A Comparative Analysis of Adaptive Digital Predistortion Algorithms for Multiple Antenna Transmitters

This paper presents a comparative study of adaptive algorithms for digital predistortion (DPD) in multiple antenna transmitters. Crossover predistorter (CO-DPD) and crosstalk canceling predistorter (CTC-DPD) were proposed to overcome the deleterious effect of RF crosstalk before the power amplifiers (PA) on digital predistortion (DPD) in multiple antenna transmitters. This paper discusses the linearization performance and computational complexity of least mean square (LMS) and recursive least squares (RLS) adaptive algorithms for CO-DPD and CTC-DPD. The adaptive predistortion algorithms for a single antenna transmitter can be extended for CO-DPD, by incorporating the DPD coefficients of more than one branch in to one filter coefficient vector of the adaptive algorithm. The adaptive CTC-DPD involves predistorters for each transmitter running in parallel with the adaptive algorithms that track the coupling between the transmitters. The computational complexity of CTC-DPD is considerably lower compared to CO-DPD, as it has lesser predistorter branches. It is estimated that the number of computations needed per sample duration, for the real-time operation of the adaptive CTC-DPD is 47% lesser compared to CO-DPD for a two-antenna transmitter, when a 9th order memory polynomial with 3 memory taps was used. The linearization performances of these adaptive predistortion techniques for two and four antenna transmitters are evaluated through simulations using QPSK, 16-QAM, UMTS, and LTE signals. It is observed that CTC-DPD performs approximately identical to CO-DPD in all the examined cases. In the presence of frequency selective crosstalk, an extension of CTC-DPD which can pre-cancel such crosstalk, performs considerably better than CO-DPD, which cannot model the frequency selectivity of the crosstalk.