Research On Key Technology Of Band - Limited Digital Predistortion System

With increasing demand on data capacity and the transmitting data rate, not only the highly efficient modulation is developed, but also the spectrum is spread in modern wireless communication system. The peak-average power radio of those signals modulated by the high efficient technology is high, which requires high linearity of radio frequency power amplifiers (PAs) and aggravates the contradiction between high linearity and high efficiency of power amplifier. Linearization techniques of power amplifier are used to alleviate the contradiction. Among those techniques, digital predistortion (DPD) has the advantage of high linearity performance, adaptable, et al, which is researching hotspots currently. However, with the spread of the signal spectrum, it causes a problem the feedback loop need high sampling rate of ADC, which increases the cost and difficulty of the DPD system’s implementation. In order to solve those problems, the paper carried out research on the wideband DPD.Firstly, in order to solve the problem of requiring very high order baseband band-limiting filter to guarantee the conventional band-limited memory polynomial model accuracy, a low-complexity band-limiting DPD is proposed. The method can reduce the required order of band-limiting filter and guarantee a high accuracy. Measurement results have shown the reduction ratios in the number of coefficients and FLOPs of the proposed model compared to those of the BLMP model are nearly 28% and 64%, respectively.Secondly, in order to solve problem of the loop delay estimation in DPD system, a loop fractional delay estimation algorithm based on the adaptive algorithm is adopted. The algorithm can improve the accuracy. The proposed algorithm is based on the adaptive algorithm, which is significance in the hardware implementation.The last, due to the conventional band-limited DPD cannot suppress the spectral regrowth outside the band-limiting filter, which the distortion of this region will interfere with the adjacent channel, and must use a narrower band-limiting filter to discard the nonlinear information at the BPF’s roll-off regions, a band-divided MP DPD is proposed. Measurement results have demonstrated the proposed model has a lower model complexity than the BLMP model. The improved adjacent channel power ratio (ACPR) of the signal was nearly 19dBc, and the improved error vector magnitude (EVM) was nearly 5.51%. What’s more, the proposed model can suppress the distortion outside the BPF.