Envelope Tracking is a power supply technique for improving the energy efficiency of Radio Frequency Power Amplifiers. It replaces the fixed DC supply voltage to the RF PA with a dynamic supply voltage, which closely tracks the amplitude, or “envelope” of the transmitted RF signal.
Traditional RF PAs are supplied with a fixed DC voltage, but are only energy efficient when they are in compression (at the peaks of the transmitted waveform). Most of the time, the supply voltage is far higher than it needs to be, and the excess power is dissipated as heat in the PA device – with OFDM signals up to 80% of the energy in the PA is typically wasted.
Envelope Tracking works by dynamically adjusting the supply voltage to the PA. This maximises the energy efficiency of the PA by keeping it in compression over the whole modulation cycle, instead of just at the peaks.
Higher data rates lead to “peakier” waveforms
As wireless data rates increase, modern digital modulation techniques such as OFDM are packing more data bits into each RF channel. This results in increasingly complex waveforms, with higher “crest factors” – the ratio of the signal peaks to the average power level, usually expressed as Peak to Average Power Ratio (PAPR).
The average transmitted power level generally stays the same, as this determines the distance (range) of the RF transmission.