Transmission System for ISDB-S

Transcription

1 Transmission System for ISDB-S HISAKAZU KATOH, SENIOR MEMBER, IEEE Invited Paper Broadcasting satellite (BS) digital broadcasting of HDTV in Japan is laid down by the ISDB-S international standard. Since it has a unique transmission technique that can use a number of modulation schemes simultaneously in one RF carrier, the most suitable modulation scheme for the broadcast content can be selected even within one RF carrier. In addition, a hierarchical modulation technique that can mitigate rain attenuation can be introduced as an application of this transmission system. This paper shows how the transmission system establishes such a versatile function. Keywords binary phase shift keying (BPSK), digital broadcasting, error correction, Integrated Services Digital Broadcasting (ISDB), MPEG-transport stream, MPEG-2, quadrature phase shift keying (QPSK), satellite broadcasting, trellis-coded 8PSK (TC8PSK), Viterbi decoding. I. INTRODUCTION Digital broadcasts using a broadcasting satellite (BS) began in Japan in December These BS digital broadcasts, as they are called in Japan, are delivered by a completely new broadcasting system focusing on HDTV and multimedia services. The system operates in the Broadcasting Satellite Service (BSS) band from 11.7 to 12.2 GHz, and has been standardized in ITU-R as Recommendation BO , titled Transmission system for advanced multimedia services provided by integrated services digital broadcasting in a broadcasting-satellite channel. The system goes by the name of ISDB-S in ITU-R, as it is the satellite segment for the ISDB family of systems. This paper is an outline of BS digital satellite broadcasting systems in Japan. II. FEATURES OF THE BS DIGITAL SYSTEM The basic configuration of the BS digital broadcasting system is shown in Fig. 1. The system consists of a source coding section that converts video, audio, and data signals Manuscript received March 15, 2005; revised July 25, The author is with the Planning Division, Engineering Administration Department, NHK (Japan Broadcasting Corporation), Tokyo, , Japan ( katoh.h-km@nhk.or.jp) Digital Object Identifier /JPROC into efficient digital signals; a multiplexing section that multiplexes the digital signals; a conditional access section that scrambles the signals and distributes unscrambling keys to subscribers; and a channel coding section that performs signal processing such as error correction and modulation. This configuration differs significantly in comparison with the conventional digital system in its capability of transmitting multiplexed signals on one satellite channel. As it aims for a system that can broadcast multiple high-definition television programs with one transponder, the channel coding section of the BS digital system was developed from a viewpoint different than that of the conventional digital satellite broadcasting systems. New technologies like the trellis-coded 8PSK (TC8PSK) modulation scheme were adopted for this section to improve transmission capacity as much as possible and to enable multiple transport streams (TSs) to be handled by one carrier. This channel coding system has consequently become a system standard known for its flexibility and extendibility. The other sections besides the ones for the channel coding were developed in adherence to the principle that they be applicable to the widest possible media cross section while taking into account compatibility with the other digital broadcasting system. For this reason, MPEG-2 Video and MPEG-2 Systems were adopted for video coding and multiplexing, and MULTI-2 was chosen for the scrambling system. In addition, the MPEG-2 advanced audio coding (AAC) system standard was chosen for audio coding with the aim of improving the coding rate. The features of the BS digital system are summarized below. 1) It adopts TC8PSK modulation and enables the broadcasting of two high-quality digital HDTV programs with one transponder by setting a wider frequency bandwidth than those of analog BSS systems. 2) It enables switchover between or joint use of multiple transmission systems so that the most optimal system can be selected in accordance with the operator s service content. 3) It enables multiple MPEG-TSs to be transmitted with one transponder, and because transmission systems /$ IEEE PROCEEDINGS OF THE IEEE, VOL. 94, NO. 1, JANUARY

2 Fig. 1. Basic configuration and features of the BS digital system. Table 1 Overview of the BS Digital System can be switched for each TS signal, it enables TS signals produced by each broadcaster to be transmitted independently. 4) It allows an operator to exert control over the transmission system such as by selecting one or changing the mixture of multiple modulation schemes and by changing its assigned ratio in each TS. This is achieved by multiplexing a transmission and multiplexing configuration control (TMCC) signal. 5) It adopts MPEG-2 Video (with MP@HL as a precondition) as its video coding system and MPEG-2 Audio (AAC) as its audio coding system. Together they achieve a high compression rate while ensuring high-quality video and audio broadcasting. 6) It enables stable reception of digital broadcasts using the same receiving antennas as those of existing analog BS broadcasts by multiplexing a burst signal for stable carrier recovery. 290 PROCEEDINGS OF THE IEEE, VOL. 94, NO. 1, JANUARY 2006

3 Fig. 2. Configuration of the channel coding section. Fig. 3. Configuration of error-correcting inner-coding device and phase mapping. III. OVERVIEW OF THE BS DIGITAL BROADCASTING SYSTEM Table 1 provides an overview of the BS digital broadcasting system. In particular, the table shows that the BS digital broadcasting system has a high transmission capacity at its maximum rate of approximately 52 Mb/s with a bandwidth per channel of 34.5 MHz. It also indicates the system s flexibility and extendibility, such as the ability to change or use more than one modulation scheme in accordance with the frame configuration. IV. CHANNEL CODING SECTION Fig. 2 outlines the configuration of the channel coding section developed for the BS digital system. The following describes each of the technical elements making up channel coding. A. Modulation and Error-Correction Schemes The channel coding section shown in Fig. 2 adopts convolutional code as the inner-code error-correction scheme. Convolutional code, which is adept at handling random errors, demonstrates high error-correction performance in combination with Reed Solomon (204,188) code, which is strong with respect to burst errors. Pragmatic code as shown by the configuration of the inner-coding device and the phase mapping in Fig. 3 is adopted for the eight-phase trellis coding. By introducing this inner-coding system with 8PSK modulation, the QPSK/BPSK convolutional inner-coding circuit can be commonly used for the eight-phase trellis coding. Furthermore, even when switching between different error-correction schemes, the convolutional encoder in the transmitter and the Viterbi decoder in the receiver can be continuously used without any degradation. B. Frame Configuration A frame configuration that specifies a fixed information length is adopted so that multiple modulation schemes and multiple TSs can be used with one transponder. As shown in Fig. 4, a 204-B signal (slot) consisting of an MPEG-TS and outer-code error correction is taken to be the minimum unit of data. A modulation scheme and TS number can be specified for each slot. A frame, in turn, consists of 48 slots and is the basic unit of channel transmission. A superframe, moreover, consists of eight frames and serves as the unit of processing for energy dispersal and interleaving. KATOH: TRANSMISSION SYSTEM FOR ISDB-S 291

4 Fig. 4. Frame signal processing. Here, as the first byte of each slot is fixed as an MPEG-TS synchronization byte (its data is represented as 47H in hexadecimal manner), channel coding will replace this byte with frame-synchronization signals or TMCC information at the beginning of each frame. Energy-dispersal signal processing is performed next with respect to the B of the superframe excluding the firstbyteof each slot. Then, as shownin the same figure, interleaving is performed in byte signal units acrosseightframesforthe203bofeachslotamongthoseslots having the same slot number in a superframe (interleave depth of eight). This makes for uniform interleaving regardless of the transmission system configuration in each slot. In this frame configuration, a baseband signal using TC8PSK can be completely transmitted in 48 slots. At the same time, a baseband signal using QPSK (coding rate ), for example, whose transmission efficiency is half that of TC8PSK, results in effective data for at most 24 slots in a frame at the time of frame configuration. The remaining slots are then treated as dummy slots and only effective information is modulated. In this way, different modulation schemes can be transmitted at the same symbol rate. The frame structure can be changed in terms of the superframe. As both block interleave and energy dispersal terminate within one superframe, no degradation of baseband signals occurs when the frame structure is changed. V. CONFIGURATION AND TRANSMISSION OF TMCC The broadcaster can set the number of slots and the modulation scheme for each TS signal independently within the assigned bandwidth (number of slots). This information is written as a TMCC signal every eight frames (superframe) and is therefore time-multiplexed within one RF carrier. The receiver demodulates this TMCC information every superframe as a basis for demodulating and decoding each TS signal, and finally selects the TS desired by the viewer and performs service decoding. A feature of TMCC signal control is that each TS signal can be regarded as transmitting a single TS independently on both the transmit side and the receive side without referring to the transmission channel. In addition, while 8 B of TMCC information are transmitted per frame, only 384 b are transmitted per superframe, since only six frames worth 292 PROCEEDINGS OF THE IEEE, VOL. 94, NO. 1, JANUARY 2006

5 Table 2 TMCC Bit Configuration Fig. 5. Relationship between TMCC transmission and burst signal transmission. of TMCC information is used per superframe with the remaining two frames used for Reed Solomon error correction of the TMCC information. Appropriate setting of the TMCC information makes the following possible for each RF carrier. 1) Transmission of multiple TSs (maximum eight TSs) 2) Selection of one of seven modulation schemes (TC8PSK, QPSK (5 coding rates), and BPSK) or their simultaneous use (maximum four schemes) Table 2 shows the bit configuration of this TMCC information. The transmit-mode/slot-information section in the figure specifies the modulation scheme for each slot, while the relative-ts-slot-information section specifies the relationship between slots and relative TS numbers allocated within the same BS channel. This information together with the table that comes next describing the correspondence between relative-ts and TS-ID enables the receiver to determine the TS ID (16 b) actually transmitted. TMCC information separately assigned at the top of each frame between two synchronization words is modulated by BPSK, as shown in Fig. 5. In terms of channel coding of TMCC, the outer code of RS(64,48) followed by energy dispersal of 15th-order M series are used. Half-rate convolutional code the same as the main signal is used as the inner code. Since this information is protected by the strong errorcorrection code and is dispersed throughout one superframe, no interleaving function is required. VI. BURST SIGNAL FOR STABLE CARRIER RECOVERY AND THE TMCC SIGNAL When a digital receiver demodulates a signal, a synchronization detection function in the form of a carrier-recovery circuit becomes necessary. In general, a stable carrier can be recovered in the modulated-wave order of BPSK QPSK TC8PSK. In BS digital broadcasting where multiple transmission systems can be used together, the optimal conditions for this carrier-recovery circuit differ according to the content and assigned ratio of the multiple transmission systems in one carrier. It is for this reason that a BPSK-modulated wave is inserted at a position decided beforehand (a four-symbol reference burst is multiplexed intermittently every 203 symbols of the main signal) in addition to the main signal and the TMCC signal described above. Using this as a reference for carrier recovery at the receiver enables stable carrier recovery even for low C/N ratios. Multiplexing a burst signal for stable carrier recovery in this way makes for stable reception even when using an old 12-GHz antenna converter, which sometimes has degraded phase noise characteristic in its down-conversion circuit. Although such antennas have no degradation when receiving analog signals, receiving digital signals using PSK becomes more sensitive to phase noise. Furthermore, while a TMCC signal is transmitted at the beginning of each frame, a synchronization signal is also transmitted before and after the TMCC information. This synchronization signal is set with a previously decided signal stream that serves as a means of fast frame synchronization when the receiver is turned on or the BS channel is changed. This situation is shown in Fig. 5. As shown in the same figure, TMCC information is transmitted in 128 symbols of every frame. This, however, equates to 8 B per frame, as this section is transmitted by BPSK (coding rate ) to improve reliability even in low CN conditions. VII. EXAMPLE OF TRANSMISSION SIGNAL Fig. 6 shows an example of signal transmission using TC8PSK (46 slots/frame) and QPSK (one-effective-slot/frame) jointly. KATOH: TRANSMISSION SYSTEM FOR ISDB-S 293

6 Fig. 6. Example of signal transmission by channel coding. In this example, one slot of information transmitted by QPSK requires twice the transmission time of one slot of TC8PSK (since the transmission efficiency of QPSK is half that of TC8PSK). This means that the transmission time must be adjusted by inserting dummy slots. Although QPSK is advantageous with respect to rain attenuation, since it affords a lower C/N cutoff limit during periods of heavy rainfall, there is a tradeoff as it also means there is a lower transmission capacity. In short, modulation schemes can be selected as desired, and transmissions can combine a modulation scheme having a large transmission capacity, like TC8PSK, with one that is robust against rain attenuation, like QPSK. The modulation scheme is arranged in a spectrum-efficient order; such as TC8PSK QPSK QPSK QPSK BPSK. Because the 294 PROCEEDINGS OF THE IEEE, VOL. 94, NO. 1, JANUARY 2006

7 Fig. 7. Configuration of a satellite digital broadcast receiver. Viterbi decoder processes from data that is received later, a robust modulation scheme needs to layout later. The Viterbi decoder resets its decoding path at the time of frame sync signal so as not to be affected by errors in the following frame. VIII. DIGITAL RECEIVER Fig. 7 shows the configuration of a receiver for BS digital broadcasts. The antenna converter shown in the figure enables existing analog BS antennas to be used for digital broadcasts as well. The output signal from the converter is passed via cable into a tuner placed inside a room, and the received signal is demodulated and decoded. The demodulation section converts each PSK signal into a digital signal, reconfigures signal frames based on information decoded from the TMCC signal, and performs reversed interleaving and energy-dispersal processing to decode multiple TS signals. The viewer selects one service that consists of video and audio from one TS signal to be decoded by the MPEG decoder. IX. CONCLUSION This paper has outlined the BS digital broadcasting system implemented in Japan. At present, with four years having passed since commencement of broadcasts, there are more than 7 million BS digital subscribers all over Japan. Moreover, thanks to commercialization of digital receivers that include terrestrial digital receiving functions, nearly everyone in Japan will soon be able to enjoy the high quality, convenience, and user-friendliness of BS digital broadcasting. REFERENCES [1] Transmission system for advanced multimedia services provided by integrated services digital broadcasting in a broadcasting-satellite channel, ITU-R Recommendation BO , Apr [2] H. Katoh, T. Kimura, N. Kawai, and A. Ohya, ISDB (Integrated Services Digital Broadcasting), presented at the Int. Symp. Broadcasting Technology (ISBT) 93, Beijing, China. [3] H. Katoh, Digital modulation scheme for ISDB in the 12 GHz band, in Proc. AIAA 15th Int. Communication Satellite Systems Conf. AIAA , pp [4] H. Katoh, A. Hashimoto, H. Matsumura, S. Yamazaki, and O. Yamada, A flexible transmission technique for the satellite ISDB, IEEE Trans. Broadcast., vol. 42, no. 3, pp , Sep [5] T. Saito and H. Katoh, ISDB (Integrated Services Digital Broadcasting) transmission system based on protection ratio study, presented at the AIAA 16th Int. Communication Satellite Systems Conf. AIAA , [6] A. Hashimoto and H. Katoh, Development of a transmission system and an integrated receiver for satellite ISDB, in Proc. IEEE 1997 Int. Conf. Consumer Electronics WAM , pp [7] H. Katoh, T. Saito, and H. Matsumura, Maximum transmission bit rate for satellite ISDB and its application in a satellite broadcasting plan, Electron. Commun. Jpn. Pt. 1, vol. 80, no. 5, pp , Hisakazu Katoh (Senior Member, IEEE) received the B.E., M.E., and Ph.D. degrees in electric and electronics engineering from the Tokyo Institute of Technology, Tokyo, Japan in 1980, 1982, and 1997, respectively. He joined NHK, Tokyo, in 1982 and has been with NHK Science and Technical Research Laboratories (STRL) since He has been engaged in research on satellite broadcasting system and digital transmission systems. From 1989 to 1990, he was also a Visiting Researcher at the University of California, Davis. From 1991, his main interest was a digital satellite broadcasting system, and he also engaged in standardization of the system both domestically and internationally. From 1998 to 2000, he joined a project for preparation of digital satellite broadcasting system in the Engineering Administration Department, NHK, to launch BS digital broadcasting in He returned to NHK STRL in 2001 and engaged in planning and coordination matters. Since 2003, he has been a Senior Associate Director of the Planning Division, Engineering Administration Department, NHK. KATOH: TRANSMISSION SYSTEM FOR ISDB-S 295