## 2 6G Key Enabling Technologies
### Digital Twins  
The concept of DTs enables the evaluation and optimization of various physical entities via digital replicas [50; 51]. Hence, the application of DTs in 6G wireless networks can be a quite challenging issue due to the integration of various cutting-edge technologies. Sixth-generation networks will be dynamic, with network operators being able to modify key network parameters and architecture in the time scale of minutes and hours, rather than months or years. In this case, DT is a virtual sandbox that can identify various network reconfigurations and leverage optimum decision-making. In real-world scenarios, for example, network failures or security attacks might not take place quite often; hence, this would significantly increase the training time of DRL agents. With DTs, however, various such occasions can be generated by deliberately injecting misfunctionalities in the DT representation. A schematic overview of the DT is depicted in Figure 5. To this end, for specific real-world physical objects, all related information is collected and digitally stored. Thisinformation is related both to the objects' technical specifications and to the way they interact with the surrounding environment. Therefore, a virtual representation can then be formulated that can accurately model the object under consideration. Data manipulation in the DT representation obviously leads to alternations in its operational behavior. All these input-output sets are used to train appropriate AI/ML models and thus provide the physical asset with a multitude of potential responses for various system configurations. In practical DT applications, however, in the context of 6G networks, mass data generated by IoT devices might pose significant challenges to communication networks [52]. Given the computing capacity required to process all real-world information for a particular object with low delay, the data gathered by IoT devices must be offloaded to more powerful servers. Hence, if this traffic is not managed correctly, it can severely degrade network performance. Moreover, the number of IoT devices interconnected in a specific DT application may be expected to grow in the future, increasing the saturation of the networks. To mitigate this effect, computations must occur closer to the data source, reducing latency and improving response times. Hence, the concept of the IEC architectural approach that was discussed in the previous subsection of blockchain can be highly applicable in DTs as well, since edge computing ensures the proper processing of IoT data. A schematic summary of the 6G key enabling technologies is depicted in Figure 6. In Figure 7, the most important key 6G requirements are presented. These can be divided into three major categories: (a) flexible architecture deployments that include CF approaches and the extensions of the SBA in 5G, (b) AI/ML integration both for threat mitigation and for resource optimization, and (c) advanced physical-layer techniques that can leverage ultra-high data rates with minimal latency. A high-level concept of 6G network architecture is depicted in Figure 8. In general, four layers can be identified, including (a) the heterogeneous resource layer, which includes the integration of all physical layer components. These include all physical resources, such as mobile phones, drones, industrial IoT equipment, industrial robots, and indoor or smart grid equipment. This layer is also responsible for proper resource allocation and power management among the participating components via AI/ML. The upper layer is the network layer, which is responsible for all network operations. These include the management of various network topologies, such as CF networks, the integration of ML approaches towards network management and reconfiguration, and open-air interfaces. Finally, subnetworks are also a key driving force in current 6G trends. To this end, smaller  
Figure 5: The digital twin concept. and more flexible groups are formulated with specific 6G components [53].