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Explanation of PCMA and CRMA

PCMA means Paired Carrier Multiple Access.

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.

Read more: satsig forum reference to Comtech "carrier in carrier" operations. This technique is applicable when both earth stations, at either end of the point to point link, are in the same satellite beam coverage area.

CRMA means Code Reuse Multiple Access.

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.


Amended 25 Feb 2015