The Research On Adaptive Predistortion Technology With Memory In OFDM System

Orthogonal frequency division multiplexing (Orthogonal Frequency Division Multiplexing, OFDM) has been recognized as one of the key technologies in 4G (B3G), which has high spectral efficiency, excellent anti-noise performance and anti-multipath fading capability. However, the PAPR (Peak to Average Power Ratio) of the OFDM signal is very large, which has great demand of the linearity of RFå'Œhigh power amplifier. If the amplifier can not meet the dynamic range of the signal changes, it will produce non-linear distortion, resulting in spread spectrum, as well as in-band signal distortion, resulting in the various sub-channel OFDM signal orthogonality between the destruction and mutual interference, deterioration in the performance of the system. In order to reduce OFDM signals affected by power amplifier nonlinear characteristics, the power amplifier pre-distortion techniques must be used to compensate for the nonlinear of power amplifier, which is one of the hot topics in OFDM now. Therefore, the application of adaptive look-up table pre-distortion technology in OFDM system is mainly analyzed and studied in the paper, and the simulation is made by MATLAB to verify the performance.First of all, the research developments of the orthogonal frequency division multiplexing, the basic concepts of the orthogonal frequency division multiplexing and the advantages and disadvantages of the orthogonal frequency division multiplexing are given. Then, the basic principles and key technologies of the orthogonal frequency division multiplexing are introduced, such as the realization of fast Fourier transform, the issue of PAPR in OFDM system. Subsequently, the measurements of non-linear power amplifier parameters are described, three kinds of non-linear power amplifier distortion are explained and the mathematical model of the power amplifier is analyzed in detail. Finally, two kinds of common pre-distortion technologies are discussed, two adaptive algorithms which used in power amplifier linearization technology are researched, and the adaptive look-up table predistorter is designed for OFDM system, which based on memory effect model of power amplifier. The loop delay estimation module is increased in the pre-distorter. The processing delay exists in the coupled feedback loop is solved, which affect the accuracy of the updated look-up table. The distortion determination module also added which judge the feedback slip signals periodically to determine whether to open lookup table refresh module for system updates. Simulation and result show that the predistorter can reduce the computation effectively and save system power significantly, effectively improve the OFDM system performance.