## 3 Spectrum Management Issues
### High-Spectrum Access
#### 3.4.3 Above 100 GHz (100-300 GHz)  
The availability of large bandwidths at the mmWave band and Terahertz frequencies (frequencies above 100 GHz) creates the possibility of replacing the end-to-end wire links of indoor and outdoor mobile networks with a wireless system [175]. Currently, the New York University team is considering allotting frequency bands above 100 GHz for licensed and unlicensed wireless communications systems [176]. Considering the fact that only approximately 1177 MHz of the international mobile telecommunications spectrum is currently being utilized, we can anticipate nearly 3-10\(\times\) increase in spectrum allocation over the next ten years [177]. The 60 GHz spectrum (between 110 GHz and 170 GHz) has the potential to be utilized in wireless fixed backhaul links, cellular and vehicular communication, radar, and health monitoring networks. In addition, various frequency spectra are degraded by different atmospheric absorptions. According to the current agreement on spectrum resources in the 60 GHz frequency band, the 120, 183, and 325 GHz bands are expected to be applied to provide optimum results for short-distance communication. The reason is that an increase in distance of a couple of meters for large channel bandwidths in this range would be attenuated quickly [178]. With the present need to satisfy user requirements, these frequency ranges are not going to be used in the current 5G plan; however, they have the potential to deliver high data rates and the ability to support high user demand for future 5G networks and beyond.  
The authors in [179] have reported that the Software-Defined Air-Interface (SDAI) design significantly improves usage efficiency and widens the horizon of spectrum accessibility. The SDAI design is proposed as a design framework for air interface; it involves controller and configurable elements, such as signal processing, coding, and modulation. It can support frequency bands below and above 6 GHz and can utilize the spectrum efficiently to provide full-spectrum service. Moreover, several other features, such as IoT connections, ultralow latency, high data rate, energy efficiency, coverage, and capacity, should be addressed by SDAI. The spectrum accessibility is increased with the help of other fundamental techniques, such as low-density parity-check codes, turbo codes, MIMO, Orthogonal Frequency-Division Multiplexing (OFDM), and other rated techniques [180; 181; 182]. Moreover, recent advances in link-level technology in terms of antenna innovations, coupled with the evolving system-level network architecture, are all expected to boost spectrum utilization efficiency in future 6G networks; such advances include the emerging M-MIMO and distributed antenna systems. Besides, MIMO antennas facilitate cooperation among different types of network terminals by allowing opportunistic access [183; 184]. The utilization of a high band spectrum, such as the mmWave band, is being considered to be the most promising frequency spectrum to fulfill the bandwidth requirements for the next-generation cellular systems [185]. However, the use of the mmWave spectrum leads to disparate fading and signal penetration losses and scattering issues [186]. These problems can be minimized comfortably if the channel properties have been identified earlier before transmitting the signal [187; 188]. The uncertain behavior of the wireless communication channel has forced many researchers to explore different propagation models so that the path loss could be predicted before implementing the methodology in a real-world scenario [189].  
The next subsections discuss some recent studies on the high mmWave spectrum band for outdoor and indoor environments.