Table of Contents
Advertisement
QMultiFlex-400™ Installation and Operating Handbook
Similarly, the IB carriers are viewed as being white noise contributors and therefore these
interfere with the OB carrier and will degrade each remote Modem's Rx Es/No. i.e. the
remotes will see the OB carrier plus satellite channel system noise, plus self-interference
from their own carriers.
If, for example the degradation to the Remote Rx Es/No is 2dB's, then the OB carrier power
can be increased (if the system is not power limited) to mitigate this additional IB Noise. If
the system is power limited, then it is not possible to increase the hub OB carrier power by
the 2dB in the example above to mitigate the effects of the self-interference by the remotes
to their own Tx carriers. In such circumstances, 2dB must be taken off the threshold C/N
for the OB carrier and if necessary, the MODCOD of the OB carrier de-rated to compensate.
i.e. the OB carrier MODCOD would need to be chosen with a threshold 2dB's lower than
the current QEF Es/No.
For example, if 16APSK-L 2/3 is used for the outbound carrier, which has an QEF Es/No
of 8.6dB, reducing this by 2dB would put the required QEF Es/No at around 6.6dB Es/No.
Therefore, candidates would be:
1.
16APSK 1/2-L having a QEF Es/No of 6.3dB.
2.
8PSK 23/36 having a QEF Es/No of 6.4dB.
In this example, a de-rated OB MODCOD of 16APSK 1/2-L would have a higher spectral
efficiency, than the 8PSK option. However, keeping the symbol rate of the carrier
unchanged would drop the OB carrier bit rate!
8.3.5
Summary
1. The hub canceller operates very well in a linear system. In various scenarios, it added
only 0.1-0.2dB of system degradation when enabled. This small degradation is caused
by the finite suppression of the received hub echo signal.
2. In systems that are not power limited, the power level of the hub OB carrier can be
increased as a countermeasure to the degradation caused by the presence of the IB
carriers under the OB.
3. In systems that are power limited, the hub OB carrier MODCOD can be de-rated to
ensure link integrity for a lower SNR received at the remote modems. This obviously
has a knock-on effect to both the OB carrier spectral efficiency and hence also its data
throughput which may or may not be an issue. In the scenario given in this document,
the OB carrier was de-rated from 16APSK-L 2/3 (QEF Es/No = 8.6dB, spectral
efficiency = 2.635236b/s/Hz) to 16APSK-1/2L (QEF Es/No = 6.3dB, spectral efficiency
= 1.972253b/s/Hz), a reduction in threshold of (8.6-6.3=) 2.3dB, an amount roughly
equivalent to the degradation by the IB carrier self-interference at the remote modem
Rx input ports.
4.
The system should always be operated under linear conditions. Consequently, the
suppression performance of the hub canceller will be at its maximum. However, if the
hub uplink-downlink loop – which is relied upon by the canceller - operates non-linearly,
then the suppression performance of the canceller may be compromised, as illustrated
in the figure below. The suppression performance will be limited to the spectral
regrowth level of the OB carrier when passed through the uplink-downlink loop. There
is justification in ensuring that the satellite channel and the hub uplink HPA operate as
linearly as possible in these applications.
8-18
Advertisement
Table of Contents
