Optimum indexing in predistorting amplifier linearizers

Because a nonlinear RF power amplifier (PA) generates intermodulation (IM) distortion as adjacent channel interference for many modulation formats, the design of linearizers has become a key technology in modern mobile and personal communication transceivers. A powerful type of linearizer is the digital predistorter (PD), which incorporates a lookup table (LUT) containing a representation of the inverse characteristic of the amplifier. However, the question of optimum nonuniform spacing has been unresolved. This paper provides the solution: a way to compute the IM for different spacing methods and a derivation of the optimum table spacing method. It demonstrates that the IM of optimum spacing is only a few dB lower than that of equal spacing by amplitude, when variations in the operating point are considered