## 1 Introduction  
Wireless communication systems have seen significant transformations since their origin. Today, advanced technologies such as autonomous vehicles, smart devices, and the Internet of Things (IoT) are ubiquitous, all hinging on wireless communication to meet the growing demand for high bandwidth and fast data rates. Over the years, mobile communication has progressed from the first generation (1G) through successive iterations--2G, 3G, 4G, and now 5G. Additionally, Wi-Fi standards (IEEE 802.11) for short-range wireless communication have rapidly advanced, particularly in terms of capacity, reliability, data rate, and medium access methods, in order to meet the needs brought about by the growing demand of data-hungry applications over the past decades, which follows the Edholm's law; this law states that the data rate and bandwidth will double every 1.5 years. Following a similar trend, total mobile network traffic is forecast to reach approximately 563 EB per month by the end of 2029. The predicted traffic increase until 2029 assumes that the early adoption of Extended Reality (XR)-type services, such as Augmented Reality (AR), Virtual Reality (VR), and Mixed Reality (MR), will occur in the latter part of the projection period [1]. This trend is causing a radio frequency crunch and forcing the research community to explore ways to acquire larger bandwidth and higher data rates. THz communications and VLC have the potential to be the feeders for this growing demand for data rates and bandwidth [2]. VLC is emerging as a promising technology to complement traditional RF communication systems, addressing the increasing demand for higher data rates and broader bandwidth. VLC utilizes the visible spectrum (400-700 nm), leveraging light-emitting diodes (LEDs) for dual purposes: illumination and high-speed data transmission. Thisdual functionality makes VLC an attractive solution for environments where RF communication is challenging or restricted, such as in densely populated urban areas, hospitals, and underwater environments [3]. Furthermore, VLC offers several advantages, including immunity to electromagnetic interference, enhanced security due to the line-of-sight (LoS) nature of light propagation, and the vast, unlicensed visible spectrum, which alleviates the spectrum scarcity issue associated with RF communication [4]. THz communication is another emerging frontier in wireless technology that operates in the frequency range between 0.1 THz and 10 THz. THz communication exploits the untapped spectrum between microwave and infrared frequencies. This enables ultra-high-speed data transmission, which is essential for next-generation applications such as ultra-high-definition video streaming, real-time VR, and advanced scientific imaging. THz waves offer remarkable advantages, including the ability to carry vast amounts of data and support extremely high bandwidth, which is crucial for mitigating the radio frequency spectrum crunch. However, THz communication also presents challenges, such as a limited transmission range and significant atmospheric attenuation. To overcome these obstacles, research is focused on developing advanced materials, innovative antenna designs, sophisticated signal processing techniques, and the use of mobile base stations [5; 6]. Due to the limitations of VLC and THz communications, these technologies are often combined with other communication technolgies such as RF, FSO, or Power Line Communication (PLC) through the use of suitable relay architectures. Many studies have integrated UAVs into these relay architectures, leveraging their ability to function as mobile base stations and adjust their positions to enhance network reliability and coverage [7; 8]. Several studies have been conducted on the interoperability of different communication technologies using DF relay architecture. In terms of VLC, this is often combined with RF, FSO, or PLC [7; 9; 10]. In [4], the authors conduct an examination of hybrid RF/VLC systems, emphasizing the integration of RF and VLC technologies.