1 closed network modes, 4 trellis coding – Mocomtech CDM-QX User Manual

Page 169

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CDM-Qx/QxL Multi-Channel Satellite Modem with DoubleTalk™ Carrier-in-Carrier® Revision

7

Forward Error Correction Options

MN/CDMQX.IOM

7–3

7.3.1

Closed Network Modes

A 220,200 code is used in transparent closed network modes, and a 200,180 code is used in framed
(EDMAC) modes. (220,200 means that data is put into blocks of 220 bytes, of which 200 bytes are
data, and 20 bytes are FEC overhead.) These two codes were chosen because they fit well into
Comtech EF Data’s clock generation scheme, and they have almost identical coding gain.

When Viterbi decoding is used as the primary FEC, an interleaver depth of 4 is used. The
increase in coding gain is at the expense of delay. The interleaving/de-interleaving delay and the
delay through the decoder itself can be as high as 25 kbps. At very low data rates, this equates to
several seconds, making it highly unsuitable for voice applications. Additionally, the de-
interleaver frame synchronization method can add significantly to the time taken for the
demodulator to declare acquisition.

A characteristic of concatenated Reed-Solomon coding is the very pronounced threshold effect. For
any given modem design, there will be a threshold value of E

b

/N

o

below which the demodulator

cannot stay synchronized. This may be due to the carrier-recovery circuits, or the synchronization
threshold of the primary FEC device, or both. In the CDM-Qx/QxL, and Rate 1/2 operation, this
threshold is around 4 dB E

b

/N

o

. Below this value, operation is not possible but, above this value, the

error performance of the concatenated Reed-Solomon system produces exceptionally low error rates
for a very small increase in E

b

/N

o

.

CAUTION

Care should be taken not to operate the demodulator near its sync threshold.
Small fluctuations in E

b

/N

o

may cause total loss of the link, with the subsequent

need for the demodulator to re-acquire the signal.

Table 6-2. Concatenated Reed-Solomon Coding Summary

FOR

AGAINST

Exceptionally good BER performance – several
orders of magnitude improvement in link BER under
given link conditions.

Very pronounced threshold effect – does not fail
gracefully in poor E

b

/N

o

conditions. Additional

coding overhead actually degrades sync threshold,
and reduces link fade margin.

Very small additional bandwidth expansion

Significant processing delay (~25 kbps) – not good
for voice, or IP applications

Adds to demod acquisition time.

7.4

Trellis Coding

In the other FEC methods described here, the processes of coding and modulation are
independent – the FEC codec has no knowledge of, or interaction with, the modulator. However,
there are schemes in which the coding and modulation are combined together where the encoder
places FEC symbols in a precise manner into the signal constellation. This can yield an overall
improvement in performance, and is used in higher-order modulation schemes, such as 8-PSK,
16-PSK, 16-QAM, etc.

When convolution coding is used, the overall coded modulation approach is referred to as
Trellis Coded Modulation (TCM). Ungerboeck was an early pioneer, and developed optimum
mapping and decoding schemes. However, the decoding scheme was seen as complex and
expensive, and Qualcomm Inc. developed a variation on the theme which uses a Viterbi
decoder at the core, surrounded by adjunct processing. The scheme is able to achieve
performance very close to the optimum Ungerboeck method but with far less complexity, and is
called pragmatic Trellis Coded Modulation.

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