The further integration of telecommunications and industry has been considerable and is expected to bring significant benefits to society and economics in 6G. It also forms some evolution trends for next-generation communication systems, including further rises in machine-type communications (MTC), uplink-dominated systems, and decentralized structures. However, the existing access protocols are not friendly to these trends. This paper analyzes the problems of existing access protocols and provides novel access technologies to solve them. These technologies include contention-based non-orthogonal multiple access (NOMA), data features, enhanced pilot design and successive interference cancellation (SIC) of diversity. With these key enablers, truly grant-free access can be realized, and some potential modifications of protocols are then analyzed. Finally, this paper uses massive and critical scenarios in digital transformations to show the great necessity of introducing novel access technologies into future communication protocols.

Multiple access scheme is one of the key techniques in wireless communication systems. Each generation of wireless communication is featured by a new multiple access scheme from 1G to 4G. In this article we review several non?orthogonal multiple access schemes for 5G. Their principles, advantages and disadvantages are discussed, and followed by a comprehensive comparison of these solutions from the perspective of user overload, receiver type, receiver complexity and so on. We also discuss the application challenges of non?orthogonal multiple access schemes in 5G.