Toshihiro Nomoto and Hajime Matsumura
NHK (Japan Broadcasting Corporation)
At present, three analog NTSC channels and one analog MUSE-HDTV channel are in service. The number of households receiving analog satellite broadcasting, including MUSE-HDTV, is more than 14 million (nearly one third of all households in Japan).
A new digital satellite broadcasting service called BS Digital Broadcasting started on December 1, 2000. Currently, BS digital broadcasting provides various services on four channels. One of the channels is used for digital simulcast with analog services; that is, the programs broadcast on the analog channels are digitally coded, multiplexed, and simultaneously broadcast on this channel in order to promote the smooth transition from analog to digital. The other three channels are used for digital broadcasting, giving a central role to digital HDTV, where each channel is shared by two or more broadcasters. "10 million households in the first one thousand days" is the target penetration rate. The MPT (Ministry of Post and Telecommunications), the broadcasting industry and the electrical appliance industry are all committed to achieving this goal.
The transmission system for BS digital broadcasting is based on ISDB
(Integrated Services Digital Broadcasting) and is called satellite ISDB
(ISDB-S). ISDB is a new type of broadcasting for multimedia services.
Handling multiple MPEG-TSs
The system systematically integrates various kinds of digital contents, each of which may include multi-program video from LDTV to HDTV, multi-program audio, graphics, texts, and so on. Most of the digital contents are nowadays encoded in the form of MPEG Transport Stream (MPEG-TS). The system therefore has to cover a wide range of requirements that may differ from one service to another.
The system can handle multiple MPEG-TSs (up to eight TSs) to ensure the independence of broadcasters. This may be regarded in terms of the number of logical channels corresponding to that of broadcasters, although the physical satellite channels are limited.
Large transmission capacity
The system employs hierarchical modulation schemes including TC8PSK, QPSK, and BPSK. The maximum information bit rate of about 52 Mbps, which is sufficient for transmitting two HDTV programs, is obtained in one satellite channel by applying TC8PSK. The occupied bandwidth (99% energy bandwidth) is enlarged to 34.5 MHz compared with 27 MHz of the conventional analog system.
High service availability
The system is robust against rainfall attenuation. For example, BPSK, with a coding rate of 1/2, achieves the required C/N of 2 dB.
Multiple modulation schemes can be used simultaneously to meet the requirements of several services integrated on one satellite channel. For example, audio, downloaded data, etc. can be transmitted by BPSK, which is robust against rainfall attenuation, while video data can be transmitted by TC8PSK, which provides the maximum transmission capacity. The information on the modulation scheme is transmitted by a TMCC signal (Transmission and Multiplexing Configuration Control) .
For source coding and multiplexing, the system employs MPEG2.
3.2 Audio coding system
We employed MPEG-2 AAC (Advanced Audio Coding) for the audio coding system. This coding technique is suitable for CD quality multi-channel audio transmission. The compression rate of MPEG-2 AAC is twice that of MPEG2-BC (Backward Compatible), which has been widely employed for conventional satellite digital broadcasting. CD quality stereo audio broadcasting can therefore be transmitted at only about 128 kbps.
3.3 Video and audio multiplexing
The system employs MPEG2 systems for data multiplexing, in which different kinds of information, such as audio, video, and data, are multiplexed.
Each broadcaster sends its signals in the form of MPEG2-TS, and they are combined at an uplink station. Therefore, each broadcaster's MPEG2-TSs are kept unchanged in order to guarantee flexible service integration for each broadcaster sharing the satellite channel.
Figure 2 is a block diagram of the channel coding. The system handles three kinds of signals in order to transmit multiple MPEG-TSs with various modulation schemes and achieve stable and easy reception. The three signals are explained below:
The input MPEG-TSs are combined into a single stream, to which ordinary signal processes for satellite systems are applied (i.e., energy dispersal, interleaving, and inner coding). The control data, which designate the modulation schemes, etc. for each TS packet, are encoded into the TMCC signal, to which a series of channel coding is applied. In most cases, some parts of the channel coding can be shared with the main signal. The burst signal is energy-dispersed to avoid line spectra in the transmission signal.
4.2 Frame Structure
To handle multiple MPEG-TSs and allow several modulation schemes to be used simultaneously, a frame structure is introduced to the main signal.
To combine the TSs, the error-protected 204-bytes packets are assigned to the "slots" in a "data frame", as shown in Figure 3. The slot indicates the absolute position in the data frame and is used as the unit that designates the modulation scheme and MPEG-TS identification. The size of the slot (the number of bytes in the slot) is 204 bytes to keep one-to-one correspondence between the slots and error-protected packets. The data frame is composed of 48 slots.
A modulation scheme can be assigned to each slot independently. Up to four modulation schemes can be assigned in a frame.
Data in different slots may be mixed by intra-frame or convolutional interleaving. It is technically inappropriate to interleave data to be modulated by different schemes. A super-frame structure is employed in order to avoid these inadequacies, as shown in Figure 4. That is, every frame within the super-frame has an identical configuration and data are byte-interleaved in the super-frame (inter-frame) direction.
TMCC carries the information of the modulation schemes and the MPEG2-TS ID, which is assigned to the slots etc. The system can deal flexibly with various requests on these transmission schemes by means of TMCC signals . The configuration for the receivers is automatically changed to demodulate the appropriate signal format. The TMCC consists of 384-bit fixed-length data.
Modulation scheme for transmission signal
The TMCC and burst signal for carrier recovery are provided by the fixed modulation scheme (i.e., BPSK), but the modulation scheme for the transmission signal can be selected arbitrarily, at least for a single slot. The information on the modulation scheme of the transmission signal is provided to the receiver by the TMCC signal.
Identification of MPEG-TS
When more than one MPEG2-TS is transmitted by one satellite transponder, the information for the identification of each MPEG2-TS is provided by the TMCC signal.
Control of transmitter and receiver
Information on whether the broadcaster is using special functions of the transmission schemes (e.g., emergency alert broadcasting) is provided to the receiver by the TMCC signal.
The frame synchronization and super-frame synchronization words are contained in the TMCC signal.
The TMCC signal requires higher reliability than the main signal because the TMCC signal is used to control the demodulation process at the home receiver. To secure reliable transmission, the TMCC signal is outer coded by RS(64,48). Finally, the TMCC is modulated by convolutional BPSK (r = 1/2). These error correction techniques allow the TMCC signal to be received until the reception C/N is around 0 dB.
4.4 Burst signal for carrier recovery
A burst signal is inserted to secure steady carrier recovery at the receiver even in low C/N conditions. The burst signal is energy-dispersed by BPSK modulation with a pseudo-random sequence. It is possible to carry some information by modulating the burst with an information signal instead of the pseudo-random sequence (although this is not included in the current standard).
Figure 5 outlines the transmission signal.
4.5 Transmission capacity
The time for transmitting the amount of bits in a frame (48 slots) differs by modulation scheme because of the different spectrum efficiencies of the schemes. When QPSK (r=1/2) is used, for example, the time is twice that of TC8PSK.
To keep the duration constant, we introduce dummy slots when modulation schemes other than TC8PSK are assigned to some slots (Figure 6). Since the bits in dummy slots are never transmitted, the actual number of bits transmitted in a frame is reduced to the number of bits that can be transmitted in a given frame duration by combining the designated modulation schemes.
When a single modulation scheme is applied throughout a frame, the transmissible bit rate is given as,
where TB: transmissible bit rate
SR: symbol rate (= 28.86 Mbaud)
SE: spectrum efficiency (3 for TC8PSK, 2 for QPSK, 1 for BPSK)
CR: coding rate (2/3 for TC8PSK, 1/2, 2/3,3/4, 5/6 and 7/8 for QPSK, 1/2 for BPSK)
The transmission signal contains parity bits for RS (204, 188) and the burst signal for carrier recovery. 188/208 denotes the ratio of the time required for transmitting the information bits by MPEG2-TS to the frame duration. When several modulation schemes co-exist in a frame, the cumulative transmissible bit rate is given as:
where TBi denotes the transmissible bit rate for the i-th modulation scheme as given by Eq. (1).
Ni is the number of slots, including dummy slots using the i-th modulation scheme.
Note that, the value of Ni takes the number listed in Table 3 to keep the frame duration constant.
The relation between large transmission capacity and high service availability is a trade-off under the limits of the satellite emission power and channel bandwidth. The transmission capacity and service availability therefore need to be selected according to the service contents and the intentions of the broadcaster.
In the digital broadcasting system, various modulation schemes - as shown in Table 1 - can be employed. From among these schemes, broadcasters can choose the most suitable modulation scheme for their services.
Here, the required C/N is evaluated with a BER below 10-11 as a quasi-error-free condition. To achieve this condition, it is theoretically necessary to have 2x10-4 at the input to the RS decoder after sufficient interleaving. For calculating the relationship, we assume reception with a 45-cm-diameter satellite dish and a satellite EIRP (Equivalent Isotropic Radiated Power) of 59 dBW.
Conventional FM-NTSC, which transmits audio signals with a digital subcarrier, begins to show impulsive noise on the screen at a C/N of around 9 dB, and the audio signal collapses at a C/N of 6 dB. Viewers are expected to continue to receive the program until the audio signal collapses, so the C/N of 6 dB is regarded as the threshold of a conventional analog system.
TC8PSK can obtain higher spectral efficiency than QPSK and BPSK, so it is appropriate for providing two HDTV programs. However, the service availability is insufficient in terms of continuity from a conventional analog system to the new digital systems. On the other hand, BPSK and QPSK with a coding efficiency of 1/2 can provide higher service availability than the conventional analog system, (although the transmission bit rate is restricted ).
A new digital broadcasting service started in Japan in December 1, 2000. The technical standard (called ISDB-S) for the transmission system was established and ITU-R approved as Recommendation BO.1408 in 1999 .
The system uses the MPEG2 standard for video and audio coding and MPEG2 systems for multiplexing programs. The most appropriate transmission scheme for each slot can be selected according to the broadcaster's requirements or program content. The system therefore has large transmission capacity, strong robustness against heavy rainfall attenuation, and high operational flexibility.
The configuration of the receiver changes automatically in order to demodulate the appropriate signal format, using the information of transmission scheme derived from the TMCC signal.
Low-cost receivers have already been put in the market by many manufacturers in Japan, so consumers can receive a variety of services inexpensively via broadcasting satellite.
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