# Free Space Optical Communication: An Enabling Backhaul Technology for 6G Non-Terrestrial Networks
## 3 FSO-Based Backhauling for NTNs
### Physical Layer Design  
FSO systems can either use IM/DD or Coherent Modulation/Coherent Demodulation (CM/CD) [97]. IM/DD utilizes the intensity (amplitude) of the optical carrier signal to represent information. In particular, IM techniques, such as On-Off Keying (OOK), Unipolar Pulse Amplitude Modulation (U-PAM), and Pulse Position Modulation (PPM), are used to encode information into the amplitude or intensity of the optical signal. On the transmitter side, Laser Diodes (LDs) or Light Emitting Diodes (LEDs) are used as the light source for generating the optical signal. The receiver includes a photodetector, such as a Positive  
Figure 11: Single-layer aerial backhaul in rural areas (adopted from [103]). Figure 12: Multi-layer aerial backhaul in urban areas (adopted from [103]). Intrinsic-Negative (PIN) diode or Avalanche Photodiode (APD), that converts the received optical signal into an electrical signal (See Figure 13a). The electrical signal then undergoes amplification, filtering, and other signal-processing techniques. A decision circuit in IM/DD usually applies a threshold to determine the intensity level of the received data. As opposed to IM/DD, coherent communication encodes information using both the amplitude and phase of the optical carrier. As illustrated in Figure 13b, a typical digital coherent optical receiver is composed of two subsystems. The receiver front-end consists of an optical front-end, a Transimpedance Amplifier (TIA), and an Analog-to-Digital Converter (ADC) followed by the Digital Signal Processor (DSP) engine. In the optical front-end, a monochromatic laser is used as the Local Oscillator (LO) to produce the reference signal. The received optical signal and LO signal are combined and fed to the photodetector. The output of the photodetectors includes I and Q components of the message signals. The output currents are converted to a voltage by the TIAs, and digitized by a set of ADCs. The resulting signals are sent to the second subsystem consisting of a chain of DSP algorithms. Coherent receivers can be implemented in different versions. If the LO frequency selected is the same as the received signal, this is called homodyne coherent detection.In heterodyne coherent detection, the LO frequency is different from that of the received signal. A second RF down-conversion is required to retrieve the message signal. CM/CD allow the use of advanced modulation schemes such as Quadrature Amplitude Modulation (QAM) and Phase Shift Keying (PSK). Additional advantages of CM/CD include an improved receiver sensitivity and the convenience of using digital signal processing for post processing. IM/DD have been widely used in both FSO and fiber optic systems, due to their low cost and ease of implementation. In the 1990s, fiber optic communication systems were loss limited. Coherent detection was considered as an alternative with a much higher  
Figure 13: Block diagrams for (**a**) IM/DD and (**b**) CM/CD. sensitivity. However, phase alignment with an optical Phase-Locked Loop (PLL) was a major problem in practical implementation. At the time, heterodyne coherent receivers were preferred over their homodyne counterparts because phase alignment could be now made at lower RF frequencies, where electrical PLL could be used. With the development of optical amplifiers such as Erbium-Doped Fiber Amplifiers (EDFAs), fiber loss was no longer an issue. Therefore, coherent detection was abandoned and OOK with direct detection became the main choice for fiber optics. In the mid-2000s, such simple modulation schemes faced limitations when data rates reached 40 Gbps. This sparked interest in the use of more complex modulation techniques such as PSK and QAM, which require coherent detection. Today, CM/CD have become the de facto standard for high-speed long-haul fiber optic networks at 100 Gbps and above. Both IM/DD and CM/CD have been considered in the literature on airborne and space FSO systems. For example, [PERSON] et al.