Comtech EF Data DMD-2050E - Manual (Page 94)

DMD2050E Universal Satellite Modem Theory of Operation
MN-DMD2050E Revision 2 3–52
exhibits inherently low decoding latency compared with TCC, and so is considered much more
desirable for 2-way, interactive satellite communications applications.
In August 1999, Comtech became the first company in the world to offer satellite modems that
incorporate TPC. Since its inception, Comtech has continued to develop and refine its
implementation of TPC in its products, and now offers a comprehensive range of code rates (from
Rate 5/16 to Rate 0.95) and modulations (from BPSK to 16-QAM). However, in the past few
years, as part of the general interest in Turbo coding, a third class of Turbo coding has emerged:
Low Density Parity Check Codes (LDPC).
LDPC is more like TPC than TCC in that it is an iteratively-decoded block code. Gallager first
suggested this in 1962 but, at the time, the implementation complexity was considered to be too
great; for decades, it remained of purely academic interest. 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. Now,
however, with silicon gates being cheap, plentiful and fast, an LDPC decoder can easily be
accommodated in a large Field Programmable Gate Array (FPGA) device.
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 U.S. 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 DMD2050E includes a newer technology version of the original
design.
3.12.1 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. Figure 3-15 graphs 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.