## 8 Optical Wireless Communication (OWC)  
The term 'optical wireless communication' (OWC) is sometimes used as a blanket term for multiple technologies, such as VLC and infrared wireless communication. However, in this paper, we explicitly use it to represent infrared wireless communication. Optical wireless communication has a long history, dating back to the 1970s [82]. The Infrared Data Association (IrDA) first standardized OWC for point-to-point links, and later extended it to WLANs. Interestingly, the first IEEE 802.11 standard also included an infrared physical layer [83]. However, it was not developed afterwards due to the convenience of using radio frequency signals in WLANs. With the recent developments in the physical layer that can exceed 10 Gbps data rates [84], OWC has attracted attention from both academia and industry again. As discussed earlier, OWC operates in the infrared region of the optical spectrum, and utilizes mature laser diodes and photodiodes to generate and detect signals [85]. The infrared region is a vast range that expands from 700 nm to 1 mm wavelengths. The OWC systems typically operate in the 780 nm to 950 nm, 1310 nm or 1550 nm ranges, which creates roughly 180 THz of bandwidth [86, 87, 88]. In comparison, recent Wi-Fi 6/7 systems operate with a maximum of 1.2 GHz of bandwidth, while 60 GHz Wi-Fi systems have access to around 8 GHz of bandwidth. Due to its significantly large bandwidth, OWC can establish point-to-point wireless links in the order of hundreds of gigabits per second [89, 90]. However, there are practical challenges in implementing WLANs with point-to-point OWC links. For instance, WLANs prefer wide beams that can cover multiple users, as the WLAN users tend to be mobile. Therefore, more practical OWC links operate in the range of a few gigabits per second to tens of gigabits per second [85, 91]. emanating from the fibre optics background, OWC links are inclined to use simple modulation schemes such as OOK as opposed to multicarrier modulation techniques. It is worth noting that due to the close proximity in the electromagnetic spectrum, VLC and OWC share similarities in their channel models and propagation characteristics. However, OWC has the advantage of invisible beams, which greatly helps in beamsteering and wavelength division multiplexing (WDM) operations. The relative positions of OWC, VLC, and microwave techniques in the electromagnetic spectrum are shown in Figure 2[92]. OWC is still in the research and development stage, pending the standardization and commercialization of products. However, parts of the OWC physical layer have already been included in VLC standards. Hence, it is safe to believe that the standardization of OWC will happen in the near future. In this section, we will discuss the recent developments of OWC and how it can serve 6G applications in the form of WLANs [93, 94]. OWC systems are typically operated as intensity modulation/direct detection (IM/DD) systems, where the transmit signal is modulated to the instantaneous optical intensity. The frequency and phase information is not used in these modes. Therefore, most of the systems employ OOK or VPPM for lower to multigigabit data rates [95, 90]. The IEEE 802.11-1997 specifically uses 16-PPM and 4-PPM schemes [83]. IM/DD systems have the advantage of simplicity in the signal processing domain compared to radio frequency systems such as Wi-Fi. On the other hand, coherent modulation schemes such as OFDM have also been trialled for OWC [96]. The main advantage of OFDM systems is its resilience to multipath effects. Although IM/DD systems work well for LOS links, under non-LOS (NLOS) scenarios, multipath effects come into play causing intersymbol interference (ISI) [97]. OFDM OWC systems are fairly robust against multipath effects and are superior in spectrum utilization compared to IM/DD systems. Nonetheless, OFDM OWC systems require complex signal processing and unipolar OFDM implementations due to non-negative nature of the optical signals.