## 4 Enabling Technologies for FSO Communication
### Modulation Schemes
#### 4.3.2 Advanced Modulation Techniques  
Spectrally efficient QAM is widely used and preferred for various applications because it modulates both amplitude and phase. In fact, QAM encompasses a family of modulation techniques with different constellation types, each corresponding to distinct arrangements of the signal points [173]. So, for spectrally efficient high-data rate transmission, higher-order modulation techniques like the QAM family--which includes rectangular QAM (RQAM), squared QAM (SQAM), hexagonal QAM (HQAM), and cross QAM (XQAM)--are gaining increased attention in wireless communication systems due to their high power and bandwidth efficiency. In SQAM, constellation points are positioned at the vertices of a square lattice, suitable only for constellations with even powers of 2. To better adapt to varying channel conditions and improve spectral efficiency, incorporating constellations with odd powers of 2 offers greater flexibility [174; 175; 176]. RQAM is typically preferred for these odd-power constellations, as it is a versatile modulation scheme, encompassing orthogonal binary FSK (OBFSK), QPSK, multilevel Amplitude Shift Keying (ASK), and SQAM as special cases. However, RQAM is not ideal for odd-power constellations; instead, a modified XQAM constellation is favored for its reduced peak-to-average-power ratio (PAPR) and enhanced power efficiency. XQAM constellations are formed by removing the outer corner points of RQAM constellations and arranging them in a cross shape, minimizing the average power of the constellation [174; 177]. Furthermore, the growing demand for high-data rates has led to the development of an optimal two-dimensional (2D) hexagonal-shaped constellation known as HQAM. HQAM features the densest 2D packing for a given optimal Euclidean distance between constellation points, resulting in improved power efficiency with lower peak and average powers as the constellation size (\(M\)) increases. Consequently, HQAM offers better BER/SER performance compared to other QAM schemes [174; 175; 177]. Depending on the trade-off between detection complexity and power efficiency, HQAM constellations are classified as regular and irregular HQAM structures. While regular HQAM has simpler detection, its power efficiency and BER performance can be further enhanced for larger \(M\) values. Irregular HQAM provides superior power efficiency and optimal performance, though it comes with increased detection complexity [174]. Additionally, carrier recovery and automatic gain control (AGC) are necessary when using SQAM with differential coding, but these are challenging to implement in practice. Carrier recovery faces significant issues, including false locks. To eliminate the need for AGC and address the false lock problem, the star QAM constellation is introduced. Star QAM, a special case of circular amplitude PSK (APSK), outperforms SQAM in peak power-limited systems. It features multiple concentric PSK circles with equal constellation points and identical phase angles. The amplitude and phase of the constellation points are mutually independent, allowing for successful differential detection rather than coherent detection, thus removing the need for precise phase tracking and channel estimation at the Rx. Due to these advantageous characteristics, star QAM has been adopted in various satellite communication standards. Conversely, the state of polarization is significantly less affected by turbulence-induced fading compared to phase and amplitude. Consequently, POLSK presents a viable alternative to optical intensity and phase modulation methods, owing to its resistance to phase noise and efficiency for long-distance operation [172]. Based on this advantage, a non-coherent binary POLSK (BPOLSK) technique has been proposed, utilizing the state of polarization of a fully polarized beam with two orthogonal channels as the information-bearing parameter [167; 178]. In this context, POLSK-based schemes have been demonstrated to offer 3 dB better sensitivity than OOK.