Answer:

• Power control is one of the main qualities in 3rd generation cellular CDMA systems. Power control reduces the intrusion in the system and thus increases the capacity.
• The problem of errors in the power control commands becomes worse during soft handoff where the terminal communicates with more than one base station at the same time. As we know, cdma 2000 and WCDMA both referred to the multicarrier mode.

• The power control is used in cdma 2000 and WCDMA because it effects the presentation of cdma 2000 and WCDMA in three ways that are:

   

1. It compensates for fast fading.
2. Compensation of fading channel leads to peaks in MS transmission power which effect the intrusion.Avoid near far problems in the uplink.
3. The Open loop Power Control algorithms are explained where the power commands are sent at a high rate and un-coded resulting in high error rate in receiving them.

Open Loop Power Control:

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

• Open loop power control is based on the sameness of the loss in the forward path to the loss in the reverse path where 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.As we know, Loss is alike on Forward channels and Reverse channels, therefore

•  

$\large{\textbf{Receive Power + Transmit Power = -73}}$ (all powers in dBm)

For example: let us assume, received power is equals to -85 dBm *  

then, Transmit power will be 

$\large{\textbf{Transmit Power = (-73) – (- 85)}}$ *  

$\large{\textbf{Transmit Power = (+12 dBm)}}$

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.

Closed loop power control:

• In cellular Frequency Division Duplex (FDD) CDMA systems, the frequency separation between connections is greatly more than the coherence bandwidth of the channel. This frequency separation has very important implications for the power control process which 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 by measuring the path loss on the forward link. This measurement idea which is used for the open loop, generally furnishes the correct average transmit power, but additional supplies 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 from each mobile. The measured Pm is 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 power by ∆p dB, if below then 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 evaluate. It should be fast enough to keep up with the fast fading. So it is the crucial part of any effective scheme to combat Rayleigh fading.