Q-Lite Satellite Modem Installation and Operating Handbook
6-89
6.2.11
ClearLinQ™ Tx Adaptive Predistorter
Screens
ClearLinQ™
is a Tx adaptive predistorter
that corrects for non-linear (and to a lesser extent
linear) distortion in the RF chain. It is applicable to all FECs and modulations including
DVB-S2X, FastLink™ and TPC (with the exception of the
low-cost
DVB-S2 option).
The predistorter compensates for non-linear AM-AM, AM-PM and memory effects
associated with an up-link HPA or satellite transponder (or both). The compensation
characteristic is acquired via a closed loop with the local receive demodulator. Transmit
charactersitsics are tuned adaptively in order to minimise the receive constellation error
(Error Vector Magnitude (EVM)). This avoids the need to input into the modem the AM-
AM or AM-PM characteristics associated with the non-linearity but does mean that the
modem must go through a one-time ‘learning’ process with respect to the particular
operational conditions. If the RF operating conditions change in any significant way
(including any change to the modulation type, Tx power level, etc.) the learning process
must be repeated. Once learning is completed then it is necessary only to switch
predistortion on. 1dB to 2dB of compensation for distortion is achievable.
The predistorter can only compensate for distortions in the signal that it is responsible for
generating. For this reason, this feature can only be used if the HPA is being used to
generate a single carrier. The transponder itself can be multi-carrier if the compensation
is aimed at removing just the effects of the HPA. Additional compensation for distortion in
the transponder is recommended only for single-carrier-per-transponder usage. The
learning process should therefore involve either a short loop around the HPA or a long
loop around the transponder to reflect which of these two types of compensation is
required.
Figure 6-40
shows the
Control
screen for predistortion. This initially has just an
Enable
button. This is used to control whether Tx adaptive predistortion is active in the modem or
not.
When enabled, the
Adapt
button becomes visible. With Tx looped to Rx through the HPA
(and potentially the transponder as well), the
Adapt
button should be pressed in order to
go through the one-time learning process that determines the required compensation
characteristics.
Figure 6-40 Edit->Tx Predistorter Screen
During the learning process a screen similar to
Figure 6-41
will be displayed.
