## 2 Advances in Multiple Access Techniques for 6G Mobile Wireless Networks  
In this section, we provide an overview of MA technologies of modern wireless communications. Then, we compare the key features of RSMA with existing MA technologies. As shown in Figure 1, since the first generation (1G) of mobile communication was introduced in the 1980s, a new generation of wireless communication systems has emerged approximately every decade. Note that the 1G system is the analog communication that supports only voice calls. In addition, since 1G was a region-specific standard, there were limitations on international roaming services. For example, the Advanced Mobile Phone System (AMPS) and C-450 were only used in the United States and Germany, respectively. As an MA technology, 1G employed frequency division multiple access (FDMA) that allocates channels to multiple users by dividing the available frequency band into non-overlapping sub-bands. The second-generation (2G) communication utilizes digital communication, enabling data transmissions such as text messages and emails. Compared to 1G, the roaming services became more widely available since many European countries employ the same standard such as Global System for Mobile Communications (GSMs). The MA technique used in 2G GSM is time division multiple access (TDMA). TDMA divides the signal into multiple time slots by sharing the same frequency band.  
The third generation (3G) refers to the era of multimedia services where the Internet and video calls are available. The standard of 3G communication is based on the Universal Mobile Telecommunication System (UMTS) and the MA technology of 3G is code division multiple access (CDMA), which allows multiple users to simultaneously share the same frequency band by using different codes. It is worth noting that there was a substantial increase in mobile communication subscribers due to the introduction of smartphones with multi-touch screens around 2007.  
The key feature of the fourth-generation (4G) communication is the true realization of mobile Internet, supporting services such as video streaming, gaming, and social network service (SNS). Note that the international standard leading 4G is Long-Term Evolution  
Figure 1: Evolution of mobile communication from 1G to 6G.  
(LTE). Two key technologies in LTE are orthogonal frequency division multiple access (OFDMA) and MIMO.  
The fifth generation (5G) was commercialized in 2019 and three key usage scenarios of 5G are eMBB, mMTC, and URLLC. To provide much higher data rates and lower latency compared to the previous generations, 5G exploits new frequency bands such as mmWave. Fifth-generation systems also adopt various innovative technologies such as massive MIMO and network slicing. Massive MIMO utilizes a large number of antennas to perform high-precision beamforming and network slicing allows for the customized networks tailored to any key usage scenarios. It is worth noting that the MA technology remains OFDMA; on the other hand, while OFDMA was only used for downlink in 4G, in 5G, it is applied to both uplink and downlink.  
The MA technologies described so far are OMA, which divides frequency and time resources to ensure orthogonality. As the number of connected devices is growing explosively, it is burdensome for OMA to serve these devices simultaneously. To address this challenge, it is necessary to enable multiple users to share the same time and frequency resources.