Harmonic Distortion in non-linear RF Systems

Non-linear systems produce harmonics for us (called harmonic distortions [shown in the OUTPUT - above figure]). This is because, instead of having one fundamental frequency, in non-linear systems, we have to consider up to at least three terms of frequency values.

The fourth frequency (Omega value) in the non-linear system can be ignored as it is insignificant (unless the system is too sensitive). 

Look at the above example (ref. - RF Microelectronics 2nd Edition B. Razavi P.16). A Transmitter in 900 MHz GSM cellphone delivers 1W of power to an antenna. How will you explain the effect of the harmonics? See the solution below

If the input has a single frequency component, the output will have components at different frequencies. This explains the harmonic distortions caused by the non-linear systems.