14-02-2013, 02:32 PM
Free Space Optical Communication
[attachment=50898]
ABSTRACT
Any Communication system needs a medium to operate. Satellite communication and optical communication are two different methods of communication. Satellite communication uses a satellite as a repeater between two points that exchange information, here medium is air and information is passed as electromagnetic waves. Satellite Communication is important where installation of cable is difficult. Satellite communication has emerged fruitful in wideband communication systems like TV Broadcasting (Direct To Home), Defense, Weather forecasting, space navigation, telemetry. Important components of a satellite communication system are transmitter (Source at earth, originating), receiver (Destination at earth), and a Transponder.
Optical Communication generally uses optical fiber as a medium and information is transmitted in the form of light. Because of their higher modulation frequency capability, the importance of light (laser) as a means of carrying information did not go unnoticed by communications engineers. Light has the information carrying capability of 10,000 times that of highest radio frequencies being used. Optical communication system consists of transmitter, channel and receiver. Transmitter consists of amplifier, modulator and a light emitting semi conductor device, where as receiver contains photo detecting devices. Primary applications of optical communication are telephony, medicine, digital audio applications. Optical communication systems have advantages over RF systems that include a wider bandwidth, larger capacity, lower power consumption, more compact equipment, greater security against eavesdropping, and immunity from interference Because of this they are expected to revolutionize space system architectures.
Introduction
Satellites are a revolution in telecommunication (in 20th century) has changed the way the people live. The need for increasing number of channels on a link has given rise to various wideband communication systems. Satellites are primarily developed for long distance telephone communications. For handling increased information, we need wideband links such as coaxial cables, microwave line-of-sight links requiring repeater stations, satellite links, waveguide links and optical fiber links Optical fiber communication, developed in the 1980â„¢s, have revolutionized the telecommunications industry and played a major role in the advent of the Information Age. Because of its advantages over electrical transmission, the use of optical fiber has largely replaced copper wire communications. The fiber optic feed, because of its quality and reliability, soon became the primary video feed. The development of laser technology was the next important step in the establishment of the industry of fiber optics.
Satellite communication:
In 1945, Arthur C. Clarke provided what most consider the initial principles for Satellite Communications. He stated that a space-station orbiting 42,000 km above the equator could act as a repeater to relay transmissions between any two points on the hemisphere beneath it. It was not until the early 1960s that the first workable communications satellite was built and launched.
By the end of World War II, the world had had a taste of "global communications." Edward R. Murrow's radio broadcasts from London had electrified American listeners. We had, of course, been able to do transatlantic telephone calls and telegraphs via underwater cables for almost 50 years. At exactly this time, however, a new phenomenon was born. The first television programs were being broadcast, but the greater amount of information required transmitting television pictures required that they operate at much higher frequencies than radio stations. To d o t h a t television broadcasting uses microwave frequency range .satellite acts as a microwave link repeater in space.
Optical communication:
The need for reliable long-distance ground communication systems has existed since the distant past. Over time, the sophistication of these systems has gradually improved, from smoke signals to telegraphs and finally to the first coaxial cable, put into service in 1940. As these communication systems improved, certain fundamental limitations presented themselves. Electrical systems were limited by their small repeater spacing and the bit rate of microwave systems was limited by their carrier frequency. In the second half of the twentieth century, it was realized that an optical carrier of information would have a significant advantage over the existing electrical and microwave carrier signals. However, no coherent light source or suitable transmission medium was available. Then, after the development of lasers in the 1960â„¢s solved the first problem, development of high-quality optical fiber was proposed as a solution to the second. Optical fiber was finally developed in 1970
SATELLITES IN GENERAL
Satellites are geostationary, it means they require 24 hours to orbit the earth. Velocity of a satellite depends on distance of it from earth. Satellites in closer orbits require less power requirements. Actual orbit velocity of geostationary satellite is 11000km/hr. Most popular band for satellite communication is 6 GHz (c-band) for uplink & 4 GHz for downlink. However radio interference limits the applications of communication satellites operating in 6/4ghzband.This problem is eliminated in second generation communication satellites that operate in 14/12ghzband ( ku-band).
COMMUNICATION SATELLITE COMPONENTS
Every communications satellite in its simplest form involves the transmission of information from a source ground station to the satellite (the uplink), followed by a retransmission of the information from the satellite back to the ground (the downlink). The downlink may either be to a select number of ground stations or it may be broadcast t o everyone in a large area. Hence the satellite must have a receiver and a receiver antenna, a transmitter and a transmit antenna, some method for connecting the uplink to the downlink for retransmission, and prime electrical power to run all of the electronics. The exact nature of these components will differ, depending on the orbit and the system architecture, but every communications satellite must have these basic components.
The amount of power which a satellite transmitter needs to send out depends a great deal on whether it is in low earth orbit or in geosynchronous orbit. This is a result of the fact that the geosynchronous satellite is at an altitude of 22,300 miles, while the low earth satellite is only a few hundred miles. The geosynchronous satellite is nearly 100 times as far away as the low earth satellite. We can show fairly easily that this means the higher satellite would need almost 10,000 times as much power as the low-orbiting one, if everything else were the same. (Fortunately, of course, we change some other things so that we don't need 10,000 times as much power.)
[attachment=50898]
ABSTRACT
Any Communication system needs a medium to operate. Satellite communication and optical communication are two different methods of communication. Satellite communication uses a satellite as a repeater between two points that exchange information, here medium is air and information is passed as electromagnetic waves. Satellite Communication is important where installation of cable is difficult. Satellite communication has emerged fruitful in wideband communication systems like TV Broadcasting (Direct To Home), Defense, Weather forecasting, space navigation, telemetry. Important components of a satellite communication system are transmitter (Source at earth, originating), receiver (Destination at earth), and a Transponder.
Optical Communication generally uses optical fiber as a medium and information is transmitted in the form of light. Because of their higher modulation frequency capability, the importance of light (laser) as a means of carrying information did not go unnoticed by communications engineers. Light has the information carrying capability of 10,000 times that of highest radio frequencies being used. Optical communication system consists of transmitter, channel and receiver. Transmitter consists of amplifier, modulator and a light emitting semi conductor device, where as receiver contains photo detecting devices. Primary applications of optical communication are telephony, medicine, digital audio applications. Optical communication systems have advantages over RF systems that include a wider bandwidth, larger capacity, lower power consumption, more compact equipment, greater security against eavesdropping, and immunity from interference Because of this they are expected to revolutionize space system architectures.
Introduction
Satellites are a revolution in telecommunication (in 20th century) has changed the way the people live. The need for increasing number of channels on a link has given rise to various wideband communication systems. Satellites are primarily developed for long distance telephone communications. For handling increased information, we need wideband links such as coaxial cables, microwave line-of-sight links requiring repeater stations, satellite links, waveguide links and optical fiber links Optical fiber communication, developed in the 1980â„¢s, have revolutionized the telecommunications industry and played a major role in the advent of the Information Age. Because of its advantages over electrical transmission, the use of optical fiber has largely replaced copper wire communications. The fiber optic feed, because of its quality and reliability, soon became the primary video feed. The development of laser technology was the next important step in the establishment of the industry of fiber optics.
Satellite communication:
In 1945, Arthur C. Clarke provided what most consider the initial principles for Satellite Communications. He stated that a space-station orbiting 42,000 km above the equator could act as a repeater to relay transmissions between any two points on the hemisphere beneath it. It was not until the early 1960s that the first workable communications satellite was built and launched.
By the end of World War II, the world had had a taste of "global communications." Edward R. Murrow's radio broadcasts from London had electrified American listeners. We had, of course, been able to do transatlantic telephone calls and telegraphs via underwater cables for almost 50 years. At exactly this time, however, a new phenomenon was born. The first television programs were being broadcast, but the greater amount of information required transmitting television pictures required that they operate at much higher frequencies than radio stations. To d o t h a t television broadcasting uses microwave frequency range .satellite acts as a microwave link repeater in space.
Optical communication:
The need for reliable long-distance ground communication systems has existed since the distant past. Over time, the sophistication of these systems has gradually improved, from smoke signals to telegraphs and finally to the first coaxial cable, put into service in 1940. As these communication systems improved, certain fundamental limitations presented themselves. Electrical systems were limited by their small repeater spacing and the bit rate of microwave systems was limited by their carrier frequency. In the second half of the twentieth century, it was realized that an optical carrier of information would have a significant advantage over the existing electrical and microwave carrier signals. However, no coherent light source or suitable transmission medium was available. Then, after the development of lasers in the 1960â„¢s solved the first problem, development of high-quality optical fiber was proposed as a solution to the second. Optical fiber was finally developed in 1970
SATELLITES IN GENERAL
Satellites are geostationary, it means they require 24 hours to orbit the earth. Velocity of a satellite depends on distance of it from earth. Satellites in closer orbits require less power requirements. Actual orbit velocity of geostationary satellite is 11000km/hr. Most popular band for satellite communication is 6 GHz (c-band) for uplink & 4 GHz for downlink. However radio interference limits the applications of communication satellites operating in 6/4ghzband.This problem is eliminated in second generation communication satellites that operate in 14/12ghzband ( ku-band).
COMMUNICATION SATELLITE COMPONENTS
Every communications satellite in its simplest form involves the transmission of information from a source ground station to the satellite (the uplink), followed by a retransmission of the information from the satellite back to the ground (the downlink). The downlink may either be to a select number of ground stations or it may be broadcast t o everyone in a large area. Hence the satellite must have a receiver and a receiver antenna, a transmitter and a transmit antenna, some method for connecting the uplink to the downlink for retransmission, and prime electrical power to run all of the electronics. The exact nature of these components will differ, depending on the orbit and the system architecture, but every communications satellite must have these basic components.
The amount of power which a satellite transmitter needs to send out depends a great deal on whether it is in low earth orbit or in geosynchronous orbit. This is a result of the fact that the geosynchronous satellite is at an altitude of 22,300 miles, while the low earth satellite is only a few hundred miles. The geosynchronous satellite is nearly 100 times as far away as the low earth satellite. We can show fairly easily that this means the higher satellite would need almost 10,000 times as much power as the low-orbiting one, if everything else were the same. (Fortunately, of course, we change some other things so that we don't need 10,000 times as much power.)