Non-orthogonal radio access technologies

The radio resource scarcity of classical orthogonal radio access-based technologies has encouraged academia and industry to search for a way out in order to support the exponential growth of data rate, system capacity, and communication latency in beyond 5G wireless communications. Recently, the concept of nonorthogonal radio accessing, where user equipment (UEs) with different services intentionally share the same radio resources, is proposed to compensate the radio resource scarcity. The focus of this chapter is to emphasize the potential of nonorthogonal access technologies in next generation networks. The fundamentals of power-domain non-orthogonal multiple accessing (PD-NOMA) are revised considering system sum-rate and user fairness and compared with conventional orthogonal frequency division multiple access (OFDMA). Furthermore, state-of-the-art NOMAbased schemes, including low-density spreading (LDS) and index modulation (IM), grant-free (GF) random access, and relatively novel concept of waveform coexistence for multiple accessing, are discussed. Finally, future directions and potential non-orthogonal radio access technologies are analyzed considering practical scenarios.