M.1.1 Background; Theory Of Operation; M.2.1 About Dpc - Comtec CDM-840 Installation And Operation Manual

CDM-840 Remote Router
Appendix M

M.1.1 Background

Satellite networks are subject to uplink/downlink degradations due to rain fade or other
environmental factors. To ensure target link availability, network designers must take into
account the rain region on both sides of a link, and design the terminals and operating point to
include the appropriate link margins.
Satellite network operators typically lease Power Equivalent Bandwidth (PEB) from satellite
owners. In clear sky operation, the terminals on both sides of the link transmit with RF power on
the ground as a means to employ the PEB of the satellite that is assigned to that link. The
presence of rain or other environmental attenuation lowers the RF power received at the
satellite relative to clear sky conditions.
In principle, terminals with sufficient RF power could increase their transmit power during rain
loss conditions, to compensate for the loss and maintain the PEB on the satellite. However, rain
fade may occur at the uplink or downlink location, which presents an ambiguity for each
transmitting modem. If power is controlled solely on receiving quality and not power-limited at
the transmitter, the satellite PEB may be exceeded – oversubscribing service level agreements
or, worse, resulting in adjacent carrier interference.
Such oversubscription is common in traditional uplink power control technology. Modems, in
general, can automatically compensate for rain attenuation, but with power constraints and set
limits not to exceed site margins. Running uplink power at margin, however, tends to flat-line
power availability. If the modems are fed sufficient information to determine which side of the
link is being impacted by rain fade, the power may exceed margin limits without compromising
PEB, whereby the terminal knows the contribution of attenuation on the downlink and the
remainder of uplink attenuation is between the transmitting terminal and the satellite. The
additional information is obtained by monitoring the hub's outbound transmission and taking
the clear sky quality reading as a zero reference, then disseminating the offset to all listening
remotes.
M.2

Theory of Operation

M.2.1 About DPC

DPC is divided into two segments: return path remote to hub/forward path hub to remote.
On the return path, power adjustments can interoperate with Adaptive Coding and Modulation
(ACM) by combining modulator output power adjustments with ACM step points. This exchange
allows the transmit power level to try and compensate for fade before it is necessary to
decrease data link capacity. The fade reporting adjustment is maintained by Adaptive Control
Loop (ACL), which reports receiving E
segments. The ACL continuously updates the link conditions for each active receiving
demodulator.
/N using messages over the LAN/WAN data transmission
S 0
M–2
MN-CDM840
Revision 3