# Revolution or Evolution? Technical Requirements and Considerations towards 6G Mobile Communications
## 7 Challenges of 6G
### THz Band  
To address the demand for future applications, the need for higher data rates, and lower latency, each generation of cellular system needs to open up new bands. Based on the laws of physics, the spectrum beyond the mmWave looks suitable for short-range communication and ambient sensing [37]. Unlike the mmWave, there is a large amount of spectrum available in the frequency range of 90 to 120 GHz, which is also known as low terahertz. Although THz band refers to the frequency range of 0.1-10 THz [125; 126], in literature, the term THz spectrum refers to the frequency range somewhere between 100 GHz and 750 GHz, which is considered as upper mmWaves that are currently used in 5G applications [127; 128]. Although the high frequencies permit high-speed data rates, the downside is the higher the frequency, the higher the signal attenuation and the shorter the communication range. However, the use of directional antennas in mmWave started with 5G, and it was shown that by properly designing a directional antenna, the communication distance can be enhanced. On the other hand, in 6G, it is necessary to make a tradeoff between data rate, communication distance, and the targeted use cases/applications. The THz is not a new frequency band; it has already been used over the past decades, for example, in medical devices and satellite radio communications. However, bringing the THz range to cellular communication is new, and different kinds of tradeoffs need to be carried out to adapt this frequency band to new use cases. On the one hand, the utilization of higher frequency bands is limited by the speed of the state-of-the-art technologies, and yet there is no cost-effective equipment for commercial usages. As THz band has been used for different applications, transistor technologies can make amplifiers up to the 1 THz range. However, combining and packaging that together with the mainstream CMOS for digital signal processing requires many steps that are advanced both in terms of technology and science [37]. Therefore, new hardware components are required, for example, for power amplification, sampling, and processing. On the other hand, long-distance communications are linked to significantly higher atmospheric absorption and propagation losses, as can be seen from Figure 11. In order to address atmospheric losses alone, the first characteristics of propagation in the THz frequency range should be identified; then, transceivers should be designed accordingly to support high-frequency band, a wide range of bandwidth, power, sensitivity, and low noise figures. Furthermore, new multipath channel models must be formed to address the frequency dispersion issue, given the wide range of bandwidth needed [129]. The currentlyavailable modulation and coding methods are also inadequate to support the THz band, while new method development is complex and challenging. Thanks to high penetration loss of THz band and consequently short-range communication, the interference issue will be limited to a shorter range, i.e., interference needs to be mitigated more locally, compared to the existing interference cancellation techniques in 4G and 5G. However, due to the trend of unitization of wider bandwidth and adaptive frequency range, the problem of interference with other coexistent technologies (THz use cases) is anticipated to be critical and should be effectively addressed before the realization of 6G. In order to provide impactful research and innovation in THz communication, first it is required to conduct a study on channel behavior and propagation characteristics of THz communications. On the other hand, it is necessary to know what parts of the THz bandwidth spectra need to be focused on for future communication and how much bandwidth is available in that frequency range.