Effect of HPA non-linearities on crosstalk and performance of digital radio systems

The performance of 64-QAM and 256-QAM systems amplified by nonlinear power amplifiers (HPAs) is investigated. Two types of HPAs are considered in the analysis. The quadrature crosstalk between the I and Q channels due to AM/AM and AM/PM conversion of the nonlinearities of the HPAs is studied by computer simulation and experimentally. The results reveal that the quadrature crosstalk is mainly caused by AM/PM conversion and that crosstalk is reduced when the output power backoff (OBO) is increased. However, for some HPAs, the crosstalk has the minimum in a certain range of OBO. The power spectra of intermodulation products caused by the nonlinearities of HPAs are shown. In addition, the probability of error performance, P(e), is computed for two types of HPAs and different OBO. A comparison of eye and constellation diagrams for TWTA operating at the different OBO is also given