Power control in CDMA sub divided into open loop and closed loop system of power control.

Open loop power control system:

The Open Loop Power Control require in CDMA is due to following reason.

• Assumes Loss is Similar on Forward paths and Reverse Paths
• Receive Power + Transmit Power = -73

1. All Powers in dBm

• Example: For a Received Power of -85 dBm

1. Transmit Power = (-73) – (- 85)
2. Transmit Power = +12 dBm

• Provides an Estimate of Reverse TX Power for Given Propagation Conditions

Open loop power control is based on the similarity of the loss in the forward path to the loss in the reverse path (forward refers to the base-to-mobile link, while reverse refers to the mobile-to-base link). Variations in the signal level due to distance and shadowing are not frequency dependent, but rather location dependent. Hence these variations are reciprocal on both the forward and reverse links. A mobile can eliminate the variations due to distance and shadowing by measuring the average received signal on the forward link and adjusting its power accordingly. Averaging eliminates variations due to multipath fading.  This is called open loop power control.

The reception of a strong signal by the mobile indicates that the mobile is either close to the base station or has good propagation conditions to the base station. This means that less mobile power is needed to produce a nominal received power at the base station. In the case of sudden improvement or degradation in the channel, the OLPC mechanism adjusts the mobile power downward or upward and thus prevents the mobile power from being too high or low at the base station.

Closed loop power control (CLPC):

In a cellular system, full duplex transmission is provided by using one frequency band from the mobile to base station. And another frequency band from the base station to mobile. These two frequencies are separated by 45 MHz in the current AMPS system to prevent interference from transmitter to the receiver. This 45 MHz separation is much larger than the coherence bandwidth of the channel, which makes the forward and reverse links fade independently. Hence, multi path fading on the reverse link cannot be estimated by the mobile. The base station measures the signal received from the mobile, compares it with a desired threshold and then sends a command to mobile asking it either to decrease or increase its power. The power control step size is usually in dB units. In addition to reducing interference, PC on the reverse link saves mobile power and hence increases battery lifetime. Reducing the transmitted power also reduces health hazards.