## 2 Physical Layer's Challenges for 6G
This includes not only energy losses during network operation but also the energy expended in the production and deployment of hardware infrastructure. Despite advancements in energy-efficient RF hardware, network operators may choose to retain older equipment to minimize costs, especially if it remains within its operational lifespan. However, this approach could delay the adoption of energy-optimizing technologies, thereby slowing their potential impact on reducing network power consumption. To emphasize the importance of designing and manufacturing more energy-efficient hardware, it is essential to evaluate the overall energy footprint of the telecommunication industry. Recent studies [53; 54] suggest that mobile networks, including components such as mobile phones and base stations, account for approximately 1% of global energy consumption, with base stations representing the largest share of this energy use. Therefore, various strategies must be implemented to improve the efficiency of RF front-end modules and reduce their energy demands. At this point, it is essential to note that digital processing devices (e.g., microprocessors, digital signal processors, microcontrollers, analog-to-digital converters, etc.) consume significantly more energy than their analog counterparts (e.g., the beamforming networks of the RF front-end system), with the exception of the RF power amplifier in the transmit chain [55]. Thus, adopting analog RF beamforming or hybrid analog-digital beamforming architectures could play a pivotal role in reducing the total energy consumption of next-generation communication systems. Addressing the challenges of 6G networks, particularly in the deployment of radio towers, requires more comprehensive strategies. The use of higher frequency bands necessitates the placement of base stations in close proximity to compensate for the reduced coverage area of mmWave frequencies. It is well established that 5G base stations require high densification, with an initial coverage radius of approximately 100 m per station, which may eventually be reduced by half. To optimize energy efficiency and performance, electronically steerable, highly directive beams are crucial. These beams enable spatial domain multiple access (SDMA) [56], which not only increases channel capacity but also ensures efficient energy utilization by directing power exclusively toward the intended user, minimizing losses in other directions. Another promising solution is the deployment of massive MIMO, where multiple radio signals are connected to a single or a small number of antenna elements (sub-arrays) [7]. This approach reduces the transmit power required from each RF module, as massive MIMO stations distribute lower-power signals across multiple antennas, offering improved energy efficiency compared to traditional antenna systems. At this point, it must be noted that high-directive antennas (beamforming) and MIMO systems present competing priorities. In sub-7 GHz frequencies, MIMO is often prioritized due to its effective handling of multipath propagation. However, at mmWave frequencies, beamforming becomes essential to overcome significant propagation losses. As a result, mmWave systems prioritize beamforming, with a viable solution being hybrid digital-analog configurations, which allow for compact MIMO implementations with minimal losses. This hybrid approach also facilitates the development of power amplifiers with improved power added efficiency (PAE). In the near future, the design of base stations is expected to transition toward integrating multiple low-power RF transceiver modules with high power amplifiers (HPA) in the transmitter and low noise amplifiers (LNA) in the receiver, rather than relying on a few high-power transmit components [57; 58]. This modular approach is already well established in modern radar systems and is also compatible with the mmWave MIMO versus beamforming compromise. A foreseen viable solution is the employment of one transmit (HPA) and one receive (LNA) RF module per MIMO port.