CDM-625 Advanced Satellite Modem
Revision 1
Forward Error Correction Options
MN-0000036 (Ref MN/CDM625.IOM)
committee adopted LDPC codes (proposed by Hughes Network Systems) as the basis for the new
DVB-S2 standard.
The LDPC method on its own produces an undesirable ‘flaring’ in the Bit Error Rate (BER)
vs.
Eb/No characteristic, and for this reason it is desirable to concatenate a short BCH code with
LDPC. This concatenation produces almost vertical BER
vs.
Eb/No curves, as can be seen in the
performance graphs that are presented later.
In order to take full advantage of the coding gain increase that LDPC provides, it became
necessary to find an alternative to 8-PSK. Comtech EF Data has therefore developed an 8-QAM
approach that permits acquisition and tracking at much lower values of Eb/No than 8-PSK.
Comtech’s implementation of 8-QAM is the subject of a US patent, granted in 2007.
Comtech EF Data chose the CDM-600 platform as the first satellite modem in which to
implement both LDPC and 8-QAM, and the CDM-625 includes a newer technology version of
the original design.
7.6.2 LDPC versus TPC
So, is LDPC better than TPC? The answer must be ‘sometimes, but not always’, and there are
issues, such as latency, that must be taken into consideration. The graph shown below illustrates
the performance of various TPC and LDPC modes relative to the Shannon Limit - the Channel
Capacity is shown for both QPSK and 8-PSK. Error free transmission is not possible for values of
spectral efficiency (capacity)
vs
. Eb/No above these limit curves. The horizontal distance to the
limit provides a metric of overall performance.
It can be seen from this graph that for Code Rates above 3/4, Comtech’s TPCs are very close (1 - 1.5
dB) to the Shannon Limit. However, at 3/4 and below, LDPCs are performing 0.7 - 1.2 dB better than
TPCs.
7–6
