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Aerosols and their impact on the future of communication

Read time: 5 mins
8 Nov 2019

Imagine just switching on your lights and downloading a movie in a second. The world demands high-speed internet connectivity at a lower price. This increasing clamour for speed and bandwidth is opening up new avenues, and one such evolving domain is LiFi - a wireless technology that makes use of light-emitting diodes (LEDs) to transmit data. Light waves are 10,000 times denser than WiFi signals, so there is vast untapped potential here. LiFi is based on Free-Space Optical Communication (FSOC), or in other words, an optical communications technology, in which information is transmitted through the atmosphere on modulated optical beams.

LiFi is also simple to set up. Just point a (LED) laser beam through the atmosphere to a photodetector kept at a distance, and voila, we have an FSOC system. Throw away the bulky copper cables and optical fibres, and do away with billions of dollars’ worth of frequency bands, and replace all this with LiFi - 10000 times the speed and bandwidth of a radio spectrum! In other words, it’s a dream come true for modern communication.

OPALS, a laser communication experiment conducted by NASA a few years ago, successfully used a high-power laser beam to transfer data from outer space to the Earth at an incredibly high data rate compared to the existing radio-frequency communication. However, some challenges need to be tackled. One such is the presence of gases and particles in the atmosphere, which absorb and scatter some light away from the beam during its transit. This absorption reduces the intensity of the light beam reaching the photodetector—a process also referred to as attenuation.

Further, atmospheric turbulence impacts the refractive index of the atmosphere and causes the light beam to oscillate around the detector. While attenuation can be measured and compensated for, managing the sheer randomness of turbulence is a colossal challenge/test for communication engineers. A better understanding of the science behind these fluctuations will help achieve this promising technology. Besides, the need to maintain a clear line-of-sight between the transmitter and the receiver poses some challenges.

The solid or liquid particles suspended in the atmosphere, commonly known as aerosols, are believed to influence FSOC systems through scattering and absorption and also due to their impact on turbulence caused by aerosol-induced atmospheric warming. A recent study by the scientists at the Centre for Atmospheric and Oceanic Sciences and Divecha Centre for Climate Change at Indian Institute of Science, Bengaluru, has revealed for the first time, the effects of aerosol-induced atmospheric warming on the turbulent fluctuations in atmospheric refractive index. It was observed that the residence time—the time during which aerosols remain suspended at a location—and the vertical distribution of aerosols, play significant roles in modulating the randomness of the scintillations.

While aerosols can decrease the surface temperature by scattering and/or absorption of solar radiation, light-absorbing aerosols increase the atmospheric temperature; the magnitude of which depends on their complex refractive index, the incoming solar radiation and the atmospheric residence time of the particles. Aerosols, in general, have a wide variety of sources ranging from oceans, deserts, vehicular and industrial emissions, volcanic eruptions and forest fires. Unlike the atmospheric gases like nitrogen, oxygen, and carbon dioxide, which are spatially homogeneous and follow a well-defined decrease in concentration with an increase in altitude, aerosols exhibit large spatio-temporal and vertical variations. This property makes it difficult to ascertain their contribution to atmospheric warming and refractive index fluctuations, at any given place, making them highly specific to regions and seasons.

The aerosol impact would thus be conspicuous in tropical environments like India, where a wide variety of aerosol sources are present. A high-altitude balloon experiment study has revealed a surprisingly large concentration of absorbing aerosols like black carbon and very low scintillations around 4.5 km altitude. High-altitude black carbon reduces the uplifting of warm air from the surface, which reduces the atmospheric turbulence. Usage of controllable optical components to compensate for the refraction of light by atmospheric turbulence will help overcome the effects of scintillations. The cost of communication systems could be thus largely reduced by making use of this property of elevated black carbon layers on FSOC.

Large concentrations of light-absorbing aerosols or absorbing aerosol layers that occur at higher altitudes can be used as a proxy to estimate altitudes with reduced turbulence, where low cost, high-performance aerial FSO links can be implemented. Such layers absorb solar radiation and warm the air around them, thereby reducing turbulence and hence decreasing the randomness of signal fluctuation in an FSO. Aerial FSO links, used between two aeroplanes or as a connecting node between two ground stations, can make this to their advantage to improve the signal to noise ratio. Besides, the usage of such aerial nodes as a connecting link between the ground stations can also solve, to a large extent, the line-of-sight requirements.

One of the main challenges in satellite remote sensing is to reduce uncertainties in the data. During clear-sky conditions, one of the main reasons for this uncertainty is the atmospheric refractive index fluctuations. Complex inversion algorithms are used to get useful data from the satellite-measured signals. Aerosol-induced warming will help get better estimates of atmospheric refractive index fluctuations, thus reducing the final uncertainties. Identifying an optimal time window will help in improving performance. Research on the combined effects of atmospheric turbulence and aerosols on FSOC systems is expected to provide a better understanding of this challenge.

Pulse broadening, or spreading of light pulses as they travel, is another severe issue in FSOC links, especially satellite-Earth communication links. High aerosol loading leads to larger pulse broadening, especially of narrower pulses, leading to channel overlapping. So incorporating the effects of aerosol may help improve the after-effects of broadening. The Indian Institute of Science's second campus at Challakere, Karnataka, houses a climate observatory where concurrent and collocated aerosol and atmospheric measurements are being carried out round the clock. Fortifying these with additional experiments on vertical profiling of atmospheric parameters are being planned to address the plethora of questions on the effect of light-absorbing aerosols on FSOC.

This article is the second of a three-part series from leading Indian researchers on the unsolved challenges in the field of mathematics and science as they find solutions to them. This article is attributed to Prof. S.K. Satheesh, Professor, Centre for Atmospheric and Oceanic Sciences Director, Divecha Centre for Climate Change, Indian Institute of Science, Bengaluru. Prof. S.K. Satheesh won the Infosys Prize 2018 in Physical Sciences. The article series has been facilitated by Infosys Science Foundation.

This article has been minoroly edited to correspond to the editorial guidelines of Research Matters.