# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 1 Introduction to Unguided Optical Communication (UOC)
### Working and Installation Principles of Unguided Optical Communications  
Over the past few decades, the current telecommunication system has gone through incredible growth and expansions in LANs, WANs, MANs, Gbps Ethernet data transmission, optical fiber communications, and UOC [12; 19]. T1 links, i.e., local loop line networks, currently have a bandwidth of 1.544 Mbps [13; 20]. Today, the fiber to the home service provides the first/last mile connectivity at a data rate of 10 Gbps. However, wherever providing fiber cable is difficult, high-data-rate bridging is needed to link the destination points with LANs, WANs, and MANs to resolve data load and first/last mile bottleneck issues to provide end-to-end broadband connectivity for, e.g., a 10 Gbps data link [22]. During the initiation phases, UOC technology was undervalued; some people even said that \"the field was not mature, even decades after its birth\" [21]. In reflection of the past, UOC was first and foremost used in the 19 th century by sending voice signals using optical beams. For example, the A. Graham Bell \"photophone\", with a range of 600 feet, was a step forward towards the development of the telephone system [14; 16]. The remarkable progress and advances made in devices and systems has made UOC a very attractive alternative option for use in civilian data communications and defense secured communication networks, as well as astrophysics, space engineering, and various underwater imaging applications. Most advancements in the UOC technology were driven by military requirements [17; 18]. Depending on the link range between Tx-Rx units and the channel characteristics, UOC is classified into OWC (mainly for indoor/short-range communication), FSO (mainly for outdoor/long-range communication), and UOWC (mainly for air-to-water/within-water communication) systems. The top-level design schematics of UOC systems of these three kinds are shown in Figure 3. In indoor/short-range systems, i.e., OWC systems, public and private backbone fiber networks are linked the data centers, which manages the data distribution to/from the internal data distribution fiber networks. The internal fiber  
Figure 2: (**a**) Acoustic wave transmission over a light beam [4]; and (**b**) today’s UOC transceiver roof-top system [5]. network is routed to all offices (1 through _N_) via connectivity control hubs. In every office, an optical head (transceiver) using IR and visible light is used to establish communication links. All appliances within the office can be connected to internal networks via OWC dongles. Provision is made in the dongle to receive and transmit data via a downlink diverged beam and uplink narrow beam, respectively. The connectivity hubs manage the data distribution from/to offices based on their requirements, as shown in Figure 3a. Most OWC installations are within the offices' rooms/floors, or within short-range outdoor access points; hence, link attenuation is not significant and can be ignored. A low-power diverged laser beam is sufficient in a OWC system over a short-range, within the office or outdoor access points, since atmospheric turbulence effects are very small; hence, transmitter power, modulation bandwidth, and the sensitiveness of the optical detector are the main factors in OWC systems. In outdoor/long-range systems, i.e., FSO systems, intense or phase-modulated IR/visible optical beams carrying multi-media-type data from multiple sources are transmitted into free space using optical antennas (telescopes) (see Figure 3b). At the receiver side, an optical antenna is used to collect incoming light for conversion into an electrical signal using a photodetector and an amplifier for further processing to recover the transmitted data stream. Note that 50 Gbps FSO systems are now commercially available for networking applications. Nevertheless, high data rates of 1.6 Tbps have been recently reported in the literature [8].