Research On Predistortion Technique In OFDM Systems

It is well known that HPAs (High Power Amplifiers) are inherently nonlinear devices. OFDM (Orthogonal Frequency Division Multiplexing) exhibits large peak-to-average power ratios, and the performance of the transceivers is very sensitive to nonlinear distortions, which arises mainly from the high power amplifier. Many researches have focused on the predistortion for memoryless HPAs. However, memory effects of HPAs can no longer be ignored when communication systems have wider bandwidth. Memoryless predistortion techniques proposed previously seldom have satisfactory effectiveness for typical wideband applications.Predistortion with memory effects has become the dominating trend in the OFDM systems. In the following sections, we will concentrate our analysis on the distortions mutually caused by nonlinearity and memory. In this paper, an improved adaptive predistortion method called 2D LUT (2-dimension look-up table) with different accuracy levels is presented to linearize HPAs with memory effects. Simulation and experimental results show that 2D LUT implements excellent performance in mitigating the signal deterioration caused by memory effects, both rectifies the signal constellation distortion and suppresses the spectrum emission. Large scale matrix computation is also avoidable in these adaptive algorithms, which makes them feasible when a real-time system is necessary.