Power Control in UMTS

Power control is necessary to keep the transmitted signal power level under control so as to minimize the interference and keep the quality of signal to a desired level. The main functions are:
1. Closed-loop power control
· Outer-loop power control
- Uplink outer-loop power control
- Downlink outer-loop power control
· Inner-loop power control
- Uplink inner-loop power control
- Downlink inner-loop power control
2. Open-loop power control
· Uplink open-loop power control
· Downlink open-loop power control

Closed-loop power control is the power control mechanism used in UMTS to solve near-far problem, minimize interference and to keep the signal quality to optimum level. Closed-loop power control is used in uplink (UL) as well as downlink (DL). However, the motive in both the cases are different. In uplink, signals from different UEs reach NodeB with different power strength, thus causing the stronger signal blocking the weaker one, resulting in near-far effect. In downlink, there is no near-far effect, but the UEs near the cell-edge or in high interference region may need extra power to overcome the increased other cell interference and weak signal due to Rayleigh fading.

Closed-loop power control can be divided into outer-loop and inner-loop power control. In case of uplink, the RNC manages the outer-loop and Node B manages the inner-loop and for downlink, UE manages the outer-loop and Node B manages the inner-loop.

Inner-loop power control (also called fast closed-loop power control), operates at 1500 times per sec (1.5 kHz) [From where did this value of 1.5 kHz come from? Answer: A UMTS 10 ms frame consists of 15 TPC commands. This results in a power control frequency of 1500 Hz (15/10ms)] and relies on the feedback information from the opposite end of the link (or channel) to maintain the signal to interference (noise) ratio to a target level set by the outer-loop power control. The transmission power is increased or decreased by a certain fixed step size depending on whether the received SIR is below or above the target SIR. Precise power control can lead to optimum use of bandwidth resulting in increase cell capacity.
The UL inner-loop power control lets the UE adjust its output power in accordance with one or more TPC commands received in the downlink direction. Remember the increase and decrease in power is limited by upper and lower bounds as defined in 3GPP TS 25.101. The upper bound, i.e. UE maximum output power, is set depending on the Power class of UE. This can also be set below the maximum capability of the UE through signaling when the link is established. The lower bound, i.e. UE minimum output power defined as the mean power in one timeslot (TS), and shall be less than -50 dB.
The DL inner-loop power control is used to control the transmission power of downlink channels at Node B as per the TPC commands received from UE. However, in some situations Node B may ignore the increase/decrease these TPC commands. For example, in case of congestion (high load scenario), the Node B can ignore the TPC commands from UE.

Outer-loop power control is used to set the target quality value for inner-loop power control, i.e it adjusts the target SIR in Node B which is used during inner-loop power control. Now the question is why do we need to adjust the target SIR? Outer-loop power control tries to keep the quality of a connection to desired value. Too high quality will waste the resources.

Open-loop power control is used to set the initial power of UE (in random access) and downlink channels. The TPC commands used in inner-loop power control are relative, so it needs a starting point and this is defined by open-loop power control. Also, this is useful in setting the power level in case of common shared channels, where it is difficult to send each UE the necessary TPC commands. In case of uplink, UE and broadcasted cell/system parameters are used to set initial access power on RACH. And in case of downlink, the measurement report of UE is used to set the initial power of downlink channels.
The open loop power control tolerance is ±9dB under normal conditions and ±12dB under extreme conditions.

[References: TS 25.214, TS 25.215, Section 7.2.4.8 of TS 25.401]