# Transmitters and Receivers for High Capacity Indoor Optical Wireless Communication
## 4 Conclusions  
We experimentally demonstrated several transmitter and receiver technologies for potential applications in indoor OWC systems for 6G networks. Due to the very high available bandwidth, the data rates in OWC exceed the limits of radio telecommunication. Furthermore, the properties of light enable the transmission to be more energy efficient and secure. For the indoor OWC transmitter, we propose a piezoelectric cantilever pair with a silicon micro-lens. This system allows for quick and direct optical beam steering towards mobile wireless users. This approach is attractive because of its minimal loss, compact size, and relatively simple implementation using commercially available resources. The use of silicon microlens in conjunction with tapered and lensed fiber, moved by the pair of piezoelectric actuators, resulted in large steering angles of 51.3\({}^{\circ}\) horizontally and 22.4\({}^{\circ}\) vertically, and a beam with 3.2\({}^{\circ}\) half-angle divergence. The observed asymmetry, between the axes, is attributed to the additional weight and fixtures associated with the horizontal actuator on top of the vertical one. Despite showing hysteresis in the actuators, the transmission angle maintains a predictable relationship with the applied voltage. By evaluating the lens and fiber combination, an error-free error transmission of 10 Gbps can be achieved up to \(\pm\)50\({}^{\circ}\) at 37.5 cm distance. The results of this prototype establish a foundation for advancing toward indoor optical wireless networks and expand it to outdoor terrestrial use. Future developments will focus on implementing comprehensive user tracking capabilities designed for typical indoor movement walking speeds. Additionally, we plan to expand the system's capacity through parallel actuator arrays, enabling simultaneous high-speed connections to multiple users. While they can be used to make a full-duplex system, the actuator is too large to integrate into common use electronics; for this a small two-dimensional VCSEL array using focal plane steering can be used, as they are compact and integrable on its implementation. We have also shown the variety of potential receivers which could be used in optical wireless communication. We demonstrated that a quad photodiode array could potentially be used for indoor OWC. It has a full-angle FoV greater than 32\({}^{\circ}\) and allows reliable error-free transmission of 16 Gbps. Because of the separation of the individual photodiodes in the array, this device could allow the localization and tracking of users or automated alignment. The transmission capacity can be increased by using more spectrally efficient modulation formats, e.g., PAM4 or DMT. We have also shown WDM transmission of 100 Gbps (four channels of 25 Gbps) received with a single surface grating coupler, and an array of surface grating couplers for increased field of view for OWC receivers. Separation of light collection and light detection, enabled by photonic integrated circuits, can enable scalability, which future receivers in the OWC will need. PICs allow light to be collected with surface grating couplers and light to be detected with high-speed photodiodes. Additionally, this separation allows processing of light in the optical domain, e.g., wavelength demultiplexing, coherent detection, or spatial beam demultiplexing. Although grating couplers may be sensitive to only one polarization state, there are many ways to increase the capacity of data transmission. Large-area and wide field-of-view grating couplers can be combined with existing fibre-pigtailed detectors as receivers for OWC, making use of the existing infrastructure. Despite promising high bandwidth, low latency and privacy, an OWC system needs to consider the use of hybrid optical/RF approach for critical systems. Due to it necessity of direct line of sight to the receiver. In critical applications, a complementary radio frequency communication path ensures continuous connectivity and reliability when optical transmission becomes obstructed.