07-04-2011, 04:53 PM
Presented by,
Karthi . K
Vasanth . J
[attachment=11883]
ABSTRACT
Optical free space communication is favourable for high data-rate, long-range point-to-point links, where the terminal size, mass, and power is limited, as for example on aeronautical or space platforms. Main advantages lie in a significantly higher data rate and the lower power/ space requirements of optical links compared to RF.
One future application for optical links is the use on high altitude platforms (HAPs), which will be positioned at altitudes of about 20 to 30 km. Inter-platform links can have a range of at least 600km at these altitudes without cloud interferences.
HAPs do have the advantage of being more cost effective compared to satellites due to lower launch costs and also the possibility of easier maintenance and reconfiguration.
Free space optics (FSO) is an emerging technology that has found application in several areas of the shortand long-haul communications space. From inter-satellite links to inter-building links, it has been tried and tested. As with any technology, FSO has worked much better in some applications than in others. In this white paper we analyze FSO from several angles, all from the perspective of finding where it can fit
into the terrestrial data page link picture.
FSO technology’s inherent strengths are its lack of use of in-ground cable (which makes it much quicker and often cheaper to install), the fact that it operates in an unlicensed spectrum (making it easier from a political/ bureaucratic perspective to install),
the fact that it can be removed and installed elsewhere (allowing recycling of equipment), and its relatively high bandwidth (up to 1 Gigabit per second (Gb/s) and beyond).
FSO technology seems most suited to is clear weather, short distance page link establishment, such as last-mile connections to broadband network backbones and backbone
links between buildings in a metropolitan area network (MAN) or campus area network(CAN)
environment. There is also significant potential for use of this technology in temporary networks, where the advantages of being able to establish a CAN quickly or being able to relocate the network in a relatively short time frame outweigh the network unreliability issues. It should be noted that tactical
implementations of this technology, or any highly-mobile implementation, are possible, but in its current state FSO has challenges providing adequate enough reliability to be considered a solution for the mobile
Warfighter without resorting to a hybrid solution of FSO paired with another transmission technology(typically Millimeter Wave). Finally, past and current implementations and tests indicate that any future implementations of FSO technology should be carefully evaluated to ensure that no potential page link interruptions are a factor before making the decision to actually implement an FSO link.
INTRODUCTION:
A fiber optic communication page link uses light sources and detectors to send and receive information through a fiber optic cable. Similarly, FSO uses light sources and detectors to send and receive information, but through the atmosphere instead of a cable (1). The motivation for FSO is to eliminate the cost,
time, and effort of installing fiber optic cable, yet retain the benefit of high data rates (up to 1 Gb/s and beyond) for transmission of voice, data, images, and video. However, swapping light propagation through a precisely manufactured dielectric waveguide for propagation through the atmosphere
imposes significant penalties on performance. Specifically, the effective distance of FSO links is limited; depending on atmospheric conditions the maximum range is 2-3 km, but
200-500 meters is typical to meet telco grades of availability. Thus, at present, FSO systems are used primarily in last mile applications to connect end users to a broadband network backbone as shown in Figure 1. Although FSO equipment is undergoing continuous development, the emphasis is on improving its application to local area networks (LAN) and, in some cases, MANs (e.g., to close a short gap in a ring network), but not to long-haul relay systems. The design goal of a long-haul transmission system is to maximize the
separation of relays in spanning distances between cities and countries. For that purpose, FSO is uneconomical compared to fiber optic or microwave radio systems .
In the following sections, FSO technology is described by comparing it to fiber optic communications for a single-link communication system. This provides a basis for understanding the direction FSO is heading relative to evelopments in fiber optics. Benefits of FSO are then considered, particularly those of military interest, such as portability and quick deployment. Drawbacks of FSO are
discussed as well as some current research to overcome them. Finally, network considerations as well as current products
and potential applications are discussed.
TECHNOLOGY DESCRIPTION:
General Framework:
Communication system design is concerned with tradeoffs between channel length, bit rate, and error performance. The generalized schema of a single-link communication system in Figure 2 provides the necessary framework to compare fiber optic and FSO technologies . Under each block are characteristics that transform its signal input to the different
physical form of the signal output.
The superscript N for each block transform represents noise contributed to the signal. For
example, the “channel” block degrades the transmitter output signal due to processes listed under the block for fiber optic
cable or FSO.Although both are optical communication systems, thefundamental difference between fiber optic and FSO systems is their propagation channels: dielectric waveguide versus the atmosphere. As a consequence, signal propagation, equipment design, and system planning are different for each type of system. The main thesis of the following discussion is that,
because of their different propagation channels, the performance of FSO cannot be expected to match that of advanced fiber optic systems; therefore FSO applications will
be more limited.
FSO Characteristics:
A generalized FSO system is shown in Figure 3, and the optical transmitter and receiver are shown in greater detail in Figure 4. The baseband transmission bit stream is an input to
the modulator, turning the direct current bias current on and off to modulate the laser diode (LD) or light emitting diode (LED) light source. The modulated beam then passes through a collimating lens that forms the beam into a parallel ray propagating through the atmosphere. A fundamental physical
constraint, the diffraction limit, comes into play at this point.
It says that the beam of an intensity modulated (non-coherent) light source cannot be focused to an area smaller than that at its source . Apart from the effects of atmospheric processes, even in vacuum, a light beam propagating through free space undergoes divergence or spreading. Recalling the single-link communication system in Figure 2,
the transmitted FSO beam is transformed by several physical processes inherent to the atmosphere: frequency-selective (line) absorption, scattering, turbulence, and sporadic
misalignment of transmitter and receiver due to displacement (twist and sway) of buildings or structures upon which the FSO equipment is mounted.
These processes are non-stationary, which means that their influence on a page link changes
unpredictably with time and position. At the distant end, a telescope collects and focuses a fraction of the light beam onto a photo-detector that converts the optical signal to an electrical
signal. The detected signal is then amplified and passes to processing, switching, and distribution stages. The basic signal processing functions of the transmitter and receiver are
shown schematically in Figure 4.
The non-stationary atmospheric processes, divergence (or beam spreading), absorption, scattering, refractive turbulence, and displacement, are the factors that most limit
the performance of FSO systems. A brief description of each is given in the following paragraphs. Divergence. Divergence determines how much useful signal
energy will be collected at the receive end of a communication link. It also determines how sensitive a page link will be to displacement disturbances (see below). Of the processes that
cause attenuation, divergence is the only one that isindependent of the transmission medium; it will occur invacuo just as much as in a stratified atmosphere. Laser light can be characterized as partially coherent, quasimonochromatic electromagnetic waves passing a point in a wave field. At the transmitter, beam divergence is caused by diffraction around the circular aperture at the end of the telescope.
Karthi . K
Vasanth . J
[attachment=11883]
ABSTRACT
Optical free space communication is favourable for high data-rate, long-range point-to-point links, where the terminal size, mass, and power is limited, as for example on aeronautical or space platforms. Main advantages lie in a significantly higher data rate and the lower power/ space requirements of optical links compared to RF.
One future application for optical links is the use on high altitude platforms (HAPs), which will be positioned at altitudes of about 20 to 30 km. Inter-platform links can have a range of at least 600km at these altitudes without cloud interferences.
HAPs do have the advantage of being more cost effective compared to satellites due to lower launch costs and also the possibility of easier maintenance and reconfiguration.
Free space optics (FSO) is an emerging technology that has found application in several areas of the shortand long-haul communications space. From inter-satellite links to inter-building links, it has been tried and tested. As with any technology, FSO has worked much better in some applications than in others. In this white paper we analyze FSO from several angles, all from the perspective of finding where it can fit
into the terrestrial data page link picture.
FSO technology’s inherent strengths are its lack of use of in-ground cable (which makes it much quicker and often cheaper to install), the fact that it operates in an unlicensed spectrum (making it easier from a political/ bureaucratic perspective to install),
the fact that it can be removed and installed elsewhere (allowing recycling of equipment), and its relatively high bandwidth (up to 1 Gigabit per second (Gb/s) and beyond).
FSO technology seems most suited to is clear weather, short distance page link establishment, such as last-mile connections to broadband network backbones and backbone
links between buildings in a metropolitan area network (MAN) or campus area network(CAN)
environment. There is also significant potential for use of this technology in temporary networks, where the advantages of being able to establish a CAN quickly or being able to relocate the network in a relatively short time frame outweigh the network unreliability issues. It should be noted that tactical
implementations of this technology, or any highly-mobile implementation, are possible, but in its current state FSO has challenges providing adequate enough reliability to be considered a solution for the mobile
Warfighter without resorting to a hybrid solution of FSO paired with another transmission technology(typically Millimeter Wave). Finally, past and current implementations and tests indicate that any future implementations of FSO technology should be carefully evaluated to ensure that no potential page link interruptions are a factor before making the decision to actually implement an FSO link.
INTRODUCTION:
A fiber optic communication page link uses light sources and detectors to send and receive information through a fiber optic cable. Similarly, FSO uses light sources and detectors to send and receive information, but through the atmosphere instead of a cable (1). The motivation for FSO is to eliminate the cost,
time, and effort of installing fiber optic cable, yet retain the benefit of high data rates (up to 1 Gb/s and beyond) for transmission of voice, data, images, and video. However, swapping light propagation through a precisely manufactured dielectric waveguide for propagation through the atmosphere
imposes significant penalties on performance. Specifically, the effective distance of FSO links is limited; depending on atmospheric conditions the maximum range is 2-3 km, but
200-500 meters is typical to meet telco grades of availability. Thus, at present, FSO systems are used primarily in last mile applications to connect end users to a broadband network backbone as shown in Figure 1. Although FSO equipment is undergoing continuous development, the emphasis is on improving its application to local area networks (LAN) and, in some cases, MANs (e.g., to close a short gap in a ring network), but not to long-haul relay systems. The design goal of a long-haul transmission system is to maximize the
separation of relays in spanning distances between cities and countries. For that purpose, FSO is uneconomical compared to fiber optic or microwave radio systems .
In the following sections, FSO technology is described by comparing it to fiber optic communications for a single-link communication system. This provides a basis for understanding the direction FSO is heading relative to evelopments in fiber optics. Benefits of FSO are then considered, particularly those of military interest, such as portability and quick deployment. Drawbacks of FSO are
discussed as well as some current research to overcome them. Finally, network considerations as well as current products
and potential applications are discussed.
TECHNOLOGY DESCRIPTION:
General Framework:
Communication system design is concerned with tradeoffs between channel length, bit rate, and error performance. The generalized schema of a single-link communication system in Figure 2 provides the necessary framework to compare fiber optic and FSO technologies . Under each block are characteristics that transform its signal input to the different
physical form of the signal output.
The superscript N for each block transform represents noise contributed to the signal. For
example, the “channel” block degrades the transmitter output signal due to processes listed under the block for fiber optic
cable or FSO.Although both are optical communication systems, thefundamental difference between fiber optic and FSO systems is their propagation channels: dielectric waveguide versus the atmosphere. As a consequence, signal propagation, equipment design, and system planning are different for each type of system. The main thesis of the following discussion is that,
because of their different propagation channels, the performance of FSO cannot be expected to match that of advanced fiber optic systems; therefore FSO applications will
be more limited.
FSO Characteristics:
A generalized FSO system is shown in Figure 3, and the optical transmitter and receiver are shown in greater detail in Figure 4. The baseband transmission bit stream is an input to
the modulator, turning the direct current bias current on and off to modulate the laser diode (LD) or light emitting diode (LED) light source. The modulated beam then passes through a collimating lens that forms the beam into a parallel ray propagating through the atmosphere. A fundamental physical
constraint, the diffraction limit, comes into play at this point.
It says that the beam of an intensity modulated (non-coherent) light source cannot be focused to an area smaller than that at its source . Apart from the effects of atmospheric processes, even in vacuum, a light beam propagating through free space undergoes divergence or spreading. Recalling the single-link communication system in Figure 2,
the transmitted FSO beam is transformed by several physical processes inherent to the atmosphere: frequency-selective (line) absorption, scattering, turbulence, and sporadic
misalignment of transmitter and receiver due to displacement (twist and sway) of buildings or structures upon which the FSO equipment is mounted.
These processes are non-stationary, which means that their influence on a page link changes
unpredictably with time and position. At the distant end, a telescope collects and focuses a fraction of the light beam onto a photo-detector that converts the optical signal to an electrical
signal. The detected signal is then amplified and passes to processing, switching, and distribution stages. The basic signal processing functions of the transmitter and receiver are
shown schematically in Figure 4.
The non-stationary atmospheric processes, divergence (or beam spreading), absorption, scattering, refractive turbulence, and displacement, are the factors that most limit
the performance of FSO systems. A brief description of each is given in the following paragraphs. Divergence. Divergence determines how much useful signal
energy will be collected at the receive end of a communication link. It also determines how sensitive a page link will be to displacement disturbances (see below). Of the processes that
cause attenuation, divergence is the only one that isindependent of the transmission medium; it will occur invacuo just as much as in a stratified atmosphere. Laser light can be characterized as partially coherent, quasimonochromatic electromagnetic waves passing a point in a wave field. At the transmitter, beam divergence is caused by diffraction around the circular aperture at the end of the telescope.
