Optimal spacing for a polar look-up table predistorter

Modern modulation standards utilize undistorted complex non-constant envelope modulation schemes for improved spectral efficiency and transmit data bandwidth. On the contrary, the increased demand of power added efficiency for longer battery life forces the power amplifiers to be operated well into their non-linear operational region. The transmitter and PA non- linearities, if uncompensated, result in many undesired effects such as spectral regrowth and degradation of BER. Look-up-table (LUT) based predistorters have been widely used to mitigate the effects of nonlinear PA´s. LUT refinements techniques such as optimal spacing have been studied for zero-hold LUT (ZOH- LUT) in the complex gain predistorter configuration. In this paper, optimal spacing for the polar LUT predistorter is studied. An optimal spacing is derived for both ZOH-LUT and Linearly interpolated LUT (Lin-LUT). An analytical study of Lin-LUT shows that the mean squared approximation error is decreased by 12 dB when the LUT size is doubled, as opposed to 6 dB for ZOH interpolation.