Linear and nonlinear filter distortion effects on MSK modulated signals

We study the effects of linear and nonlinear filter distortions on MSK modulated signals. The linear filter distortions considered include bandlimiting and impairments described by a polynomial decomposition of the amplitude and phase response, i.e., linear amplitude, parabolic amplitude, residual amplitude, parabolic phase, cubic phase and residual phase. The nonlinear distortions considered are memoryless AM/AM and AM/PM conversions. The class of nonlinear filters that can be modeled by a linear distortion followed by memoryless AM/AM and AM/PM conversions can be understood by studying the combined effects of these individual distortions. This model can be used for modeling TWTAs and other nonlinear amplifiers. Using this approach we show how certain linear filter distortions can precompensate for subsequent nonlinear distortions. Specifically, we demonstrate that when the phase shift vs. input power curve exhibits a negative slope, i.e. AM/PM<0, the positive parabolic phase tends to reduce the net phase variation as compared to the negative parabolic phase. Conversely, for AM/PM>0, the negative parabolic phase reduces the net phase variation as compared to the positive parabolic phase.