## 2 Physical Layer's Challenges for 6G  
The immense demands of next-generation mobile communication have introduced numerous challenges in both the physical and cyberspace domains that must be addressed to enable the full deployment of 6G. This study focuses on the physical challenges arising from the need for increasingly higher throughput, particularly within the 6G eMBB scenario. This scenario is driven by the growing demand for ultra-high data rates, where any lack of connectivity, whether in remote locations or while in motion, is considered unacceptable. Achieving seamless connectivity becomes particularly challenging in the case of low Earth orbit (LEO) satellites, which operate at altitudes ranging from 200 to 2000 km and complete polar orbits within a few hours. These satellites traverse the sky at extremely high speeds, reaching several kilometers per second [19]. Consequently, such velocities result in Doppler shifts of hundreds of kilohertz for communication systems operating at frequencies above 10 GHz. Addressing these challenges effectively is crucial for the seamless integration of terrestrial and satellite communication networks. The expected peak data rates for 6G are projected to reach nearly 1 terabit per second (Tbps), more than 100 times the capacity of current 5G networks [5]. To meet these growing demands, ultra-wideband systems are essential. However, the sub-6 GHz spectrum is heavily congested, significantly limiting the available bandwidth. Consequently, mmWave and Terahertz communication technologies have been proposed to enhance the capacity, robustness, and security of mobile networks from a physical layer perspective [20,21]. In practical terms, it is necessary to explore new frequency bands to support these advanced services, as existing bands such as C, Ka, and Ku are heavily utilized by satellite communications. A promising solution lies in the largely unexploited FR-3 sub-bands, which offer significant potential for integration into advanced 5G and future 6G systems. By definition, the mmWave band includes all frequencies corresponding to wavelengths between 1 to 10 mm, or equivalently, 30 to 300 GHz. The mmWave bands currently under investigation for mobile communications include the Q band (33-50 GHz), V band (50-75 GHz), W band (75-110 GHz), D band (110-170 GHz), and G band (170-300 GHz). Notably, the Q band overlaps with part of the Ka band, as it has not been officially defined by the International Telecommunication Union (ITU) or the Institute of Electrical and Electronics Engineers (IEEE) [22]. Among these bands, the D and G bands are particularly promising for future applications, as they remain largely unallocated and offer significant potential for ultra-high data rates due to their wide bandwidth availability. It is well known that increasing the central frequency of transmission results in greater path loss, as described by the Friis equation representing energy diffusion due to the free space loss model [23]. Beyond traditional propagation losses, mmWave and, more critically, terahertz frequency bands are highly susceptible to atmospheric effects due to their short wavelengths. One of the key challenges is attenuation caused by energy absorption from atmospheric gases such as oxygen and water vapor, which varies significantly across different frequency bands, as shown in Figure 2 (attenuation measured in dB/km). The propagation of electromagnetic waves in the Q and V bands is particularly affected by atmospheric absorption [24]. However, there are specific frequency windows where absorption is minimized, making them viable for mobile communications. One such example is the 96 GHz window, where oxygen attenuation reaches a local minimum, highlighting its potential for applications such as sensing and communications in commercial or military operations. Additionally, the 77-81 GHz range is already licensed for automotive radar applications. Rain is another critical factor contributing to signal attenuation (absorption by water droplets), especially in higher mmWave bands.