Comtech EF Data CDM-625A - Manual (Page 685)

CDM-625A Advanced Satellite Modem
Appendix M
M–5
M.4 An Existing Satellite ACM Scheme
The best-known scheme for ACM-over-Satellite is addressed in the DVB-S2 specification (EN 302307)
ratified by ETSI in March 2005. Hughes Network Systems (Germantown, MD) has commercially
deployed DVB-S2 with ACM-over-Satellite.
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.
M.5 Disadvantages of DVB-S2
While the scheme defined by DVB-S2 is very effective for many broadcast and higher data rate
applications, it is by no means 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. Most
importantly, however, LDPC codes can now be designed that yield almost equivalent coding
gain, but with considerably shorter block lengths.