Compensating Imperfections in RF-DAC Based Transmitters Using LUT-Based Predistortion

This paper provides a model for the frontend of RF-DAC based transmitter architectures, which suffer from the imperfections of mixers, amplifiers and DACs. The model considers gain and phase errors as present in mixers, nonlinear compression due to limited output resistance as in amplifiers and element mismatch as in DACs. The model parameters are determined by measurement comparison with a prototype IC. Sveral compensation methods are compared, resulting in an IQ-LUT based predistortion as the most promising method. The LUT values are determined using LMS algorithm, resulting in a performance improvement from 10.7 % EVM to 2.7 % for IEEE802.11 transmission. As LMS is costly to implement in a digital design, another approximation method is evaluated, using only linear operations. This method results in an EVM of 2.4 %. Reducing the LUT coefficients from 256 to 16 and using linear interpolation for the missing coefficients is possible without degrading the overall system performance.