## 2 Vision and Key Features for Future 6G Networks
### Extremely Large Bandwidth (THz Waves)  
The wireless data traffic volume and magnitude of connected things are expected to leap by many folds in 6G, specifically in the region of hundred(s) of equipment in a given cubic meter. Moreover, data-hungry apps such as sending holographic videos need a spectrum bandwidth that is currently unavailable in the millimeter-wave spectrum. This situation presents difficult challenges related to area or spatial spectral efficiency and the needed frequency spectrum bands for connectivity. Hence, a bigger radio frequency spectrum bandwidth has become necessary and can only be found at the sub-THz and THz bands, often referred to as the gap band between the microwave and optical spectra (Figure 6) [29]. Figure 5: Key features for future 6G. Figure 6: Electromagnetic spectrum and wavelength of terahertz and millimeter waves [29]. Using basic wave equations, the THz waves are denoted as having small wavelengths and radio frequencies higher than millimeter waves. Thus, THz waves principally convey more data faster, but a strategy must be devised to overcome the inherent short propagation capabilities. Therefore, the advent of THz waves into a mobile network portfolio could provide solutions for those applications where 5G technology failed to meet the high data throughput requirement or ultra-reliable low latency regimes [30]. Accordingly, service providers harnessing THz waves are expected for communications in areas with many devices or large amounts of data. Especially with the increasing adoption of smart homes, buildings, cities, and societies, 6G will fulfil the need for human-to-machine and M2M communications that will arise, particularly with the development of robotic and self-directed, unmanned aerial vehicle systems. This technology is enshrined in the concept of the Internet of Everything (IoE). One can infer that 6G will convey an ultra-dense network capability encouraging super network flexibility with the capacity to cleverly assimilate divergent techniques to instantaneously meet the numerous applications conditions. Conversely, an extended THz band communication technology is equipped with the capability to concurrently assist macro-scale and micro-scale services, such as terabit WLAN and nano-sensor networks [31]. References [28; 32] present an extensive and exhaustive review of THz band within the range of 0.1-10 THz for assisting Tbps high-speed communications. However, utilizing that spectrum presents many challenges that must be addressed. One of the possible techniques entails the arrangement of the THz spectrum bandwidth based on the absorption and reflection attributes of its sub-bands to optimize and reuse for communications and provide support for many other services. For instance, in scenarios assisting various divergent applications, extending harmonic attributes should be allowed, and this condition can be met by meticulous frequency planning. Detecting weak signals as sensitivity decreases is extremely difficult in the THz regions. Consequently, the THz radio spectrum can be boldly partitioned into favorable spectrum windows between atmospheric absorption peaks above 500 GHz (Figure 7) and below the 500 GHz bands [33]. Figure 6 indicates that the rise in free space loss is negligible when moving into the THz region from 30 GHz onwards. If we assume that the transmitting and receiving antenna area is maintained, the free space loss can be reduced by deploying an antenna with high gain. Apart from the free space loss, another issue in the higher frequencies that must be tackled is the increased complexity and parallelism in RF hardware and the reduced beam width that creates problems with signal acquisition and beam tracking in mobile applications [34]. Moreover, the signal penetration and reflection capability of various surfaces are two of the essential parameters that require further consideration when categorizing the radio spectrum in the presence of technological boundaries.