Non-Orthogonal Multiple Access Based M-Ary CDMA Systems

Code division multiple access (CDMA) is missing in 4G and 5G systems, albeit it possesses many salient technical advantages over OFDMA, such as its unique multipath diversity gain with RAKE receiver. This work aims to propose a way to vitalize the performance of CDMA systems with M-ary code modulation (i.e., M-ary CDMA). In particular, non-orthogonal multiple access (NOMA) technique is leveraged to augment spectrum efficiency. Built upon the foundation of a high spectrum efficiency of M-ary CDMA systems, NOMA is integrated with M-ary CDMA for enhancing its user capacity further. Specifically, this work showcases the designs of downlink transmitter and receiver structures for a NOMA based M-ary CDMA system, and validates the effectiveness of the proposed scheme in terms of improved user capacity and bit error rate via simulations. Finally, the challenges and limitations faced by the proposed scheme are identified and the directions for future research are given.