## 6 The 5G Spectrum Standardization  
The International Telecommunication Union (ITU) was founded in 1865 as a result of a global agreement between the private and public sectors on the visibility of enabling the connection of communication networks, allocation of radio frequency bands, and development of international technical communication standards [56]. The main responsibility of the ITU is to create standards for unified communications that will allow networks and technologies to be integrated globally, thus enabling unrestricted access to telecommunications services. In relation to IMT systems, the ITU regulates each element of the radio communication system. The ITU is also in charge of harmonizing and standardizing the frequency band requirements for mobile communication technologies to promote cross-platform interoperability between current and emerging technologies. The ITU oversees, assesses, and develops the requirements for 5G technology [153].  
The ITU Radio regulations outline the spectrum assignments that ITU-R has been working on over the years for various applications and other services. There are several spectrums identified for IMT systems. A list of these bands and the corresponding radio communication conference that approved them are contained in [154]. Although some preferences are being established, 5G theoretically can be deployed in any of the specified bands.  
The USA is willing to use the 3.5 GHz (3550-3700 MHz) shared band as well as the 600 MHz licensed spectrum (617-652/663-698 MHz). The radio frequency below 1 GHz  
\begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Platform** & **Type** & **Altitude Range** & **Orbit** & **Beam Footprint Size** \\ \hline GEO satellites & Spaceborne & 35,786 km & Fixed position based on elevation, with respect to a given point on Earth & 2000–3500 km \\ \hline MEO satellites & Spaceborne & 7000-25,000 km & Circular around Earth & 100–1000 km \\ \hline LEO satellites & Spaceborne & 300–1500 km & Circular around Earth & 100–1000 km \\ \hline UAVs & Airborne & 8–50 km & Fixed position based on elevation, with respect to a given point on Earth & 5–200 km \\ \hline HAPs & Airborne & 20 km & Fixed position based on elevation, with respect to a given point on Earth & 5–200 km \\ \hline \hline \end{tabular}
\end{table}
Table 9: NTN platform classifications [151, 152].  
that Europe has selected for 5G is the 700 MHz band (694-790 MHz), but the leading pioneering 5G spectrum should be 3.4-3.8 GHz. To enable more downlinks, the 1.5 GHz band (1427-1452/1492-1518 MHz) is also being considered [12; 155]. Other nations are investigating the optimal band to roll out their 5G services. Japan is considering the frequencies of 3600-4200 MHz and 4400-4900 MHz for the testing 5G deployment. Nigeria is considering the 3500 MHz and 26,000 MHz frequencies, while in China, the frequencies being considered are the 3300-3600 MHz and 4800-4990 MHz bands [64; 123; 154].  
Due to the steadily increasing data traffic, more spectrum resources are required to support upcoming mobile communication networks. Furthermore, it is significant to remember that each nation will have different specifications for its national spectrum. Therefore, this section covers discussions on the different spectrum allocations and assignments for the 5G network and the sub-6 GHz and above-6 GHz frequencies. Furthermore, the worldwide trials and the different deployment modes of the 5G network are presented.