1、 Rec. ITU-R BO.1784 1 RECOMMENDATION ITU-R BO.1784 Digital satellite broadcasting system with flexible configuration (television, sound and data) (Question ITU-R 3/6) (2007) Scope This Recommendation is intended for the digital broadcasting-satellite service (BSS), when high flexibility in the syste
2、m configuration and broadcasting interactivity is of importance allowing for a wide-ranging trade-off between operation under minimal C/N levels or maximum transmission capacity. The ITU Radiocommunication Assembly, considering a) that the digital multiprogramme television systems for use by satelli
3、tes have been developed in Recommendations ITU-R BO.1408 and ITU-R BO.1516, which are referred to as the current systems; b) that recent developments in the field of channel coding and modulation have produced new techniques with performances approaching the Shannon limit; c) that these new digital
4、techniques would offer better spectrum and/or power efficiency, in comparison to the current systems, whilst maintaining the possibility to be flexibly configured to cope with the specific satellite bandwidth and power resources; d) that the recommended system makes use of such techniques and thus a
5、llows for a wide-ranging trade-off between operation under minimal C/N levels or maximum transmission capacity, achieving appreciable gain over DVB-S (System A in Recommendation ITU-R BO.1516) depending on the selected DVB-S2 mode; e) that the recommended system was developed to cover not only broad
6、casting, but also interactivity and contribution applications, such as contribution TV links and digital satellite news gathering (DSNG); f) that a system covering all these application areas while keeping the single-chip decoder at reasonable complexity levels, would enable the reuse of the develop
7、ment for the mass market products for contribution or niche applications; g) that the new adaptive coding and modulation (ACM) technique offered by the recommended system would allow a more efficient spectrum utilization for unicast applications in connection with a return path, through the optimiza
8、tion of the transmission parameters (i.e. modulation and coding) for each individual user, dependent on path conditions; h) that the recommended system accommodates any input stream format, including single or multiple Motion Picture Experts Group (MPEG) Transport Streams (characterized by 188-byte
9、packets), IP as well as asynchronous transfer mode (ATM) packets and continuous bit-streams; j) that the recommended system would be capable to handle the variety of advanced audiovisual formats currently available and under definition, 2 Rec. ITU-R BO.1784 further considering a) that an ITU system
10、Recommendation helps the market in establishing services based on standardized systems, thus avoiding the proliferation of proprietary developments, which is of benefit to both the end users and the industry in general; b) that, in spite of the success of the current systems, a new specification to
11、enable delivery of a significantly higher data rate in a given transponder bandwidth than the current systems are able to do, is appreciated by many satellite broadcasters, operators and manufacturers around the world; c) that the requirement to offer high-definition television (HDTV) services will
12、force broadcasters to look for more efficient methods of carrying these services within the existing transponders; d) that the inherent flexibility of the recommended system would provide means to alleviate the influence of the atmospheric attenuations at the higher broadcasting-satellite service (B
13、SS) bands (such as the 17 GHz and the 21 GHz BSS bands), which are intended to be used for HDTV services; e) that the recommended system comprises backwards-compatible modes, allowing existing BSS receivers to continue working, recommends 1 that the DVB-S2 system specified in ETSI EN 302 307 V 1.1.2
14、: http:/www.itu.int/ITU-R/study-groups/docs/rsg6-etsi/index.html (see Attachment 1) may be considered as a suitable system for the development of a system for satellite broadcasting with flexible configuration.1NOTE 1 A description of the recommended system (System E) is provided in Annex 1, while A
15、nnex 2 contains comparison tables which list the recommended system (System E) along with the systems contained in Recommendation ITU-R BO.1516 (Systems A, B, C, D). Annex 1 Main characteristics of the DVB-S2 system (referred to as System E) DVB-S2 is the second-generation specification for satellit
16、e broadband applications developed by the DVB (Digital Video Broadcasting) Project in 2003 and became ETSI standard EN 302 307 in 2004. EN 302 307 specifies framing structure, channel coding and modulation for different types of satellite applications: broadcasting of standard definition and high-de
17、finition TV (SDTV and HDTV); interactivity (including Internet access) for satellite broadcasting applications (for integrated receivers-decoders (IRDs) and personal computers); contribution applications, such as digital TV contribution, distribution and news gathering; data content distribution and
18、 internet trunking. 1The word “shall” in this ETSI standard should be considered as “should” in this ITU-R Recommendation. Rec. ITU-R BO.1784 3 To be able to cover all the application areas while still keeping the single-chip decoder at reasonable complexity levels, DVB-S2 is structured as a tool-ki
19、t, thus enabling the use of mass market products also for contribution or niche applications. The DVB-S2 system has been specified around three concepts: best transmission performance, approaching Shannon limit, total flexibility and reasonable receiver complexity. To achieve the best performance-co
20、mplexity trade-off, achieving an appreciable capacity gain over DVB-S for conventional broadcast applications, DVB-S2 benefits from more recent developments in channel coding and modulation: low-density parity check (LDPC) codes are adopted combined with quadrature phase shift keying (QPSK), 8-PSK,
21、16-APSK (amplitude and phase shift keying) and 32-APSK modulations, for the system to properly work on the non-linear satellite channel. Framing structure allows maximum flexibility for a versatile system and synchronization also in worst-case configurations (low signal-to-noise ratios, SNR). For in
22、teractive point-to-point applications such as IP unicasting in connection with a return path, the adoption of the ACM functionality allows to optimize the transmission parameters for each individual user on a frame-by-frame basis, dependant on path conditions, under closed-loop control via the retur
23、n channel (connecting the receiver to the DVB-S2 uplink station via terrestrial or satellite links, signalling the receiver reception condition). This results in a further increase of the spectrum utilization efficiency of DVB-S2 over DVB-S, allowing the optimization of the space segment design, thu
24、s making possible a drastic reduction of the cost of satellite-based IP services. DVB-S2 is so flexible that it can cope with any existing satellite transponder characteristics, with a large variety of spectrum efficiencies and associated SNR requirements. Furthermore it is designed to handle the va
25、riety of advanced audio-video formats currently under definition by the international bodies. DVB-S2 accommodates any input stream format, including single or multiple MPEG Transport Streams (characterized by 188-byte packets), IP as well as ATM packets and continuous bit-streams. Backwards-compatib
26、le modes are also available, allowing existing legacy IRDs to continue working. The DVB-S2 system structure The DVB-S2 system is composed of a sequence of functional blocks, as described in Fig. 1. Signal generation is based on two levels of framing structures: BBFRAME at baseband (BB) level, carryi
27、ng a variety of signalling bits, to configure the receiver flexibly according to the application scenario; PLFRAME at physical layer (PL) level, carrying few highly-protected signalling bits, to provide robust synchronization and signalling at the physical layer. 4 Rec. ITU-R BO.1784 Depending on th
28、e application, DVB-S2 input sequences may be single or multiple MPEG transport streams (TS), single or multiple generic streams, either packetized or continuous. The block identified as Mode Adaptation provides input stream interfacing2, input stream synchronization3(optional), null-packet deletion4
29、(for ACM and transport stream input format only), CRC-8 coding for error detection at packet level in the receiver (for packetized input streams only), merging of input streams (for multiple input stream modes only) and slicing into data fields. A baseband header is then appended in front of the dat
30、a field, to notify the receiver of the input stream format and Mode Adaptation type to notify the receiver of the input stream format and Mode Adaptation type: single or multiple input streams, generic or transport stream, constant coding and modulation (CCM) or ACM, and many other configuration det
31、ails. Thanks to the forward error correction (FEC) protection (covering both the header and the data payload) and the wide length of the FEC frame, the baseband header can in fact contain many signalling bits without losing transmission efficiency or ruggedness against noise. It should be noted that
32、 the MPEG multiplex transport packets may be asynchronously mapped to the baseband frames. Stream Adaptation is then applied, to provide padding in case the user data available for transmission are not sufficient to completely fill a BBFRAME, and baseband scrambling. 2Input sequences may be single o
33、r multiple TSs, single or multiple generic streams (packetized or continuous). 3Data processing in DVB-S2 may produce variable transmission delay. This block allows to guarantee constant-bit-rate and constant end-to-end transmission delay for packetized input stream. 4To reduce the information rate
34、and increase the error protection in the modulator. The process allows null-packets reinsertion in the receiver in the exact place where they originally were. Rec. ITU-R BO.1784 5 Forward error correction (FEC) encoding carries out the concatenation of BCH (Bose-Chaudhuri-Hochquenghem) outer code an
35、d low density parity check (LDPC) inner codes (rates 1/4, 1/3, 2/5, 1/2, 3/5, 2/3, 3/4, 4/5, 5/6, 8/9, 9/10). Depending on the application area, the FEC coded blocks (FEC frames) can have a length of 64 800 or 16 200 bits. When variable coding and modulation (VCM) or ACM are used, FEC and modulation
36、 mode are constant within a frame but may be changed in different frames; furthermore, the transmitted signal can contain a mix of normal and short code blocks. For backwards-compatible modes, the bit-stream at the output of the FEC encoder is further processed together with the DVB-S signal accordi
37、ng to a specified procedure. Bit interleaving is then applied to FEC coded bits for 8-PSK, 16-APSK and 32-APSK to separate bits mapped onto the same transmission signal. Mapping can be chosen among QPSK, 8-PSK, 16-APSK and 32-APSK constellations (see Fig. 2), depending on the application area. QPSK
38、and 8-PSK are typically proposed for broadcast applications, since they are virtually constant envelope modulations and can be used in non-linear satellite transponders driven near saturation. The 16-APSK and 32-APSK modes, mainly targeted to contribution applications, can also be used for broadcast
39、ing, but these require a higher level of available C/N and the adoption of advanced pre-distortion methods in the uplink station to minimize the effect of transponder non-linearity. Whilst these modes are not as power efficient as the other modes, the spectrum efficiency is much greater. The 16-APSK
40、 and 32-APSK constellations have been optimized to operate over a non-linear transponder by placing the points on circles. Nevertheless their performances on a linear channel are comparable with those of 16-QAM and 32-QAM respectively. By selecting the modulation constellation and code rates, spectr
41、um efficiencies from 0.5 to 4.5 bits per symbol are available and can be chosen dependant on the capabilities and restrictions of the satellite transponder used. Physical layer framing has been designed to provide robust synchronization and signalling at the physical layer. Thus a receiver may synch
42、ronize (carrier and phase recovery, frame synchronization) and detect the modulation and coding parameters before demodulation and FEC decoding. The DVB-S2 physical layer signal is composed of a regular sequence of frames (see Fig. 3): within a frame, the modulation and coding scheme is homogeneous,
43、 but may change (in the adaptive coding and modulation configuration) in adjacent frames. Every frame is composed of a payload of 64 800 bits in the “normal frame” configuration, 16 200 bits in the “short frame” one, corresponding to an FEC code block. A header of 90 binary modulation symbols preced
44、es the payload, containing synchronization and signalling information, to allow a receiver to synchronize 6 Rec. ITU-R BO.1784 (carrier and phase recovery, frame synchronization) and detect the modulation and coding parameters before demodulation and FEC decoding. The first 26 binary symbols (the se
45、quence 18D2E82HEX) of the PL header identify the start of the PL frame (SOF, Start Of Frame), the remaining 64 symbols are used for signalling the system configuration. Since the PL header is the first entity to be decoded by the receiver, it could not be protected by the FEC scheme (i.e. BCH and LD
46、PC). On the other hand, it had to be perfectly decodable under the worst-case link conditions (SNR of about 2.5 dB). Therefore, to minimally affect the global spectrum efficiency, the signalling information at this level has been reduced to 7 bits, 5 of which are used to indicate the modulation and
47、coding configuration (MODCOD field), 1 for frame length (64 800 or 16 200 bits), 1 for presence/absence of pilots to facilitate receiver synchronization (as explained below). These bits are then highly protected by an interleaved first-order Reed-Muller block code with parameter rates (64, 7, t = 32
48、), suitable for soft-decision correlation decoding. Independently from the modulation scheme of the PLFRAME payload (FEC code block), the 90 binary symbols forming the PL header are /2-BPSK modulated; this variant of the classical BPSK constellation introduces a /4 rotation on even symbols and /4 on
49、 odd symbols, thus allowing a reduction of the radio-frequency signal envelope fluctuations. The PL frame payload is composed of a different number of modulated symbols depending on the FEC length (64 800 or 16 200 bits) and the modulation constellation, but (excluding the optional pilots) the payload length is always a multiple of a slot of 90 symbols (see Fig. 3), thus showing periodicities which can be exploited by the frame synchronizer in the receiver: once the current PL header has been decoded, the decoder knows exactly the PL frame length and thus the position of th
