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).

Open loop control sets the sum of transmit power and receive power to a constant, nominally -73, if both reverse and forward powers are in dBm.  A reduction in signal level at the receive antenna will result in an increase in signal power from the transmitter.

For example, assume the Forward received power from the base station is -85 dBm.  This is the total energy received in the 1.23 MHz receiver bandwidth. It includes the composite signal from the serving base station as well as from other nearby base stations on the same frequency.

The open loop transmit power setting for a received power of -85 dBm would be +12 dBm. Thus open loop power control adjusts the transmit power of the phone to match the propagation conditions that the phone is experiencing at any given time.

By the TIA/EIA-98 standard specification, the open loop power control slew rate is limited to roughly match the slew rate of closed loop power control directed by the base station. This eliminates the possibility of open loop power control suddenly transmitting excessive power in response to a receiver signal level dropout.

Closed loop power control

In cellular Frequency Division Duplex (FDD) CDMA systems, the frequencyseparation between links is greatly more than the coherence bandwidth of the channel.This frequency separation has very important implications for the power controlprocess: It causes multipath fading on the forward and reverse links to be independent.

This means that the mobile cannot measure the path loss of the reverse link bymeasuring the path loss on the forward link. This measurement technique, which isused for the open loop, usually provides the correct average transmit power, butadditional provisions must be made for the effects of asymmetric Rayleigh fading.

Therefore, the mobile transmitter power is also controlled by its cell. Each cell site demodulator measures the received signal power Pm(or SINRmin case of SINR basedCLPC) from each mobile. The measured Pmis compared to the desired power level Pd for that mobile and a power control adjustment command is sent accordingly. If the measured value is above the set point, then a one bit command is sent; to lower powerby ∆p dB, if below, the one bit command is sent to raise power by ∆p dB. The transmitter adjusts its power up or down, relative to the open loop estimate.

The closed loop power control (CLPC) is a sort of “fine tuning” on the open loop power estimate. It should be fast enough to keep up with the fast fading. So, it is the crucial component of any effective scheme to combat Rayleigh fading.