Open Loop vs. Closed Loop Power Control

Cellular systems rely on effective communication between Base Stations (BS) and Mobile Subscriber Stations (MSS). A Base Station serves multiple mobile subscribers (MSs) within its coverage area. Power control is crucial for optimal performance in these systems due to several factors:

• Maintaining Signal-to-Noise Ratio (SNR) at the receiver for reliable communication.
• Compensating for signal attenuation caused by path loss and other impairments between the transmitter and receiver. Transmit power needs to increase if the received power drops below the noise floor.
• Minimizing co-channel interference by reducing transmit power where appropriate. (See “RF interference” for details on co-channel and other RF interference types.)
• Equalizing power levels from different mobile subscribers at the Base Station, especially important in CDMA and other cellular systems.

Two main types of power control are used: Open Loop and Closed Loop. Let’s examine the differences between them.

Open Loop Power Control

Image alt: Open Loop Power Control

In Open Loop Power Control, there is no feedback mechanism, either from the mobile to the BS or vice versa.

Consider a CDMA system where a dedicated pilot channel is transmitted by the base station to all subscribers for channel estimation. The mobile unit receives this pilot channel and estimates its power strength. Based on this estimate, the mobile unit adjusts its transmit power accordingly.

This approach assumes a correlation between the forward link (BS to MS) and the reverse link (MS to BS). In other words, it assumes that if the signal from the BS to the MS is weak, then the signal from the MS to the BS will also be weak.

Closed Loop Power Control

Image alt: Closed Loop Power Control

In Closed Loop Power Control, feedback is used to adjust the transmit power level.

Let’s look at how this works in the forward link. The BS receives the mobile’s signal. Based on the received power level and other parameters like SNR and Bit Error Rate (BER), the BS determines the optimal power level the mobile needs to transmit for effective communication.

This estimated power level is then communicated to the mobile via a control channel. The mobile adjusts its power level based on this feedback from the BS.

Often, the mobile also estimates the base station’s power level and communicates this information to the base station. This allows the base station to adjust its power level for optimal reverse link performance.