Meaning
A physical characteristic describes the deviation from a linear relationship between the input power and the output power of an active RF amplification stage. This power amplifier non linearity leads to amplitude distortion and phase distortion when the amplifier operates close to its saturation point. In wireless communication systems, this behavior causes spectral regrowth and high error vector magnitude, which degrades the overall signal quality.
Distortion Mechanism
At low input levels, the gain of the amplifier remains constant, but as the input signal increases, the output begins to compress. This compression generates third and fifth order intermodulation products that lie within and adjacent to the desired communication band. In orthogonal frequency division multiplexing systems, these non-linear effects cause subcarriers to interfere with each other, destroying the orthogonality of the signal.
Digital Correction
Digital pre-distortion algorithms mitigate the distortion by applying an inverse non-linear function to the baseband signal before it reaches the digital to analog converter. The pre-distorted signal compensates for the predictable compression of the amplifier, resulting in a highly linear output after amplification. This digital approach allows the hardware to operate closer to its saturation region, which increases power efficiency without violating spectral emission masks.
Continuous feedback from the amplifier output allows adaptive algorithms to adjust the pre-distortion coefficients dynamically as the temperature of the board changes during operation.
Operational Limit
The one decibel compression point defines the boundary where the actual gain is one decibel lower than the small signal gain. Engineers use this metric alongside the third order intercept point to characterize the dynamic range of the transmitter. Operating the system below these critical points prevents the generation of out of band emissions that interfere with adjacent communication channels.
