# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 7 UOC Near-Future Network Scenarios and Research Challenges
### Proposed Near-Future Data Communication Network Scenario  
Over the past few decades, communication technologies have undergone significant revolutions to satisfy the needs of rapidly growing applications [82; 486]. UOC systems were primarily developed, demonstrated, and projected as an alternative/replacement for existing RF communication systems to meet high data transportation requirements along with resolving interference and security issues. The Tx/Rx optics at both stations control the characteristics of modulated optical beams and enables the possible prevention of the beam from the damage of the UOC channel. A suitable Ro-FSO and/or RF-FSO must be configured as a hybrid approach to take the advantages (strengths) and compensate for the limitations of both (RF and UOC) channels, particularly for NGN applications such as intelligent and highly protected data transportation, telesurgery systems, cloud data handling, and MIMO battlefield environment monitoring, etc. [487]. Figure 41 depicts the proposed architecture of a next-generation communication network. UOC transceivers are used in all terrestrial links, space satellite communication links, ground stations, and underwater points to achieve fast data transmission. Establishing a heterogeneous (RF, FSO, coaxial cable, and optical fiber) network becomes mandatory to enable ultra-high bandwidth wireless networks to connect the ground, space, and water [488]. The optoelectronic components used in large-bandwidth fiber-optic communication can also be employed in UOC systems, because modulation techniques at baseband and the transmission protocols are almost similar in both technologies. Fiber optics and UOC technologies mostly consist of a 100 Gbps (or greater) optical source, optical detector, DWDM, MZM modulator, and other components. This makes it possible to integrate UOC systems into other high-speed communication networks [489]. The design of full UOC links with DWDM is essential nowadays for data transfer for a range of services [490], including WLAN, 5G/6G cellular services, terrestrial digital television, and new wireless services. The design of next-generation wireless networks using DWDM is depicted in Figure 42. The next generation of UOC networks require three main subsystems: (i) a dedicated high-bandwidth point-to-point, ground-to-space, or air-to-water data link; (ii) a redundant and highly reliable mesh architecture with the ability to easily add/remove nodes; and (iii) limited nod broadcasting, i.e., single-input multiple-output (SIMO) architecture. Optoelectronic components for next-generation UOC networks are now commercially available, with highly matured and massive production subsystems. Because the majority of currently available UOC subsystems are optoelectronic devices used in fiber-optic communication, they are compatible with all existing communication protocols/networks. For high-data-rate carriers, interoperability inside diverse networks with their distinct communication protocols is an important and challenging open topic for research. Some commercial systems use an asynchronous transfer mode (ATM), while most UOC systems use a protocol-independent, protocol-transparent approach for flexible deployment and cheaper service costs to attract more nodes [486; 487; 488]. Existing network infrastructure must be upgraded with UOC data lines, and/or adequate UOC communication systems must be built to satisfy future data transfer demands. According to research on NGNs, mesh network architectures based on ring and star architectures' combined advantages can improve reliability. The performance of rectangular, partially coherent, flat-topped beams used in UOC data links was recently investigated. This analysis used numerical values for SNR, BER, and power-in-bucket (PIB) that accounted for atmospheric losses due to absorption, scattering, and turbulence [487; 488]. An advanced method of mitigating atmospheric-turbulence-induced fading was demonstrated in [488], using all optical relaying and intermediate repeaters.