What is Envelope Tracking?

Envelope Tracking is a technique used to enhance the efficiency of a power amplifier used in wireless systems. In this technique, the amplitude envelope of the RF signal is tracked and the bias voltage is adjusted accordingly to ensure that the PA operates with maximum efficiency at any instantaneous power level.

An RF Power Amplifier operates most efficiently when it is operated at peak output power, where the gain enters compression. Envelope Tracking takes advantage of this principle, it ensures that the amplifier operates at a point close to compression as often as possible by regulating the supply/bias voltage. So, if the amplifier is required to operate at a lower power level, the supply voltage is adjusted to lower the compression point of the amplifier which makes it more efficient for low power levels. The alternative to this would be to let the amplifier operate at a constant power level with a static supply voltage in all cases, however in this scenario the efficiency of the amplifier for low power levels will be very low. And thus if used in a battery operated device, would drain the battery very quickly.

In modern communication technologies like W-CDMA/HSPA+/LTE, the signal waveforms have more extreme "peaks" and "valleys" in comparison to older technologies like GSM. Figure below.

If we take the case of LTE signal (OFDM modulation), the "peaks" and "valleys" are more prominent so that higher data throughput can be achieved with limited spectrum resources. See image below.

During the peaks, the amplifier requires a higher voltage to deliver the required power output with maximum efficiency and at lower power levels the amplifier needs a lower voltage. 

However, if we do not use Envelope Tracking and the amplifier operates at a constant voltage at all times i.e for both high and low power levels, the extra power which is not required during low power operation will be dissipated into the device as heat. This will result in reduced efficiency of the amplifier.

In the images above, the supply voltage is not regulated based on the RF signal i.e No Envelope Tracking

An envelope tracking system, tracks the amplitude envelope of the RF signal and then uses this information to drive the power supply to the RF power amplifier (see image below). This way the power supply matches the voltage requirement of the RF amplifier and less heat is dissipated.

In the images above, the supply voltage is regulated based on the RF signal i.e Envelope Tracking Enabled

Here is a video from Keysight that How to Use Envelope Tracking to Improve Power Amplifier Efficiency