Mocomtech CDM-QX - Manual (Page 170)

CDM-Qx/QxL Multi-Channel Satellite Modem with DoubleTalk™ Carrier-in-Carrier® Revision 7
Forward Error Correction Options MN/CDMQX.IOM
7–4
As more and more high power transponders are put in to service, Intelsat recognized that the
transponders are no longer power limited, but bandwidth limited. In order to maximize
transponder capacity, 8-PSK was looked at as a method of reducing the occupied bandwidth of a
carrier, and adopted Qualcomm’s pragmatic TCM, at Rate 2/3.
A Rate 2/3 8-PSK/TCM carrier occupies only 50% of the bandwidth of a Rate 1/2 QPSK carrier;
however, the overall coding gain of the scheme is not adequate by itself; accordingly, Intelsat’s
IESS-310 specification requires that the scheme be concatenated with an outer R-S codec. When
combined, there is a threshold value of Eb/No of around 6 dB, and above approximately 7 dB, the
bit error rate is better than 1 x 10
-8
.
The detractions of the concatenated R-S approach apply here also, along with more stringent
requirements for phase noise and group delay distortion – the natural consequences of the higher-
order modulation. The modem fully implements the FEC, but not the framing of the IESS-310
specification at data rates up to 18 Mbps.
In accordance with the specification, the R-S outer code can be disabled. Performance curves for
both cases are shown in the figures provided at the end of this chapter.
Table 6-3. 8-PSK/TCM Coding Summary
FOR AGAINST
Exceptionally bandwidth efficient compared
to QPSK Needs concatenated Reed-Solomon outer codec to give
acceptable coding gain performance
Demod acquisition threshold much higher than for QPSK
8-PSK is more sensitive to phase noise and group delay
distortion than QPSK
7.5 Turbo Product Codec (Hardware Option)
7.5.1 Introduction
Turbo Coding is an FEC technique developed within the last few
years that delivers significant performance improvements, as
compared to more traditional techniques. Two general classes of
Turbo Codes have been developed: Turbo Convolutional Codes (TCC) and Turbo Product Codes
(TPC, a block coding technique).
Comtech EF Data has chosen to implement an FEC codec based on TPC. A Turbo Product Code is
a 2- or 3-dimensional array of block codes. Encoding is relatively straightforward, but decoding is a
very complex process requiring multiple iterations of processing for maximum performance to be
achieved.
Unlike the popular method of concatenating a Reed-Solomon codec with a primary FEC codec,
Turbo Product Coding is an entirely stand-alone method. It does not require the complex
interleaving/de-interleaving of the R-S approach and, consequently, decoding delays are
significantly reduced. Furthermore, the traditional concatenated R-S schemes exhibit a very
pronounced threshold effect – a small reduction in Eb/No can result in total loss of demod and
decoder synchronization. TPC does not suffer from this problem – the demod and decoder remain
synchronized down to the point where the output error rate becomes unusable. This is considered to
be a particularly advantageous characteristic in a fading environment. Typically, in QPSK, 8-PSK