# On End-to-End Intelligent Automation of 6G Networks
## 2 6G: Preliminaries and Enabling Technologies
### Enabling Technologies
#### 2.2.1 Wireless Communication Technologies  
One essential aspect of 6G networks is the underlying wireless communication technologies to be used. These technologies are the cornerstone that enables the achievement of the target performance requirements of 6G networks. To complement the currently deployed wireless communication technologies, new technologies such as millimeter-wave (mmWave), terahertz (THz), and optical wireless communication (OWC)/visible light communication (VLC) are being proposed as supporting technologies in 6G networks. * Millimeter-Wave (mmWave) Communication: mmWave communication is defined as communication using the frequency band ranging between 30 GHz and 300 GHz [26]. Due to the frequency band used, mmWave communication has the ability to offer Gbps speeds [27]. Accordingly, it has already been proposed as one of the technologies for 5G networks, particularly using frequencies below 50 GHz [28]. For example,  
Figure 2: The 6G evolution requirements compared to 5G in terms of KPIs and concepts/paradigms. mmWave has been proposed for potential deployment for enhanced local area access in 5G, achieving a peak data rate exceeding 10 Gbps [29]. Similarly, it has been proposed for cellular access for distances up to 200 m, achieving a 20\(\times\) increase in capacity compared to 4G when utilizing directed antennas [29]. However, to support 6G and meet its performance requirements, further bands (such as above 100 GHz) will be needed to achieve the target throughput and latency values. More specifically, mmWave is expected to play a crucial role for short-distance communication scenarios such as in autonomous/connected vehicles and smart factories communication [30]. It is worth noting that adopting mmWave introduces some challenges pertaining to the potential channel blocking by nearby entities (e.g., nearby vehicles in the case of autonomous/connected vehicles), which results in lower channel reliability. * Terahertz (THz) Communication: THz communication is defined as communication using the frequency band from 0.1 to 10 THz [31]. Compared with mmWave, the benefit of using THz communication is two-fold. Firstly, there is an abundance of available spectrum in this frequency band (larger than that of mmWave communication), thus allowing for more users. Secondly, THz communication has the ability to support even higher throughput rates, reaching Tbps [32]. This can be done without the need for any additional techniques for spectral efficiency enhancement [33]. Moreover, it offers improved system reliability, better energy efficiency, and higher robustness/adaptability to changing propagation scenarios [33]. Accordingly, THz communication is being proposed for multiple throughput-demanding delay-sensitive applications such as sensing, imaging, and localization [33]. Additionally, in a similar fashion to mmWave communication, THz communication is being proposed for a multitude of applications and use scenarios, such as in local area networks, military communication applications, wireless data center networks, and space communication networks [34]. Despite the many benefits offered by THz communications, many corresponding challenges also arise. For example, one major challenge is the transceiver design and the high associated cost, particularly for the digital-to-analog and analog-to-digital converters (DACs and ADCs) [35]. Another challenge is the channel and noise modeling at the THz spectrum. This is because the channel characteristics in this frequency band significantly differ from other bands [35]. Similarly, due to the wavelength size, THz signals suffer from noise resulting from molecular absorption [35]. Hence, these concepts need to be considered when using THz communication for 6G. * Optical Wireless Communication (OWC)/Visible Light Communication (VLC): As the name suggests, optical wireless communication (OWC) is defined as communication using the optical frequency band, namely between 300 GHz and 30 GHz [36].