Can any one tell me about Introduction of FSO
Free-Space Optical communication (FSO) has become most economical and attractive technology and as an alternative to radio frequency communication over last twenty years. Incredible advances made in the designing of electro-optical components and systems coupled with huge progress achieved in the information processing capabilities helped improve the optical communication transceivers. Now a day’s many state of the art optical wireless systems being engaged in many short-range to long-range terrestrial, ground-space and space-space communication application scenarios. This thesis presents Fog effects on terrestrial Free-Space Optical (FSO) links. Due to dense fog FSO links may have a complete loss of signals. Moreover, sudden changes in temperature and pressure causing scintillations may distort the amplitude of the target signal. Therefore, any optical wireless system that relies on the dissemination of optical beams in free-space must have been designed to take these factors into account. The main emphasis of this work has. Free-space optical wireless communications, also referred to as FSO, is an effective way of communication and is very poplar subject in today’s communication. The methods that are used in optical transmission existed in their primitive form for centuries throughout the known human history; well before the work of Claude Chappe, a French inventor who initially introduced a practical semaphore system. in year 1792. His system was a series of semaphores located on buildings, where Human Operators transmitted information from one building to another. However, the information through optical mean are remained limited until the establishment of optical telegraph at the beginning of 19th century. At the start of 19th century the major work on the telecommunication is started and people are enabled to speak over long distances. Due to the changing nature of atmosphere the quality of service (QoS) of transmitter and receiver remains the issue. Optical transmission was started for the telecommunication systems after the laser as a light source was invented. In 1905 Albert Einstein has invented First working laser as a coherent light source being not in a nature. By the formation of Ruby Laser in 1960 by Theodore Maimann, a milestone for the optical technology has started and another solution to telecommunication systems as optical telecommunication based on optical fiber technology. In 1970-71 a wired optical communication is started with low attenuation incessant mode optical fibers and semiconductor lasers. Optical fiber transmissions indubitably dominates fields of terrestrial long-distance transmissions, starting at about 10 km for metropolitan networks and up to several thousand kilometers as for cables and the trans-oceanic transmissions and thus it turn into an integral and essential element of highway system across the whole world. The network transportation, based on silica single mode glass fibers in covered cables, is already installed for most countries, e.g. Europe, North America or Japan. Longer distance directive radio links for terrestrial applications are only used for very special cases, but radio frequency is used as the main medium for shorter distances non-directive applications. The access segment networks are successful examples of the digital cellular telephone network (GSM, UMTS), WLAN for data transmission.
They are usually used In smaller cells, or DVB-T for digital television broadcasting. The communication capacity of wired systems from telegraph systems to Dense Wavelength Division Multiplexing (DWDM) based state of the art fiber systems has been increasing constantly. With several Advancements in electro optics equipment, to perform system functions analogous to those carried out in electrical and electronic domain, and the introduction of single mode fibers has further accelerated this trend at an average rate of doubling every 24 months. In past electrical coaxial cables were used for the transmission of data signals at a maximum bit rate of 565 Mbit/s over 1 km distance between electrical regenerators. But with the single-mode fibers and lasers the transmission capacity reached up to 2.5 Gbit/s for the distances of 100 km without regenerators. Another achievement was made in 1990s with the development of optical amplifiers Erbium Doped Fiber Amplifiers (EDFA). After that the DWDM systems were designed that may combine many optical channels in the frequency domain, allowing the simultaneous amplification of optical signal from all the channels. Finally, the transmitted stream signal is demultiplexed at the receiver end.
FSO has a potential to solve troubles for bridging the networking gap and provide broadband internet services access to rural areas. It has very high frequency slot of 300 THZ allows high data rates for the optical communication. Advantages of FSO
Permit free communication, easy equipment system, keep away from electromagnetic pollution and wiretapping protection are big compensation of the optical communication systems. web browsing, electronic exchange, streaming audio and video, teleconferencing, Real-time medical imaging transfer, enterprise networking, work sharing, and high speed interplanetary are some possible applications of FSO links. [7] Previously optical communication was considered as a key to the last mile access problem but at present used for data transfer applications because of its high speed by creating links between fixed and mobile platforms[19] ,The popularity of optical communication systems are hindered by its vulnerability to certain weather conditions and atmospheric variations. In the communication systems, communication is inclined by the spread channel, and the spread channel for FSO communication is free space.
2.2 Disadvantage of FSO
The major disadvantage of FSO is propagation error produced due to interaction of the optical wave with the earth ambiance. To avoid propagation impairments special techniques have been used to cope up with the effects. Absorption and scattering on atmospheric particles as fog, rain, snow and clouds result in such effects as wave front distortion, beam wandering and Beam spreading that in turn are responsible for significant signal loss [35].
5
Fig I-1: Atmospheric Effects on FSO Links[6]
Due to air turbulence, propagation impairments bit error rate (BER) is increased. Optical disorder decreases exponentially with altitude and the major turbulent found up to 25 km altitude by the measurements.
The overall effects of optical turbulence are stochastic in space and time, producing signal fades on a timescale of several milliseconds in fixed applications Fade levels can exceed 20 dB in extreme cases depending on the propagation path and on the link range. In case of continental fog [3] the attenuation can be up to 130 dB/km up to 480 dB/km in dense maritime fog. [18].
It is interesting to note that though this technology has been named Free-space Optics, much of its utilization has been in terrestrial atmosphere where light Communication suffers all sorts of channel degradation effects which are usually absent in actual free-space channel scenario This factor is perhaps the only drawback associated with FSO communications and one of the main reasons for its limited deployments in many regions of the world.
Light attenuation through terrestrial atmosphere is caused by various physical phenomena absorption, scattering, beam wandering, beam spreading and wave front distortion etc. Absorption and Scattering of light occurs when transmitted
6
FSO beam interacts with particles of smoke and dust, droplets of fog, haze, smog and rain, flakes of snow etc., suspended in the atmosphere. During this interaction of light with particles and droplets, light is both absorbed and scattered in different directions. As a result of these interactions, light continuously loses energy while Traveling in the forward direction, thus restricting the maximum length of the link as well as presenting new challenges for optimum detection of received signal. Absorption can be controlled by transmitting light at such chosen bands of spectrum where absorption effect is minimum and we can ignore this loss without Affecting the practical results significantly. Because of Fog in atmosphere the light Undergoes scattering by droplets and it cause attenuation due to fog is biggest challenge to FSO communication today on the transmitted optical signals.
This attenuation is caused in major part by scattering by fog droplets or aerosol; and attenuations by molecular/aerosol absorption and molecular scattering effects are negligible in comparison.
3. Weather Effects on FSO
The main effects on FSO in the form of precipitation are fog, rain, snow and clouds. When an optical beam propagates through clear sky conditions, even then some optical signal attenuation takes place due to the absorption by atmospheric particulates (gaseous constituents of the atmosphere in particular Oxygen and water vapors). The presence of precipitation in the Earth atmosphere considerably increases the amount of optical signal attenuation and is dependent on the size of the particles, particle concentration and form, and the chemical composition.
The terrestrial FSO links are mostly affected by low clouds, fog and snow.Fog being the most detrimental of all effect with attenuation levels as high as 480 dB/km in dense maritime fog environments [18], around 120 dB/km in continental fog environments [3]. Optical attenuations may easily exceed 50 dB/km in low clouds [24], while through rain the attenuations are not that significant as heavy rain showers can cause specific attenuations up to 30 dB/km at a rain rate of 150 mm/h [16]. However, measurements showed that optical attenuations through falling dry snow can easily reach 45 dB/km [17]. This potentially limits the achievable optical link range with very high reliability to less than 500 m during such conditions and thus requires a detailed characterization of each climatic conditions for terrestrial FSO link of different path lengths Rain, Snow, Haze, smoke and Fog may effects the FSO. Among all atmospheric effects, fog is the most critical weather condition.
Figure I.2: Different Types of Attenuation on FSO Links [5]
Fog consists of several physical parameters such as liquid water content, particle size distribution, fog temperature and humidity. Since the size of fog particles is comparable to the transmission wavelength of optical and near infrared waves, it causes attenuation due to Mie scattering, which in turn reduces availability for considerable amount of time.
8
4. Optical Scattering – Single Scattering based Microphysical Model
Fog attenuations can be predicted based on method given by Mie scattering theory based on microphysical parameters of fog particle such as DSD. In order to apply Mie scattering towards prediction of optical attenuations for fog conditions, following assumptions are made that does not seem to have a large impact on the accuracy of the results calculated [15],
• The scattered light has the same wavelength as of the incident light
• The fog particles are spherical in shape and are acting independently with a complex refractive index in free-space
• Only single scattering takes place and the multiple scattering effects are negligible
If individual particles don’t have a high forward scattering efficiency and their number concentration is low enough, path attenuation can be calculated through the single scattering theory, which assumes that the energy absorbed or scattered by a particle is definitely lost. However, if this is not the case, the contribution of scattered light to the amount of energy transmitted through the medium cannot be neglected. If γ is the attenuation of the total extinction coefficient per unit length then it can be represented in the form as,
(1.1)
Where and are coefficients of molecular gas and aerosol absorption and are the molecular gas and aerosol scattering coefficients, respectively. in order to calculating the influence towards overall fog attenuation, the molecular gas scattering and absorption coefficients attenuation is negligible, and aerosol absorption coefficients can be ignored when compared with the attenuation
9
contribution by fog droplets (aerosol scattering coefficient), so: index of refraction (especially its real part), and its isotropy (the property of molecules and materials of having identical physical characteristics in all directions). In order to predict the accurate scattering pattern the above mentioned patterns must be known. The size of the atmospheric particle defines the type of symmetry of the scattered energy with reference to the direction of propagation of the incident light beam. We can further subdivide the scattering process into three distinct scattering regimes, as discussed below, on the basis of the size of particle and its interaction with the incident radiation. If the particle size is equal (comparable) to the wavelength of the incident light the scattering by the particle presents a large forward lobe and small side lobes that start to appear and this type of scattering is called Mie scattering.
The backward lobe becomes larger and the side lobes disappear if the size of the particle becomes smaller. If the particle size is approximately 10 percent the size of the wavelength of the incident beam the backward lobe is symmetrical with the forward lobe. This type of scattering is termed as Rayleigh scattering. The third type of scattering is referred as Geometrical scattering and occurs if the particle size is 10 times greater than the incident wavelength
In this type of scattering some very complex and irregular shape scattering patterns are formed as a result of incident wavelength and atmospheric particle interaction. when particle sizes exceed the incident wavelength forward scattering becomes more significant[11]. Here, is the specific attenuation measured in it is calculated by adding attenuation effect of all individual fog droplets present per unit volume per unit of radius increment. is the real
10
part of the complex refractive index of the fog particles and is the normalized Mie scattering cross-section and the factor is introduced here for denormalizing with respect to the geometrical cross-sectional area of the fog droplets
The modified gamma drop size distribution (hereafter MGDSD) profiles for moderate and dense continental as well as maritime fog conditions are plotted. The corresponding MGDSD parameters of continental fog only are listed in Table 1.1[18]. The parameters and LWC in show the fog droplets number density and the liquid water content, respectively. Whereas, and are the three parameters of MGDSD representing shape parameter, the intercept or the normalization constant and the slope or gradient of the fog particles DSD, respectively for moderate continental fog.
Table(1.1) Standard values of modified gamma distribution parameters for continental fog conditions [11]
Under continental fog conditions there are small indications of wavelength dependent attenuation for optical links i.e., at longer wavelengths the attenuations are smaller when compared with shorter wavelengths.
5. Scattering
In the context of free-space optical beam propagation, scattering is defined as the beam dispersal of light particles into a range of directions as a result of its Fog Type M ᴧ Nr (cm-3) LWC (g/m3) ᵧ(db/km)
Moderate
6
606.5
3
20
0.016
37.66
Dense Continental
6
2.37
1.5
100
0.063
124.82
11
physical interactions with the atmospheric particles. When an atmospheric particle intercepts an electromagnetic wave, part of the wave energy is removed by the particle and re-radiated into a solid angle centered at it. The re-radiation or scattering behavior depends on the characteristics of the particle like size of the particle in relation to the incident wavelength, its complex Index of refraction especially its real part), and its isotropy (the property of molecules and materials of having identical physical characteristics in all directions). If all these abovementioned parameters are known, the scattering pattern of the particle can be predicted with sufficient accuracy [21].
The size of the atmospheric particle defines the type of symmetry of the scattered energy with reference to the direction of propagation of the incident light beam. The scattering process divided into three distinct scattering regimes, on the basis of the size of particle and its Interaction with the incident radiation.
If the size of the particle is equal or comparable to the wavelength of the incident light , the scattering by the particle presents a large forward lobe and small side lobes that start to appear and this type of scattering is called Mie scattering. As the size of the particle become smaller, the backward lobe becomes larger and the side lobes disappear. When the size of the particle is approximately 10 percent the size of the wavelength of the incident beam the backward lobe is symmetrical with the forward lobe This type of scattering is termed as Rayleigh scattering[22]. The third type of scattering is referred as Geometrical scattering and occurs if the size of the particle is 10 times greater than the incident wavelengt
1
Chapter-I
INTRODUCTION TO FSO Free-Space Optical communication (FSO) has become most economical and attractive technology and as an alternative to radio frequency communication over last twenty years. Incredible advances made in the designing of electro-optical components and systems coupled with huge progress achieved in the information processing capabilities helped improve the optical communication transceivers. [37] Now a day’s many state of the art optical wireless systems being engaged in many short-range to long-range terrestrial, ground-space and space-space communication application scenarios. This thesis presents Fog effects on terrestrial Free-Space Optical (FSO) links. Due to dense fog FSO links may have a complete loss of signals. Moreover, sudden changes in temperature and pressure causing scintillations may distort the amplitude of the target signal. [7] Therefore, any optical wireless system that relies on the dissemination of optical beams in free-space must have been designed to take these factors into account. The main emphasis of this work has been effect of fog on terrestrial FSO links. 1. Brief History of FSO Free-space optical wireless communications, also referred to as FSO, is an effective way of communication and is very poplar subject in today’s communication. The methods that are used in optical transmission existed in their primitive form for centuries throughout the known human history; well before the work of Claude Chappe, a French inventor who initially introduced a practical semaphore system
2
in year 1792. His system was a series of semaphores located on buildings, where Human Operators transmitted information from one building to another. However, the information through optical mean are remained limited until the establishment of optical telegraph at the beginning of 19th century. At the start of 19th century the major work on the telecommunication is started and people are enabled to speak over long distances. Due to the changing nature of atmosphere the quality of service (QoS) of transmitter and receiver remains the issue [31].
Optical transmission was started for the telecommunication systems after the laser as a light source was invented. In 1905 Albert Einstein has invented First working laser as a coherent light source being not in a nature. By the formation of Ruby Laser in 1960 by Theodore Maimann, a milestone for the optical technology has started and another solution to telecommunication systems as optical telecommunication based on optical fiber technology [31]. In 1970-71 a wired optical communication is started with low attenuation incessant mode optical fibers and semiconductor lasers. Optical fiber transmissions indubitably dominates fields of terrestrial long-distance transmissions, starting at about 10 km for metropolitan networks and up to several thousand kilometers as for cables and the trans-oceanic transmissions and thus it turn into an integral and essential element of highway system across the whole world [1]. The network transportation, based on silica single mode glass fibers in covered cables, is already installed for most countries, e.g. Europe, North America or Japan. Longer distance directive radio links for terrestrial applications are only used for very special cases, but radio frequency is used as the main medium for shorter distances non-directive applications. The access segment networks are successful examples of the
3
digital cellular telephone network (GSM, UMTS), WLAN for data transmission.
They are usually used In smaller cells, or DVB-T for digital television broadcasting [1]-[31].The communication capacity of wired systems from telegraph systems to Dense Wavelength Division Multiplexing (DWDM) based state of the art fiber systems has been increasing constantly. With several Advancements in electro optics equipment, to perform system functions analogous to those carried out in electrical and electronic domain, and the introduction of single mode fibers has further accelerated this trend at an average rate of doubling every 24 months. In past electrical coaxial cables were used for the transmission of data signals at a maximum bit rate of 565 Mbit/s over 1 km distance between electrical regenerators. But with the single-mode fibers and lasers the transmission capacity reached up to 2.5 Gbit/s for the distances of 100 km without regenerators. Another achievement was made in 1990s with the development of optical amplifiers Erbium Doped Fiber Amplifiers (EDFA). After that the DWDM systems were designed that may combine many optical channels in the frequency domain, allowing the simultaneous amplification of optical signal from all the channels. Finally, the transmitted stream signal is demultiplexed at the receiver end [21].
2. Overview of Optical Wireless Communication
FSO has a potential to solve troubles for bridging the networking gap and provide broadband internet services access to rural areas. It has very high frequency slot of 300 THZ allows high data rates for the optical communication [7].
4
2.1 Advantages of FSO
Permit free communication, easy equipment system, keep away from electromagnetic pollution and wiretapping protection are big compensation of the optical communication systems. web browsing, electronic exchange, streaming audio and video, teleconferencing, Real-time medical imaging transfer, enterprise networking, work sharing, and high speed interplanetary are some possible applications of FSO links. [7] Previously optical communication was considered as a key to the last mile access problem but at present used for data transfer applications because of its high speed by creating links between fixed and mobile platforms[19] ,The popularity of optical communication systems are hindered by its vulnerability to certain weather conditions and atmospheric variations. In the communication systems, communication is inclined by the spread channel, and the spread channel for FSO communication is free space.
2.2 Disadvantage of FSO
The major disadvantage of FSO is propagation error produced due to interaction of the optical wave with the earth ambiance. To avoid propagation impairments special techniques have been used to cope up with the effects. Absorption and scattering on atmospheric particles as fog, rain, snow and clouds result in such effects as wave front distortion, beam wandering and Beam spreading that in turn are responsible for significant signal loss [35].
5
Fig I-1: Atmospheric Effects on FSO Links[6]
Due to air turbulence, propagation impairments bit error rate (BER) is increased. Optical disorder decreases exponentially with altitude and the major turbulent found up to 25 km altitude by the measurements.
The overall effects of optical turbulence are stochastic in space and time, producing signal fades on a timescale of several milliseconds in fixed applications Fade levels can exceed 20 dB in extreme cases depending on the propagation path and on the link range. In case of continental fog [3] the attenuation can be up to 130 dB/km up to 480 dB/km in dense maritime fog. [18].
It is interesting to note that though this technology has been named Free-space Optics, much of its utilization has been in terrestrial atmosphere where light Communication suffers all sorts of channel degradation effects which are usually absent in actual free-space channel scenario This factor is perhaps the only drawback associated with FSO communications and one of the main reasons for its limited deployments in many regions of the world.
Light attenuation through terrestrial atmosphere is caused by various physical phenomena absorption, scattering, beam wandering, beam spreading and wave front distortion etc. Absorption and Scattering of light occurs when transmitted
6
FSO beam interacts with particles of smoke and dust, droplets of fog, haze, smog and rain, flakes of snow etc., suspended in the atmosphere. During this interaction of light with particles and droplets, light is both absorbed and scattered in different directions. As a result of these interactions, light continuously loses energy while Traveling in the forward direction, thus restricting the maximum length of the link as well as presenting new challenges for optimum detection of received signal. Absorption can be controlled by transmitting light at such chosen bands of spectrum where absorption effect is minimum and we can ignore this loss without Affecting the practical results significantly. Because of Fog in atmosphere the light Undergoes scattering by droplets and it cause attenuation due to fog is biggest challenge to FSO communication today on the transmitted optical signals.
This attenuation is caused in major part by scattering by fog droplets or aerosol; and attenuations by molecular/aerosol absorption and molecular scattering effects are negligible in comparison.
3. Weather Effects on FSO
The main effects on FSO in the form of precipitation are fog, rain, snow and clouds. When an optical beam propagates through clear sky conditions, even then some optical signal attenuation takes place due to the absorption by atmospheric particulates (gaseous constituents of the atmosphere in particular Oxygen and water vapors). The presence of precipitation in the Earth atmosphere considerably increases the amount of optical signal attenuation and is dependent on the size of the particles, particle concentration and form, and the chemical composition.
The terrestrial FSO links are mostly affected by low clouds, fog and snow.
7
Fog being the most detrimental of all effect with attenuation levels as high as 480 dB/km in dense maritime fog environments [18], around 120 dB/km in continental fog environments [3]. Optical attenuations may easily exceed 50 dB/km in low clouds [24], while through rain the attenuations are not that significant as heavy rain showers can cause specific attenuations up to 30 dB/km at a rain rate of 150 mm/h [16]. However, measurements showed that optical attenuations through falling dry snow can easily reach 45 dB/km [17]. This potentially limits the achievable optical link range with very high reliability to less than 500 m during such conditions and thus requires a detailed characterization of each climatic conditions for terrestrial FSO link of different path lengths Rain, Snow, Haze, smoke and Fog may effects the FSO. Among all atmospheric effects, fog is the most critical weather condition.
Figure I.2: Different Types of Attenuation on FSO Links [5]
Fog consists of several physical parameters such as liquid water content, particle size distribution, fog temperature and humidity. Since the size of fog particles is comparable to the transmission wavelength of optical and near infrared waves, it causes attenuation due to Mie scattering, which in turn reduces availability for considerable amount of time.
8
4. Optical Scattering – Single Scattering based Microphysical Model
Fog attenuations can be predicted based on method given by Mie scattering theory based on microphysical parameters of fog particle such as DSD. In order to apply Mie scattering towards prediction of optical attenuations for fog conditions, following assumptions are made that does not seem to have a large impact on the accuracy of the results calculated [15],
• The scattered light has the same wavelength as of the incident light
• The fog particles are spherical in shape and are acting independently with a complex refractive index in free-space
• Only single scattering takes place and the multiple scattering effects are negligible
If individual particles don’t have a high forward scattering efficiency and their number concentration is low enough, path attenuation can be calculated through the single scattering theory, which assumes that the energy absorbed or scattered by a particle is definitely lost. However, if this is not the case, the contribution of scattered light to the amount of energy transmitted through the medium cannot be neglected. If γ is the attenuation of the total extinction coefficient per unit length then it can be represented in the form as,
(1.1)
Where and are coefficients of molecular gas and aerosol absorption and are the molecular gas and aerosol scattering coefficients, respectively. in order to calculating the influence towards overall fog attenuation, the molecular gas scattering and absorption coefficients attenuation is negligible, and aerosol absorption coefficients can be ignored when compared with the attenuation
9
contribution by fog droplets (aerosol scattering coefficient), so: index of refraction (especially its real part), and its isotropy (the property of molecules and materials of having identical physical characteristics in all directions). In order to predict the accurate scattering pattern the above mentioned patterns must be known. The size of the atmospheric particle defines the type of symmetry of the scattered energy with reference to the direction of propagation of the incident light beam. We can further subdivide the scattering process into three distinct scattering regimes, as discussed below, on the basis of the size of particle and its interaction with the incident radiation. If the particle size is equal (comparable) to the wavelength of the incident light the scattering by the particle presents a large forward lobe and small side lobes that start to appear and this type of scattering is called Mie scattering.
The backward lobe becomes larger and the side lobes disappear if the size of the particle becomes smaller. If the particle size is approximately 10 percent the size of the wavelength of the incident beam the backward lobe is symmetrical with the forward lobe. This type of scattering is termed as Rayleigh scattering. The third type of scattering is referred as Geometrical scattering and occurs if the particle size is 10 times greater than the incident wavelength
In this type of scattering some very complex and irregular shape scattering patterns are formed as a result of incident wavelength and atmospheric particle interaction. when particle sizes exceed the incident wavelength forward scattering becomes more significant[11]. Here, is the specific attenuation measured in it is calculated by adding attenuation effect of all individual fog droplets present per unit volume per unit of radius increment. is the real
10
part of the complex refractive index of the fog particles and is the normalized Mie scattering cross-section and the factor is introduced here for denormalizing with respect to the geometrical cross-sectional area of the fog droplets
The modified gamma drop size distribution (hereafter MGDSD) profiles for moderate and dense continental as well as maritime fog conditions are plotted. The corresponding MGDSD parameters of continental fog only are listed in Table 1.1[18]. The parameters and LWC in show the fog droplets number density and the liquid water content, respectively. Whereas, and are the three parameters of MGDSD representing shape parameter, the intercept or the normalization constant and the slope or gradient of the fog particles DSD, respectively for moderate continental fog.
Table(1.1) Standard values of modified gamma distribution parameters for continental fog conditions [11]
Under continental fog conditions there are small indications of wavelength dependent attenuation for optical links i.e., at longer wavelengths the attenuations are smaller when compared with shorter wavelengths.
5. Scattering
In the context of free-space optical beam propagation, scattering is defined as the beam dispersal of light particles into a range of directions as a result of its Fog Type M ᴧ Nr (cm-3) LWC (g/m3) ᵧ(db/km)
Moderate
6
606.5
3
20
0.016
37.66
Dense Continental
6
2.37
1.5
100
0.063
124.82
11
physical interactions with the atmospheric particles. When an atmospheric particle intercepts an electromagnetic wave, part of the wave energy is removed by the particle and re-radiated into a solid angle centered at it. The re-radiation or scattering behavior depends on the characteristics of the particle like size of the particle in relation to the incident wavelength, its complex Index of refraction especially its real part), and its isotropy (the property of molecules and materials of having identical physical characteristics in all directions). If all these abovementioned parameters are known, the scattering pattern of the particle can be predicted with sufficient accuracy [21].
The size of the atmospheric particle defines the type of symmetry of the scattered energy with reference to the direction of propagation of the incident light beam. The scattering process divided into three distinct scattering regimes, on the basis of the size of particle and its Interaction with the incident radiation.
If the size of the particle is equal or comparable to the wavelength of the incident light , the scattering by the particle presents a large forward lobe and small side lobes that start to appear and this type of scattering is called Mie scattering. As the size of the particle become smaller, the backward lobe becomes larger and the side lobes disappear. When the size of the particle is approximately 10 percent the size of the wavelength of the incident beam the backward lobe is symmetrical with the forward lobe This type of scattering is termed as Rayleigh scattering[22]. The third type of scattering is referred as Geometrical scattering and occurs if the size of the particle is 10 times greater than the incident wavelength In this type of scattering some very complex and irregular shape scattering patterns are
12
formed as a result of incident wavelength and atmospheric particle interaction. In general, forward scattering becomes more significant when particle sizes exceed the incident wavelength [17]. Figure shows illustration of the re-distribution of radiation once a particle is hit by an electromagnetic wave.
Figure I.3: Illustration of Scattering Patterns for Three Different Scattering
It is interesting to note that most of the light that reaches our eyes comes indirectly by means of scattering process. Fine atmospheric particles standing in the path of free-space optical beams scattered energy from the incident wave.
As a result reradiate in different directions. The received power at a distance d from the transmitter is given by the Beer-Lambert law and the total transmittance can be re-written as,
13
In this equation, is the attenuation of the total extinction coefficient per unit of length. This parameter is dependent on the transmission wavelength and the density of the atmospheric constituents along the line of sight. From above the loss L in dB that an optical beam experiences is given [21]
(1.3)
Generally, is given by,
(1.4)
where ) are molecular and aerosol absorption coefficients and ) are molecular and aerosol scattering coefficients respectively.
5.1 Rayleigh scattering
Rayleigh scattering refers to scattering by molecular and atmospheric gases of sizes much less than the incident wavelength. It varies as the fourth power of the incident wavelength with overall constant dependent on the index of refraction[17]. The scattering by the atmospheric gas molecules (Rayleigh scatter) contributes to the total attenuation of the electromagnetic radiation.
The expression for the molecular scattering coefficient is given by:
[ ] * +
5.2 Mie Scattering
The determination of the properties and optical characteristics of atmospheric aerosols as well as clouds and fog requires a good understanding of the interaction of light with particles. The theory of electromagnetism (arrived at its maturity in the late 19th century) provides a standard treatment for simple cases, namely the interaction of spherical, homogeneous and isotropic light particle with the
(1.5)
14
propagating light beam. This specific case was treated by Gustav Mie [38]. It should be noted that scattering and absorption are dynamic, local phenomena and tend to vary widely from location to location in severity. Scattering caused by relatively larger size particles of fog, rain, snow and clouds etc., is classified as Mie scattering. Mie theory best describes the scattering phenomena by aerosol particles of sizes comparable to the incident wavelength. For practical purposes, optical losses due to Rayleigh scattering are considered almost negligible when compared to losses due to Mie scattering in the wavelength range 0.5 mm to 2 mm.
Generally, according to the Mie theory, the aerosols scattering coefficient depends on the particle concentration, size distribution, cross-section, and incident radiation wavelength. Basically, Mie theory is an analytical approach to describe the propagation of light in the scattering medium by combining processes of scattering and absorption variations. The calculated parameters such as scattering cross sections and absorption can characterize the properties of scattering and absorption of the medium for a given wavelength. In the treatment of Mie scattering we assume that the frequencies of incident radiation are identical and distributed. All effects involving transitions by quantum scattering centers will not be considered. We also assume that particles are independent. This assumption means that the scattering centers are not positioned on a structure or a specific order which Prevents the systematic phase shifts give rise to interference phenomena. In clouds, fog and rain, it is usually considered that the particles are distributed randomly in the volume studied, or, in other word, there is no phase consistency between the fields absorbed or scattered by individual particles. Furthermore, the concentration of droplets is generally low enough that we can consider that particles are not coupled together. Finally, we assume that the
15
scattering caused by particles is just times larger than the scattering caused by single particle. The interaction of electromagnetic radiation with an absorbing sphere whose size is greater or equal to the wavelength is well described by Mie theory, which was particularly well presented in several references in the literature[15]-[38]. We employ the Maxwell’s equations and the principle of energy conservation to determine the effective scattering cross section, absorption and extinction and the scattering efficiency, absorption and extinction as a function of the wavelength of the radiation, the particle size and its complex refractive index. For example, consider the free-space optical transmissions through fog.
All the fog particles actually consist of an accumulation of water spheres with a particular drop size distribution, the effect of the individual drop must be summed over a unit volume keeping in mind the assumptions already mentioned above. Then, from Mie theory the aerosol scattering co-efficient ba(l) is given by:
∫ ( ) [ ]
where is the real part of the refractive index of the fog particles, l is the incident wavelength in is the radius of fog particle in . The scattering efficiency is defined as the scattering cross-section normalized by the particle cross-sectional area, and is the particle size distribution which usually is the modified Gamma distributions. In order to simply describe the action of aerosol in the atmosphere for the free space optical communication,[21] it is convenient to use the following practical relationships commonly used to describe the scattering coefficient , in horizontal path with constant aerosol concentration:
[ ]
where and are constants that depend upon the aerosol characteristics like
(1.6)
(1.7)
16
density, particle size distribution and is the wavelength of interest in . The constant is related to the atmospheric visibility range and varies from (from poor visibility to clear line-of-sight). The constant is related to the visual (ormeteorological) range (in kilometers) as:
[ ]
Since the Earth atmosphere has maximum transmission at 0.55 wavelength, therefore visual range is measured at this wavelength. The scattering coefficient then becomes:
( ) [ ] (1.9)
5.3 Geometrical Scattering
Geometrical or Non-selective scattering applies to particle sized larger than the incident wavelength. In this case, mie theory is approximated by the principles of reflection, refraction and diffraction. The name non-selective refers to the fact that scattering is independent of wavelength [21]. Geometrical or non-selective scattering occurs in snow, hail, rain and cloud droplets, where the size of the particles is much higher than the incident light wavelength. Type Radius Size Parameter Scattering Process
Air Molecule
0.0001
0.00074
Rayleigh
Haze Particle
0.01-1
0.074 – 7.4
Rayleigh – Mie
Fog Droplets
1-20
7.4 – 147.8
Mie – Geometrical
Rain
100-10000
740 – 74000
Geometrical
Snow
1000-5000
7400 – 37000
Geometrical
Hail
5000-50000
37000 – 370000
Geometrical
(1.8)
17
Table I.2: scattering processes for typical atmospheric particles with their radii
6. Atmospheric Effects Attenuation Modeling
In the recent years, there is a growing trend to investigate various possibilities of modeling the free-space atmospheric channel for different communication scenarios in order to enhance the capabilities of the FSO links [18]. In late 90’s, when FSO started gathering momentum as a potential access technology, the research mostly revolved around studying the influence caused by fog attenuations on the terrestrial FSO links by comparing fog attenuations for different wavelengths [ 7 ] and proposing better models relating visibility range to fog attenuation . The phenomenon of fog has been actively re-searched for many decades but with varying interests [24]. However, driven by niche applications of FSO technology in many ground & space and fixed & mobile communication scenarios with the need of more bandwidth demands the research activity soon started towards characterizing various atmospheric
Phenomena considered detrimental (like fog, rain, snow, clouds and atmospheric turbulence) to the propagation of optical signal in free- space [ 24].
Since terrestrial FSO systems are more vulnerable to certain fog conditions, therefore, the more focus remained on characterizing different fog conditions for terrestrial FSO link scenarios by investigating the time-series analysis of fog attenuations [23 ]. In this chapter, optical attenuation results are discussed for terrestrial FSO links in radiation and advection fog conditions in further detail with possible comparison with measurement attenuation data available from other locations. In addition, results about optical attenuations measured in rain and dry snow conditions are also presented and analyzed. Moreover, with ever growing
18
demand of ground-space optical links, clouds play a decisive role on the optical link availability. Because of the un-availability of optical attenuations measured through clouds, optical attenuations are simulated for different cloud types using predictions generated using Mie scattering theory and the prediction model given in ITU-R P. 1622 recommendation.
The terrestrial FSO links must deal with the atmosphere just above the surface of the earth, where it has maximum density due to the gravitational force. Atmospheric attenuation can be distinguished as molecular absorption, Rayleigh-scattering, and aerosol- scattering. Molecular absorption is an effect of electron and nucleus-resonance of atmospheric molecules, Rayleigh-scattering is caused by the atmospheric molecules acting as dipole-antennas, and aerosol-scattering is caused by droplets and particles that are larger than the wavelength that is influenced. Atmospheric attenuation of FSO systems is typically dominated by fog but can also be due to low clouds, rain, snow, dust, and various combinations thereof. Molecular absorption may be minimized by appropriate selection of the optical wavelength.
7. Applications of FSO
During Last two decades free-space optical (FSO) Communications becomes attractive technology. Based on the environmental conditions and range over which an FSO link operates, it is subject to different impairments. Directed laser beams are used to transmit data over Long-range links and can be used for building-to-building, ground-to-aircraft, or ground-to-satellite communication[ 33]-[ 1].
19
Figure I.4: Optical communication model
Communication links may operate over long ranges for several kilometers or longer, and often their primary impairment is atmospheric turbulence, which causes phase and intensity fluctuations in the received signal [6]. For Short-range links we often use infrared or visible light emitting diodes (LEDs) [ 6], and can be used indoors for data communications or outdoors for vehicular communication. Short range links operate over meters to hundreds of meters and are not strongly affected by atmospheric turbulence. Whereas Outdoor links may be subject to impairment by environmental effects, such as rain, fog and haze. Several studies have been made on the impact of these effects on FSO links [1], but these studies only take account of the attenuation caused by them. In particular, these studies do not take account of how rain, fog and haze may degrade the performance of an imaging receiver. FSO technology is one of the most promising and emerging access technology having tremendous potential to offer fiber-like bandwidth and the lowest cost per bit of information transmitted. In order to successfully design and field an optical wireless system, a number of critical design tradeoffs related especially to the free-space atmospheric channel, the specification of a particular wavelength for transmission and the
20
transmitter and receiver design must be resolved
8. Parameters for performance of optical communication
The performance of optical communication systems is depending upon the two important parameters which are given below:
8.1Internal parameters
8.2 External Parameters
8.1 Internal Parameters
Internal limitations and parameters related to design include wavelength, transmission bandwidth, optical power, Bit error rate, divergence angle, receiver bandwidth, optical loss e.t.c.
8.2 External parameters
Environmental conditions, visibility range and atmospheric attenuation, scintillation or atmospheric turbulence is considered as external parameters[ 32].
21
Chapter II
Literature Review FSO
In the communication world, FSO is one area which need be researched comprehensively. The theory behind free-space optical communications is well represented in the literature review. The effect of fog and other impairments on the performance of the systems have remained an actively researched area and different kinds of solutions have been proposed to combat such conditions. The selection of an appropriate wavelength to provide optimum performance has been a major point of disagreement [14]. Availability and reliability analysis for FSO systems has been extensively carried out [14, 5] to prove their importance in the evolving broadband network. Hybrid FSO-RF links promises very high availabilities [12], and the development of an intelligent switch-over is an open problem [3]. Currently, optical wireless systems are mainly being used for short range communication applications like the Last-mile access. FSO has demonstrated the application potential in many areas such as line-of-sight communications, satellite communications and the last mile solution in a fiber optics networking. Optical wavelength having wavelengths 0.8 μ m, 1.5 μ m, 3.5 μ m and 10 μ m are now commonly available and are currently being used in state-of-the-art free-space laser communication systems [19, 120]; unfortunately the system performance is imposed by atmospheric turbulence [16]. To reduce atmospheric effect in free-space laser communication systems, several techniques have been used, such as adaptive optics, aperture averaging, coding techniques and multiple transmitters [21, 3]; however, significant improvement has not been achieved yet. Such systems are in use for long distances and the most accurate solutions with acquisition and tracking for space
22
application.
free-space optical links engaging imaging receivers in the presence of misalignment and atmospheric effects, such as haze, fog or rain. A detailed propagation model based on the radiative transfer equation. compare the relative importance of two mechanisms by which these effects degrade link performance: signal attenuation and image blooming. It has been observed that image blooming controls over attenuation, except under medium-to-heavy fog conditions.
High-bandwidth optical wireless transmission links are of prime importance and have tremendous potential to serve for the next generation optical access networks. Currently optical wireless or Free Space Optics(FSO) communication products supporting data rates over 1Gbps are available from several communication equipment manufacturers. One obvious accompanying disadvantage of this technology comes from its transmitted optical beam interaction with the earth atmosphere. Notably high attenuation are inflicted by fog, clouds and snow conditions, that result in substantial loss of optical signal power over the communication channel.
In today’s access network, the optical wireless (OW) or Free Space Optics (FSO) technology is playing an increasing complementary role to the existing radio frequency (RF) based techniques. This is attributable to
its prime fundamental feature of point to point huge bandwidth that is comparable to that obtainable from optical fibre but with an added advantage of lower deployment cost and time [1]. This technology recently
has found niche applications both in military and commercial services sectors as well. The earlier doubts about FSO’s usefulness, its declining acceptability by service providers and slow market penetration are now
23
rapidly fading away, judging by the number of service providers, organisation, government and private establishments that now incorporate FSO links into their network infrastructure [2, 3].
the attenuation of free space optical (FSO) communication systems operating at selected wavelengths of 0.83 μm, 1.31 μm and 1.55 μm in the controlled laboratory based fog and smoke environments are compared. The fog and smoke are generated and controlled homogeneously along the indoor atmospheric chamber of length 6 m.
The rising need for high bandwidth transmission capability link along with security and ease in installation has led to the interest in free space optical communication technology. It provides highest data rates due to their high carrier frequency in the range of 300 THz. FSO is license free, secure, easily deployable and offers low bit error rate link. These characteristics motivate to use FSO as a solution to last mile access bottleneck.
24
Chapter III
The Phenomenon of Fog
1. Fog
Fog consists water droplets composed by concentration of water vapors on a pre-existing aerosol distribution of different nature (dust, smoke, volcanic ash etc) suspended in the air near the surface of the earth under the effect of gravity. The presence of fog acts to disperse the light and thus trim down the visibility close to the land. Whenever the flat visibility is less than 1 km a mist layer is reported and the relative moisture of the air is brought to the saturation point (close to 100%). The water density in mist is normally around 0.05 g/m3 having a visibility of about 300 m and 0.5 g/m3 normally but for the dense mist having visibility range is about 50 m [1]. A fog layer can widen vertically up also at the altitude of 300m to 400m above the earth up to the altitude of the layer where high temperature inversion is present. Optical reduction is extremely interconnected with mist concentration, and it is for the most part exaggerated by the concentration and the volume of mist droplets. As a general rule fog droplets tend to cluster around 5 μ m to 15 μ m in diameter.
1.2 Formation of Fog
Generally, fog is formed below a broad range of scenarios in consequence of super saturation formed by cooling, moisturize or combination of air parcels, closer the land, that have different temperatures. The existence of small suspended water droplets and ice crystals can make an thing difficult to distinguish to a far-away
25
viewer and end result is poor visibility circumstances. Throughout the formation phase of mist, simultaneous increases in liquid water content, droplet concentration and significant mist drop size have been seemed to take place slowly but surely. After fog formation stage i.e., during the mature stage almost droplet concentration and Liquid Water Content (LWC), while a constant decreasing size in droplet of mist has been observed.
Mist dissipation stage usually occurs as the droplet concentration, significant droplet size, and LWC all reduce with the passage of time [ 35]. Droplet absorption and bulk dependent dispersion losses of the propagating optical beam cause poor visibility that occurs in the course of a lessening in the intensity difference between a point and its conditions and through the blurring effect of onward dispersion of light due to the existence of the water crystals. Mist is mostly occurring in climatic environments having large concentrations of aerosols characterized by a little activation super saturation (level of super saturation at which aerosol particles spontaneously grow to become cloud drops). It is very difficult to distinguish between inactivated and activated fog droplets in different fog types.
The existence of mist and its microphysical characteristics have influence on the capacity, ease of use and dependability of FSO systems by attenuating the power of transmitted optical signal propagating through it [22]. Optical signal attenuation through fog is mainly caused by scattering of electromagnetic wave on the spherical water droplets suspended in the air and interacting independently. The physical mechanism of fog attenuation is similar to rain attenuation, but since the
26
typical fog droplet radius is much smaller than the rain droplet, therefore the impaired wavelengths are shorter in fog as compared to rain. The role of typical fog droplet radii is very important in case of terrestrial FSO links as there exist several types of fog in different regions and seasons. Typical fog droplet radii vary from environment to environment and hence the fog influence on the optical signal propagation varies. In order to forecast the visual signal attenuations due to fog the knowledge about evolution, dissipation of fog along with its spatial and of time circulation and microphysical characteristics are quite useful. Especially, among the microphysical characteristics of concern, the droplet size distribution (DSD) of mist and amount of liquid water content (LWC), common particle size and number per air volume are crucial. [22] These properties together with the awareness of the dispersion characteristic of single droplets permit to estimate mist reduction. The measurement of mist LWC and DSD is nontrivial and dedicated information is still scarce in the text. Additionally, these microphysical parameters undergo spatial and of time changes and are dependent on the full of atmosphere circumstances like relative dampness, hotness, and the surroundings e.g. continental or maritime and the microclimate. Based on visibility range estimate Eldridge explained three comprehensive types of shorter visibility climatic conditions; fog for visibilities less than 500m, mist for visibilities between 500 and 1000m and haze for visibilities greater than 1000m. These zones are based on changes in observed element volume distributions and changes in the wavelength selectivity of calculated reduction coefficients. Mist is primarily made of microscopic well dust or salt or tiny droplets of a few microns to a few tenths of micron sizes.
27
2. International Codes For visibility
Nowadays, an international code of visibility is adopted in order to differentiate between different climatic conditions based on visibility range estimate as given in Table III.1 Description Visibility Range (KM) Attenuation(db/Km)
Dense Fog
0.04-0.07
250-143
Thick Fog
0.07-0.25
143-40
Moderate Fog
0.25-0.5
40-20
Light Fog
0.5-1.0
20-9.3
Thin Fog
1.0-2.0
9.3-4.0
Haze
2.0-4.0
4.0-1.6
Light Haze
4.0-10.0
1.6-0.5
Clear Sky
10.0-25.0
0.5-0.1
Very Clear Sky
25.0-50.0
0.1-0.04
Extremely Clear Sky
50.0-150.0
0.04-0.005
Table III.1: International code of Visibility Range
Intricate relationships exist between aerosols plus fog. Fog may appear under the influence of particular aerosol characteristics like number concentration, size distribution and chemical composition that have a large impact on the occurrence of a particular type fog. These aerosol characteristics strongly influence fog microphysics and hence the overall life cycle of a particular type fog [25]. Therefore, as a result the LWC, DSD, and visibility range of a fog layer are affected and modified according to the characteristics of the aerosols contained in the air mass. All the optical characteristics of aerosols and in particular those of fog are connected to the atom size division, which is the mainly important parameter
28
allowing us to compute the optical properties of a quantity of droplets. Commonly, this division is represented by systematic functions such as log-normal division in the case of aerosols and the modified gamma distribution for mist, which is widely used to model a variety of types of mist with cloud and is given by:
)
In this equation, gives the add up to of particles per volume unit and per increment element of the particle radius , and are parameters which characterize the particle size distribution. Fog is usually characterized by and with [ ] for opaque mist these limitation values are and .
3. Classification of FOG
Radioactive humidification is the main physical mechanisms that result in fog formation. When radioactive mechanism dominates the fogs are termed as radioactive fogs and as adjective fogs when the second mechanism is prominent. Many processes are caught up in the Formation and development of mist, including the microphysics, primary role of radiation, turbulence, and humidity transport over various terrain to produce the saturation of air [35]. The virtual significance of each process varies from case to case and also temporally during a mist occurrence. As per mechanisms, Jiusto listed fourteen factors that influence the creation and dissipation of a mist layer. The limited level factors mostly occupy dampness accessibility, radioactive equilibrium of the unambiguous and overcast air, disorderly mixing, heat and moisture transfer in the dirt medium and microphysical processes besides warmth and humidity flat
(III-1)
29
advections and erect shift associated with large level and/or musicale circulations [28].
4. Types of Fog
Fog occurs naturally in many different types dependent upon the mechanism of their formation, the location dependent climatic conditions and the geographical location. Some major types of fog are:
4.1 Radiation or continental fog
4.2 Advection or maritime fog
4.3 Precipitation or frontal fog
4.4 Upslope fog
4.5 Valley fog
4.6 Arctic or steam fog
4.7 Cloud base lowering fog
4.1 Radiation or Continental Fog
Radiation fog in moderate areas generally occurs in winter and is formed near the ground plane beneath unambiguous sky conditions in sluggish air in connection with an atmospheric high pressure. The favorable conditions for radiation fog formation are very little pace winds, lofty moisture and unambiguous sky. Under this circumstances, radiation mist creation has been found to be insightful to the united dynamical and thermodynamically constitution of the developing no tunnel frontier coating over ground [29 ]. The typical mist droplets are considered round in form having diameter ranges from to for any type of fog [26] depending upon the geographical location.
30
In the case of reasonable continental mist droplets, the style is about , whereas for thick continental fogs the mode size is about . As a contrast, a obscure may include a good amount of very small water droplets, but the radii of the drops that lead annihilation and spreading are in the collection of , whereas, the restrictive liquid particle span of a cloud is of the order of , bigger drops than this include sprinkle or rain.
4.2 Advection or Maritime Fog
Advection mist is shaped by the progress of damp and humid air lots having contrasting temperature properties above the colder naval or worldly surfaces. The air in get in touch with the surface is cooled below its dew spot, causing concentration of water vapor. It is characterized by a liquid water substance reaching up to 0.5 g/m3 against a visibility range of 50 m. The maritime fog particle diameter is normally larger in size to continental fog particles. The mode in case of moderate maritime fog lies around 8 μ m and for dense maritime fog case it is around 10 μ m with 20 fog particles in unit volume cm3 for both types [21 ].
Research’s have exposed that the source and record of air lots in the coastal areas are significant factors in the practical inconsistency and in the spatial allocation of maritime mist in addition to the mist microphysical uniqueness [35]. The details about remaining fog types can be found in much detail in [29 ]. The fog droplet distributions for the moderate maritime and continental fogs and dense maritime and continental fogs. The fog particle concentrations for the continental or emission mist (on left) and naval or advection mist (on right). it was observed that the attenuation of FSO links in the troposphere are in lofty relationship with the mist strength, and
31
optical attenuations are chiefly affected by the thickness and allocation of the fog particles [23 ].
5. Effect of fog on FSO link
In order to apply an open-air short-range optical wireless connection is the atmospheric reduction, caused by the amalgamation and dispersion. Water particles and carbon dioxide chiefly cause the inclusion of optical signals, while mist, rain, snow and clouds cause the dispersion of optical signals transmitted in free gap. Due to the dispersion portion of the light beam travelling from a one souse to another may deflect away from the projected recipient [16]. Among various moody belongings on FSO statement, mist is the nearly all prevention attenuating factor. vapor causes significant attenuation of the optical signals for significant amount of time and is thus highly deterrent for achieving high availability in FSO transmissions. The main reason of significantly high attenuations due to dispersion in dissimilar mist conditions is that the size of mist particles is comparable to the broadcast wavelengths of optical and near infrared waves. This dispersion process is classified as Mie scattering process and the optical attenuations in case of mist droplets can be exactly predicted by applying Mie scattering theory [38]. However, it involves complex computations and requires detailed information of fog parameters like particle size, refractive index, particle size distribution that may not be readily available at a exacting site of fitting.
An exchange draw near is to expect mist attenuations based on the visibility range information. The visibility range is the distance to an object where the image difference drops to 5 % of what it would be if the object were close by in its place.
32
The 550 wavelength is usually used to measure the visibility range as it is extensively established as a visibility range location wavelength. The visibility range is regularly calculated with 550 wavelength at meteorological stations or at the airports as at this wavelength the atmosphere has utmost transmittance. [29],
6. Fog Modeling
To forecast the effects of attenuations based on visibility range approximation, three models are extensively used and they are the Kruse, Kim and Al-Naboulsi models and purposed new models.[14]- [17]- [5].
6.1 Kruse Fog Model
6.2 Kim Fog Model
6.3 Al-Naboulsi Model
6.4 New Proposed Model
6.1 Kruse Fog Model
The diminution coefficient for both Kim and Kruse copies is same and is
(III-2)
Here stands for prominence range, stands for program of air drops to ratio of pure sky, in nm views for wavelength, as prominence variety mention (550 nm) and q is the size spreading coefficient of sprinkling. Permitting to Kruse model [8 ]-[9], q limitation is,
33
q={ (III-3)
6.2 Kim Fog Model
Permitting to Kim [13]-[ 23] the value of the q limitation is,
q={
Eqn. 2 indicates that for any atmospheric circumstances, there will be fewer for upper wavelengths. Therefore, diminution of 1550 nm is estimated to be smaller than diminutions at smaller wavelengths. Kim cast offs such wavelength addiction of visual diminutions for low prominence variety circumstances as in thick fog.
6.3 Al-Naboulsi Fog Model
Al-Naboulsi [10] typifies advection and pollution fog discretely and planned two dissimilar copies for advection and pollution fog. The advection fog is made by the activities of misty and warm air multitudes overhead the cooler marine faces while Pollution fog is associated to the ground freezing by pollution over interior surfaces. Permitting to Al-Nabulus the advection fog type is[15]
αADV(λ)=
and for the fog radiation model is,
(III-4)
(III-5)
(III-6)
34
αRAD(λ)=
The specific attenuation for the advection and radiation for both are simply calculated as given below,
AFog(dB/km)= (α(λ))
6.4 New Proposed Model
The imitation with dignified data in earlier segment shows that any present exemplary cannot be preferred. This irritates the need to offer a model that can deliver more exact expectation of diminution in terms of visibility.
A new exemplary is suggested here that has the minimum SSE The summation of square mistakes is the summation of square mistakes [24]between diminution expected by a exemplary for a prominence and diminution measured alongside that prominence. The model is given as shadows for 850 nm wavelength. If f is the definite diminution in dB/km and x is the prominence in meters
Coefficients (with 95% confidence bounds):
(III-7)
(III-8)
(III-9)
(III-10)
(III-11)
35
The model planned for 950 nm wavelength is assumed as follows:
Coefficients (with 95% confidence bounds):
7. Fog Attenuation Effect on FSO Link
For the FSO link, we existing here a judgment of essentially measured reasonable interior fog diminutions and the attenuation prices expected by Kim, Kruse and Al Naboulsi models that are pretend for 850 nm and 1550 nm wavelengths[13]- [10]. The choice of pretending 850 nm is for the purpose that our amount set up also uses this wavelength for diminution amounts and the 1550 nm is nominated for imitation due to its signifance in future message applications and eye protection concerns. The value of diminution has been taken as 120 dB/km beside a prominence variety of 100 m. This value is closer to diminution value as expected by Kruse model. Giving to [30]-[28], the atmospheric diminutions are wavelength helpless that means system working at extensive wavelengths has better variety routine than systems at
(III-12)
(III-13)
(III-14)
(III-15)
(III-16)
(III-17)
36
smaller wavelengths. The imitations show that this results additional importantly detected for Kruse model as the Kim ideal does not show any wavelength needy diminution behavior along side a prominence variety lesser than 500 m. The Al Naboulsi pollution fog ideal is also pretend for the two declared wavelengths (850 nm and 1550 nm) and it was detected that this ideal too does not show any substantial wavelength helpless attenuations for adequate interior fog (radiation fog) conditions.
37
Chapter IV
Simulations & Discussions
In this section, the proposed kruse, kim and Al Naboulsi models and new proposed models for the fog attenuation based on visibility range estimation are simulated. For FSO link we have simulate and present comparison of moderate continental fog and attenuations values predicated by kim, Kruse and Al Naboulsi models which are simulated at different wavelengths that are 550 nm and 1550 nm [13]. The choice for taking simulations at 550 nm is due to the reason that most of the setups for measurements uses this wavelength [9 ]. The 1550 nm is selected for simulation due to its importance in future application for communication and for eye safety concerns. The models are simulated and compared in these sections. All the simulations have been made in MATLAB 9.0 (R2009a).
1. Simulation for KIM models at 550nm, wavelengths and visibility range of 200 meters
In the figures given below the values of wavelengths taken 550nm against a visibility range of 200 meters . This value is closer to attenuation value as predicted by Kim model. According to [22], the atmospheric attenuations are wavelength dependent that means system operating at longer wavelengths has better range performance than systems at shorter wavelengths. The simulations show that this effect is more prominently observed for the Kim model does not show any wavelength dependent attenuation behavior against a visibility range lower than 500 m.
38
Figure IV-1: Simulation For Kim Model at q=1.6 if V> 50 Km
Figure IV-2: Simulation For Kim Model at q=1.3 if 6km < V< 50 Km
0 20 40 60 80 100 120 140 160 180 200
-2.2
-2
-1.8
-1.6
-1.4
-1.2
-1
-0.8
-0.6
visibility in meters
Specific attanuation in dB/Km
0 20 40 60 80 100 120 140 160 180 200
-2.2
-2
-1.8
-1.6
-1.4
-1.2
-1
-0.8
-0.6
visibility in meters
Specific attanuation in dB/Km
39
Figure IV-3: Simulation For Kim Model at q=0.16 V + 1.34 if 1km < V< 6 Km
Figure IV-4: Simulation For Kim Model at For q= V – 0.5 if 0.5km < V< 1 Km
0 20 40 60 80 100 120 140 160 180 200
-2.2
-2
-1.8
-1.6
-1.4
-1.2
-1
-0.8
-0.6
visibility in meters
Specific attanuation in dB/Km
0 20 40 60 80 100 120 140 160 180 200
-2.2
-2
-1.8
-1.6
-1.4
-1.2
-1
-0.8
-0.6
visibility in meters
Specific attanuation in dB/Km
40
Figure IV-5: Simulation For Kim Model at q= 0 if V< 0.5 Km
Figure IV-6: Comparison of kim model at different vales of q for Wavelength 550nm
2. Simulation for KIM models at 1550 nm wavelengths
In the figures given below the values of wavelengths taken 1550nm against a
visibility range of 50 kilometers. This value is closer to attenuation value as predicted
0 20 40 60 80 100 120 140 160 180 200
-2.2
-2
-1.8
-1.6
-1.4
-1.2
-1
-0.8
-0.6
visibility in meters
Specific attanuation in dB/Km
0 20 40 60 80 100 120 140 160 180 200
-12
-10
-8
-6
-4
-2
0
visibility in meters
Specific attanuation in dB/Km
41
by KIM model. The fig IV-7 is simulate for the distance of 200meters and wavelength
of 1550 nm for q=1.6 if visibility is greater than 50 KM.
Figure IV-7: Simulation For Kim Model at q=1.6 if V> 50 Km at 1550nm
Figure IV-8: Simulation For Kim Model at For q=1.3 if 6km < V< 50 Km at 1550nm
0 20 40 60 80 100 120 140 160 180 200
-0.45
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
visibility in meters
Specific attanuation in dB/Km
0 20 40 60 80 100 120 140 160 180 200
-0.45
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
visibility in meters
Specific attanuation in dB/Km
42
Figure IV-9: Simulation For Kim Model at q=0.16 V + 1.34 if 1km < V< 6 Km at
1550nm
Figure IV-10: Simulation For Kim Model at q= V – 0.5 if 0.5km < V< 1 Km at 1550nm
0 20 40 60 80 100 120 140 160 180 200
-0.45
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
visibility in meters
Specific attanuation in dB/Km
0 20 40 60 80 100 120 140 160 180 200
-0.45
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
visibility in meters
Specific attanuation in dB/Km
43
Figure IV-11: Simulation For Kim Model at q= 0 if V< 0.5 Km at 1550nm
Figure IV-12: Simulation For Kim Model Comparison at 1550nm
3. Simulation for Kruse models at 550nm wavelengths
and visibility range of 200 meters
In the figures given below the values of wavelengths taken 550 nm against
different visibility ranges and different distribution size q . This value is closer
0 20 40 60 80 100 120 140 160 180 200
-0.45
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
visibility in meters
Specific attanuation in dB/Km
0 20 40 60 80 100 120 140 160 180 200
-12
-10
-8
-6
-4
-2
0
visibility in meters
Specific attanuation in dB/Km
44
to attenuation value as predicted by Kruse model. According to [20], the atmospheric attenuations are wavelength dependent that means system operating at longer wavelengths has better range performance than systems at shorter wavelengths. The simulations show that this effect is more prominently observed for Kruse model as the Kim model does not show any wavelength dependent attenuation behavior against a visibility range lower than 500 m.
Figure IV-13: Simulation For Kruse Model at q=1.6 if V> 50 Km at 550nm
100101102103-2.2-2-1.8-1.6-1.4-1.2-1-0.8-0.6Visibility in metersSpecific attanuation in dB/Km q=1.6 if V>50km
45
10
0
10
1
10
2
10
3
-2.2
-2
-1.8
-1.6
-1.4
-1.2
-1
-0.8
-0.6
Visibility in meters
Specific attanuation in dB/Km
q=0.585V1/3 if V<6Km
Figure IV-14: Simulation For Kruse Model at q=1.3 if 6km < V< 50 Km at
550nm
Figure IV-15: at
550nm
10
0
10
1
10
2
10
3
-2.2
-2
-1.8
-1.6
-1.4
-1.2
-1
-0.8
-0.6
Visibility in meters
Specific attanuation in dB/Km
q=1.3 if 6km<V<50km
46
10
0
10
1
10
2
10
3
-4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
Visibility in meters
Specific attanuation in dB/Km
Comparsion
Figure IV-16: Simulation for Kruse Model Comparison at 550nm
4. Simulations for Kruse models at 1550nm wavelength
In the figures given below the values of wavelengths taken 1550nm against
different visibility range. This value is closer to attenuation value as predicted by
Kruse model. According to [28], the atmospheric attenuations are wavelength
dependent that means system operating at longer wavelengths has better range
performance than systems at shorter wavelengths. The simulations show that
this effect is more prominently observed for Kruse model as the Kim model does
not show any wavelength dependent attenuation behavior against a visibility
range lower than 500 m.
47
Figure IV-17: Simulation For Kruse Model at q=1.6 if V> 50 Km at 1550nm
Figure IV-18: Simulation For Kruse Model at q=1.3 if 6km < V< 50 Km at 1550nm
10
0
10
1
10
2
10
3
-0.45
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
Visibility in meters
Specific attanuation in dB/Km
q=1.6 if V>50km
10
0
10
1
10
2
10
3
-0.45
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
Visibility in meters
Specific attanuation in dB/Km
q=1.3 if 6km<V<50km
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Figure IV-19: at
1550nm
Figure IV-20: Simulation For Kruse Model Comparison at 1550nm
10
0
10
1
10
2
10
3
-0.45
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
Visibility in meters
Specific attanuation in dB/Km
q=0.585V1/3 if V<6Km
10
0
10
1
10
2
10
3
-1.4
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
Visibility in meters
Specific attanuation in dB/Km
Comparsion
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5. Simulations for Al Naboulsi Model at different wavelengths and Visibility Ranges
In this section we have simulate the Al Naboulsi fog model at different wavelengths and visibility ranges. In the figure IV-21 we have taken the wavelength of 1000 nm and 1550 nm visibility range of 10 to 250 meters.
Figure IV-21: simulation for Al Naboulsi Fog Model at =1000 nm
In figure IV-22 we have taken the =1550 nm with visibility range of 330 meters and plot the fog model
050100150200250024681012 AlNaboulsi Model at Lambda=1000 & Visibility 10 to 250 meters
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Figure IV-22: simulation for Al Naboulsi Fog Model at =1550 nm
05010015020025030035000.511.522.533.544.5x 104 AlNaboulsi Model at Lambda=1550nm & Visibility 10 to 330 meters
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6. Simulation for New Proposed Model
Figure IV-23: simulation for proposed new model
2003004005006007008009001000110012005060708090100110Visibility in metersattenuation in db/km Proposed Model
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Chapter V
Conclusion & Future work
1. Conclusions
Free-space optical communication is a emergent technology. It is mount in communications networks to provide for the last mile access. it provides very high data rates comparable to fiber optics with greater economic efficiency. The FSO has successfully established itself in several areas of the world of telecommunications such as secure connections for military applications, high definition television and temporary broadband connections (“Fiber backup”). However, this technology is subject to certain number of constraints and challenges imposed mainly by the environmental factors, which degrade its link performance, and hence limit its spread to most telecommunications networks. Although in the last few decades, much research work has been done on various aspects of modeling the atmospheric free-space channel for different environmental factors, but significant breakthrough could not be achieved in overcoming, especially, the deleterious fog effects. The main difficulties towards theoretical characterization of the free-space atmospheric channel are
• the unavailability of extensive and accurate weather parameters database
• the need to identify main influencing parameters of the meteorological effects and study of their impact on the propagation of optical signals in free-space
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• the unavailability of enough experimental data of optical attenuations (especially for different fog environments)
Further difficulty is experienced in the validation of empirical models through some microphysical models: comparisons between models and measurements cannot be explicitly made mainly due to the unavailability of simultaneous measurement of microphysical parameters along with the measured fog attenuations at the site of FSO deployment. Moreover, the microphysical characteristics of a particular fog type change with the location to location, and even at a particular location it change with time.
2. Achieved Work
The free-space optics work in the field of optical waves in the visible and near infrared spectral range for which the optical components used to provide broadband. To enhance the robustness of FSO link improved atmospheric effect (attenuation and scintillation) mitigation is required. So, we are interested to study the attenuation of optical radiations (in this spectral range) during propagation through the free-space atmosphere in general and through fog in particular. Different aspects of optical wireless communication systems have been investigated in depth in this thesis. The physical phenomena that occur during propagation of radiation through the atmosphere and causing the extinction, namely the absorption and scattering were detailed from a theoretical point of view. In particular, the problem of scattering of light by spherical particles was studied from the theory of Mie scattering. We have evaluated various models
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that allow us to calculate the atte
