Comtech EF Data CDM-600/600L - Manual (Page 127)

CDM-600/600L Open Network Satellite Modem Revision 3
Forward Error Correction Options MN/CDM600L.IOM
6–7
cheap, plentiful and fast, an LDPC decoder can easily be accommodated in a large Field
Programmable Gate Array (FPGA) device. Further interest in LDPC was stimulated in 2003,
when the Digital Video Broadcasting (DVB) 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.
E
b
/N
o
characteristic, and for this reason it is desirable to concatenate a short BCH code with
LDPC. This concatenation produces almost vertical BER vs. E
b
/N
o
curves, as can be seen in the
performance graphs that are presented later in this chapter.
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 E
b
/N
o
than 8-PSK. A
discussion of this approach follows.
Comtech EF Data chose the CDM-600 platform as the first satellite modem in which to
implement both LDPC and 8-QAM, followed by the CDM-600L.
6.7.2 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. E
b
/N
o
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 to
1.5 dB) to the Shannon Limit. However, at 3/4 and below, LDPCs are performing 0.7 to 1.2 dB
better than TPCs: