SATELLITE OPTICAL COMMUNICATIONS full report
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SATELLITE OPTICAL COMMUNICATIONS
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.
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.
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.
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.)
One of the biggest differences between a low earth satellite and a geosynchronous satellite is in their antennas. As mentioned earlier, the geosynchronous satellite would require nearly 10,000 times more transmitter power, if all other components were the same. One of the most straightforward ways to make up the difference, however, is through antenna design. Virtually all antennas in use today radiate energy preferentially in some direction. An antenna used by a commercial terrestrial radio station, for example, is trying to reach people to the north, south, east, and west. However, the commercial station will use an antenna that radiates very little power straight up or straight down. Since they have very few listeners in those directions (except maybe for coal miners and passing airplanes) power sent out in those directions would be totally wasted.
The communications satellite carries this principle even further. All of its listeners are located in an even smaller area, and a properly designed antenna will concentrate most of the transmitter power within that area, wasting none in directions where there are no listeners. The easiest way to do this is simply to make the antenna larger. Doubling the diameter of a reflector antenna (a big "dish") will reduce the area of the beam spot to one fourth of what it would be with a smaller reflector. We describe this in terms of the gain of the antenna. Gain simply tells us how much more power will fall on 1 square centimeter (or square meter or square mile) with this antenna than would fall on that same square centimeter (or square meter or square mile) if the transmitter power were spread uniformly (isotropically) over all directions. The larger antenna described above would have four times the gain of the smaller one. This is one of the primary ways that the geosynchronous satellite makes up for the apparently larger transmitter power which it requires.
DTH services were first proposed in India in 1996, but due to concerns of national security and a cultural invasion -- from mainly the Western countries -- it did not pass approval of the Indian government. In 1997 the government even imposed a ban when the Rupert Murdoch-owned Indian Sky Broadcasting was about to launch its DTH services in India. Finally in 2000 DTH was allowed. But the new policy mandated that all operators can only broadcast using Indian satellites. Clearly the first company to jump into the fray was the state-owned TV broadcasting giant Doordarshan (DD) (using C-band transponder). And even as DD penetrated 4.6 million homes in no time, triggering DTH interests of private players, not many could start operations owing to lack of availability of KU transponder capacity in
Currently besides DD, the other DTH operators are a broadcaster known as Dish TV that is owned by a local TV channel and Tata Sky, a joint venture between the local Tata group and Rupert Murdoch's Star group .And waiting in the queue are, reportedly two very large local players, the sun network and Anil Ambani of reliance group.
Optical communication is any form of telecommunication that uses light as the transmission medium. The light forms an electromagnetic carrier wave that is modulated to carry information. An optical communication system consists of a transmitter, which encodes a message into an optical signal, a channel, which carries the signal to its destination, and a receiver, which reproduces the message from the received optical signal. The most commonly used optical transmitters are semiconductor devices such as Light-emitting diodes (LEDs) and laser diodes. (The difference between LEDs and laser diodes is that LEDs produce incoherent light, while laser diodes produce coherent light). Semiconductor optical transmitters are compact, efficient, and reliable, operate in an optimal wavelength range, and can be directly modulated at high frequencies, making them well- suited for fiber-optic communication applications. Laser functions as a source of electromagnetic waves at light frequencies and are able to form a narrow pencil like beam which does not spread over long distances. Laser beam may be modulated at a rate much
faster than other light sources.
Optical fiber is the most common type of channel for optical communications, however, other types of channels are earthâ„¢s atmosphere, tube filled with suitable gas. The signal encoding is typically simple intensity modulation. Types of optical fibers are step index(single mode,multi mode) and graded index. The main component of an optical receiver is a photo detector that converts light into electricity through the photoelectric effect. The photo detector is typically a semiconductor-based photodiode, such as a p- n photodiode, a p- i-n photodiode, or an avalanche photodiode. Metal-semiconductor- metal (MSM) photo detectors are also used due to their suitability for circuit integration in regenerators and wavelength-division multiplexers.
Fiber-optic cable is used by many telecommunications companies to transmit telephone signals, internet communication, and cable television signals, sometimes all on the same optical fiber. Itâ„¢s in communications where fibers have made the most significant advances. Long distance telephone cables, sometimes several inches in diameter and containing hundreds or even thousands of paired wires, have been replaced by a single- fiber cable. Because the light transmitting fiber is immune to electronic noise the fiber can carry thousands more conversations with better sound quality Optical technologies â€œ including knowledge of photonics and optical glasses and fibers â€œ is a related field, and essential in laser systems, where telephone optical fiber networks are increasingly used for transmitting voice, image and computer data. The latest advance in the telecom and informatics industries owes much to the development of fiber optics and associated optoelectronic devices. With the worldwide expansion of telephone optical fiber networks for transmission of voice, images and computer data, today there is wide availability of high- quality optical and associated electronic components at competitive prices.
The light weight and no corrosiveness of the fiber make it very practical for aircraft and automotive applications. A single fiber can handle as many voice channels as a 1500-pair cable can. The spacing of repeaters from 35 to 80km for fibers, as opposed to from 1 to1.5km for wire, is a great advantage. Fiber is immune to interference from lighting, crosstalk, and electromagnetic radiation and has enormous potential bandwidth.
Limitations and Remedies:
The transmission distance of a fiber-optic communication system has traditionally been limited primarily by fiber attenuation and second by dispersion. Various frequency components of a modulated carrier wave travel at different velocities. This is referred to as dispersion. This results in distortion.
The solution to this has been to use opto-electronic repeaters. Because of their high complexity, especially with modern wavelength-division multiplexed signals, and the fact that they had to be installed about once every 20km, the cost for these repeaters was very high. An alternative approach is to use an optical amplifier, which amplifies the optical signal directly without having to convert the signal into the electrical domain. Made by doping a length of fiber with the rare-earth mineral erbium, and pumping it with light from a laser with a shorter wavelength than the communications signal (typically 980 nm), amplifiers have largely replaced repeaters in new installations
Free space optical communication between satellites networked together can permit high data rates between different places on earth. The use of optical radiation as a carrier between the satellites permits very narrow beam divergence angles. Due to the narrow beam divergence angle and the large distance between the satellites the pointing from one satellite to another is complicated.
Because of fiber optic technologyâ„¢s immense potential bandwidth, 50 THz or greater, there are extraordinary possibilities for future fiber optic applications. Already, the push to bring broadband services, including data, audio, and especially video, into the home is well underway. Broadband service available to a mass market opens up a wide variety of interactive communications for both consumers and businesses, bringing to reality interactive video networks, interactive banking and shopping from the home, and interactive distance learning. The last mile for optical fiber goes from the curb to the television set top, known as fiber-to-the- home (FTTH) and fiber-to-the-curb (FTTC), allowing video on demand to become a reality.
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