| Most of the wireless communication systems are required to achieve least adjacent channel emissions and high bandwidth efficiency, that is to say signals must be transmitted in a limited band. However, distortion caused by nonlinear devices certainly will disturb other channels. In order to take good advantage of the limited band and contain more channels, modern communication systems often use linear modulation and wideband transmission techniques. Typical modulation techniques, such as QAM (Quadrature Amplitude Modulation) and QPSK, have varying envelopes. Devices working near saturation will produce inter-modulation distortion while those kind of signal flowing through nonlinear power amplifier, specifically the distortion brought by the AM-AM and PM-PM character of the amplifier which will disturb other users. In modern mobile communication systems, the adjacent channel interference requirements range from -45 to -60 dBc relative to the in-band carrier level. The linear modulation signal which has the advantage of high band efficiency is sensitive to the nonlinear distortion of the system because of its nonconstant envelope. In order to obtain high output power, amplifier has to work at saturation situation, while the nonlinear distortion becomes severer as the power is increasing.People have done widely research on the linearization technique of RF power amplifier in modern wireless communication systems, such as GSM, IS-95 serial, CDMA2000 and WCDMA.This theme adequately analyses several main types of linearization techniques people proposed: feed-forward, feedback, and predistortion. The application of the digital signal processing combined with the RF circuit makes the predistortion algorithm more efficient and the system more flexible, stable and better performance.At last, based on the actual testing results, the existing problems are pointed out, as well as the possible method for improvement. |
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