ss can be theoretically modeled as a multiple-access channel and a broadcast channel, respectively. The capacity region of the Gaussian broadcast channel can be achieved by power-domain superstition coding with a successive interference cancellation (SIC) receiver. Meanwhile, the capacity region of Gaussian multiple-access channel corre- sponds to CDMA, where different codes are used for the different transmitters, and the receiver decodes them in an SIC manner. In general, a user with poor-channel condition tends to allocate more transmission power, SO this user would decode its own messages by treating the co-scheduled user's signal as noise. On the other hand, a user with good chan- nel condition applies the SIC strategy by first decoding the information of the poor- channel user and then decoding its own, removing the other users' information. The results of studies in 3GPP suggest that using a non-SIC receiver results in negligible per- formance degradation in many cases [23]. Relaxing the need for an SIC receiver signifi- cantly would reduce the decoding complexity for the downlink case as the others' codebooks are no longer required. In addition to the orthogonal MA scheme, the 3GPP NR may support an uplink non- orthogonal transmission (Note: at the time of publication of this book, 3GPP has decided not to specify non-orthogonal multiple access in Rel-16 and instead only specify a two-step RACH) to provide the massive connectivity that is desperately required for applications in mMTC as well as other scenarios. 3GPP has further studied grant-free uplink multiple-access schemes for mMTC scenarios. Since there is no need for a dynamic and explicit scheduling grant from gNB, latency reduction and control signaling minimization could be expected. For uplink non-orthogonal multiple access, network information theory suggests that CDMA with a SIC receiver provides a capacity achieving scheme. However, securing uplink non- orthogonal multiple access gain requires further system design enhancement. As the number o