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Explanation of PCMA and CRMA |
This technique allows the transponder bandwidth to be reused by superimposing
carriers on top of each other. This has the potential for major
savings in space segment rental costs. It is possible when, in the
system design, the fractional transponder bandwidth resource is being used up
first, before the power resource is fully utilised. This can occur
in networks where not all earth stations are of the minimum possible size, from
the point of view of receive G/T. In such cases, with modern high
sensitivity/high power satellites, it is difficult to simultaneously and
efficiently exploit the satellite power and bandwidth resources. The
tendency is to run out of bandwidth and resort to abnormally high power spectral
density carriers such as 8 PSK or 16 QAM or similar. In PCMA, an
earth station knows what it is transmitting and by suitably delaying and then
subtracting a weak version of the transmitted signal from the received signal
the 'interference' arising from carrier superimposition is eliminated.
This technique involves many earth stations transmitting the same kind of
signal bursts on the same frequency, but all starting at different times, so
avoiding interference with each other. For example, to transmit a
binary 1, an earth station transmits a carefully predefined burst and to
transmit a binary 0, transmits a same length but completely different carefully
predefined burst.
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