On the Use of PAM versus QAM Constellations in Rate-Splitting Multiple Access

A comparison between the use of PAM and QAM constellations in Rate-Splitting Multiple Access (RSMA) is presented. Previous studies of RSMA with discrete or finite input signaling have considered either QAM or PSK. We propose the use of PAM constellations for every stream, with a phase rotation of 90 degrees applied to the common stream to achieve orthogonality with private streams. Numerical results based on data rate and Symbol Error Rate (SER) analysis indicate that PAM is superior to QAM when there is a significant disparity between channels for a two-user scenario. This finding indicates that PAM constellations can extend the operational region of RSMA over other Multiple Access (MA) strategies.