A Reduced Complexity Generic NOMA MAP Receiver for Wideband MIMO Channels

Multiple input and multiple output (MIMO) technology and non-orthogonal multiple access (NOMA) are crucial methods for future wireless systems as they provide many advantages over conventional systems. Power domain NOMA methods are investigated in MIMO systems extensively, whereas there is little work on the integration of practical code domain NOMA schemes and MIMO, which is the subject of this study. We propose a generic maximum a posteriori (MAP) receiver, where sparse code multiple access (SCMA) and multi-user shared access (MUSA) can be directly adapted. The proposed technique combines the index modulation in code domain with channel division multiple access (ChDMA) on which a novel receiver architecture is structured at significantly reduced complexity for highly dispersive channels. It outperforms conventional methods with very limited number of radio frequency (RF) chains. Furthermore, we provide bit-error rate performance simulations and analysis in terms of achievable information rate (AIR) showing mismatched decoding capacity under different code lengths and overloading scenarios for uplink transmissions where flexible structure of MUSA is exploited.