## 4 Enabling Technologies for FSO Communication
### Modulation Schemes
#### 4.3.3 Advanced Optical Spatial Modulation Techniques
This innovation enables the use of multiple Txs, leading to increased data rates and a flexible optical chain. This category of OSM is known as an _advanced OSM_ system, which employs multiple light sources simultaneously for data transmission, resulting in higher data rates and more effective utilization of available spectrum bandwidths [158]. Advanced techniques for OSM include Optically Enhanced SM (OESM), Optical Generalized SM (OGSM), and Optically Improved Quadrature SM (OIQSM) [202; 203; 204; 205; 206; 207; 208; 209; 210]. In OESM, the number of active light sources varies based on the signal constellation. If the size of the primary constellation is \(m\), where \(m=\log_{2}(M)\), then OGSM employs \(N_{opt}\) optical chains, where \(N_{opt}\) light sources are activated by \(N_{L}\) light sources. The optimal \(N_{opt}\) for achieving maximum SE should lie between \(\left\lfloor\frac{N_{L}}{2}\right\rfloor\) and \(N_{L}\). For enhanced SE, OIQSM separates two symbols from a constellation into their real and imaginary components, which are transmitted independently using two sets of light sources. Table 4 summarizes the SEs and optical chains required for these advanced SM techniques. Overall, OIQSMprovides the highest SE with the smallest constellation size, requiring one optical chain for OSM and four for OIQSM. Although, OSM system improves SE, it also leads to higher hardware costs and increased power consumption. These associated costs vary across different schemes. For instance, the power consumption, \(P_{tot}^{()}\), number of switches, \(N_{sw}^{()}\), and total hardware expenses,\(C_{(\cdot)}\), for OSM, OESM, OGSM, and OIQSM are expressed as [199]  
\[P_{tot}^{OSM}= \,P_{oc}+\alpha_{tr}P_{t}+P_{sw}N_{sw}^{OSM}, \tag{34a}\] \[P_{tot}^{OESM}= \,2P_{oc}+\alpha_{tr}P_{t}+P_{sw}N_{sw}^{OSSM},\] (34b) \[P_{tot}^{OOSM}= \,P_{oc}N_{opt}+\alpha_{tr}P_{t}+P_{sw}N_{sw}^{OSSM},\] (34c) \[P_{tot}^{OIQSM}= \,4P_{oc}+\alpha_{tr}P_{t}+P_{sw}N_{sw}^{OIQSM}. \tag{34d}\]  
\[N_{sw}^{OSM}= 2^{(\eta_{SE}-log_{2}(M)-1)}, \tag{35a}\] \[N_{sw}^{OSOSM}= \,2^{(\eta_{SE}-log_{2}(M))/2-1},\] (35b) \[N_{sw}^{OSGM}= \,1/4\Big{(}1+\sqrt{1+4\times 2^{((\eta_{SE}-2 log_{2}(M))+1)}} \Big{)},\] (35c) \[N_{sw}^{OIQSM}= \,1/4\Big{(}1+\sqrt{1+4\times 2^{((\eta_{SE}-2 log_{2}(M))/2+1)}} \Big{)}. \tag{35d}\]  
\[C_{OSM}= C_{o}+C_{S/P}+C_{sw}N_{sw}^{OSM}, \tag{36a}\] \[P_{tot}^{OSM}= \,P_{oc}+\alpha_{tr}P_{t}+P_{sw}N_{sw}^{OSM}, \tag{34a}\]0 (36b) \[P_{tot}^{OSM}= \,P_{oc}+\alpha_{tr}P_{t}+P_{sw}N_{sw}^{OSM}, \tag{34a}\]1 (36c) \[P_{tot}^{OSM}= \,P_{oc}+\alpha_{tr}P_{t}+P_{sw}N_{sw}^{OSM}, \tag{34a}\]2  
where \(P_{t}\) represents the total transmitted optical power, \(P_{oc}\) and \(P_{sw}\) denote the power consumption of the optical chain and the optical switch, respectively. \(\alpha_{tr}\) denotes the slope-dependent load factor, \(\eta_{SE}\) represents the SE, and \(C_{S/P}\), \(C_{o}\), and \(C_{sw}\) are the costs associated with the serial-to-parallel converter, the optical chain, and the optical switch, respectively. The quantity of switches needed for OGSM, OESM, and OIQSM is significantly lower than that for OSM at a given SE and M-ary modulation scheme. Consequently, the power consumption and switch costs are much lower for higher SEs in OESM, OGSM, and OIQSM. As SE increases, the number of switches in OSM grows exponentially, while in the other schemes, the growth is linear. This results in the total power consumption of OSM exceeding that of the advanced OSM schemes because of the high switch demand. It is important to note that the modest rise in power consumption due to multiple optical chains in advanced OSM schemes is balanced by the significantly higher power consumption of switches in OSM at extremely high SE levels. Additionally, the overall cost of OSM exceeds that of other