Comtech EF Data CDM-625A Installation And Operation Manual page 647

CDM-625A Advanced Satellite Modem
Appendix M
Existing Satellite ACM Scheme
M.4 An
The best-known scheme for ACM-over-Satellite is covered in the DVB-S2 specification (EN 302307)
ratified by ETSI in March 2005. While primarily for digital video broadcast, the DVB-S2
specification also encompasses two-way interactive services.
A summary of the main transmission aspects follows:
•
Four modulation types are defined: QPSK, 8PSK, 16APSK and 32APSK.
• The primary FEC type is low-density parity check coding (LDPC), concatenated with a
short BCH code.
•
The 8PSK, 16APSK and 32APSK modulation types use interleaving.
•
There are ten code rates defined: 1/4, 1/3, 2/5, 1/2, 3/5, 2/3, 3/4, 4/5, 5/6, 8/9 and 9/10
which depend on the modulation type and other system requirements.
•
A single FEC block may be 64800 bits (normal, referred to as 64k blocks) or 16200 bits
(short, referred to as 16k blocks)
•
Adaptive coding and modulation is defined for optimizing satellite transponder capacity.
Hughes Network Systems (Germantown, MD) have commercially deployed DVB-S2 with ACM
over satellite.
M.5 Disadvantages of DVB-S2
While the scheme defined by DVB-S2 is undoubtedly very effective for many broadcast and
higher data rate applications, it is definitely not a 'one size fits all' solution. Some of the
disadvantages are as follows:
•
Excessive latency. The so-called short blocks are too long for low latency IP applications
at low data rates. This is exacerbated by the addition of interleaving.
• Overly complex in its implementation. The design of DVB-S2 dictated that all FEC blocks
should be constant in bits. This means that for each ModCod, there are a varying
number of symbols. This then makes the task of synchronization a much more
demanding task. Also, because of the limitations of tracking the higher-order
modulations in a very low SNR environment, so-called pilot symbols were added in
order to aid tracking.
•
Concatenated BCH code added to mitigate the problem of error rate 'flaring' and
'flooring'. This is no longer necessary. Since the introduction of the original LDPC/BCH
scheme an enormous amount of research has been done on the design of LDPC codes.
M–5
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