Power Control is used in CDMA systems for two reasons:

1. To prevent the Near Far problem.
2. To increase the CDMA system capacity.

Accordingly power control is implemented as follows:

1. Power control at mobileso that all the transmissions from the mobiles are received at the base stations's receiver at approximately the same strength(within 1dB).This prevents the near far problem.

2. Power control at base stationso that only the minimum RF power required for reliable communication is allowed from the base station transmitter.This maximises the number of users sharing a cell and hence increases the CDMA system capacity.

    Power control at the mobile

To implement power control at the mobile,CDMA systems use two techniques,namely

1. Open Loop Control
2. Closed Loop Control

• In open loop control, the mobile estimates the loss between itself and the base station and then uses it to make a coarse adjustment in its transmitted RF power.The mobile estimates the path loss by measuring the received signal level from the base station. The mobile then combines this reading with the power control information sent by the base station during some initial signalling transactions.This information includes transmit power from the base station and some parameters that can be used to adjust the open loop power control for different sized cells and different cell ERP and receiver sensitivities.
• In closed loop power control mode, the cell site receiver forms an estimate of the received SNR of the mobiles signal every 1.25ms. The SNR measurement is compared with the predetermined threshold value of SNR. If the received SNR is too high, a decrease power command is sent to the mobile and if the received SNR is low, an increase power command is sent to the mobile. This power control command is sent every 1.25ms, providing an 800bps power control rate.The mobile responds to the power control bits (1-decrease power, 0-increase power) by making small adjustments of 2dB in its RF power output until the received SNR is within acceptable limits.

Power control at Base Station

To implement power control the base station continuosly lowers it's transmit power in small steps, until the responding mobile, signals the base station for more power. This process is performed individually for each of the CDMA channels on a forward CDMA waveform.