This paper proposes an iterative algorithm for multi-user detection of a synchronous CDMA communication system in the presence of channel distortion based on a maximum a posteriori (MAP) estimation. The proposed algorithm employs the known finite alphabet of discrete symbols as a priori information to improve symbol estimation accuracy. This paper extends our previous work (Blunt and Ho, 2002) to develop a MAP estimator that takes into account both the structure of the particular symbol constellation and the transmission diversity resulting from channel distortion to increase dramatically the accuracy of multi-user symbol estimation.