Why Does The Hub Canceller Es/No Seem Unstable - Teledyne QMultiFlex-400 Installation And Operating Handbook

QMultiFlex-400™ Installation and Operating Handbook
Figure 8-23 – Cumulative Doppler Shift of Intelsat-12 As Seen from London
Figure 8-22 shows the delay Doppler varying sinusoidally over the course of 24 hours
(86400 seconds), as does the frequency offset Doppler in Figure 8-23. However, when the
rate of change of the RTD Doppler is at a maximum in Figure 22 at t=0, the rate of change
of the frequency offset Doppler is zero. So, the maximum and minimum rates of change of
RTD and frequency offset Doppler occur 4 times a day spaced about 6 hours apart.
Provided the satellite orbit inclination is less than ±2.7°, the canceller will be able to maintain
tracking. Thankfully, very few satellites have inclinations so large and the canceller will work
reliably with most.
3. The Peak to Average Power Ratio (PAPR) of the composite carrier is too high for the
satellite communications system to cope with. When two signals are combined together,
as in the case with the Hub Canceller, the PAPR of the composite signal is higher compared
to the PAPR of just one of the signals making up the carriers. For example, if you have a
16APSK signal with 5% roll off, the PAPR of the waveform might be say 7dB. If two such
waveforms are combined to make an overlapped carrier pair, then the average power will
increase by 3dB, but the sum of the peaks will increase by 6dB: a net 3dB increase in
PAPR. Unless the satellite communications system has enough dynamic range to pass
such signals, clipping or saturation on the peaks of the combined waveform may occur
which could in turn compromise the suppression performance or tracking reliability of the
canceller. In such circumstances, it might be beneficial to apply more back-off to amplifiers
to prevent saturation or, alternatively, select a higher roll-off factor for the signals, which
will reduce their individual and combined PAPR.

8.3.6.4 Why does the Hub Canceller Es/No Seem Unstable?

If you are operating at low symbol rates, e.g. below 512kBaud, the tracking bandwidth of
the canceller becomes narrow. As a consequence, it becomes less easy for the canceller
to track large phase noise variations in the received echo, if they exist. Elevated levels of
phase noise sometimes occur in satellite communications links comprising low cost
8-23