Digital Signal Processing Reference
In-Depth Information
shown in the figures for the respective constellation diagrams (QPSK ...
32APSK). Compared with DVB-S, the minimum C/N ratio necessary in
DVB-S2 is much more dependent on the modulation method and can also
be varied by the code rate.
Q
10
quadrant
00
quadrant
1
1
LP bit
0
0
I
HP bit
0
0
01
quadrant
11
quadrant
1
1
Fig. 14.30.
Hierarchical QPSK modulation
Some rough comparisons from the DVB-S2 draft standard are given be-
low:
Table 14.1.
Minimum C/N ratio necessary in DVB-S and DVB-S2
Modulation method
min. required C/N [dB]
DVB-S QPSK
appr. 3…7.5
DVB-S2 QPSK
appr. -2.4…6.5
DVB-S2 8PSK
appr. 5.5…11
DVB-S2 16APSK
appr. 9…13.1
DVB-S2 32APSK
appr. 12.7…15.6
Unlike DVB-S, DVB-S2 has a frame structure. There is an FEC frame
and a physical layer frame. An FEC frame firstly contains the data to be
transmitted which are either data which have an MPEG-2 transport stream
structure or data which are quite independent of this, so-called generic
data. This data field is followed by an 80 bit long baseband header. The
data block with the baseband header is then padded to a length dependent
on the selected code rate of the error protection and then provided with the
BCH code plus the LDPC code. Depending on the mode, an FEC frame
then has a length of 64800 or 16200 bits. The FEC frame is then divided
into a physical layer frame composed of n slots. The physical layer frame
starts with the one-slot-long physical layer header in which the carrier is
/2 shift BPSK modulated. This is followed by slot 1 ... slot 16. Slot 17
may be a pilot block if pilots are transmitted (optional). This is followed
Search WWH ::
Custom Search