Comtech EF Data SLM-5650A - Manual (Page 403)

SLM-5650A Satellite Modem Revision 8
Appendix D MN-SLM5650A
D–9
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 8PSK. Comtech EF
Data has therefore developed an 8QAM approach that permits acquisition and tracking at much
lower values of Eb/No than 8PSK. Comtech’s implementation of 8QAM is the subject of a U.S.
Patent, granted in 2007.
D.8.2 LDPC versus TPC
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. 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.
Figure D-1 graphs the performance of various TPC and LDPC modes relative to the Shannon
Limit – the Channel Capacity is shown for both QPSK and 8PSK. 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. It is clear that, in
order to provide the best possible performance over the range of code rates from 1/2 to 0.95, both
an LDPC and a TPC codec need to be offered.
Figure D-1. TPC & LDPC Modes Performance (Relative to Shannon Limit)
In order to provide a robust set of LDPC codes with varying degrees of latency, the SLM-5650A
has implemented three different block sizes:
The Ultra Low Latency (ULL) code provides the lowest latency, but worst performance of the
three block sizes. This code is available in the data rate range from 32 kbps to 2 Mbps and is
compatible with the CDM-625 ULL code.
The Low Latency (LL) code provides modest latency and performance over the data rate range
of 32 kbps to 5 Mbps.