A Combined Approach to Digital Predistortion and Crest Factor Reduction for the Linearization of an RF Power Amplifier

A combined approach to digital predistortion (DPD) and crest factor reduction (CFR) is proposed. The new CFR is structured similar to DPD and is implemented by introducing a steady-state offset into the DPD coefficients. The DPD and CFR coefficients are estimated using separate adaptive processes but applied to the transmission path in a common module. The DPD/CFR module provides the means to exploit margins in the transmitter performance, allowing the tradeoff between peak-to-average-power ratio (PAPR), error vector magnitude, and adjacent channel power ratio (ACPR). The proposed approach applies CFR to the predistorted signal instead of the input signal. This allows the envelope clipping module, which is typically present to protect the power amplifier (PA), to be removed, thereby avoiding divergence problems during the iterative closed-loop estimation of the DPD coefficients. Results show that post-CFR lowers the PAPR of the predistorted signal by 5 dB, which reduces the stress on the peaking transistor in a Doherty PA. The combined DPD/CFR reduces the ACPR of the transmitter by 21 dB compared with the unlinearized PA.