ISDB-T consists of 13 OFDM segments. One OFDM segment corresponds to a frequency spectrum having a bandwidth of B/14 MHz (B means the bandwidth of a terrestrial TV channel: 6, 7 or 8 MHz), so one segment occupies bandwidth 6/14 MHz (428.57 kHz), 7/14 MHz (500 kHz) or 8/14 MHz (571.29 kHz). Television broadcasting employs 13 segments with a transmission bandwidth of about 5.6MHz, 6.5 MHz or 7.4 MHz.
ISDB-T has three transmission modes having different carrier intervals in order to deal with a variety of conditions such as the variable guard interval as determined by the network configuration and the Doppler shift occurring in mobile reception. In Mode 1, one segment consists of 108 carriers, while Modes 2 and 3 feature two times and four times that number of carriers, respectively. Table 2 lists the basic parameters of each mode in ISDB-T system.
A digital signal is transmitted in sets of symbols. The active symbol duration is the reciprocal of the carrier spacing – this condition prevents carriers in the band from interfering with each other. The guard interval is a time-redundant section of information that adds a copy of the latter portion of a symbol to the symbol’s “front porch” with the aim of absorbing interference from multi-path-delayed waves. Accordingly, increasing the guard-interval duration in the signal decreases the information bit rate. An OFDM frame consists of 204 symbols with guard intervals attached regardless of the transmission mode. The time interleaving duration in real time depends on the parameters set at the digital-signal stage and on the guard-interval duration, and consequently the values shown in Table 3 for these parameters are approximate.
The error-correction scheme uses concatenated codes, namely, Reed-Solomon (204,188) code for the outer code and convolutional code for the inner code. The information bit rate takes on various values depending on the selected modulation scheme, inner-code coding rate, and guard-interval ratio. The range shown in Table 2 reflects the minimum and maximum values for 13 segments.
A mixture of fixed-reception programs and handheld reception programs is made possible through hierarchical transmission achievable by band division within a channel. “Hierarchical transmission” means that the three elements of channel coding, namely, the modulation scheme, the coding rate of convolutional error-correcting code, and the time interleaving duration, can be independently selected. Time and frequency interleaving are each performed in their respective hierarchical data segment.
As described above, the smallest hierarchical unit in a frequency spectrum is one OFDM segment. Referring to Fig. 20, one television channel consists of 13 OFDM segments, and up to three hierarchical layers (Layers A, B, and C) can be set with regard to these segments. If the OFDM signal is transmitted using only one layer, the layer is A. If the signal is transmitted using two layers, the center “rugged” layer is A and the outer layer is B. If the signal is transmitted using three layers, the center “rugged” layer is A, the middle layer is B, and the outer layer is C. Taking the channel-selection operation of the receiver into account, a frequency spectrum segmented in this way must follow a rule for arranging segments. In addition, one layer can be set for the single center segment as a partial-reception segment for handheld receivers of one-segment services. In this case, the center segment is Layer A. Using the entire band in this way is called ISDB-T. Audio broadcasts and one-segment services feature a basic one-segment format as well as a three-segment expanded format, both referred to as ISDB-TSB.
ISDB-T service examples and transmission signals
1.8.3 Outline of ISDB-T
Figure 21 shows ISDB-T system configuration. This system uses MPEG-2 Video coding and MPEG-2 advanced audio coding (AAC) for source coding. Moreover, it adopts MPEG-2 Systems for encapsulating data streams. Therefore, various digital content such as sound, text, still pictures, and other data can be transmitted simultaneously. For channel coding, transmission parameters may be individually set for each layer, making for flexible channel composition. Furthermore, to achieve an interface between multiple MPEG-2 Transport Streams (TSs) and the Channel coding, these TSs are re-multiplexed into a single TS. In addition, transmission control information, such as channel segment configuration, transmission parameters, etc., are sent to the receiver in the form of a transmission multiplexing configuration control (TMCC) signal.