Comtech EF Data CDM-625 - Manual (Page 350)

CDM-625 Advanced Satellite Modem Revision 15
DoubleTalk Carrier-in-Carrier Option MN-CDM625
10–20
10.6 Carrier-in-Carrier Automatic Power Control (CnC-APC)
10.6.1 About CnC-APC
A number of Comtech EF Data modems, including the CDM-625, offer Automatic Uplink Power
Control (AUPC) to mitigate the effects of rain fading and other link impairments.
It is important to note that the simple system employed in AUPC (whereby distant-end Eb/No is
monitored, and local power is increased when a degraded link is detected) has an undesirable
characteristic, which leads to some operators not permitting its use on their transponders: The
inability of ‘classic’ AUPC to determine at which side of link the fade (or other impairment) has
occurred. More specifically:
• If the fade is at the local side, all is well – the drop in Eb/No at the distant site corresponds
exactly to the drop in power (due to excess attenuation) on the local uplink; when uplink
power is increased, the power at the transponder does not exceed its clear-sky value.
• Conversely, if the fade occurs at the distant side, the AUPC system increases power in
exactly the same way – but now, there is no excess uplink attenuation and, as a result, the
clear-sky power at the satellite will be exceeded.
In a transponder with many carriers using AUPC, a fade event at the ‘wrong’ side can cause
many carriers to simultaneously increase power beyond their authorized maximums, leading to
intermod problems, adjacent channel interference (ACI) issues, and in some extreme cases,
saturation of the transponder.
When considering power control in the Carrier-in-Carrier case, not only must the issue of
exceeding allocated power limits be respected, but the problem of driving CnC ratios outside
working limits during fading must also be taken into account.
10.6.2 AUPC and Carrier-in-Carrier in the CDM-625
The CDM-625 currently permits ‘classic’ AUPC when Carrier-in-Carrier mode is in operation, but
this does nothing to stop the problem of exceeding power limits when the fade is on the ‘wrong’
side. To limit the impact of this, you are constrained to 3 dB of permitted power increase.
Depending on the satellite band, and the depth and rapidity of the fade, this constraint may
curtail the effectiveness of the system.
10.6.3 The CnC Automatic Power Control Algorithm
In addressing the shortcomings of ‘classic’ AUPC, from its studies of the unique problems of
power control in CnC systems, Comtech EF Data has determined that there is sufficient
information available (CnC ratio, power level, Eb/No, RSL, etc.) on the local and distant sides to
control power at each end without exceeding the total composite power allocated to each
carrier in the CnC pair. Furthermore, the power control algorithm developed by Comtech EF
Data ensures that the CnC ratio remains within the correct working range.