Interleave division multiple access (IDMA) is a Non-Orthogonal Multiple Access (NOMA) technique regarded as an enabling technology for next generation wireless systems. IDMA can improve the system efficiency by supporting multiple access for a large number of stations. For high spectral efficiency transmission in IDMA, previous works proposed systems in the context of Superposition Coded Modulation (SCM) where multiple layers of BPSK or QPSK modulated symbols are transmitted simultaneously. However this method has a very high complexity due to the high number of streams that need to be separated in the multi-user detection of the receiver. In this paper, instead of SCM, we employ QAM modulation up to 256-QAM for high spectral efficiency transmission. We show our receiver architecture which uses a soft demapper that significantly decreases the receiver detection complexity. While a maximum number of users that can be accommodated in the proposed system is slightly less than the conventional, our proposed system is much more suited in modern multi-mode transceivers aside from the fact that it needs about 25% complexity compared with SCM-QPSK.
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