## 1 Introduction  
In recent years, the dramatic increase in the data traffic carried by telecommunication networks due to the continuous emergence of new high bandwidth services, smart devices and variable sophisticated applications has generated a significant spectrum congestion with the existing radio frequency (RF) systems. Initially, optical fiber systems provided the means to increase the amount of data to be transported. Despite their success, their installation is not always an easy, flexible or even viable task, for they cannot provide wireless connectivity and solve the last mile bottleneck problem. Complementing the existing wireless RF solutions, optical wireless communications (OWCs), commonly known as free space optical communications (FSOs) which mainly operate in the infrared (IR), visible light and ultra-violet bands, as well asmillimeter wave (MMW) communications that operate within 30-300 GHz have been initially developed [1, 2, 3, 4]. Even more recently, the impressive advances in both wireless communication devices and high-directional antenna design technologies have paved the way to the exploitation of terahertz bands that areast the cross-section of the higher end of microwaves and the lower end of optical frequency bands, and thus, it can offer their distinct advantages [5]. Indeed, in comparison with MMW communication links, terahertz (THz) links achieve increasing capacities owing to their larger atmospheric transmission window [6; 7; 8]. This is very critical since the sixth generation (6G) per user bit rate is expected to be 1 Tb/s in different fields of application, which the existing MMW links for 5G cannot accommodate, contrary to the emerging THz links which have the potential to reach such demands [5; 9]. Moreover, potential applications of THz links also include virtual reality, augmented reality, holographic projection in 6G wireless communication, terabit wireless personal area networks, terabit wireless local area networks, cellular networks, transmission of high-definition television (HDTV) signals with low latency, wireless back-haul for data and voice communications, wireless extensions of broadband access to fiber optical networks to address the last-mile problem, machine-to-machine communications among sensor networks and critical biomedical applications such as health monitoring systems [5; 7; 9; 10]. Additionally, THz outperforms MMW in terms of unregulated frequency spectrum, higher security level, lower transmitted power as well as higher directional information-bearing beams [10]. At the other side of the spectrum, when compared to IR FSO links, THz links are much less vulnerable to signal scintillations and signal attenuation under the presence of dust, clouds or atmospheric turbulence [11; 12; 13; 14]. Likewise, THz links do not suffer from the stronger ambient infrared noise, whereas THz power transmissions are not constrained by eye-safety infrared emission power limits. Nevertheless, rain attenuates THz radiation to a greater degree, which is a critical natural obstacle for THz deployment [15; 16; 17]. In this respect, the major limiting factor for the performance of THz links has been reported to be the THz signal attenuation due to humid air that results from molecular absorption mainly due to the presence of water vapor, and to a much lesser degree due to the presence of oxygen [18]. Apart from attenuation due to weather conditions and molecular absorption, the inevitable transmitted energy distribution over a large area, as the emitted THz signal is propagating towards the receiver side, brings about the attenuation due to FSPL. Thus, this unavoidable loss always exists and further degrades the THz links' performance [19]. Additionally, even in a clear sky temperature, pressure inhomogeneities bring about random variations of the refractive index along the atmospheric channel, which give rise to the atmospheric turbulence effect.