## 2 State-of-the-Art
### The Era beyond 5G  
As discussed briefly in the Section 1, the next generation of mobile networks, known as \"6G\" or \"Next G\" is under development. More specifically, the first activities in North America are focused on research conducted by academic bodies in conjunction with standards developing organizations (SDOs). Europe has also granted research funds under the Horizon plan. However, it is notable that eastern countries, such as China and Japan have set up research teams for the research and development of 6G networking infrastructures and have funded multi-billion R&D projects. Therefore, the research community has already published the first wave of research works in the field. In the research work of [PERSON] et al. in 2020 [42], which has been supported by Nokia, the authors examine the key characteristics of 6G based on the recent technological advances. It is worth mentioning that 6G, besides the hardware infrastructure, will be heavily based on the artificial intelligence air interface management and optimization as well as on more dense networking as a result of the IoT. In [43] [PERSON] et al. have conducted a comprehensive survey on 6G. One of the key outcomes of this survey is that 6G will enable new dimensions on mobile networking, and more specifically in the fields of extended reality (XR), TI, digital twin, pervasive intelligence, intelligent transportation/logistics, enhanced communications, multi-sense experience, and global ubiquitous connectivity. Similarly, in [44], the authors have also performed a survey on 6G. However, in this publication the authors mostly focus on the deployment challenges. Concretely, for the deployment of 6G, it is necessary to transition to the frequency band of THz (Terahertz), design more dense antenna networks, as well as define the communication protocols. Modulation techniques are also a cornerstone, in order to link 6G to the previous 5G and 4G. Among the most common and effective modulation techniques lie discrete Fourier transform (DFT) precoded OFDM and the OFDM which is already applied for the link between 4G and 5G (for a definition of OFDM please refer to Figure 1). A key aspect in the development and deployment of 6G is the redesign of the so-called air interface. Briefly, air interface can be realized as the communication link between two devices, e.g., the mobile device and the antenna. Currently, in 5G, the applied interface is open radio access network, abbreviated as RAN. In [45], the focus is concentrated on the deployment of 6G in subnetworks where reliability and availability of the network is critical. Thus, by default, the connection is achieved through wired connection. However, 6G is a suitable candidate for wireless connectivity. On the other hand, it becomes evident that the key challenges in such deployments are interferences, including jamming attacks as well as impulsive noise. Figure 9: The various latency components of a typical TI system [41]. Based on the lessons learnt from the deployment of 5G networks, specifically in the field of software support and network security, the authors in [46] have conducted research which emphasizes the environmental impact of 6G in terms of energy consumption. Under the spectrum of energy efficiency, one of the key goals for the deployment of 6G is to become approximately 100 (one hundred) times more efficient in comparison to 4G networks. The recent advances in 5G networks have indicated that software support is fundamental for the achievement of the goals set for this mobile network generation. Notably, the utilization of energy intensive servers, in contrast to the chips used in virtualized radio access networks (also known as RAN), has enabled 5G networks to deliver higher bandwidth at the cost of increased energy consumption. Despite the key challenges and issues identified for the successful deployment of 6G networks in the previous paragraphs of this section, the capabilities offered are greater than any previous mobile network generation.