Automatic Gain Control-AGC

This page describes Automatic Gain Control(AGC) algorithm basics which monitors received signal and controls gain automatically in a receiver.

AGC which stands for Automatic Gain Control is very useful in any receiver especially in the handset or mobile side. As the signal transmitted from Base Station reaches to the mobile or handset through different paths or terrains leads to variations in the signal received at the mobile/handset. Also wireless mobile devices moved by people at various speeds leads to variations in the signal strength.
In this situation of time varying signal, need arises to control the level or gain of the signal before it is passed to baseband processing chain of the device for data decoding.
The intelligence built in the mobile device to monitor and control the gain automatically is called Automatic Gain Control. AGC is very important module in signal processing chain of the receiver.

Automatic Gain Control

As shown in the figure, normal receiver will have shaded modules. For Automatic Gain Control to be implemented VGA (Variable Gain Amplifier) is inserted and AGC algorithm either on FPGA or DSP is to be implemented. LNA also to be changed to variable gain type if already not provided on the circuit. Also as AGC algorithm works on the digital data, additional ADC is also provided on board.

Automatic Gain Control Algorithm continuously monitor RSSI and/or received I and Q baseband signals and decide how much gain to be changed in LNA and VGA modules.
RSSI stands for Received Signal Strength Indication, which is the voltage equivalent to the received instantaneous input power. This is calculated using the device called RF power detector, which is also the additional module, or otherwise RSSI will be calculated using I2 + Q2 of the received baseband signal vector.
Usually variable gain LNA and VGA will have either analog attenuator or digital attenuator which can be controlled by analog voltage/digital word. This is how their gains are changed by applying attenuations.

Useful RF Links

heterodyne vs homodyne Receiver   LNA and LNB basics   RF attenuator
Automatic Time offset correction/compensation, refer page on ATC.
Automatic Frequency offset correction/compensation, refer page on AFC.

RF and Wireless Terminologies