Linearizing a Power Amplifier Using Predistortion

Abstract

Linear power amplication is important in radio communications to achieve high bitrates. Strong regulations of the frequency spectrum force manufacturers to use complex modulation schemes to increase the capacity. More linear ampliers can use more complex modulation schemes like the 64 Quadrature Amplitude Modulation (QAM). In addition will a linearized amplier allow for increased output power and better efficiency. Digital predistortion is one of the more successful methods of amplier linearization. By distorting a signal before it reaches the PA, the nonlinearities of the PA may repair the distorted signal instead of damaging it. A behavioral model is created using 16QAM modulated excitation signals. The PA model is based on empirical measurements. An open loop predistorter based on lookup tables corrects the I and Q channels in base band. The predistorter is created and simulated using AWR VSS. A predistorter with a fairly small look-up table of 1008 entries shows an average reduction of third order IM levels of 18,3 dB, and fth order IM levels of 9,3 dB. Even when using mismatched lookup tables, an average reduction of 10,3 dB for IM3 and 4,4 dB for IM5 is achieved.