POWER CONTROL IN WCDMA NETWORKS

 Spread Spectrum technology such as WCDMA, CDMA where network users’ transmissions are distinguished using Code is Interference Limited, which affects the Coverage, Capacity and Quality of such networks. Power control helps to maintain link quality in the uplink and downlink by adjusting transmit power level of UE, NodeB and ensuring that only the required amount of power is being transmitted within the system.

Power control is a central mechanism used in WCDMA system to:

1.      Prevent near-far effect (a situation which describe the difference in received power between two WCDMA users using the same transmit power) by providing minimum power level for each connection.

2.       Help minimize interference and thereby increase capacity and quality. It is also used for Resource allocation and helps in interference management by adapting UE transmit power to changing channel and interference conditions.

3.      Provides protection against shadowing and fast fading and helps to extend MS battery life.

Open Loop Power Control is used for setting initial uplink and downlink Transmit power when the UE is accessing the network.  It is mainly used at the beginning of a connection until more accurate control information is available. The UE estimates the average path loss in the downlink and adjusts the uplink transmission power accordingly.

Closed loop power control is of two types (Inner Loop Power Control & Outer Loop Power Control)

·        Inner Loop Power Control (Fast Closed Loop Power Control) is used in the Uplink to adjust UE Tx power based on the TPC (Transmit Power Control) commands received in the downlink so as to keep received uplink Signal-to-Interference Ratio (SIR) measured over one Time slot duration (577μs) at a given SIR target and vice versa. Inner Loop PC frequency is 1500Hz.

Inner loop PC uses the following rule:

if SIR[Measured] > SIR[target] TPC command to be sent is 0

 if SIR[Measured] < SIR[target] TPC command to be sent is 1

In simple terms: The receiving end of the connection compares the SIR value of the received signal to the target SIR value of the received signal and then control bits are sent back to the transmitting end indicating that the Tx power should either be increased if (received_SIR < target_SIR) or decrease (received_SIR > target_SIR) by a certain amount in 1dB step.

·        Outer Loop Power control measures the Bit Error Rate (BER) or Frame Error Rate(FER) and is used to maintain channel quality at the level of bearer service quality requirement while using as low power as possible. It reflects how the User perceives service quality and is used both in the Uplink and Downlink.