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QMultiFlex-400™ Installation and Operating Handbook
downlink LNB's or uplink BUC components. If the canceller cannot adequately track the
phase noise, the echo suppression performance of the canceller may become
compromised, resulting in residual noise being passed on by the canceller to the
downstream demodulator. This in turn can cause the received Es/No to become less stable.
To mitigate this potential problem, it is advised that narrowband satellite communications
links (i.e., less than 0.5MBaud) incorporating a canceller should be equipped with a low
phase noise LNB in the downlink and a low phase noise BUC in the uplink. In particular,
the use of LNB's and BUC's, with the ability to lock their internal local oscillators to an
external reference supplied by the modem or some other low phase noise reference signal,
is highly recommended, not only from a phase noise standpoint but also from a carrier
frequency accuracy standpoint. Supplying LNB's and BUC's with external references
ensures the phase noise associated with the uplink and downlink signals is minimised and
reduces the phase noise tracking burden that would otherwise be placed on the canceller.
8.3.6.5 Why is the Es/No Worse when the Hub Canceller is enabled?
Most of the time, the Hub Canceller will provide robust operation and present only a small
system degradation, in the order of 0.1 - 0.2dB.
However, signals of higher order modulations, e.g., 16APSK, 32APSK and so on, need to
operate with higher signal to noise ratios in order to achieve error free link performance. As
the operational signal to noise ratio increases, the residual noise exiting the canceller, due
to its finite suppression performance, forms a higher proportion of the net noise passed on
by it to the demodulator. Consequently, at higher SNR's, the canceller degradation will most
likely increase.
There is a second potential cause of excessive degradation when the Hub Canceller is
enabled. The Hub Canceller is essentially an echo canceller. Here, a reference copy of the
uplink signal is suitably delayed, frequency, phase and amplitude aligned with the received
unwanted echo to suppress it and reveal the signal of interest from the remote earth-
stations. The amount of echo suppression - and hence canceller degradation - depends on
how well the local reference correlates with the received echo. If, for whatever reason, the
echo does not correlate very well with the local reference, then the suppression
performance of the canceller will be compromised causing degradation to the received
signal of interest. For example, if the communications link is operating non-linearly (e.g.,
one or more amplifiers in the communications link are being driven into saturation or
clipping), then the received echo will be subjected to non-linear distortion for which the
canceller is incapable of resolving: the canceller is a linear system. Hence, significant
canceller degradation can arise if the satellite communications link is working non-linearly.
8.3.6.6 RTD Optimisation
If the Hub Canceller is locked, the modem will periodically log the canceller delay as a
function of time. The RTD is displayed graphically in View > Graphs > PCMA > PCMA
Delay. Observe how the RTD has varied over the course of several days. Note the
maximum and minimum RTD and use these values to refine the acquisition delay sweep
specification of the canceller in menu Edit > Paired Carrier. Refining the canceller's
acquisition delay sweep will help minimise acquisition/re-acquisition times.
8.3.6.7 Hub Canceller vs No. of Rx channels
All QMultiFlex-400™ hub modems are fitted with hardware to support:
A) 14 Rx channels with a hub-canceller enabled or
B) 16 Rx channels (an extra two Rx channels) without the hub-canceller being enabled.
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