## 1 Introduction
### Literature Survey and Research Motivation  
The process of modulating the information signal is an important aspect accomplished to achieve connectivity between the transceivers. The choice of modulation technique has a very critical role in determining the quality of the received signal and, subsequently, the information extraction at the receiver. Out of the various modulation techniques available, on-off keying (OOK) is most popular in FSO systems because of its sheer simplicity and cost-effective setup [3; 4; 5; 13]. OOK-enabled FSO systems, however, perform miserably bad as channel conditions (turbulence) severity increases from weak to strong. Since atmospheric turbulence can cause unpredictable fluctuations in the intensity of the received signal; hence, amplitude modulated systems experience high bit error rates (BER) in such conditions [1; 2; 3]. The BER performance improvement in FSO links has been shown with the use of hybrid modulation schemes like pulse-position modulation (PPM) and sub-carrier modulation (SIM); however, the requirement of strict receiver coherence through carrier synchronization significantly add to system complexity and cost escalations [14; 15; 16]. Adaptive sub-carrier and polarization modulation schemes have also been reported to deliver seamless FSO link connectivity, but this virtue is limited only to moderately noisy and weak turbulence channels only [17; 18]. Use of line coding techniques like a return to zero (RZ), non-return to zero (NRZ) and alternate mark inversion (AMI) along with modulation schemes like amplitude shift keying (ASK), OOK, PPM, etc. has also been proposed as a complementary measure to overcome the disastrous effect of channel turbulence [19; 20]. Some higher-level modulation techniques like quadrature amplitude modulation (m-ary QAM) schemes with coherent detection have been reported for significant improvement in BER over turbulence FSO link; however, the main issue with coherent links is a requirement of carrier synchronization at the receiver, which adds to system complexity [21]. Apart from the choice of modulation schemes, diversity techniques have also  
Figure 3: Formation of turbulence eddies leading to random fluctuation and ruptured wavefront in FSO links. been found to be an effective tool in mitigating the effect of atmospheric turbulence in FSO links. As in radio-frequency (RF) based transmission, diversity refers to the transmission of modulated information over multiple channels to improve the link reliability. The temporal diversity, for example, involves the transmission of the same information multiple times with the objective of creating redundancy and the option to choose the best information packet at the receiver. This technique not only requires longer signal processing times but also leads to inefficient use of communication resources owing to redundant transmission required each time. Spatial diversity, on the other hand, overcomes this issue by using multiple transmitters and receivers, which will engage in simultaneous transmission of information, thus overcoming the delays experienced in time diversity systems. On the receiver side, there are three different techniques to combine the diversity signals, i.e., equal gain combining (EGC), maximal ratio combining (MRC) and selection combining (SC). Both EGC and MRC require precise phase information in order to ensure the constructive addition of received signals, thus making this system a complicated one. SC meanwhile uses the receiver branch with the highest signal-to-noise ratio (SNR) among all the branches, which is equivalent to selecting the branch with the highest light intensity making it much easy, simple and less complex. [PERSON] et al. in their paper [22], proposed the use of OOK FSO links with spatial diversity and reported performance enhancement over weak and moderate turbulence regimes; however, it was also reported that for overcoming turbulence regimes, the number of receivers should be increased which in turn may increase system complexity. [PERSON] et al.