## 4 Enabling Technologies for FSO Communication
### Modulation Schemes
#### 4.3.3 Advanced Optical Spatial Modulation Techniques  
In light-based communication systems, LEDs have a relatively low modulation bandwidth, which greatly restricts their transmission rates. Additionally, conventional SISO  
Figure 9: Basic setup for OFDM signal transmission. C: Collimator, PD: Photodetector, ADC: analog-to-digital converter, DAC: Digital-to-Analog Converter, FFT: Fast Fourier Transform, IFFT: Inverse Fast Fourier Transform, CP: Cyclic Prefix, S/P: Serial-to-Parallel, P/S: Parallel-to-Serial, L: Laser, Tx: Transmitter, Rx: Receiver. techniques do not provide spatial diversity gains. To tackle these challenges, MIMO technology, similar to its use in RF communication, has been integrated into various OWC systems. A MIMO setup using SM can also enhance SE. However, as previously mentioned, the development of multiple antenna transmission schemes encounters several challenges. The challenges associated with conventional modulation schemes and MIMO setups can be addressed by a novel technique known as OSM, where only a single transmit aperture (LED/laser) is active at any moment [158; 199; 200]. In these configurations, light sources replace antennas, and optical chains substitute for RF chains. During transmission, information is transmitted through both the signal and antenna spaces simultaneously. Specifically, a portion of the incoming bits is utilized to select a LED/laser, while the remaining bits choose the symbols to be transmitted from the selected LED/laser. By incorporating the LED/laser index as part of the symbol, additional bits can be transmitted, enhancing the system's SE without resorting to higher-order PAM or PPM. A particular case of OSM is optical space shift keying (OSSK), where transmit aperture indices alone are used to transfer information, simplifying the system compared to OSM. Since only one laser is active during each signaling period, issues related to ICI and antenna synchronization are completely avoided in OSM/OSSK, thereby reducing the design complexity of MIMO transceiver chains [152; 167; 184]. These inherent advantages make OSM/OSSK an ideal modulation technique for light-based communication systems such as underwater OWC (UOWC), visible-light communication (VLC), and FSO communication [201]. Furthermore, in OSM, the constellation size dictates the number of bits transmitted per symbol; larger constellation sizes can enhance SE but may also increase Rx complexity. Additionally, research indicates that the error performance of OSM surpasses that of alternative MIMO transmission techniques with equivalent SE, making it a viable option for FSO communication [152; 167; 184]. OSM can reduce the number of required constellation levels for a given SE without compromising power efficiency and while simplifying Rx complexity in comparison with MIMO SM techniques. This requirement can be minimized even further by incorporating the polarization dimension. Moreover, utilizing polarization alongside the Tx index and symbol dimensions could enable a corresponding MIMO system with half the number of Txs and Rxs, enhancing space efficiency [167]. Consequently, researchers have begun exploring FSO-OSM systems in combination with coherent detection, capitalizing on the seamless integration of their advantages. This exploration has led to significant improvements in both SE and transmission capacity [158]. An innovative coherent MIMO OSM architecture, which integrates the spatial modulation (SM) principle with MPOLSK, termed SM-MPOLSK, has been proposed. By allocating one bit to the additional polarization dimension, this new OSM approach necessitates smaller constellations or fewer transmit apertures than previous methods. Additionally, due to POLSK's inherent resilience to channel scintillation, this OSM method, which employs the polarization dimension along with multiple transmit/receive apertures, demonstrates enhanced error performance in AT conditions [167]. Researchers have recently created new OSM structures to enhance SE.