## 8 Optical Wireless Communication (OWC)
Recent OWC developments have an optical fibre heritage, as opposed to earlier systems [98, 99, 100], and are biased towards using laser diode transmitters rather than LED transmitters. Although LEDs provide good diffusion of the signal, the modulation bandwidth is far too limited to implement anything near the gigabit range. Therefore, laser diodes, with their well-established fibre optic techniques, have taken the place of transmitters. The receivers are built using photodiodes that are suitable for the operating wavelength, while optical concentrators and filters are used at the receivers to enhance the quality of the incident signal at the photodiode. One of the physical layer aspects of OWC that is of particular interest is the beam-steering operation. Since OWC links are very narrow beams, it is necessary to steer them towards the users to have a useful SNR value. Further, using narrow beam widths and precision beamsteering, it is possible to operate multiple overlapping channels in the same coverage area without causing interference [98, 100]. Based on the developments in the physical layer, novel upper layer (data link and network layers) approaches are required to efficiently use the recent OWC physical systems [101]. As mentioned earlier, IrDA and IEEE included OWC in their older standards. IrDA introduced the advanced infrared (Alr) standard to establish indoor WLANs with nondirected wide-angle transceivers (\(\pm 60^{\circ}\)). This particular standard was able to achieve data rates in the range of 250 kbps to 4 Mbps using 4 PPM with variable repetition encoding. Variable rate encoding is supported to improve the SNR when the link quality degrades. Doubling the repetition rate improves the SNR by 3 dB [102]. The MAC protocol proposed for this standard is called the Alr-MAC, which is based on the CSMA/CA mechanism. This MAC protocol supports both reliable and unreliable modes of transmissions [103]. Similarly, the IEEE 802.11 standard also facilitates optical wireless communication at speeds of 1 Mbps and 2 Mbps using 16 PPM and 4 PPM, respectively. The MAC protocol and remaining upper layer protocols were the same as for the RF physical layer. These upper layer protocols, however, are not suitable for recent physical layer developments of OWC, due to their complex operation and user requirements. In a study on OWC upper layers under the OMEGA-Home Gigabit Access Project [104], a technology-independent MAC layer, shown in Figure 9, was introduced, which can be transparently used for any multi-gigabit WLAN technology. This MAC layer is called Inter-MAC, and it paves the way to heterogeneous WLANs with radio frequency and optical technologies. The Inter-MAC architecture has a split plane architecture, where the data plane is directly interfacing the transmission technologies, while the control and management planes are transparent to the underlying physical layer characteristics. A practical demonstration involving the Inter-MAC and OWC techniques is shown in [105], with high-data-rate VR headsets. A more comprehensive split-plane architecture for OWC was presented in [106, 107, 108]. As shown in Figure 10, the presented architecture has a wide control beam that covers the whole coverage area. On the other hand, the data beams are narrow and cover a specific set of users. Each of these data beams has separate processing threads in the central office, such that they operate independently of each other. Furthermore, the fibre links attached to the access points ensure that there will not be any capacity bottlenecks with the increasing traffic load. Hence, an AP in this architecture can easily support the high data rates expected in 6G networks. Furthermore, the MAC protocol in operation has a significant impact on the performance of the network. While WLANs typically prefer contention-based MAC protocols, WiFi is moving into more contention-free mechanisms with recent standards. Hence, both contention-based and contention-free MAC protocols have been investigated for OWC.