## 1 Introduction
### Preliminaries
The third and most significant application will be  
Figure 1: Block diagram of signal modulation, propagation and detection in free-space optics (FSO) link. that of cost-effective data connectivity to residential/individual users. Here as well, in case C, the FSO operating in the 780-1600 nm range can be downconverted to the VLC spectrum range, i.e., 375-700 nm, thus providing end-users optical connectivity. Thus, in the different examples shown in Figure 2, FSO links can be used to extend the optical carrier used in the existing high-capacity fiber backbone and core network to the end-users with having the need to install an expensive light-wave network. Although 5G networks are being extensively being worked out to serve the needs of futuristic smart city systems, however, increasing bandwidth crunch will mean shifting towards millimeter-wave configuration. In such architectures, the RF links become increasingly directional, power-hungry and a reduction in serviceable range/area. As a result, the actual data rates may drop significantly as compared to theoretical ones [12]. The proposed link is, however, aimed at serving heterogeneous devices and networks through its ability to converge with backbone networks. The FSO link will, therefore, not only provide cost-effective connectivity on account of the use of unlicensed spectrum but also deliver data rates that, in a true sense much higher than the ones that can be achieved from 5G networks. Despite the array of features and potential applications, FSO systems are yet to see large-scale commercial deployment. This is primarily because FSO is a LOS communication link where the atmosphere serves as the propagation medium, and thus, the presence of meteorological phenomena like rain, fog, snow, haze, and dust can result in significant signal degradation on account of absorption and scattering [1; 2; 3; 4]. While the absorption of information-bearing photons leads to loss of power delivered at the receiver, scattering may lead to loss of LOS configuration, thus triggering erroneous data recovery at the receiver in both cases. Furthermore, even on clear sunny days with negligible aerosols, FSO links can still suffer from a severe impairment. This impairment can be attributed to random fluctuations in the refractive index of the propagating medium and is commonly known as atmospheric turbulence. Temperature  
Figure 2: Smart city plan with optical connectivity to end-users. fluctuations and/or mixing of hot and relatively cold air masses causes the formation of pockets of varying density known as eddies along the propagation path. The turbulence eddies, as shown in Figure 3, are capable of causing intensity and phase fluctuations in information-carrying photons, thus, at times leading to irreparable loss of information [7]. Over the years, various mathematical models like gamma-gamma, log-normal, k-distributed and negative exponential models have been investigated and proposed to characterize the impact of turbulence atmosphere on FSO signal [1; 2]. Depending upon the severity of intensity scintillations caused to the presence of turbulence eddies, the turbulence may be categorized as weak, moderate or strong. While the log-normal model of best suited for received signal characterization during weak turbulence conditions, the negative exponential model is best suited for strong turbulence conditions only; however, the gamma-gamma model has been experimentally found to be approximately ideal for turbulence conditions ranging from weak to strong [1; 3]. For the same reason, the performance analysis of the FSO link proposed here has been investigated, assuming that the channel is characterized by the gamma-gamma model.