Predistortion

Predistortion is a method for enhancing the linearity of radio transmitter amplifiers. In most telecommunications systems, these amplifiers must be linear, meaning they need to accurately reproduce their input signal. If an amplifier compresses its input or has a non-linear input-output relationship, the output signal can spill into adjacent radio frequencies, causing interference on other channels.

Various specifications measure power amplifier linearity, including P1dB, inter-modulation distortion (IMD), AM-to-PM distortion, spectral regrowth, and noise power ratio (NPR). For a truly linear system, these measures are essentially equivalent: an amplifier with low IMD will also have low spectral regrowth and low AM-to-PM distortion. There are two equivalent ways to conceptualize how predistortion works: correcting gain and phase distortions, or cancelling inter-modulation products. While one approach is often preferred when designing predistortion circuitry, the result is generally the same. A predistorter designed to fix gain and phase non-linearities will also improve IMD, and one targeting inter-modulation products will also reduce gain and phase issues.

By combining a predistorter with the target amplifier, the overall system becomes more linear and exhibits less distortion. Essentially, an "inverse distortion" is added to the amplifier’s input, canceling out any non-linearity the amplifier possesses. This technique saves costs and improves power efficiency. Radio power amplifiers become more non-linear as their output power approaches their maximum rating. Predistortion allows for more usable power from the amplifier without needing a larger, less efficient, and more expensive unit. Another key design consideration for RF power amplifiers is the memory effect, or amplifier nonlinear dynamics. Predistortion can be implemented in either analog form or as digital pre-distortion.