Outline of the Specification for ISDB-T

ISDB (Integrated Services Digital Broadcasting) is a new type of broadcasting for multimedia services. It systematically integrates various kinds of digital contents, each of which may include HDTV, SDTV, sound, graphics, text, and so on. Since ISDB contains a variety of services, it must cover a wide range of requirements that may differ from one service to another. For example, a large transmission capacity is required for HDTV service, while mobile reception is required for sound and data services. ISDB-T (Terrestrial Integrated Services Digital Broadcasting) systems are characterised by:

• MPEG-2 interface: the input signals to the system and output signals from the system conform to the MPEG-2 TS specifications.
• flexible use of modulation schemes: digital contents can be simultaneously transmitted with the appropriate modulation schemes and appropriate bit-rates for each type of content integrated in the ISDB stream.
• use of a control signal that informs the receiver of the multiplexing and modulation configuration.
• partial reception: some of the services can be received by a lightweight, inexpensive narrow-band receiver.

ISDB-T can be applied to either DTTB (Digital Terrestrial Television Broadcasting), DSB (Digital Sound Broadcasting), a mixture of these services, or full multimedia services.

Tokyo pilot station is experimentally broadcasting from Tokyo tower. This pilot broadcasting will be extended to nationwide trials, ranging from the northern island of Hokkaido to the southern island of Okinawa in 1999, using eleven experimental stations located around the country and connected to the program distribution centre in Tokyo.

An outline of the specifications for ISDB-T are described below. Various systems suitable for different applications and spectrum conditions can be derived by selecting an appropriate number of segments.

The specifications describe a transmission system that delivers an IF signal with input signals of MPEG-2 transport streams, which includes the re-multiplexing, channel coding, modulation, transmission control and the corresponding functions of the receiver.

1. Baseline of ISDB-T

By using MPEG-2 coding and systems in the multiplexing process, ISDB-T provides elements in operation and reception that are common to those in digital satellite broadcasting and communications. It also provides flexible multi-program editing for different reception conditions by hierarchical transmission in a transmission channel. The transmission parameters for each hierarchical layer can be selected independently of each other.

A Transport Stream is re-multiplexed and arranged into data groups (Data Segments) prior to OFDM (Orthogonal Frequency Division Multiplexing) framing. After channel coding, data segments are forming into OFDM segments, each having a bandwidth of Bo/14 MHz (Bo = 6, 7, or 8 MHz), by adding pilots.

1.1 Hierarchical Transmission

In ISDB-T, the transmission parameters of the modulation scheme of OFDM carriers, the coding rates of inner code, and the length of the time interleaving can be independently chosen for each data segment. Hierarchical transmission of ISDB-T is achieved by transmitting OFDM segment groups having different transmission parameters in a channel. A maximum of three layers (three different segment-groups) can be transmitted in one channel at the same time.
It should be noted that partial reception is regarded as one hierarchical layer.

1.2 Partial Reception

By limiting the range of frequency interleaving within a segment itself, it is possible to separate a segment independently from the remaining segments in the transmitted signal. In such a way, partial reception of services contained in a transmission channel can be obtained using a narrow-band receiver that has a bandwidth of one OFDM segment.
It should be noted that partial reception uses a dedicated segment ,which is positioned to be the central one among OFDM segments.
Figure 1 shows an example of hierarchical transmission and partial reception.

1.3 Multiplexing for Hierarchical Transmission

Multiplexing in the ISDB-T system complies with MPEG-2 systems. For the hierarchical multiplexing in ISDB-T, in principle, a single transport stream (TS: defined in MPEG-2 systems) is transmitted in a transmission channel bandwidth of NsLBo/14 MHz (Ns indicates the number of segments used in the system), whether or not a hierarchical transmission is in operation. For this reason, division and synthesis of the TS is necessary and this process is performed at both the transmission and reception sides.
It should be noted that because a signal for partial reception is part of the whole signal in a channel, part of a TS is received in partial reception.

2. Transmission Parameters (ISDB-T for 6 MHz)

Table 1 Segment Parameters for ISDB-T (6 MHz)

Table 2 Transmission Parameters for ISDB-T (6 MHz)

Table 3 Information Rates per Segment for ISDB-T (6 MHz)

3. Channel Coding and Modulation

In the byte-wise interleaving, the delay caused in the interleaving process differs from stream to stream of different layers depending on its properties (i.e. modulation and channel coding). In order to compensate for the delay difference including de-interleaving in the receiver, the delay adjustment is carried out prior to the byte-wise interleaving on the transmission side.

The frequency interleaving consists of inter-segment frequency-interleaving and intra-segment frequency-interleaving. The inter-segment interleaving is carried out among differential modulation (DQPSK) segments and among coherent modulation (QPSK, 16QAM, and 64QAM) segments. The intra-segment frequency-interleaver is a random interleaver.