Wide bandwidth adaptive digital predistortion of power amplifiers using reduced order memory correction

Adaptive digital predistortion (DPD) with memory correction is made challenging when the observation bandwidth used to measure the output signal is narrower than the discernable intermodulation (IMD) products generated by the input signal and power amplifier nonlinearity. This is likely to occur when the input signal has a wide bandwidth and the PA has severe nonlinearities, as in the case of a Doherty PA. Wide bandwidth signals tend to stimulate the memory effects within the PA requiring memory correction to be included in the DPD. However, if in the pursuit of additional memory correction the number of basis waveforms used is over-specified relative to the measurable output signal bandwidth, the estimation of the memory coefficients will be ill-conditioned. This paper proposes a method based on eigenvalue decompositions to reduce the order of the memory coefficient estimation. It also shows how measurements from multiple narrow bandwidth observations can be combined to increase the available rank of the estimation and improve the overall cancellation performance. Experimental results obtained using a RF test bench comprising a Doherty PA and a 20 MHz input signal demonstrate the relationship between the available order of the estimation, the number of observation bandwidths used, and the performance of the wide bandwidth IMD cancellation.