# Fibre Wireless Distributed Antenna Systems for 5G and 6G Services
## 1 Introduction  
The goal of fifth-generation (5G) wireless networks is to increase bandwidth, end-to-end latency, and energy efficiency. The radio access network, centralized as base stations (BS), have expanded exponentially [1]. As it improves scalability and requires less network maintenance, a centralized radio access network (C-RAN) minimizes capital expenses. Optical Fronthaul (OFH) links baseband units (BBU) and remote radio heads (RRH) are required to support C-RAN. For advanced 5G and beyond applications, the fronthaul link must have characteristics such as high speed, ultra-low latency, traffic protection, and safe data privacy. Optical Fronthaul is a critical component of radio access network (RAN) operations for macro and small cells, distributed antenna systems with RAN sharing, and the trend toward RAN openness and virtualization. Numerous approaches based on cutting-edge and upcoming technology, including intelligent surfaces, drones, and satellites, have been proposed to address mm-Wave and THz coverage concerns [2; 3]. These innovative plans will most likely coexist with the established fibre-optic-based signal distribution systems. To fully utilize the potential of the upcoming mobile telecommunication technologies, the mobile network will become extensively integrated with the fibre-optic network [4; 5; 6; 7]. Numerous studies have been conducted on the fibre-assisted mobile network using high-frequency bands. It has beenshown that 5G mm-Wave services with analog-optic transmission are capable of massive throughput [8; 9]. Notably, 6G is the next generation of wireless networks, following the current 5G networks. While 5G technology is just starting to be deployed, research and development into 6G are already underway; 6G aims to build on the capabilities of 5G and provide even faster speeds, lower latencies, and a greater capacity for a growing number of connected devices. Furthermore, 6G is also expected to support new technologies and use cases, such as ultra-high definition video, virtual and augmented reality, and the Internet of Things. It is too early to say exactly what 6G will entail, but it is expected to be a major step forward in wireless technology. To investigate the viability of a 6G network, the mobile data transmission in an orthogonal frequency division multiplexing (OFDM) across a terahertz range was shown in [10]. In [11], the utilization of a terahertz amplifier-aided Kramers-Kronig receiver was realized, which was the longest feasible attained link [11]. A THz system using multiple antennas, such as a 2 \(\times\) 2 MIMO configuration, was proposed in [12]. This system was later improved in terms of its data rate, reaching 2 \(\times\) 300 Gb/s, through the use of coherent 64 QAM-OFDM transmission and probabilistic shaping [13]. The fibre-wireless convergence network may be implemented using a distributed antenna system (DAS), depending on the design [14; 15]. To increase the mobile service coverage, the DAS uses optical transport networks to distribute broadband mobile data over a defined geometric region. The fundamental benefit of DAS is that it is simple to design and deploy, making it extremely scalable and adaptable [13]. The standard protocol for the evolved common public radio interface (eCPRI) is used in the digitization and sampling of the analogue waveforms for the optical transport of mobile data [14]. However, a broadband digital optical link is necessary for this protocol. For instance, a 4 \(\times\) 4 MIMO requires 120 Gb/s to convey an 800 MHz bandwidth [15]. When developing indoor DAS networks that handle mm-Wave and THz services, radio over fibre (RoF) innovation is a noteworthy substitute for digital transmission in the optical domain. As shown in Figure 1, optical fronthaul (OFH) links, such as Radio over Fiber links, constitute the mainstay in OFH. The RRH is wirelessly connected with the donor antenna, which is further connected to the main hub unit. The main hub unit is then connected to the wireless connectivity, thus creating remote antenna units.