## 2 Advances in Multiple Access Techniques for 6G Mobile Wireless Networks
### Comparison of RSMA Technology with Existing Multiple Access Technologies  
By adjusting the ratio and power of common and private messages, RSMA can include NOMA, SDMA, OMA, and multicasting techniques as special cases. As shown in Figure 2, through a two-user message construction example, we can show that existing MA technologies can be regarded as special cases in RSMA. Recall that RSMA splits each user'smessage into a common and private part. For example, user 1's message \(W_{1}\) is divided into a common part \(W_{c,1}\) and a private part \(W_{p,1}\). Similarly, user 2's message \(W_{2}\) is divided into a common part \(W_{c,2}\) and a private part \(W_{p,2}\). Here, \(W_{c,1}\) and \(W_{c,2}\) are combined to form a common message \(W_{c}\), while \(W_{p,1}\) and \(W_{p,2}\) become the private message for each user. In the case of SDMA, the power for the common message is set to zero in RSMA, which means that users' messages consist of only private messages. In other words, user 1's message \(W_{1}\) consists entirely of the private part \(W_{p,1}\), and user 2's message \(W_{2}\) consists entirely of the private part \(W_{p,2}\). For NOMA, one user has only the common message and the other user has only private message. For example, if user 1 has a higher channel gain than user 2, then user 1's message \(W_{1}\) becomes entirely the common message \(W_{c}\), while user 2's message \(W_{2}\) becomes entirely the private message \(W_{p,2}\). In this case, user 1 completely decodes user 2's common message and removes this interference through the SIC operation. Meanwhile, user 2 decodes its own message by regarding user 1's private message as noise. In OMA, since a single user occupies a wireless resource at a time, only user 1's message \(W_{1}\) is used entirely as the private message \(W_{p,1}\) and transmitted while user 2's message \(W_{2}\) is not transmitted. In a different time slot or frequency, user 2's message \(W_{2}\) is used entirely as the private message \(W_{p,2}\) and fully transmitted, while user 1's message \(W_{1}\) is not used. In the case of multicasting, the same message is transmitted to both users as the common message. Similar to RSMA, multicasting uses a common message that all users decode. The difference between multicasting and RSMA lies in how the common message is composed. In general, the common message in multicasting is generated based on service demands, such as video streaming. In contrast, RSMA designs the common message to mitigate interference between users. It is worth noting that RSMA is robust to imperfect CSI compared to existing MA techniques. When perfect CSI is available at the transmitter, linear precoding techniques such as SDMA can effectively manage interference between users. However, since SDMA treats interference as noise (TIN), it can suffer from significant performance degradation under imperfect CSI conditions. SDMA achieves near-optimal performance when the inter-user interference is weak in the wireless communication systems with perfect CSI at the transmitter (CSIT). However, in general, perfect CSIT cannot be guaranteed in practical wireless communication systems due to the limited feedback resources. In such a wireless environment, TIN is far from optimal. This encourages the use of more aggressive types of interference management schemes such as a joint decoding (JD) type. As a JD type of interference management strategy, NOMA achieves near-optimal performance when one interference link is much stronger than the other. It is worth noting that RSMA can embrace both TIN and JD types of interference management strategies. Since RSMA can include  
Figure 2: Resource allocation for various MA technologies. both SDMA (TIN) and NOMA (JD) as special cases by adjusting the common message's rate, power, and content, RSMA can outperform existing MA techniques in all scenarios [5]. Specifically, when the rate of the common message is zero, RSMA reduces to SDMA.