## 1 Introduction  
Wireless communication has grown rapidly, leading to the development and widespread use of 5G networks. However, 6G wireless systems aim to achieve much higher ambitions, targeting ubiquitous ultra-high-speed connectivity, ultra-low latency, and full integration with artificial intelligence and extended reality services. Meeting these objectives requires overcoming profound challenges at the physical layer, particularly regarding efficient spectrum utilization, severe propagation losses, energy sustainability, and scalable system design. Critical technologies such as beamforming, massive MIMO, RIS, terahertz communications, wireless energy harvesting, and digital twins are pivotal. Although many studies have been conducted, a comprehensive and systematic review consolidating these advancements has been lacking. The rollout of 5G, especially in the sub-6 GHz spectrum, has significantly enhanced network performance and reliability, offering scalable support for the pre-existing Internet of Things (IoT) ecosystem [1; 2]. The importance is highlighted by the European Union's allocation of 150 million euros to support further research and advancements in the field of IoT [3]. Despite the efforts of service providers and manufacturers, 5G has not fully achieved its true capabilities. For example, in most European countries, the mmWave band (FR-2) has not yet been deployed [4]. However, numerous articles can already be found in the literature proposing the structure and functionalities of next-generation mobile communications, namely, those beyond 5G and 6G [5; 6; 7; 8; 9]. A motivation behind the rush for the standardization of 6G is the emerging Society 5.0 and its demand for broadband, low-latency, and ultra-reliable communications [10]. The early generations of mobile communications (1G-3G) primarily focused on expanding and upgrading communication infrastructure. In contrast, long-term evolution (LTE) or 4G transformed mobile communications into an integral part of daily life by delivering significantly higher data rates. 5G represents a significant evolution from a technology that enhances daily life (4G) to a comprehensive social infrastructure, enabling connectivity not only between people but also with animals and machines through the IoT [11]. As envisioned by many researchers, time and space will become controllable at a high level in the Beyond 5G/6G era [5]. This implies that, through emerging technologies such as augmented reality and digital twins, users will be able to transcend certain limitations of the physical world. Specifically, the real world (physical space) will seamlessly merge with cyberspace (the Internet), unlocking novel capabilities that were unattainable with previous generations of mobile communications [12; 13]. The term physical space encompasses all communication infrastructures, extending beyond terrestrial networks [5]. On the other hand, cyberspace refers to all software-driven methods used to process and manage the vast amount of data collected from the physical space. In such a cyber-physical system, humans, animals, and machines will seamlessly interact through cyberspace, fostering enhanced connectivity and automation. Moreover, cyber-physical systems are expected to become deeply integrated with various social activities, contributing to the development of a more \"human-centered\" society. In fact, many experts foresee that communication networks will soon serve as the foundational infrastructure of society, functioning as its neural network [5; 14]. The complexity and abstractness of 6G has led to an ongoing debate among service providers, infrastructure manufacturers, researchers, and governments worldwide about how to proceed with the transition to this new generation. For this reason, a wide range of white papers has been published, offering key insights and guidelines on recent research and developments in the field [14; 15].