Digital Predistortion Techniques for Rf Power Amplifiers with Cdma Applications

With the increasing importance of spectral efficiency in mobile communications, linearity of the RF PA has become a critical design issue for non-constant-envelope digital modulation schemes.1 This issue is particularly significant in spread spectrum applications such as CDMA and wideband CDMA (W-CDMA) base stations, where the peak-to-average ratio of modulated RF signals can vary over a range of 3 to 12 dB. The concern for linearity is primarily due to the stringent restrictions on intermodulation products and out-of-band power emission requirements. Furthermore, amplification of multicarrier (multichannel) signals requires adequate amplifier linearity in order to avoid significant cross modulation. Additionally, for bandwidth-efficient modulation the amplifier nonlinearity can produce substantial signal distortion and, hence, increased bit error rates (BER).2 Linearity is achieved, in part, through the use of more linear amplifiers such as class A amplifiers, and by operating the amplifier backed off from the saturation range so that the signal level is confined to the linear region of the amplifier characteristics. However, this approach results in low DC-to-RF conversion efficiency, which is particularly costly in base station applications. Furthermore, low DC-to-RF conversion necessitates high current operating points, resulting in undesired thermal effects. DIGITAL PREDISTORTION TECHNIQUES FOR RF POWER AMPLIFIERS WITH CDMA APPLICATIONS