Comtech EF Data CDM-550T, Manual

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CDM-550T Satellite Modem Forward Error Correction Options
Rev. 1.3 7–5
OQPSK operation. These are excellent choices for applications where maximum
bandwidth efficiency is desired, in combination with high coding gain. However, with
Firmware Version 1.15 (and later) Comtech EF Data has introduced two further code
rates - Rate 21/44 BPSK (very close to Rate 1/2) and Rate 5/16 BPSK (very close to Rate
1/3). These two rates were developed to address an entirely different case, namely that of
transmission from very small antennas, with limited transmitter power. For a dish
antenna, the gain is directly proportional to its area, and the lower the gain, the less
directional the antenna becomes. Thus, in satellite transmission, even though the dish
may be perfectly pointed at the desired satellite, if the beamwidth is wide enough,
adjacent satellites in the orbital arc will also be illuminated. This is a potential source of
interference, and for this reason the ITU (International Telecommunications Union) place
strict limits on the power spectral density (also referred to as flux density) of signals
arriving at adjacent satellites.
One obvious method to reduce the level is to spread the transmitted signal over as wide a
bandwidth as possible. In the past, this has sometimes been achieved using a Spread
Spectrum modulation format, but this has severe penalties as far as demodulator
complexity is concerned. However, by using BPSK modulation, and high FEC code rates
(up to Rate 1/3, for example) the power spectral density may be reduced. Taking Rate 1/2
QPSK as a baseline, moving to Rate 5/16 BPSK Turbo Product Coding gives an
immediate reduction in power spectral density of 5.0 dB.
Furthermore, the increased coding gain of this FEC method allows a further reduction in
transmitter power. Using Rate 1/2 Viterbi with concatenated Reed-Solomon as a baseline
example, Rate 5/16 provides 1.5 - 2.0 dB improvement in coding gain. Putting these two
factors together yields an overall reduction in power spectral density of approximately
7.0 dB. This simultaneously permits a smaller antenna, and reduced transmitter power.
The disadvantage is the increased spectral occupancy of the carrier, and it will depend on
the particular satellite operator to determine if this poses a severe economic problem.
When operating at these higher code rates (21/44 and 5/16), the demodulator is forced to
operate in a region where the Ebt/No (also referred to as Es/No) is negative - in other
words, there is more noise than signal. For this reason it is more difficult for the
demodulator to accurately measure the Eb/No, and so users may see more variation than
normal when viewing the Eb/No on the RX PARAMETERS screen, and when operating
near threshold. This is most pronounced in the case of Rate 5/16.
One unexpected advantage of the Turbo Product Coding method is that, unlike
concatenated Reed-Solomon methods, there is no pronounced threshold effect. The
demodulator and FEC codec can stay locked down to very low values of Eb/No, but at
this level the output BER becomes unusable (around 1 x 10-2).