# Revolution or Evolution? Technical Requirements and Considerations towards 6G Mobile Communications
## 6 Enablers for 6G and Relevant Technical Preparation and Implementation
### New Multiple Access Techniques  
The previous generations of cellular networks have adopted radically different multiple access techniques with one common theme in mind: to have orthogonal signals for different users at the receiver side. In view of emerging applications and in order to fulfil the need for massive numbers of connections with diverse requirements in terms of latency and throughput, 6G networks are experiencing a paradigm shift in design philosophy, shifting from orthogonal multiple access (OMA) to non-orthogonal multiple access (NOMA) design. NOMA superposes users at the same time-frequency resource and distinguishes them in the power domain. Compared to the conventional OMA schemes, NOMA allows users to simultaneously share the same resource blocks that are beneficial for supporting many connections in spectrally efficient communications. To date, a variety of NOMA technologies, including power domain NOMA (PD-NOMA), pattern division multiple access (PDMA), sparse code multiple access (SCMA), multi-user shared access (MUSA), and so on, have been proposed [106]. Recently, rate-splitting multiple access (RSMA) [107] has emerged as a powerful and robust non-orthogonal transmission technique and interference management strategy for cellular networks.  
In today's wireless networks, access points commonly employ more than one antenna, which opens the door to multi-antenna processing. The key building block of the downlink of multi-antenna networks is the multi-antenna (Gaussian) broadcast channel (BC). NOMA was initially designed for a single antenna (single-input single-output; SISO) BC. Contrary to the SISO BC that is degraded and where users can be ordered based on their channel strengths, the multi-antenna BC is non-degraded, and users cannot be ordered based on their channel strengths. This is the reason that superposition coding NOMA is not capacity-achieving [108]. RSMA is a more general and more powerful multiple access scheme that relies on linearly precoded rate-splitting with successive interference cancellation (SIC) to decode part of the interference and treat the remaining part of the interference as noise. This provides an opportunity for rate and QoS enhancements and complexity reduction. However, the information and communication theoretic analysis in specific scenarios, to understand benefits in terms of spectral efficiency, reliability, and CSI feedback overhead reduction over conventional strategies used in 5G, is still lacking at the moment.  
Moreover, despite several emerging proposals focusing on multiple access techniques, there is very little understanding of the performance of these techniques to fulfill 6G requirements. Furthermore, exploitation of potential opportunities to infuse AI/ML into new multiple access techniques has not yet been attempted. At the moment, several multiple access schemes have been devised as candidates for the 6G waveforms. However, recent literature has suggested the RSMA as the most promising multiple access candidate that is expected to fulfill the 6G requirements [109]. Figure 9 presents the classification of the potential 6G multiple access techniques.