Carrier-In-Carrier Automatic Power Control (Cnc-Apc; Overview - Comtech EF Data CDM-570A Installation And Operation Manual

CDM-570A/570AL Satellite Modem with Optional Packet Processor
Appendix G
G.5.2.3 Use a Spectrum Analyzer to Estimate PSD Ratio
You can also estimate PSD Ratio or CnC Ratio by using a
Spectrum Analyzer capable of integrating the signal power in a
given bandwidth.
•
CnC Ratio (in dB) = Power
• PSD Ratio (in dB) = (Power
= CnC Ratio – 10 log (BW
If the two carriers have same Symbol Rate / Bandwidth, then the CnC Ratio is same as the PSD
Ratio.
G.6

Carrier-in-Carrier Automatic Power Control (CnC-APC)

G.6.1

Overview

A number of Comtech EF Data modems, including the CDM-570A/AL, 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.
(in dBm) – Power (in dBm)
C1 C2
– 10 log BW (in Hz)) – (Power
C1 C1
/ BW )
C1 C2
G–19
MN-CDM570A
– 10 log BW (in Hz))
C2 C2
Revision 3