Power Control in WCDMA

Why Power Control?

Its objective is to maximize capacity by minimizing power and interference.

To avoid the interference between the users because each one use same frequency and bandwidth at same time

Minimise Uplink Interference- To adjust the transmit power of the UE so that the signals received at the Radio Base Station (RBS) from all UEs have equal Signal to- Interference Ratio (SIR) at the same bit rate (the near-far problem). Because the UE at the cell edge will suffer from a higher Path Loss than another UE that is close to BS.

Cont… Minimise Downlink transmit Power- Downlink

Power Control is used to minimize the transmitted power of the RBS. Because there is a shared resource in WCDMA.

Type of Power Control

Open Loop Power Control

Inner Loop Control

Outer Loop Power Control

WCDMA Power Control loops

Open Loop Power Control

Used for initial power setting of the UE at the beginning of the connection.

This open loop power control function ensures that the random access does not cause too much interference.

1. UE measures the received power of the CPICH.

2. UE reads the power control information from Broadcast channel in which it reads BS transmit power of the CPICH.

Cont…

3. From these values UE estimates the minimum transmit power necessary to access the cell.

4. Attempt to access the cell at estimated value.

5. If this attempt is fail then there is no response from BS, then it will attempt with a increase power in steps and re-try.

Cont…

Response from BS

Attempt to access cell

Inner Loop Power Control It takes place over a dedicated channel once it is setup

1. This is required to avoid UE transmitting too high a power level as they move towards the BS and vice versa.

2. The system must ensure that the UE transmits only sufficient power level s to be

received and avoid unnecessary interference to other users.

3. Once a dedicated channel is established , the UE (Uplink) power can be controlled be the BS by sending power control messages (TPC bits, are used for downlink power control as well) as UE moves far or near from the BS.

4. Power can be adjusted in steps of less than 1dB at a rate of 1500times per second.

5. Conclusively, overall idea here is that every UE in the cell shoulfd requires same Signal to Interference (SIR) ratio at base station.

Cont…

Moving far

Coming towards

Outer Loop Power Control

It is used to keep the quality of communication at the required level by setting SIR target.

The Outer Loop aims at providing the required quality , no worse, no better.

- If the received quality in UL is better than the required then SIR target is decreased.

- If not then SIR target is increased.

Cont…

Cont… The Serving Radio Network Controller (SRNC) will constantly

monitor the CRCs of the uplink Transport Blocks (TB) and estimate the Block Error Rate (BLER).

If it discovers that this BLER is either higher or lower than what is required for the RAB, it will change the SIR target for Inner Loop Power Control.

In doing so the uplink BLER for the service is maintained, regardless of UE environment and mobility.

The UE must perform the same function on the downlink TBs and change the target for the Downlink Inner Loop Power Control. This will maintain the downlink BLER for the service regardless of the UE environment and mobility.

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