The achievable rate of stationary rayleigh flat-fading channels with IID input symbols

In this work, we derive a new upper bound on the achievable rate of stationary Rayleigh flat-fading channels with i.i.d. input symbols. The novelty lies in the fact that this bound is not restricted to peak power constrained input symbols like known bounds, e.g., in [1] or [2]. Therefore, the derived upper bound can also be used to evaluate the achievable rate with i.i.d. proper Gaussian input symbols, which are capacity achieving in the coherent case. The derivation of the upper bound is based on the prediction error variance of the one-step channel predictor.