Power Control mechanisms in wireless networks has been systematically studied since 1970s.
One of the earliest approach of power control is manual tuning where amplifier power is manually tuned up and down. This manual tuning may help in cases where the distance between transmitter and receiver does not change.
In cases where the distance between transmitter and receiver vary, two approaches are defined : Closed loop power control and Open loop power control.
Closed loop Power Control
One of the earliest approach of power control is as follows: i) Transmitter sends a signal to the receiver, ii) Receiver measures the power of the received signal, iii) If the measured power is too low, receiver sends a special command to the transmitter saying “increase power” and if the measured power is too high, receiver sends a special command to the transmitter saying “decrease power”.
With this mechanism transmitter can change its power dynamically. This mechanism is called closed loop power control and the special command used for this purpose is called Transmit Power Control(TPC) command.
Transmitter sends a signal, receiver receives it, sends feedback to transmitter. Receiver retunes itself by the feedback. This control loop is called a closed loop in control system theory.
This power control mechanism happens very frequently (1000/1500 times per second). This mechanism is based on the assumption that there is already a call is setup between transmitter and receiver.
This closed loop power control mechanism cant be used when the mobile phone is turned on to communicate with base station as there is no established connection between.
Open loop Power Control:
Mobile has to determine the proper transmit power which would be strong enough to be properly decoded by base station and weak enough not to interfere with the other mobile devices communicating with base station.
Below mechanism is used in the above cases:
- Base station transmits certain reference signal with a fixed power value.
- Base station includes the power information of the signal and maximum allowed power in the reference signal.
- UE decodes the reference signal and measures the power.
- UE calculates the path loss.
- UE know how much power allowed for it and it can figure out how much power it can transmit.
This process is called Openloop power control.
