Analysis of the effects of linearity and efficiency of amplifiers in QAM systems

In this paper, we provide a comprehensive study on the performance of the quadrature amplitude modulation (QAM) in the presence of nonlinear amplifiers. The study includes both in-band and out-of-band distortion by incorporating nonlinearity induced adjacent channel interference (ACI). Two objective functions proposed in the literature are studied: 1) total degradation and 2) total bit energy-to-noise power density ratio for a target bit error rate. We also consider both fixed and adaptive DC bias controlled amplifiers. The two objective functions result in identical designs in the case of fixed bias amplifiers but not in the case of adaptive bias controlled amplifiers. In the later case the second objective function is more useful.