Premier Mounts CDM-600 specifications Ldpc versus TPC

CDM-600 Satellite Modem	Revision 7
Forward Error Correction Options	MN/CDM600.IOM

it remained clear that the Shannon Limit was still an elusive target. Berrou’s work on Turbo Codes showed, through the use of an ingeniously simple approach (multiple, or iterative decoding passes) that it is possible to achieve performance close to the Shannon Limit. Berrou’s early work dealt exclusively with iteratively-decoded convolutional codes (Turbo Convolutional Coding, or TCC), but in time the iterative approach was applied to a particular class of block codes called Product Codes - hence Turbo Product Coding (TPC). TPC 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, on a commercial basis, 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, namely Low Density Parity Check Codes (LDPC). It 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, and for decades it remained of purely academic interest. 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. 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. 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. A discussion of this approach follows.

Comtech EF Data has chosen the CDM-600 platform as the first satellite modem in which to implement both LDPC and 8-QAM.

7.7.2LDPC 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.

7–7