## 1 Introduction  
With the rapid proliferation of wireless devices, the increasing demand for ultra-high data rates, and the emergence of new applications like augmented reality (AR), Internet of Everything, and autonomous vehicles, the future of communication networks is rapidly approaching the era of 6G networks. The journey towards 6G is marked by a series of evolutionary leaps, each building upon the foundations of its predecessor. As we transition from fourth-generation (4G) and fifth-generation (5G) networks to the next generation, 6G networks promise to redefine the boundaries of wireless communications, ushering in a new era of hyper-connectivity, massive data exchange, and seamless integration of physical and virtual worlds. While 4G networks provided significant improvements in data rates and network capacity, 5G networks introduced revolutionary technologies such as M-MIMO, beamforming, flexible frame structure for orthogonal frequency division multiplexing, non-orthogonal multiple access (NOMA), reconfigurable intelligent surfaces (RIS), and network slicing, enabling enhanced user experiences and supporting a diverse range of services [1, 2, 3]. Compared with 5G, 6G networks aim to exhibit substantial enhancements in peak data rate, latency, SE, EE, reliability, and security. Applications and technologies driving 6G, target requirements, and potential services that 6G can offer are briefly discussed in [1]. 6G is projected to achieve peak data rates of up to 1 Tbps and reduce air-interface latency to as low as a few hundred nanoseconds. This is essential for real-time and mission-critical applications, including industrial automation, AR, remote healthcare, and tactile internet. 6G aims to enhance SE and EE gains by 100 times in \(\text{bps}/\text{Hz}/\text{m}^{3}/\text{Joules}\) compared to current 5G networks. 6G networks are envisioned to enhance end-to-end reliability significantly, by 99.99999%. 6G networks are expected to incorporate stronger security measures, such as post-quantum cryptography and quantum key distribution, to mitigate the evolving threat landscape and safeguard critical communication systems [4]. To analyze channel modeling and characteristics in 6G wireless networks effectively, it is imperative to incorporate diverse technologies and disciplines. This integration should address high mobility, uncertain motion trajectories, and the non-stationary time or frequency or spatial domains [5]. The realization of B5G and 6G networks relies on breakthrough advancements in physical layer technologies. Potential technologies that have garnered significant attention include RIS, terahertz communication, and cell-free M-MIMO [4, 6, 7]. MIMO techniques offer several enhancements, such as spatial multiplexing, spatial diversity, and beamforming, and these have been incorporated into contemporary wireless standards. M-MIMO plays a vital role in 5G physical layer network design. Unlike traditional MIMO, M-MIMO utilizes several hundred antennas at the transmitter and/or receiver. The large number of antennas offers more flexibility to overcome dynamic channel conditions. Additionally, M-MIMO provides benefits like channel hardening, improved reliability, higher SE, EE, reduced multiuser interference, low power consumption, low latency on the air interface, and enhanced security [8, 9, 10, 11]. M-MIMO antenna arrays, featuring hundreds of elements at base stations (BS), can be effectively implemented using millimeter waves (mmWave), which hold the promise of significantly improving performance [12]. RIS is an emerging technology with great promise for revolutionizing wireless communication systems, including B5G and 6G networks. RIS consists of passive electromagnetic surfaces embedded with reconfigurable elements, such as tunable metamaterials or semiconductor devices, which can dynamically control the propagation environment. RIS operates by manipulating the phase and amplitude of impinging electromagnetic waves as they pass through or reflect off the surface.