Modeling and compensation of CDMA power amplifiers.

Abstract

The efficient operation of a linear high power amplifier (HPA) often results in enhancing its inherent nonlinear behavior. This nonlinear behavior causes co-channel and adjacent channel interference due to intermodulation distortion and spectral regrowth of the sidebands. These deleterious effects, especially the sideband regrowth, are of great concern in present second generation cellular telephone systems such as IS-95 and will have an even greater impact in the new wideband third generation cellular telephone systems such as IS-2000.

Compensation (linearization) allows an HPA to operate efficiently while mitigating the effects of its nonlinear behavior. A digital baseband predistorter based on the "fixed point" approach (FPPD) is developed to compensate the wideband HPA in a CDMA cellular telephone system. The FPPD incorporates a memoryless nonlinear HPA model derived from samples of the HPA's input and output signals to overcome shortcomings in previous FPPDs. The FPPD's performance is evaluated using an IS-2000 forward link simulator. The link performance for a perfectly linear HPA, nonlinear HPA, and compensated HPA are compared. The FPPD's effect on reducing sideband regrowth, as compared to just backing off the HPA's input signal, is also examined.