Misalignments Feedforward Transmitter Correction Design for Nonlinear Distortion Cancellation in OFDM Systems

摘要

The Feedforward linearization technique is a well-known power amplifier linearization method to reduce out-of-band distortion that can approach wideband signals with high efficiency. The Feedforward structure consists of two loops, where an accurate adjustment between them is necessary to ensure a correct behaviour. This adjustment must remain along the time to compensate for temperature, environmental and operating changes. Amplitude and phase imbalances of the circuit elements in both loops produce mismatched effects that lead to degrade the performance of the Feedforward technique. A method is proposed to compensate for these mismatches, introducing two complex gain coefficients calculated by means of a genetic algorithm. The method is applied to broadband systems based on OFDM modulation schemes. The OFDM transmission is an efficient way to deal with multipath, being its implementation less complex than traditional equalizers. This together with an efficient implementation, using the Fast Fourier Transform algorithm, has made it ideal for new technology. However, it is also very sensitive to nonlinear distortions due to its greatly variable envelope and high peak to mean envelope power ratio values. Therefore, Feedforward is a good linearization method for OFDM systems because simultaneously offers wide bandwidth and good nonlinear distortion suppression. The method functionality has been verified by means of simulation.