Digital predistortion (DPD) of a nonlinear power amplifier (PA) is made challenging when the observation bandwidth (OBW) used to measure the output signal is narrower than the intermodulation distortion (IMD) spectrum generated. Wide bandwidth input signals stimulate memory effects within the PA thereby requiring the model order of the DPD to be expanded to include memory correction coefficients. However, if the model order is over-specified relative to the OBW, the estimation of the DPD coefficients will be ill-conditioned and the distortion cancellation will be degraded. This paper proposes a method of selecting the best model order of the DPD coefficient estimation for the available OBW. Adjacent channel power ratio (ACPR) is used as an independent measure of the distortion cancellation for a given model order. Experimental results demonstrate the relationship between the model order of the estimation, the OBW, and the ACPR performance of the wide bandwidth IMD cancellation.
