Study On Analog Predistortion Of RF Power Amplifier Based On OFDM Signal

The current trend in mobile terminal devices is to become smaller and lighter while targeting a longer operation time. The non-constant envelope digital modulation schemes, especially having high peak-to-average ratio (PAPR) such as OFDM, are used in many modern wireless systems. It makes the Radio Frequency (RF) Power Amplifier (PA) design crucial to satisfy good linearity and high power efficiency at the same time.This research is mainly about analog predistortion in OFDM system.Firstly, two nonlinear characteristics have been studied, AM-AM conversion and AM-PM conversion. And then some descriptive measures of RF PA nonlinearity have been shown. Furthermore, we introduced some basic linearization techniques:feedforward, feedback, LINC and predistortion.Secondly, we focused on analog predistortion technique and single diode predistorter, including series diode predistorter, the parallel diode predistorter and varactor diode predistorter.In this thesis, we propose to use an analog predistorter to compensate medium-level nonlinearity of PA in quasi-saturation region. In order to model the complicated nonlinearity in quasi-saturation region, the Rapp model is employed to fit both the ideal clipping device and FET device represented by Curtice FET model.The linearizer we employed is series-diode configured type, which can optimize the linearization performance by controlling the bias voltage of the diode. From simulation results, the third-order inter-modulation distortion ratio (IMD3) can be reduced by-10 dB on average when a two-tone signal is input.Finally, the bias-controlled predistorter has been used in OFDM system. IEEE 802.11a OFDM signal is adopted as system input. It is also concluded that the predistorter can effectively decrease the out of band emission in OFDM signal.