1 Introduction

The FSOC system is among the most well-known wireless technologies, with a huge increase in attention and advancement over the previous decade [1]. Optical wireless communication (OWC) [2], without fiber communication [3], and Laser Communication (Lasercom) [2] are several additional terms for FSOC. FSO is recognized as a potential option to make available broadband communication services to last part customers who have a special desire for point-to-point connections in clean air condition among transmission and reception. FSOCs are fundamentally the identical as fiber optic communication technologies. The distinction is that the output of an FSO is collimated and communicated via the atmosphere, whereas the optical signal is sent via an optical fiber [4]. In comparison to current technologies, FSO presents a complete solution that can handle the rapidly expanding need for bandwidth. Most crucially, FSO systems can substitute optical fiber cable in situations where lines cannot be deployed or when large capital expenditure (CAPEX) is required [5–6]. Channel capacity, immunity to electromagnetic interference, no spectrum registration, and greater data speed are only a few of the benefits of FSOC. An FSOC, on the other hand, is vulnerable to non-ideal circumstances caused by inclement weather. To put it another way, a sole beam FSOC is vulnerable to meteorological circumstances like weather instability, dust, drizzle, fog, and smog [7], which cause light source attenuation. As a result, the weather conditions for the FSO system should be fully evaluated to guarantee the sufficient power reception. Alternate approach to accomplish longer FSOC reaches, multiple and single beam system with multiple channels is demonstrated in to suppress the effects of weather instabilities [7]. Different studies are reported in literature to quell the effects of atmospheric turbulences but results are not up to the desired level [8–9]. As per our best knowledge, certain researchers have looked at FSO transmission circumstances in their studies. As a result, this paper will go through the features of FSOC link performances in tropical circumstances.

In addition, choosing the right wavelength for an FSO is a crucial decision. The safety issue of the FSO beam’s impact on human vision and skin needs to be taken into account. The most widely utilised wavelength range for optical communication is 0.85–1.55 μm. FSO installers use a lot of 780 nm, 850 nm and more recently 1550 nm beams. 1550 nm wavelength is more eye-safe and delivers more power than 780 nm and 850 nm wavelengths. Light can focus on the cornea and lens at wavelengths shorter than 1400 nm, potentially endangering human vision. In contrast, the lens and cornea absorb wavelengths longer than 1400 nm. Thus, the eye is better safeguarded. Furthermore fog, smog, and haze can all be penetrated by a higher wavelength laser. Compared to 850 nm and 780 nm, the laser light with a wavelength of 1550 nm experiences less air attenuation [10].

One of the most significant environmental and developmental threats to natural ecosystems and socioeconomic systems is climate change. Consequently, natural ecosystems and human cultures are already being impacted by climate change, according to the Intergovernmental Panel on Climate Change (IPCC 2014). A system’s vulnerability is a theoretical concept since it is an unobservable and unmeasurable state (Hinkel 2011). It shows that a natural environment or a socioeconomic system is more likely to suffer negative consequences [11].

Meteorological risks can result in fatalities as well as harm to buildings, infrastructure, and crops. Examples of these risks include strong winds, heavy rains, and hail. There are different types of indicators to reduce the impact of vulnerabilities. The selection of indicators depend on different hazardous weather like hail storm, heavy rainfall and windstorms. Indicators like roof slope, roof material, window glazing, presence and type of shutters, number of floors etc. [12].

Homeowners are able to evaluate the building’s risk and make appropriate adjustments. When making decisions, local authorities can utilise a comprehensive understanding of the physical vulnerability and how it varies geographically within a community. The information produced can be used by emergency services, engineers, scientists, and insurance firms to optimise building design, determine premiums, and look into the relationship between structures and natural processes. However, indicator-based vulnerability analysis is still in its infancy for buildings susceptible to meteorological warnings [12].

The following sections constitute: introduction about FSOC given in Sect. 1, general working of FSOC is elaborated in Sect. 2, detailed description of FSOC components is discussed in Sect. 3, hybrid FSOC systems are listed in Sect. 4 followed by Sect. 5 which includes conclusion and future scope.

2 Detailed Discussion of FSOC Systems

A modulated narrow optical laser beam transports the digital data from the transmitter to the receiver via the free space environment before being processed at the receiving station [13]. The ability of the transmitter and receiver at both networking locations to see each other is referred to as “line of sight.” Unlike microwave systems, which require additional route clearance to allow for the development of Fresnel zones [14], IR beams have a less strict line of sight requirement since they propagate and extend in a linear manner.

Fig. 1
figure 1

Fresnel zone installation of RF systems (a) Correct (b) Incorrect [13]

Full size image

The Fresnel zone is taken into account while modeling any free space channel [15]. Figure 1 depicts several instances of proper and inappropriate wireless antenna placement on towers in the Fresnel zone. Radio waves spread outward from the antenna into the Fresnel Zone, which is the area around the visible line of sight. Fresnel Zones are circular cross sections between two wireless devices that must be clear of any obstacles or impediments to prevent signal loss [16]. Because the Fresnel Zone’s size is proportional to the signal’s wavelength, it’s been observed that the longer the wavelength, the larger the Fresnel Zone (the area that must be clear). The 5 GHz Fresnel Zone is estimated to be around half the size of the 2.4 GHz Fresnel Zone, requiring less free space between the two locations [17] to avoid difficult calculations.

2.1 FSO Transmitter and Receiver Modules

In a static configuration, the FSOC consists of three basic components: transmitter, space medium, and recipient, which are further subdivided into several components [18], as shown in Fig. 2. At the transmitter, a modulated laser beam converts an electrical signal to an optical signal. The transmitter is made up of four primary components, as indicated in Fig. 2: a laser modulator, a pulse generator, a cw laser source, and a transmission antenna [98].

Fig. 2
figure 2

FSOC transmitter and receiver (Transceiver) between two buildings [18]

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Binary data stream is passed to electrical pulse linecoder and through intensity modulator and cw laser, signal is converted into optical signal. There are utmost important two types of intensity modulation that are incorporating only laser without intensity modulator and with the intensity modulator [19]. Performance of output pulse shape after external modulation is better than direct modulation because of higher efficiency of intensity modulator. For light pulse stream train, two sources can be incorporated in the system such as Laser and Light emitting diode (LED). Former one has directionality, coherence, narrow beam size and later one has broad spectrum, non-coherence, non-directionality. In order to support higher speed, high capacity and better performance, laser should be used in the system. Diverse models of lasers are available in the market such as Vertical Cavity Surface Emitting Laser (VCSEL), Distributed-Feedback Laser (DF), and Fabry-Perot (FP) [20].

The telescopic lens collects LD’s photon stream and transmits binary data into the wireless medium channel. After transmitter, the channel is next component, with the open air environment serving as the propagation medium for the FSO system. This medium has been shown to have a number of concentric gas layers all across the cosmos. The troposphere, stratosphere, and mesosphere are the three basic layers that make up the hemisphere. These strata can be differentiated by their temperature gradients in relation to their elevations. FSOC communicates inside troposphere, and following operation performed like scintillation, geometrical losses, physical abstractions, absorption, and weather instability, take place in the troposphere. After signal transmission across the FSO channel, the signal is received and processed using an optical receiver as shown in Fig. 2 [21].

A telescope collects and focuses the incoming optical light from the channel, which is then directed towards the photo detector (PD). Receiving multiple uncorrelated radiations, averaging, and concentrating the radiation onto the PD all need a wide detector telescope’s aperture. The averaged signal is then linked to optical filters, which filter out unwanted wavelength components and minimise solar irradiation significantly. The photodetector’s detecting component is a photo diode, which is a semiconductor device that converts the photon energy of light into an electrical signal by releasing and accelerating current conducting carriers inside semiconductors. The two most often used photodiodes are the pin photodiode (PIN) and the avalanche photodiode (AVL) (APD). The possibilities are limited to only two kinds commercial-off-the-shelf (COTS) [21–22] due to their high quantum efficiency, semiconductor design, and widespread availability from commercial-off-the-shelf sources.

FSOC have amplifiers for a variety of reasons, including the following:

  • The optical preamplifier can be used to enhance the intensity of optical signals that have been weakened by a range of weather instabilities.

  • To obtain better eye opening.

  • To effectively improve receiver sensitivity by suppressing the limiting impact of the electrical amplifier and thermal noise.

The demodulator’s design is determined by the modulator’s symbol set [23]. Its goal is to identify the phase of a received signal and map it back to the symbol it represents, allowing for the recovery of original data. On the other hand, the receiver must be able to match the received signal’s phase to that of a reference signal [24]. Lastly, the received pulse is integrated over a one-bit period, sampled, and especially in comparison to a threshold value to decide if a ‘one’ or ‘zero’ bit is present on the demodulator side. The bit error rate (BER) is reduced to the absolute minimum using the maximum likelihood detector [25].

2.2 FSOC Classes

Single beam FSOC and multiple beam FSOC are two types of the prominently used types. Positive and negative characteristics can be found in the system categories. Figure 3 depicts a FSOC system with multiple beams with purpose of transmitting data using just one beam. The major disadvantage of a FSOC system with single beam is dispersing of light in outer space owing to weather instability caused by haze and rain. As a result, under these scenarios, the likelihood of a low power beam at receiver occurs. Therefore, employment of multiple beam is preferred to single beams FSOC system [7].

Fig. 3
figure 3

Multibeam technique

Full size image

Despite its shortcomings, the economical nature of single beam FSOC is used for low cost operations and short reach systems [26]. The use of a multiple lasers approach in a multibeam FSOC decreases the deterioration of power reduction caused by atmospheric turbulence [27]. Also, notably in FSOC, the special effects of weather instabilities are reduced using this technique. One of the best examples is that it may be used to resolve detector beam spreading and power loss [28]. The functioning of a FSOC beams of multiple transmitter was assessed in terms of loss of transmitter and receivers, connection boundary, and power of receiver [29], regardless of atmospheric losses [30]. This technique created a “fail-safe” condition and also reduced the effects of intermittent physical impediments such scintillation, snow, rain, insects, and birds [7]. In coastal places where reduced visibility owing to fog is regarded a limited phenomena, system with multi-beams, in meticulous, is proven to provide greater connection ease of use.

Article [7] computed the geometrical losses for a beam of single transmitter, two transmitters, three transmitters, and four transmitters and power received were 21.93 dB, 23.20 dB, 24.92 dB, and 27.959 dB, respectively. It is perceived that multiple beams increase system performance. FSOC system with multiple beams was reported at 1 Gbps and covered 1200 m. BER of 10 − 9 was obtained for 1140 m link distance. Distance of 833 m covered with one transmitter, 991 m for two and 1075 m for three transmitter beams.

2.3 Modulations in FSOC Systems

A literature review looks into hybrid modulation techniques (Table 1), a novel modulation method employed in FSO communion systems. According to [31], modulation differential phase shift keying and its incorporation with multi pulse position modulation and polarization division multiplexed was performed to enhance the performance of system. Due to hybridization of the modulations, bandwidth efficiency of M-ary pulse position modulation MPPM becomes more than binary phase shift keying. The hybrid arrangement complication is decreased at the disbursement of bit error rate performance when compared to coherent demodulation of PDM-QPSK-MPPM.

This strategy involves installing a series of relay nodes between the sender and receiver [31]. The signal is received and relay node accept it and sent to the next node, and so on until bits reach at end destination. The goal of deploying a large no. of units of relays was to reduce turbulence and increase the FSO link’s dependability. The joint modules of multiple dimensions have modulated the laser signals in various ways, improving the FSO system’s bandwidth usage efficiency and BER performance.

Table 1 Literature of different modulations in FSOC systems
Full size table

Because the FSO system is widely employed in today’s communication systems, FSO networks have become a hot issue for development for networks with link lengths ranging from a meters to kilometers [37,38,39,40]. In wireless communication technology, free space signal transmission is categorized in three different flavours such as (a) laser satellite networks (LSN) (b) Laser terrestrial wireless networks (LTWN) and Laser wireless indoor networks (LWIN) [41]. Figure 4 [42] represents aforementioned categories in single place having LSN, LTWN and LWIN [43,44,45,46].

Fig. 4
figure 4

Conceptual diagram of OWC

Full size image

LSN

The LSNs are constructed in such a manner that they give end-users with high speed wireless optical access by utilizing satellites in outer space that cover up significant regions of globe [, 40,41,42,43,44,45,46,47,48,49]. As given in Fig. 4, line of sight is essential requirement for satellites to establish link between them and can cover earth geographical areas using constellations. Deployment of lasers in space for satellite communication has replaced RF communication due to many advantages such as security, speed, capacity and absence of electromagnetic induction (EMI). LSNs are extensively deployed in the different earth orbits such as Lower earth orbits (LEO) ranges from 160 km to 1,000 km, Medium earth orbit (MEO) extends from 10,000 to 20,000 km from earth, and Geosynchronous earth orbit (GEO) cover distance greater than 35,786 km [50].

LTWN

LTWN is replacing the RF communication because of its premier advantages and nowadays, it also acting as backup of optical fiber communication due its potential to provide fiber like data rates. Optical fiber is tedious to deploy in the area like hills, intar-universities, military stations etc. The expansion in the transfer speed and limit necessities prompts a shift from RF to optical correspondence. The benefits like high velocity of requests Gbps, enormous data transfer capacity, unlicensed range, and high security expands its use in huge applications. Regardless of the various benefits, FSO has some major issues, for example, climatic condition has extraordinary deteriorative impact its exhibition [11, 51,52,53]. Irrespective of the advantages of FSO, it is encountered by some serious issues such as weather turbulences like rain, dust, fog, snow which limits the data rates, distance reach, and performance etc. It is also evident that even if atmosphere is clear, even though non-homogeneity arises due to temporal and spatial effects and introduces fluctuations in the refractive index which puts different effects in different wavelengths. This non-homogeneity is termed as the scintillation and it is experienced by signal at receiver due to weather through which it travelled.

LWIN

Indoor communication is inevitable part of current generation and wireless fidelity is widely used which is based on RF technology [54] Optical indoor communication is an ultimate solution to RF communication due to immunity to electromagnetic interferences, high security, potential to cater high data rates, wide bandwidth and particularly a great last-mile solution [55]. Developments in optical communication is accelerating day by day and light emitting diodes are being used instead of traditional incandescent bulbs as well as fluorescent lights. LEDs has numerous advantages like ubiquitous data transmission way out, extended lifetime, economical, high switching, and power efficiency which makes it optimal for optical indoor communication typically VLC [56]. Transmission of information in VLC takes place through modulating the illumination intensity of light emitting diode more rapidly as compared to the response time of eye of human which provide high speed transmission and also at the same time solve illumination purposes. Visible light spectrum does not need regulation, no harmful to human and open up a new window for broad bandwidth services.

3 FSOC Parameters

Internal and external parameters are the two types of system parameters used by FSO.

3.1 Internal Parameters

3.1.1 Link Margin

One of the most critical aspects of system design is generating an exact link budget, which determines how well an FSO connection will function under given weather conditions [57]. A link budget, to put it another way, is a tool for estimating the margin or additional power in a connection under specific operating conditions. The projected available power based on free space losses is then calculated using this increased available power and a static model of air attenuation. It is required to construct a link budget equation before appropriately setting an FSO system.

where Pe(i) and Sr(j) denote the ith transmitter power and the jth receiver sensitivity, respectively, and αtgoe(i, j), αatmo(i, j), and αsys(i, j) denote geometrical, atmospheric, and system losses, respectively, between link I j). The expressions for the parameters αtgoe(i, j), αatmo(i, j), and αsys(i, j) are as follows:

$$ LM\left(i, j\right)={P}_{e}\left(i\right)+\left|{S}_{r}\left(j\right)\right|-[10{log}_{10}{\propto }_{tgoe}\left(i,j\right)-\left[{\propto }_{rain}\left(i,j\right)\times d\left(i,j\right)\right]-{\propto }_{sys}(i,j)$$
(1)

3.1.2 Launched Power and Sensitivity of Receiver

The amount of transmitted power is the amount of optical energy launched by the FSO system. On the other hand, detector sensitivity is defined as the smallest optical input magnitude that yields a given output energy with a certain BER at the FSO system receiver. The measurement point might be at the transmitter (laser source) or detector (FSO receiver) apertures, and peak or average power is used to evaluate these two features. When recording a measurement from a laser or receiver spot, any additional losses suffered by the optical power that propagates over the entire system must be taken into account [58–59].

3.1.3 Bandwidth

This study presents new findings from research in the field of Free Space Optics (FSO) for the certification of various wavelengths for future communication (including space) [60]. Its purpose is to connect well-established technologies such as Wavelengths of 850 nm and 1064 nm, as well as 1550 nm, are now being developed, with future technologies such as 10 m wavelengths in the works.

3.1.4 BER in FSOC

The FSO system’s performance may be evaluated in a variety of ways, including by looking at the BER and Q-factor. The bit error rate is defined as the number of bit mistakes detected in the receiver divided by the number of bits delivered by the transmitter (BER). When a digital transmission contains noise (unwanted signals), the receiver makes erroneous judgements, resulting in a high BER rating. For assessing optical performance and measuring the BER, Q is often one of the most essential quality variables [61]. The bit error rate (BER) and signal to noise ratio (SNR) are two metrics used to evaluate the quality of optical communication networks. Using the approach suggested by, the SNR with turbulence is stated in terms of the mean signal and noise intensity I0 and in [62].

$$ SNR\left(dB\right)=10\text{l}\text{o}\text{g}\left[\frac{1}{0.31{C}_{n}^{2}(\frac{2\pi }{\lambda }{)}^{\frac{7}{6}}{L}^{\frac{11}{6}}}\right]$$
(2)

where λ is the wave length, L is the transceiver link distance, and C2n is the index of refraction structural parameter For light to high turbulence, C2n is considered to be constant, with average values of 10− 16 to 10− 13.

3.1.5 Power Loss

The geometrical loss of any FSO connection is totally determined by the optical transmitters’ beam width, transmitter route length, and detector aperture area.

$$ {P}_{r}=4{A}_{r}{P}_{t}/\pi (l\theta {)}^{2}$$
(3)

The power of transmitter Pt which is transmitted over a total area of π(lθ)2/4.

The beam often extends to a radius bigger than the receiving aperture, causing the excess energy to dissipate. The multi-beam FSO system, wide detector apertures, or modest transmission divergence are employed to minimize geometrical losses. In general, under a uniform transmitted power distribution, for a single and multi-beam FSO system [63–64].

3.1.6 Misalignment Losses

Because the FSO system uses a narrow-beam (Gaussian distribution) emitter and operates in a wireless medium, alignment loss from the transmitter to the receiver is quite common. The requirement of alignment in an FSO system may be well-achieved when the Gaussian power distribution centre is positioned at the detectors’ centre. This is due to the detector’s inability to accurately gather light at the beam’s margins, where the intensity is low. This is due to the detector’s inability to accurately gather light at the beam’s margins, where the intensity is low. The principal cause of misalignment in a The base motion (building sway) of buildings is a viable FSO system, especially for FSO systems installed on skyscrapers, which are prone to sway. It’s possible to add an automatic pointing and tracking system with an FSO system to reduce misalignment loss, with the tracking system adjusting the FSO system for optimal alignment on a regular basis [65,66,67,68].

3.2 External Parameters

3.2.1 Rain

On FSO modulated light travelling through free space, atmospheric properties such as absorption, scattering, and non-selective scattering have a significant impact [14]. The wavelength characteristic of absorption-based attenuation is largely dependent on wavelength, with UV wavelengths experiencing the most severe absorption (below 200 nm). Aside from that, wavelength has a considerable impact on light scattering, with rain adding non-selective scattering due to large rain particles [69–70]. Fog and heavy snow are common in the European region, which has a direct influence on an FSO link [7]. The Rainfall Type Stratiform is This sort of rainfall is caused by frontal weather systems. This type of rain forms in low-pressure zones where warm air collides with chilly air. Convective clouds produce convective rainfall. These clouds form when a wet atmosphere becomes overheated in compared to the ambient temperature. The rain cell is an area of space made up of connected locations when the rainfall rate exceeds a certain threshold [71,72,73,74,75,76,77,78]. A variety of studies have employed radar to assess the size and shape of rain cells [79,80,81,82,83,84,85,86]. Signal attenuation in FSO communication can also be caused by rain. The radius of the raindrops is larger than the wavelength of the FSO source. The connection may be disrupted if it rains heavily. Rain attenuation, on the other hand, has a minor effect when compared to fog. At frequencies less than 10 GHz, rain causes less specific attenuation. Rain is the primary attenuation factor for RF communications when the frequency exceeds 10 GHz. For frequencies greater than 40 GHz, the attenuations for RF connections are greater.

3.2.2 Haze

Particulate matter, often known as haze, is created when dust, smoke, and other particles are spread in the atmosphere. Mie Scattering is the name given to the phenomena because the size of the haze particles is roughly equal to the wavelength of the transmitted signal.

3.2.3 Fog

Because fog causes both dispersion and absorption, it is the most problematic communication barrier for FSOs. The density of the fog varies according to the particle size. Fog can be classified as thin fog, light fog, moderate fog, or dense fog. The fog density varies with altitude, making modelling challenging. The link is switched to RF due to the difficulty of sustaining the communication link in extreme fog. Increase the transmission power under mild fog conditions to improve the system’s performance. The power budget of the system can be increased when the visible range is limited by using a multi-hop link [87].

3.2.4 Atmospheric Turbulence

Atmospheric turbulence is a random phenomena caused by changes in air temperature and pressure. The atmosphere behaves like various cells called eddies due to variations in the refractive index. The light transmission route is deflected by these eddies. The refractive index structure coefficient is used to measure turbulence. The value depending on the time of day, it changes. Another significant component reduces the optical signal along the FSO channel. Atmospheric turbulence is this factor. Atmospheric turbulence is caused by differences in refractive index along the FSO route. The FSO receiver then experiences optical scintillation. Various statistical models have been developed through time to represent atmospheric turbulence channels of varying intensities. The K distribution was chosen as a useful model for severe turbulence channels because it provides high agreement between theoretical and experimental evidence. Three forms of air turbulence are used to test the laser beam. The three are scintillation, beam wandering, and beam spreading. Random fluctuation develops in the beam when the size of the cells refractive indices exceeds the beam size, creating deflection in the beam’s propagation direction. Beam wandering has an impact on signal quality. Beam spreading is the spread of a propagating optical beam in the atmosphere. To prevent beam dispersion, the average aperture radius is raised.

4 Market Growth of FSOC

Fiber optics is a fantastic solution for high data transmission, low piece errors, and serving as the backbone for web structures. The metro area has been developed to a large extent as a result of the ongoing burrowing to lay fibre. They have given deeply of their frameworks, but less liberally at the framework boundaries. In the last mile bottleneck, this lopsidedness has happened. When capital uses are required, service providers are challenged with the necessity to deliver assistance immediately and at a reasonable cost. There are a few options for addressing this last-mile network bottleneck from a development standpoint, but most aren’t viable.

Fibre Optic Cable: Optical communications are fully dependent on optical mediums, and optical fibre is a capable and promising medium that is frequently used due to its numerous advantages. Despite the numerous benefits of optical fibres, there are also drawbacks, such as time spent excavating and trenching, rights of way, and licence requirements. Optical fibre cannot be relocated once installed, and joining fibres in MAN networks can cost as much as $100 000-$200 000 per kilometre, with trenching and drilling accounting for 85% of the total cost. RF communication: RF communication is used in a variety of geographical areas and is a mature technology that requires a significant investment. Although RF may reach greater distances than FSO, the capacity constraint in RF is a primary reason why FSO is preferred. RF systems can handle 622 Mbps, whereas FSO can handle several gigabits per second. FSO makes financial sense for telecom firms when it comes to extending optical communication systems.

Copper cables are practical and available in almost every place for digital subscriber loop connectivity. Even though optical fibre is much thinner than copper cable lines, hybrid systems that combine optical fibre and copper wire are not the best choice. The largest issue is bandwidth scalability, and copper wires can give a temporary answer by providing 2–3 Mbps per user.

Several factors have accelerated the need for FSO. To begin with, the end client is requiring more transmission capacity, which means that more information must be provided. In reality, by the end of 2005, the number of web users will have risen to around 796 million. It has been proved that using FSOs is not only less expensive than using fibre optics, but also less expensive than using other common improvements like DSL or link modem services. It’s difficult and expensive to provide last-mile availability. In metropolitan areas, 95% of structures are planned to be within 1.5 km of fiber-optic infrastructure. In any case, they are currently unable to access it. Burrowing and digging roads are expensive and generate traffic congestion. Trees are dislodged as a result of the weather conditions. FSO supports 100 Mbps and can cater to LOS receivers. This office is a node on a metropolitan-territory ring that is linked to a territorial ring using standard fiber-optics components.

5 Advantages

Optical communication in free space provides a number of benefits over radio frequency communication. The wavelength of an RF signal is longer than that of an optical signal. The fact that FSO is more favourable than RF is demonstrated by the wavelength difference.

License Free Operations

The fundamental distinction between RF and FSO is spectrum licencing. FSO does not require spectrum licence, resulting in a simple and cost-effective implementation. To avoid interference, Spectrum licencing is required for RF. The visible and infrared light spectrums can both transfer data. However, because the infrared zone does not require a licence, it is simple and inexpensive. The radiation level has been set by the International Electrotechnical Commission (Standard IEC60825-1) and must not be exceeded. The unlicensed operation is primarily due to a lack of line of sight.

Bi-Directional Communication

FSO systems have the ability to communicate in both directions as well as in a single direction. They’re utilised in LANs and Mans to give high capacity and data speeds through bi-directional communication.

Large Bandwidth

As the carrier frequency grows, so does the data rate of transmission. The optical carrier frequency in optical communication is higher than in RF transmission. Because of the tiny beam divergence, the optical intensity of the transmitted beam power is greater at the receiver than at the RF, requiring less power. The lesser wavelength of the FSO results in a smaller antenna when compared to the RF.

High Security

Because the optical beam cannot pass through walls, data communication is safe. Unlike RF, FSO beams cannot be identified using a spectrum analyzer. Without trenching and at a low cost: The procedure of laying optical fibres in the earth is known as trenching, and it is a costly operation. These free-space frameworks cost around a quarter as much as comparable ground-based fiber-optic advancements. In this way, FSO operates at a low cost.

Rapid Deployment

Free-space optics enables designs to quickly set up broadband access. If the FSO can be installed below windows rather than on housetops, introducing an FSO framework should be remarkably fast, if not much faster. To communicate in free space, sender and receiver need maintain a clear vision.

6 Limitations

In Free Space Optical Communication, the optical wave propagates over free space, which is susceptible to numerous disturbances. Disturbances such as absorption, scattering, and turbulence cause the wave to be attenuated. The electromagnetic characteristics, shape, and direction of the beam are all affected by these interruptions, lowering the overall performance of the optical connection. Weather circumstances such as fog, rain, and haze affect the FSO connection distance [88]. Some of the challenges include the following:

  1. a)

    Physical obstructions: when flying birds, trees, or towering structures occur in the line of sight (LOS) of the FSO system’s transmission, they can momentarily block a single beam.

  2. b)

    Scintillation: Due to the heat emanating from the ground and man-made drives such as heating ducts, temperature fluctuations between distinct air packets would occur. Temperature differences can induce signal amplitude changes, resulting in “image dancing” at the FSO receiving end. The scintillation effect is handled by Light Pointe’s multibeam technology.

  3. c)

    Geometric losses: The spreading of the beam, which diminishes the signal’s power level as it travels from the transmitter to the receiver, causes geometric losses, also known as optical beam attenuation.

  4. d)

    Absorption: Water molecules floating in the terrestrial atmosphere are responsible for absorption.

  5. e)

    Air turbulence: weather and environmental structure cause atmospheric disturbance.

  6. f)

    Atmospheric attenuation: In most cases, fog and haze cause atmospheric attenuation.

  7. g)

    Scattering: When an optical beam and a scatterer clash, scattering occurs. It’s a wavelength-dependent phenomena in which the energy of an optical beam remains constant. However, only directional redistribution of optical energy occurs, resulting in a decline in beam intensity over longer distances.

7 Applications

The FSO communication connection is now in use for a variety of services in a variety of locations. These are detailed descriptions below:

  • High security military communication systems.

  • “Last-Mile” solution.

  • Re-establish connectivity when fiber fails.

  • Satellite communication.

  • Indoor communication.

  • Accelerate speed.

  • Application in CCTV systems.

  • In universities, science parks, industries.

8 Comparison of Different Reported Studies

S. No

Author, Publisher and Year [reference]

WDM Channels Distance and data rate

Modulation and technique

Results

Limitations

1

K. Murugan et al., Springer and 2020 [88]

4 channels Ro-FSO, 15 km and 20 Gbps/channel

4-QAM-OFDM and MDM-WDM

80 Gbps MDM-WDM system covered 15 km over FSO at

SNR 20 dB

Two consecutive LG modes were used which increased mode coupling and deteriorate system performance

2

M. Singh et al., Springer and 2020 [89]

2 × 40 Gbps–40 GHz, Ro-FSO system, 40 km and 40 Gbps/channel

RZ-DPSK, AMI, and NRZ-DPSK, Hermite Gaussian (HG) modes (HG00 and HG01)

Demonstrated system covered 40 km over FSO and NRZ-DPSK found best and offered Q = 11.38

Capacity was low as only 2 channels were used

3

S. Chaudhary et al., Taylor and Francis and 2019 [90]

2 channel Ro-FSO, 14 km and 2.5 Gbps

MDM-AMI and MDM-NRZ

Achieved 14 km over FSO at SNR 16.42 For AMI and 10.5 for NRZ

Very low data rate

4

A. Thakur et al., Springer and 2018 [91]

4 channel spectrum slicing FSO using highly nonlinear fiber, 5 km and 2.5 Gbps and EDFA, RFA, SOA performance investigation

NRZ, RZ, CSRZ

Investigated system can covered 5 km using CSRZ EDFA and found best with Q factor 9.57

Data rate and distance covered are less

5

A. Thakur et al., Springer and 2018 [92]

4 channel spectrum slicing FSO using highly nonlinear fiber, 5 km and 2.5 Gbps

NRZ and Spectrum sliced FSO system four wavelengths were generated from single laser source using HNLF

Maximum link range is observed in case of clear weather (> 8Km) and system works for 1.5 Km in case of Fog at BER 10− 9.

Capacity was less

6

M. Kaur et al., IJCRT and 2017 [93]

4 channels,

FSO with spectrum slicing using HNLF,

at 2.5

Gbps

NRZ, RZ, CSRZ, MDRZ

covered 5 km at BER 10− 9 using MDRZ.

Data rate was less

7

Shaina et al., Elsevier and 2016 [94]

Single channel FSO under 3 wavelength windows, 500 m and 2.5 Gbps

NRZ considered

System obtained 500 m distance over FSO under worst weather 70 dB/km at BER 10− 5 for 850 nm, 10− 4 for 1310 nm, 10− 3 for 1550 nm

Distance was very low

8

H. Zhou et al., Elsevier and 2015 [95]

8 WDM channels Radio over-FSO, 1 km and 10 Gbps

NRZ

Got distance of 1 km at BER 10− 9

Distance was very short

9

A. Malik et al., Springer and 2014 [96]

32 WDM channels, 47 km and 2.5 Gbps

NRZ, two system investigated i.e. WDM-FSO, Multiple Tx/Rx FSO

Investigated system achieved distance 47 km at BER 10− 10 in Tx/Rx FSO system and 31.7 km im WDM FSO

Data rate per channel was less

10

F. Hossain et al., IEEE and 2013 [97]

Multiple Tx/Rx FSO using SOA, 1 km and 10 Gbps

NRZ, 1 Tx/Rx to 8 Tx/Rx

Obtained distance of 1 km at BER10− 9 and with the increase in Tx/Rx, result improved

Distance was very short

9 Conclusion

FSO is a wireless communication technology that uses light to transport data in free space. FSO has two advantages: unlicensed spectrum and greater bandwidth. The benefits and drawbacks of the FSO system, as well as the issues that FSO and other channel types face, are discussed. The main challenges in FSO are turbulence, absorption, and scattering. To increase the quality of FSO communication, a variety of mitigation strategies are employed. It is necessary to choose and model channel distributions based on turbulence conditions in order to deploy mitigation techniques. Air turbulence mitigation, cost reduction, and improved performance are the most pressing challenges in the near future. To solve these issues, polarisation multiplexing, the best spectrum slicing approach, optical code division, multilayer modulations, the best amplifier and channel model, and other techniques can be utilised. The next-generation FSO network must be built for great scalability, with the capacity to scale up to 64 or 128 channels, providing connection to multiple end users.