Table C-1Dynamic Power Control Parameters

Parameter

Default

Significance

 

 

 

 

 

 

Target Range

0.2 dB

Target range for no power adjustment; No

 

 

adjustment occurs when power is within this range

 

 

 

Speed Up

5 dB

DPC and PLDM process speeded up from default

Eb/No

 

(1/min) if Eb/No is below this amount

 

 

 

The start point for all enabled modulators is the Nominal Power Level. This level is measured during initial system turn-up and is the power level at which all modems will initially transmit when a call is established. Once communica- tion is established and data starts to flow, the modulator begins receiving DPC packets from the demodulator and adjusting its transmit power level accord- ingly.

The transmit power level is compared to the received Eb/No. Depending on whether the transmit power is high or low, the modulator transmit output level is adjusted in steps (step size set by parameters Max Step Up/Down Power) until the Eb/No is within the range set by the Target Range for no power adjust- ment.

If the received Eb/No should, for any reason, fall below a minimum value set by the Speed Up Eb/No parameter then the PLDM messages will be transmitted at a faster rate until the nominal level is restored.

Adjustment for Data Rate

The maximum output power level is scaled according to the data rate at which the modulator is transmitting. Consider, for example, a system that normally runs at a data rate of 512 kbps with a Nominal Power Level of -10 dBm and a Maximum Power Level of -5 dBm. If a voice switch occurs that sets the data rate to 32 kbps, the power requirements are much lower than the nominal -10 dBm level. With the above process, it is theoretically possible for the power to be raised all the way to -5 dBm, which is past the saturation point for such a small data rate. To prevent the use of excessive power that will drown out the signal and cause damage to the satellite, the modulator calculates a maximum power that is appropriate for its current data rate.

The net result of the DPC algorithm is that the transmit power of all video carri- ers is adjusted such that the receiving modems see a constant Eb/No value. This optimizes receive signal quality and use of system resources since it can lead to a reduction in power amplifier intermodulation and also a conserving of satellite transmit power.

Appendix C - Dynamic Power Control

C-3

Page 117
Image 117
Cables to Go CDM-570L manual Adjustment for Data Rate