Power control in CDMA

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written 7.9 years ago by

teamques10 ★ 68k

• modified 5.4 years ago

Since CDMA has near-far effect which is from other users transmitting in the same frequency band at the same time, therefore is is important to implement good power control.
Near-far effect occurs in the absence of power control that is if all mobiles were to transmit at the same power level, the mobile closest to the base station will overpower all others (since the signal power drops exponentially with the distance).
Another reason for power control is battery life—if the mobile station were to continuously transmit at a power higher than that needed to maintain an acceptable SIR, the battery lifetime is reduced.
Using power control, each mobile station may transmit using the minimum power needed for maintaining the required SIR ratio, thus conserving its battery life.
Using frame error rate for power control decisions is preferred over using signal strength used in other systems.
Power control is especially important on the Reverse Link where non-coherent detection is employed.
Two types of power control are implemented:

a) an Open loop b) a Closed loop.
Before a traffic channel is assigned, there is no closed power loop control in CDMA because it involves feedback from Base station, and to prevent the sudden fall of signal strength an open loop power control is implemented.
Here, power transmitted is inversely proportional to received signal strength of pilots of base station.
Open loop power control: Starts when the first mobile attempts to communicate with the base station.
This power control is used to compensate for the slow variables shading effects. This power control fails or too slow for fast Rayleigh fading channels.
Closed loop power control: The power of closed loop control is used to compensate for the rapid Rayleigh discoloration. This time, the mobile transmit power is controlled by the base station. For this purpose, the base station continuously monitors the reverse link signal quality. If the quality of the connection is low, it tells the mobile to increase its power; and if the quality of the connection is very high, the mobile base station controller reduces its power.

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