## 2 Physical Layer's Challenges for 6G
Rain-induced attenuation increases with frequency but eventually plateaus, becoming largely independent of rainfall intensityat higher frequencies. Similar attenuation effects are observed with cloud cover, where vaporized water further contributes to energy absorption. To mitigate these losses, various strategies must be implemented to enhance antenna and system gain while ensuring optimal efficiency levels. Techniques such as beamforming [9; 25], the use of highly efficient high-power amplifiers based on GaN [26; 27; 28], MIMO systems with adaptive channel coding [29; 30; 31; 32], and dynamic link budget control based on meteorological data can help overcome atmospheric effects [33; 34]. Additionally, the integration of reconfigurable intelligent surfaces (RIS) can further optimize performance [35; 36; 37; 38]. The key enabler of mmWave communications is the ability to maintain highly directional antenna radiation beams through beamforming. This capability serves as the fundamental mechanism underpinning the aforementioned techniques and is essential for ensuring efficient operation. Interestingly, mmWave bands also hold significant potential for applications such as inter-satellite, moon-to-satellite, or interplanetary communications due to their broad available bandwidth. Furthermore, the absence of weather-related effects, such as rain and clouds, in space further enhances their suitability for these long-range, high-data-rate applications [39]. Another significant challenge lies in massive machine-type communications (mMTC) within IoT systems, where millions of devices periodically transmit data such as status updates or sensor readings [40; 41]. This results in vast amounts of data across extensive areas, requiring efficient management through either terrestrial base stations or satellite networks. In mMTC applications, lower operational frequencies are preferred, as they align with the relatively lower data rate requirements compared to the high-speed demands of eMBB scenarios. Promising frequency bands for mMTC include the C and S bands [42; 43], which offer several advantages over mmWave frequencies. These bands are more resilient to weather conditions, support communication through multiple propagation paths, penetrate walls effectively, provide wider coverage areas, and benefit from well-established communication protocols such as Wi-Fi and LoRa. A key characteristic of this scenario is its diversity, encompassing a wide range of devices, data types, and geographic locations. Examining the evolution of telecommunication hardware, the efficiency of RF components has not advanced as expected, resulting in 5G devices and base stations that are excessively power-hungry [44; 45; 46]. This challenge is expected to intensify with the advent of 6G, where the number of base stations and connected devices will significantly increase to ensure seamless broadband communication [47; 48; 49]. The widespread deployment of hundreds of thousands of IoT devices acting as sensors or actuators will require robust systems capable of supporting their communication demands within the machine-type communication framework of 6G. To enable efficient wireless data transfer, these devices must incorporate  
Figure 2: Atmospheric losses in dB/km for the range 1-350 GHz. efficient telecommunication technologies. Achieving high efficiency is critical, as many IoT devices will depend on energy harvesting (EH) or wireless power transfer (WPT) for their operation [50; 51; 52]. These technologies can potentially eliminate the need for conventional batteries in certain applications, such as ultra-low-power electromagnetic energy harvesting (EM-EH) techniques, which operate with minimal energy requirements. This approach not only enhances system versatility but also significantly reduces maintenance costs by minimizing battery-related replacements and environmental impact. A holistic assessment of power requirements must consider all aspects of energy consumption across every stage of development and operation.