High-Capacity Coherent FSO

A paradigm-changing revolution on the area of high-speed wireless communication systems is currently being forced by the unstoppable increase on bandwidth demand. Due to spectrum exhaustion, traditional radio frequency (RF) communications can hardly cope with the foreseen requirements for next-generation cellular and satellite communications. This is motivating the pursuit of alternative wireless communication technologies, exploiting new spectral bands and transmission paradigms. In this sense, optical wireless communications, also widely known as free-space optics (FSO), are nowadays perceived as the most promising technology to ensure a future-proof solution for the upcoming generations of wireless networks. Among their multiple advantages, FSO systems are particularly appealing for their virtually unlimited bandwidth, potentiating ultrahigh-capacity communications. Following this premise, this chapter starts by reviewing the most relevant application scenarios and enabling technologies that are fueling the development of novel high-capacity FSO systems. Special attention will be dedicated to the role of digital coherent optics in enabling the recent demonstration of record-breaking multi-terabit optical wireless communications. Subsequently, the issue of time-varying power fadings generated by atmospheric turbulence is discussed, including methodologies for its statistical modeling and experimental procedures for controlled turbulence emulation. Motivated by this challenge, the following sections cover the development of tailored channel modeling and estimation techniques, which can be employed for the optimization of advanced modulation schemes, enabling time-adaptive data-rate provisioning in FSO systems. Finally, this chapter is concluded with the discussion of an ultrahigh-capacity FSO field trial, exploiting the use of digital coherent optics and wavelength-division multiplexing (WDM) technology to achieve up to 5 Tbps data-rate transmission over 1.8 km.