Look-up table (LUT) predistortion is the most promising technique to linearize RF power amplifiers (PAs). Before it can be employed successfully, loop delay errors must be estimated and compensated accurately. In this paper, the effect of loop delay errors on LUT predistortion is analyzed. It is shown that LUTs fluctuations happen to both amplitude and phase tables. Then, a novel loop delay estimation algorithm, with a high degree of robustness and accuracy, is presented. To further improve the robustness of LUT predistortion to loop delay errors, the smoothing filter (SMF) method to polish LUTs fluctuations is proposed here. LUT predistortion can suffer bigger loop delay errors because of the proposed SMF method, but retains good linearization performance close to the optimal one. The performance of the proposed loop delay estimation algorithm is investigated by comparative study with a state-of-the-art algorithm. Its robustness is also demonstrated under a noisy feedback path by simulations. The effectiveness of the SMF method is assessed by both simulations and experiments. Comparison among different predistortion systems is also presented. The results clearly show that the proposed techniques are useful for LUT predistortion.
An analytical method to design a power amplifier (PA) with an optimized power added efficiency (PAE) trajectory for envelope tracking (ET) architecture is proposed. To obtain feasible matching solutions for high-efficiency performance of the PA in the dynamic supply operation, hybrid continuous modes (HCM) architecture is introduced. The design space for load impedances of the HCM PAs with nonlinear capacitance is deduced mathematically using the device’s embedding transfer network, without the necessity of using load-pull. The proposed design strategy is verified with the implementation of a GaN PA operating over the frequency range of 1.9 GHz to 2.2 GHz with PAE between 67.8% and 72.4% in the 6.7 dB back-off power region of the ET mode. The ET experimental system was set up to evaluate the application of the PA circuit. Measurement results show that the ET PA at 2.1 GHz reaches the efficiency of 61%, 54%, 44% and an error vector magnitude (EVM) of 0.32%, 0.60%, 0.67% at an average output power of 34.4 dBm, 34.2 dBm, 34.1 dBm for 6.7 dB peak-to-average power ratios (PAPR) signals with 5 MHz, 10 MHz, and 20 MHz bandwidths, respectively. Additionally, tested by a 20 MHz bandwidth 16 quadrature amplitude modulation (QAM) signal, 41.8% to 49.2% efficiency of ET PA is achieved at an average output power of 33.5 dBm to 35.1 dBm from 1.9 GHz to 2.2 GHz.
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