# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 1 Introduction to Unguided Optical Communication (UOC)
### A Brief History of Communication Technology  
Communication technologies have evolved over time, reaching their status and capabilities for use in a multitude of applications in many areas. The capability to cover a huge amount of data information at very high speed with high quality and minimum latency is the main feature of the modern communication technology, which underpins today's telecommunications services. We have seen a tremendous advance from pre-historic smoke-based pre-coded signal transmission system to the 21 st century's smart mobile phones with a multitude of features and capabilities not seen before [1; 15]. Prehistoric man used smoke signs, firelight, and some sound sources to convey pre-defined (coded) messages (i.e., celebrations, war, etc.) over line-of-sight (LoS) and short coverage areas. The terrestrial signal towers used in the Great Wall of China and coastal signal towers in the Mediterranean region demonstrated the use of light (fire)-based communications over longer transmission ranges, which are still being used [16]. Greece invented the hydraulic semaphore in the 4 th century before Christ (BC), and also used it during the time of Punic war, which was in the 3 rd century BC, for communication purposes. In the 5 th century BC, the first network of pigeon messengers is thought to have been established in Assyria and Persia by Cyrus the Great. In 2000 BC, they were carrying messages to warring groups in Mesopotamia. The great Persian Empire of Cyrus in the 6 th century BC employed relays of mounted messengers, served by posthouses. The system was favorably described by the Greek historians Herodotus and Xenophon. Following these early developments, the Egyptians, Romans, and Chinese introduced new systems [1; 2; 3]. In the 18 th Century,  
Figure 1: Categorical revolution roadmap of UOC up to today’s technology. the French introduced navy communications through flag-holding semaphore, conveying information between ships, which was an advanced version of the systems first introduced by the Romans. In 1672, Robert Hooke demonstrated speech transmission with the help of an acoustic telephone and showed that sound signals could pass through a wire medium. In 1790, after the navy flag messaging system, the Chappe brothers demonstrated an initial optical/light telegraph scheme in Europe. Pendulum-like mechanical arm structures were built on a tower, which were used for the data transfer between towers [3,4]. In 1838, S.B. Morse, V. Alfred, and G. Leonard came up with the idea of a telegraph recording process, where a button-based data transmission scheme was proposed and named as Morse code [4]. In 1858, wired telephonic communications between Britain and the US were demonstrated successfully. A man named Cyrus Field from New York wanted to lay the first transatlantic telephone cable to connect England and the United States by telegraph. This project, though it was met with many setbacks, was finally completed in August of 1858 [3]. In 1867, British Admiral Philip Colomb established a lamp-based code-signaling mechanism for inter-ship communications. The year 1876 was a significant one for Alexander Graham Bell. Having come to the U.S. as a teacher for the deaf, he had been trying to figure out a way to transmit speech electronically. Despite little support from his friends, he successfully invented the telephone in March of 1876 [3]. In 1877, Thomas Alva Edison demonstrated a phonograph system using an acoustic mechanism for recording sound signals. In the 19 th century, A. Graham Bell demonstrated the \"photophone,\" as shown in Figure 2a, where light beams were used for the transmission of acoustic signals; this was not taken further but set the foundation for wireless transmission using light [16]. In 1893, Nikolai Tesla was able to transmit wireless RF signals successfully. Following that, in 1896, Marconi was successful in transmitting RF signals over a 2 km distance [17].