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Difference between Open Loop Power Control vs Closed Loop Power Control

Cellular system basically composed of two main entities viz. Base Station (BS) and Mobile subscriber Station (MSS). Base Station serves more than one mobile subscribers(MSs) in its coverage area. Power control is very essential in cellular communication system due to following features.

•   To maintain Signal to Noise Ratio at the receiving end of a communication link for effective communication. It is required to increase the transmit power in the conditions when it goes down below the noise power level. This is due to transmitted power will get attenuated due to path loss as well as other factors on the path from transmitter to the receiver.
•   It is often required to reduce the transmit power to minimize the co-channel interference. Refer RF interference to understand the co-channel and other RF interference types.
•   As BS in CDMA and other cellular systems receive transmissions from different mobile subscribers, it is required to equalize the power level from these subscribers.

There are two main types of power controls viz. open loop and closed loop. Let us examine the difference between Open Loop Power Control vs Closed Loop Power Control used in the cellular system.

Open Loop Power Control

Open Loop Power Control

In the Open Loop Power Control, there is no feedback either from mobile to BS or from BS to mobile. Let us take example of CDMA system wherein there is dedicated pilot channel provided for channel estimation. It is transmitted by the base station to all the subscribers. The mobile unit receives the pilot channel and estimates the power strength. Based on this estimate, the mobile unit adjusts the transmit power accordingly. During this open loop control, it is assumed that both forward link (from BS to MS) and reverse link (from MS to BS) are correlated.

Closed Loop Power Control

Closed Loop Power Control

In the Closed Loop Power Control, feedback is used for adjusting the transmit power level. Let us see how this is done in the forward link. BS receives the mobile signal. Based on this received power level as well as other parameters such as SNR and BER, Base station determines what is the optimum power level mobile need to transmit to achieve effective communication link performance. This estimated power level is communicated to the mobile by the BS over control channel. Mobile adjusts the power level accordingly using the feedback provided by the BS.

Often Mobile estimates the base station power level and communicates base station to adjust its power level to achieve effective reverse link performance.

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