Power Control is used for two reasons:
- To prevent the near far problem.
- To increase the system capacity.
Accordingly power control is implemented as follows:
- Power control at mobile so that all the transmissions from the mobile are received at the base station’s receiver at approximately the same strength(within 1 dB). This prevents the near far problem.
- Power control at base station so that only the minimum RF power required for reliable communication is allowed from the base station transmitter.
To implement power control at mobile, CDMA systems use two techniques, namely
- Open Loop Control and
- 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 receiving 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.
In closed loop control mode, the cell site receiver forms an estimate of the received SNR of the mobiles signal every 1.25 ms. The SNR measurement is compared with the predetermined threshold value of SNR. If the received SNR is too high(low), a decrease(increase) power command is sent to the mobile. This power control command is sent every 1.25 ms, providing an 800 bps power control rate. The mobile responds to the power control bits(1-decrease power, 0-increase power) by making small adjustments of 2 dBin its RF power output until the received SNR is within acceptable limits.
To implement power control, the base station continuously lowers its 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.