Unified Framework Towards Flexible Multiple Access Schemes for 5G

Non-orthogonal multiple access(NOMA) schemes have achieved great attention recently and been considered as a crucial component for 5G wireless networks since they can efficiently enhance the spectrum efficiency, support massive connections and potentially reduce access latency via grant free access. In this paper, we introduce the candidate NOMA solutions in 5G networks, comparing the principles, key features, application scenarios, transmitters and receivers, etc. In addition, a unified framework of these multiple access schemes are proposed to improve resource utilization, reduce the cost and support the flexible adaptation of multiple access schemes. Further, flexible multiple access schemes in 5G systems are discussed. They can support diverse deployment scenarios and traffic requirements in 5G. Challenges and future research directions are also highlighted to shed some lights for the standardization in 5G.