered pilots and continual pilots are transmitted at boosted power level of 16/9 whereas the data subcarriers are normalized to 1, and modulated according to the PRBS (Pseudo Random Binary Sequence) sequence (X11+X2+1). The TPS pilots are used for signaling parameters related to transmission scheme, i.e. to channel coding and modulation. The TPS pilots are defined over 68 consecutive OFDM symbols and transmitted in parallel on 17 TPS subcarriers for 2k mode and 68 for 8k mode. Each OFDM symbol conveys one TPS bit which is differentially encoded in every TPS subcarrier. The TPS information contains frame number, constellation, hierarchy, code rate, guard interval, FFT mode, and BCH error protection code, respectively. Unlike continual and scattered pilots, TPS
pilots are transmitted as the normal power level of 1 with DBPSK modulation.
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 [8]. 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 block diagram of DVB-H codec and transmitter is as shown in Fig. 1.2.
IP
Fig. 1.2 Block diagram of DVB-H codec and transmitter
As we can see the DVB-H codec is composed of the MPE, MPE-FEC, and time slicing.
In order to satisfy the low power issue in battery-powered terminals, a time-multiplexed transmission of different service is exploited. This technique, called time slicing, allows for selective access to desired data and results in a large battery power saving effect. The burst duration of time slicing is in the range of several hundred ms whereas the off-time may amount to several seconds. The lead time for power-on and resynchronization is assumed to be less than 250ms. Depending on the duty/turn-off ratio, the resulting power saving may be more than 90%. 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.
As for the physical layer, the DVB-H is compatible with the DVB-T standard except some additional points. First, the DVB-H provides new TPS pilots which exploit the reserved TPS subcarriers defined in the DVB-T standard. The new contents of the TPS pilots provide the information about MPE-FEC and time slicing. Besides, an additional OFDM transmission mode and a new symbol interleaving method within the inner interleaver, 4k mode and in-depth interleaving, are also provided by the new TPS pilots. DVB-H provides an intermediate 4k mode with 4096-point FFT in the OFDM modulation. The 4k mode represents a compromise solution between the 2k and 8k mode to satisfy long distance transmission and mobile reception. The in-depth interleaving allows the symbol interleaver operates at 8k interleaving length while the 2k or 4k mode is applied to improve the interleaving performance. Besides, the DVB-H also supports 5MHz transmission channel bandwidth. The parameters for 8MHz channel bandwidth in DVB-H standard are listed in Table 1-2.
Table 1-2 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