# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 1 Introduction to Unguided Optical Communication (UOC)
### A Brief History of Communication Technology
Alexander Graham Bell was back in the history books again after he made the first coast-to-coast call by phone to his assistant in January of 1915. It was the first long-distance call made in history from a landline. It has significance because it made long-distance communication all over the country a reality [3]. In 1927, the first wireless communication using the telephone was established between England and the USA [5,18]. In 1930, the company named \"American Telephone & Telegraph\" demonstrated the videophone. In 1934, wireless communications were established between the USA and Japan. In 1936, in Germany, videophone connectivity was used for communal purposes. In 1946, the introduction of a voice mobile phone calling system was demonstrated. In 1956, a telephone link with 36 circuit cables was established between Scotland and Newfoundland. In 1962, the first wireless space satellite communications were demonstrated for the first time. In 1964, George Hockham and Charles Kao verified voice communications using light over fiber-optic cables for the first time. Following the invention of the laser, fundamental and ground-breaking steps were taken in terrestrial long-distance communications [3,19]. The availability of coherent laser sources opened new opportunities for light-based communication. In 1965, the picture phone trial was initiated, while in 1969, the first internet service between a few computers in four universities in the USA under the Advanced Research Projects Internet Advanced Research Projects Agency Network (ARPANET) was demonstrated. Inventor Martin Cooper placed the first cellular mobile call in 1973 to his rival at Bell Labs, Joel Engel. The first mobile phone had a maximum talk time of 30 min, and it took a year for the battery to recharge. The phone would eventually be a prototype for Motorola's first mobile phones [3,5,7, 3]. In 1979, because of the judgment and necessity of INMARSAT, naval boats were connected for security and safety communications. In 1981, Japan deployed a communal automatic mobile network. This was followed by a secured and limited amount of internet data transmission for the emergency services, corporations, and researchers of a few universities in 1982 [5,6]. On 1 January 1983, the Internet was officially born. ARPANET officially switched its old network control protocols and Transmission Control Protocol/Internet Protocol became standard [3; 8]. The first canopy of 64 satellites was put into place by a company called Iridium in 1998. They also produced the first hand-held satellite phones, which were smaller and less cumbersome than the earlier \"bag\" phones. This revolutionized mobile telecommunications and would lead to the modern smartphone [3; 4; 5; 6; 7]. The availability of real-time communications has had an enormous impact on society at large. Today's commercial UOC systems are more compact and with much reduced installation costs (see Figure 2b). The integration of massive connectivity, with the aim of providing voice and video services, text messaging, internet connections, entertainment, third-party-service access, online applications, etc., using a single device is possible if and only if a beam of light is used as the carrier signal [20; 21]. Furthermore, the rising demand for large bandwidth backhaul or mobile backhaul networks necessitates the need for a complementary optical wireless technology with a huge bandwidth for use in many indoor and outdoor applications including aerial vehicles, satellite, space craft, and UAVs. According to a recent study, UOC has reached a 400 Gbps of data rate; hence, it is a better option for high-altitude platforms' (HAP) digital data up/down links [8; 10].