The purpose of this paper is to analyze and determine analytically the BER (Bit Error Ratio) performance for multi-amplitude constellations with in-phase and quadrature components, thus avoiding the need for extensive, lime-consuming computer simulations. This paper describes new theory approach, which leads to original solution, for calculating the BER performance of a 1Tx-MRx "1 Transmitter - M Receivers" scheme with channel estimation errors and flat Rayleigh fading. Results are related to 4PAM, 8PAM, 16QAM and 64QAM modulation schemes, with Gray coding.