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Quantum and Evolution Series Installation and Operating Handbook
The steps involved in using the predistorter are as follows:
1. Configure the modem in a closed-loop setup as described above.
2. Ensure the modem is in 16 QAM mode for both Tx and Rx.
3. Configure the modem Tx output power to the required level in the Edit/Tx-
Rx/Common tab.
It is recommended this level should not exceed -10dBm as some power headroom is
needed for the predistorter to apply its expanding nonlinear characteristic.
4. In the Edit/Tx-Rx/Common/Predistorter tab, select the predistorter algorithm adaption
method.
There are two choices: the algorithm can adapt and automatically stop when
convergence is detected or, alternatively, the user can program the algorithm to
iterate a specific number of times. Both methods work well. However, best
performance is achieved if the algorithm is allowed to run for several hundred
iterations.
5. Pressing the
Start
This starts with a special undistorted 16QAM constellation being transmitted into the
channel. The signal is used to sound the link for the presence of spectral inversions
and receive constellation phase ambiguities. Note that while this constellation is
button will initiate the adaption process.
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The predistorter is adaptive, i.e. no prior
knowledge of the nonlinearity is required.
The transmit constellation is adaptively
tuned to optimize the Rx constellation. The
predistorter operates in a closed loop that
contains the nonlinear amplifier(s) to be
compensated. For example, the loop could
be closed around the up-link HPA by
feeding the down-converted HPA signal
back into the modem's demodulator.
Alternatively, for some applications, it may
be feasible to simply demodulate the
received satellite signal, thereby allowing
ground and space segment nonlinearities to
be out-tuned. Since the predistorter only has
visibility of its own transmit and receive
signals, it is best suited to single channel
per HPA or single channel per transponder
applications.
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