# Key Technologies for 6G-Enabled Smart Sustainable City
## 4 Key Candidate Technologies for Future SSC
### Evolution of Mobile Communication Systems  
After the introduction of the first analog communication in the 1980s, a new generation of communication systems has been introduced almost every decade. The first generation refers to analog communication capable of only voice calls. The second generation (2G) refers to the digital communication era that allowed for data transmission, such as email and text messages. The third generation (3G) refers to the multimedia communication era that enabled various services such as video and digital music via mobile phones. In the fourth generation (4G), long-term evolution (LTE) provided maximum transmission speeds of up to 1 Gbps in the downlink. It is worth noting that LTE brings the popularization of smartphones and enables high-definition videos and network gaming even while on the move.  
Recently, 5G was commercialized globally in 2019 [11]. The fifth generation can support transmission speeds of up to 20 Gbps, which is 20 times faster than 4G. It is well known that 5G has three key usage scenarios: eMBB, mMTC, and URLLC. Key features of 5G include the use of mmWave bands such as 28 GHz and network slicing, where each slice of a network can be employed for different purposes. Further, 5G imposes on the integration of mobile communication with vertical industries such as smart factories, connected vehicles, and smart cities.  
Sixth-generation mobile communication is expected to be commercialized around 2030. Considering the trend of innovative technology adoption in mobile communication across several generations, it is anticipated that 6G will offer significantly enhanced performance and groundbreaking services compared to 5G. In June 2023, ITU-R WP 5D completed the draft of the new recommendation for IMT-2030 (6G) [10]. In the 6G vision, there are six usage scenarios in total. Among these, three usage scenarios are extended versions of the eMBB, mMTC, and URLLC, and also three new usage scenarios are introduced. These include (1) immersive communication, (2) massive communication, (3) hyper-reliable and low-latency communication (HRLLC), (4) ubiquitous connectivity, (5) integrated sensing  
\begin{table}
\begin{tabular}{p{14.2 pt} p{142.3 pt}} \hline \hline
**6G Technology** & **Main Contents** \\ \hline Big Data & [ITU-T Y.3602] describes operations for big data provenance and provides the functional requirements for a big data service provider to manage big data provenance [24]: [ITU-T Y.3603] describes the general concept of metadata and its utilization in a big data ecosystem and provides requirements and a conceptual model of metadata for the data catalog [25]. \\ \hline IoT & [ITU-T Y.4210] specifies requirements for a universal communication module that is an important part of mobile IoT devices [26]; [ITU-T Y.4212] specifies the requirements and capabilities of network connectivity management in the IoT [27]. \\ \hline AI & [ITU-T M.3080] provides a framework of AI-enhanced telecom operation and management (AITOM) and describes the functional framework of AITOM to support telecom operation management [28]; [ITU-T M.3384] provides definitions, classifications, object selection, and an automatic evaluating mechanism for the evaluation of the intelligence levels of AITOM systems [29]. \\ \hline \hline \end{tabular}
\end{table}
Table 4: _Cont._and communication, and (6) AI and communication. To make these scenarios a reality, we need to support NTN as well as the terrestrial network (TN) for a wide range of coverage and mobility. In addition, by integrating sensing and AI with enhanced communication, the 6G network is expected to improve sensing accuracy, detection probability, and AI-related inference capabilities. This will boost productivity and enhance living standards in 6G-enabled SSC.