The envelope tracker in the Note was developed by Qualcomm, and it’s the first commercial implementation of the technology that I know of. What it does is essentially match the power pushed through the phone’s signal amplifier to the actual power needed to transmit a signal. That might sound obvious, but in practice it’s a very difficult thing to accomplish.
A mobile signal is a waveform riddled with peaks and valleys, so a typical phone amp maintains a uniform “envelope” of power that can capture its highest amplitudes. The problem is particularly pronounced with LTE, which suffers from what’s known as a high peak-to-average ratio.
The best explanation was given to me last year by Jeremy Hendy, VP of sales and marketing at envelope tracking pioneer Nujira: LTE is classical music. 3G is heavy metal. Classical music has long quiet interludes punctuated with wild crescendos, while heavy metal is pretty uniform in loudness. Heavy metal is going to sound just as good (or bad) on any amp, but classical music requires much higher power to capture its nuance.
“You need a high-powered amp for LTE otherwise the signal is distorted,” Hendy said. “That’s why the power on an LTE [handset] is so bad. For every 4 watts you put in you only get 1 watt out.”
So if you’re wondering why your new LTE phone has such crappy battery life (or comes with an enormous lithium-ion caboose) compared to your old 3G model, you have your answer. What does envelope tracking do about it? It basically wraps the power envelope around the waveform tighter than a latex bondage suit, resulting in relatively little power loss.