The effective signal-to-noise and distortion ratio (SNDR) at the output of a nonlinear amplifier is defined through the decomposition of the nonlinear output into correlated output and uncorrelated distortion. The analysis is based on the orthogonalisation of the nonlinear behavioual model that allows the accurate estimation of the effective in-band (or co-channel) distortion and hence determination of SNDR. Fundamental issues regarding the evaluation of the effective in-band distortion and its effect on digitally modulated signals are discussed. Simulations of in-band distortion and SNDR of WCDMA signals are verified experimentally using feed-forward cancelation. 2 Orthogonal behavioural model development Many nonlinear models are available for behavioural modelling of nonlinear circuits; however, the developed model presented here is intended to provide insight into how nonlinear in-band distortion contributes to signal-to-noise and distortion degradation. To this end, the behavioural model is developed by orthogonalising the nonlinear response in respect to the input signal and each branch of the model such that each component at the output of the model is orthogonal to each other. In the proposed model, a complex power series behavioural model of the nonlinear
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