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ETSI
4.2 System architecture
According to figure 1, the DVB-S2 System shall be composed of a sequence of functional blocks as described below.
Mode adaptation shall be application dependent. It shall provide input stream interfacing, Input Stream
Synchronization (optional), null-packet deletion (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. For Constant Coding and Modulation (CCM) and single
input Transport Stream, Mode Adaptation shall consist of a "transparent" DVB-ASI (or DVB-parallel) to logical-bit
conversion and CRC-8 coding. For Adaptive Coding and Modulation (ACM), Mode Adaptation shall be according to
annex D.
A Base-Band Header shall be appended in front of the Data Field, to notify the receiver of the input stream format and
Mode Adaptation type. To be noted that the MPEG multiplex transport packets may be asynchronously mapped to the
Base-Band Frames.
For applications requiring sophisticated merging policies, in accordance with specific service requirements (e.g. Quality
of Service), Mode Adaptation may optionally be performed by a separate device, respecting all the rules of the DVB-S2
specification. To allow standard interfacing between Mode and Stream Adaptation functions, an optional modulator
interface (Mode Adaptation input interface) is defined, according to clauses I.1 (separate signalling circuit) or I.2
(in-band signalling).
Stream adaptation shall be applied, to provide padding to complete a Base-Band Frame and Base-Band Scrambling.
Forward Error Correction (FEC) Encoding shall be carried out by the concatenation of BCH outer codes and LDPC
(Low Density Parity Check) 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 block shall have length n
ldpc
= 64 800 bits or 16 200 bits. When VCM and ACM is
used, FEC and modulation mode may be changed in different frames, but remains constant within a frame.
Mapping into QPSK, 8PSK, 16APSK and 32APSK constellations shall be applied, depending on the application area.
Gray mapping of constellations shall be used for QPSK and 8PSK.
Physical layer framing shall be applied, synchronous with the FEC frames, to provide Dummy PLFRAME insertion,
Physical Layer (PL) Signalling, pilot symbols insertion (optional) and Physical Layer Scrambling for energy dispersal.
Dummy PLFRAMEs are transmitted when no useful data is ready to be sent on the channel. The System provides a
regular physical layer framing structure, based on SLOTs of M = 90 modulated symbols, allowing reliable receiver
synchronization on the FEC block structure. A slot is devoted to physical layer signalling, including Start-of-Frame
delimitation and transmission mode definition. This mechanism is suitable also for VCM and ACM demodulator
setting. Carrier recovery in the receivers may be facilitated by the introduction of a regular raster of pilot symbols
(P = 36 pilot symbols every 16 SLOTs of 90 symbols), while a pilot-less transmission mode is also available, offering
an additional 2,4 % useful capacity.
Base-Band Filtering and Quadrature Modulation shall be applied, to shape the signal spectrum (squared-root raised
cosine, roll-off factors 0,35 or 0,25 or 0,20) and to generate the RF signal.