Radyne OM20 User Manual download pdf (Page 38)

Fig. 4.8: Stream format at the output of the Mode Adapter [EN 302 307]

In Figure 4.8 is shown how upper layer stream formats (either generic continuous or

packetized stream) partitioned into the BBFRAME DATA field of the newly build Baseband

Frame (BBFRAME), as well the different filled-in BBHEADER fields. E.g. MATYPE (2

bytes) describes the used input stream format, the type of mode adaptation (CCM / ACM) and

the transmission roll-off factor. Furthermore the User Packet Length (UPL) and the Data

Field Length (DFL) is stored.

Stream Adaption Subsystem

As next step stream adaptation provides padding to complete a constant length BBFRAME.

bch

- DFL - 80 zero bits will be appended after the DATA field as trailer. The resulting

BBFRAME has now a constant length of K

bch

bits, where the value K

bch

depends on the FEC

coding (see Table 4.1 and 4.2 – BCH Uncoded Block K

bch

). This entire BBFRAME will then

be randomized (scrambled).

Fig. 4.9: BBFRAME at the output of the Stream Adapter [EN 302 307]

FEC Coding

DVB-S2 uses a very powerful FEC coding based on concatenation of BCH (Bose-Chaudhuri-

Hocquenghem) outer with LDPC (Low-Density Parity-Check) inner coding. The result is a

performance which is only 0.7 dB to 1.2 dB from the Shannon limit

[MORELLO]. The FEC

parameter options depend on the system requirements, rather with adaption modes VCM and

ACM, the FEC code rates can be changed dynamically.

This Shannon Bound is given by 1 / log

e (this limit is approximatively -1.6 dB)

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Radyne OM20 User Manual Page 38