Combined open and closed loop power control in a communications satellite
View Patent ↗In a telecommunication network where a wireless path is provided between a transmitter and a receiver via a satellite relay forming a multi-hop relayed signal path, a method and system are provided for furnishing adequate signal at a user terminal, which may be in motion, along the propagation path, the method including an open loop component that uses measurements of fading in a received signal at a first terminal to adjust transmit power from the first terminal and further includes a closed loop component that uses measurements of fading in the received signal at a second terminal along with an acknowledgement message and power correction.
1. In a satellite communications network, wherein a power controlling hub terminal and a plurality of user terminals communicate through a communications relay satellite, a method of power control for a transmitter of a user terminal comprising:
transmitting along a forward link from the hub terminal via the satellite to generate a forward link receive signal at the user terminal, the forward link receive signal experiencing forward link fading; thereafter
transmitting along a return link from the user terminal via the satellite to generate a return link receive signal at the hub terminal, the return link receive signal experiencing return link fading;
estimating the return link fading by monitoring the forward link receive signal at the user terminal;
using a speed estimation during movement of the user terminal over a preselected distance to examine the forward link receive signal to correct for errors in estimation processes and to generate an open loop power correction value;
transmitting an acknowledgement message over the forward link from the hub terminal to the user terminal; thereupon
monitoring at the user terminal for whether or not the acknowledgement message has been received at the user terminal; and
a) if the acknowledgement is received, updating a closed loop power correction at the user terminal with a received closed loop power correction value contained in the acknowledgement message; and
b) if the acknowledgement message is not received, incrementing a closed loop power correction at the user terminal; thereupon
adding the open loop power correction to the received closed loop power correction or the incremented closed loop power correction to yield a total power correction at the user terminal; and
using the total power correction to adjust power level of the return link signal from the user terminal to the hub terminal.
2. The method according to claim 1 wherein the speed estimation is used to determine how long the forward link receive signal is to be observed.
3. The method according to claim 1 wherein the forward link receive signal is observed for a predetermined time period.
4. The method according to claim 1 further including estimating return link signal-to-noise ratio E b /N 0 by introducing periodic return link transmission outages during which no terminals are transmitting on the return link.
5. The method according to claim 1 wherein return link open loop power correction, C o , is obtained as
C
o
=
(
1
Δ
^
)
2
where
{circumflex over (Δ)} is the return link open loop amplitude fade estimate obtained from monitoring the forward link given by the ratio:
Δ
^
=
SNR
estimate
SNR
nominal
where SNR estimate is the estimate of the SNR, and SNR nominal is the nominal SNR corresponding to α=1.
6. The method according to claim 1 further including the step of causing periods of outage of transmission on the return link in order to allow observations of noise levels in the return link.
7. A system for controlling signal power output of a transmitter of a user terminal in a satellite communications network having a power controlling hub terminal and a plurality of user terminals communicating through a communications relay satellite, the system comprising:
a hub transmitter in the hub terminal for transmitting along a forward link from the hub terminal via the satellite to generate a forward link receive signal at the user terminal, the forward link receive signal experiencing forward link fading;
a user terminal transmitter in the user terminal for transmitting along a return link from the user terminal via the satellite to generate a return link receive signal at the hub terminal, the return link receive signal experiencing return link fading;
a receiver in the user terminal for estimating the return link fading by monitoring the forward link receive signal at the user terminal;
a control system in the user terminal for using a speed estimation during movement of the user terminal over a preselected distance to examine the forward link receive signal to correct for errors in estimation processes and to generate an open loop power correction value;
a controller in the hub terminal for transmitting an acknowledgement message over the forward link from the hub terminal to the user terminal;
a subsystem in the receiver in the user terminal for monitoring at the user terminal whether or not the acknowledgement message has been received at the user terminal; and
a) if the acknowledgement is received, for updating a closed loop power correction with a received closed loop power correction contained in the acknowledgement message; and
b) if the acknowledgement message is not received, for incrementing a closed loop power correction at the user terminal;
a subsystem in the user terminal for adding the open loop power correction to the received closed loop power correction or the incremented closed loop power correction to yield a total power correction at the user terminal; and
a subsystem in the user terminal for using the total power correction to adjust power level of the return link signal from the user terminal to the hub terminal.