# Performance Analysis of Hard-Switching Based Hybrid FSO/RF System over Turbulence Channels
## 1 Introduction  
Over the last two decades, we have seen the spread of wireless communication system over the entire globe. The number of wireless devices increases much rapidly than we have imagined. Radio-Frequency (RF) has always played a major role in our traditional wireless communication networks. The major problems of RF links are their low capacity and their high cost. RF is a costly source of communication because its frequency sub-bands are licensed. The exponential increase in the demand for high data rates and availability of frequency band for communication has pushed researchers to a limit where they must now explore other means of communication. The new wireless devices that are being added to our network demand for frequency band which is surmounting the capacity of the RF spectrum. It's the necessity of time that we start digging deeper into other frequency bands that are available in the electromagnetic spectrum. Free-space optics has been proposed as a viable solution for higher data rates and emerged as a competitor for traditional communication systems. FSO operates over an unregulated band with the ability to provide services up to a few kilometers. FSO has astonished us with its capabilities and the speed it offers. Recently in 2018, German aerospace center, i.e., DLR, in collaboration with ADVA optical networking has set a record of 13.16 terabits/sec operational over 10.45 km [1]. Although FSO system can achieve very high data rates at a low cost, it has its own demerits as well. Thus, the FSO cannot completely replace the RF system everywhere. The impact of the atmosphere is not the same on FSO and RF. Attenuation is mainly caused by scattering and absorption. Clouds, snow and rain are the causes for scattering (also known as aerosol scattering), whereas, absorption of FSO signal ismainly due to carbon dioxide and water particles. The reason behind significant attenuation of FSO link due to scattering is the size of fog particle, which is comparable to the operating wavelength of the optical signal. This kind of scattering is called Mie scattering and are generally applicable to horizontal links close to the earth's surface. FSO signal is not sensitive to rain but degrades significantly in fog. For RF signal, frequencies below 10 GHz are merely affected by the fog droplets, whereas, for frequencies above 10 GHz faces significant scattering [2]. RF signal is not very sensitive to the fog, but its performance deteriorates in rain. So, FSO and RF can be good complimentary systems to be deployed together. For the last few years, researchers have been working on hybrid systems with FSO to enhance its performance and capabilities. In [3], hybrid power line communication (PLC) and visible light communication (VLC), also known as FSO, was studied by practical demonstration. This hybrid system is analyzed based on the maximum range, the connectivity and speed. In [4], possibilities of hybrid FSO/RF systems were investigated. Concept of receiver diversity is proposed in which the receiver takes the copies of the data and combines it to recover the originally transmitted signal. These copies are taken from parallel working FSO and RF link simultaneously. In [2], the effects of different weather conditions for hybrid FSO/RF link were studied. A setup for hybrid FSO/RF system was installed to experimentally measure the real-time measurements of attenuation. Comparative study for attenuation was evaluated by a model by [PERSON], [PERSON] and [PERSON]. In [5], the authors analyzed the availability of FSO-only system and hybrid FSO/RF system. They practically showed that FSO-only system can achieve 99.999%, i.e., five nines, availability for a distance of 140m only, whereas, a hybrid FSO/RF system could increase the availability distance for five nines. In [6], hybrid FSO/RF was tested in a practical environment using adaptive combining. The receiver retrieved the original data out of two links by using a maximal ratio combining (MRC) technique.