Ofdm System Power Amplifier Pre-distortion Technology

Orthogonal frequency division multiplexing (OFDM) system uses a multi-carrier modulation method with orthogonal sub-carriers. It is of great flexibility, resistant to fast fading. And it can also achieve a higher spectral efficiency than the single carrier scheme. So it is widely used in high speed communication systems. However, because of its high peak-to-average power ratio (PAPR), OFDM is sensitive to nonlinear distortion, and requires power amplifier with high linearity. Nonlinear distortion of power amplifier is one of the major factors that degrade the performance of OFDM systems. In order to reduce nonlinear distortion of power amplifier and improve system performance, it is necessary to do pre-distortion processing for OFDM signals to compensate the nonlinearity of power amplifier.Frequently used nonlinear models of power amplifier are introduced in this paper, and the effect of nonlinear power amplifier is analyzed theoretically for OFDM systems. The existing technologies used to reduce nonlinear distortion of power amplifier are summarized, and compared with each other on power efficiency, adaptability and realization cost. Then by combining the advantages of look-up table based pre-distortion and polynomial based pre-distortion, a new pre-distortion technology is researched in this paper. This technology divides the nonlinear function of power amplifier into several segments, and then estimates each segment respectively with a lower order polynomial. And then amplitude transformer and phase transformer are used to compensate the nonlinearity of amplifier according to the estimated parameters. Simulation results show that this method can improve amplifier's performance to 4.3 dB.In order to reduce the extra RF devices used for direct feedback,this paper propose a new conception of indirect feedback pre-distortion. That is, instead of estimating nonlinearity at transmitter as direct feedback pre-distortion, this paper estimates the nonlinearity of amplifier at the receiver. Then these parameters are transferred to transmitter through backward link, and used for pre-distortion processing. The problem is that nonlinear and multi-path dispersion mix together at receiver end. And their parameters are generally unknown, so conventional method doesn't work well. In this paper, an algorithm that estimates both the nonlinear parameters of the power amplifier and the impulse response of the wireless channel is proposed. Multi-level PN sequences are transmitted to construct a Vandermonde matrix, which separates the nonlinearity and linearity into different subspaces. Finally, indirect feedback pre-distortion is processed at transmitter end. Simulation results show that indirect feedback pre-distortion can work well over fading channel.