individual links are limited to about 50 km. If 100 such links are chained to form a 5000-km channel, communications theory shows how much the signal-to-noise ratio (SNR) must be improved in each link so that the total performance is that of an original single link. For an analog system the link SNR must be improved by 20 dВ while for a digital system operating at an error rate of ~105 only 2 dB is required.
Satellite channels, on the other hand, are marked by lower power and wide bandwidth. These qualities predispose the channel to digital transmission in another way, as we shall develop.
These channels have a special character that tends to favor digital transmission, but a digital format in any communications network makes it easier to guarantee a given data error rate or fidelity of reproduction. For analog signals that have been digitized, the fidelity of reproduction is set almost entirely by the fineness of the digitizing, and it is easily controlled throughout the system. The error rate in a digital channel generally obeys a threshold rule: Beyond a certain signal power, the receiver error rate falls very rapidly so that as long as signal power exceeds this threshold, a performance level can be virtually guaranteed throughout a system, even if many links and processors are chained. If error performance is not sufficient in one link, digital error correction methods can be used to improve it; in addition, performance of this coding will improve rapidly as a threshold signal power is exceeded.
Cost. In cases where one can choose between digital or analog means to transmit information, digital transmission may be cheaper. This may be true because of the availability of wide bandwidth, the low cost of manufacturing digital equipment, difficulties with error control, or customer factors, like compatibility, flexibility, or need for security.
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Text 3C
The Satellite Channel
No event has more strongly motivated digital transmission research than the communications satellite. This channel consists of a relatively linear high-power amplifier on the ground that feeds an “uplink” and a highly nonlinear low-power amplifier in the satellite that feeds a “downlink” to the ground. Microwave carrier frequencies are normal. The satellite offers an attractive star network topology; it allows communication over long distances at wide bandwidths at relatively low cost.
It appears that digital modulation will eventually predominate in this channel. Compared to other channels, the satellite transmission path is characterized by wide bandwidth, a nonlinear amplitude response, and low power, that is, by very long distances. These factors all point to digital transmission, but the most telling constraint is the low-power one.
As transmitter power drops it becomes increasingly necessary to resort to bandwidth-spreading modulations, including digital ones. Simple analog modulation has at best the same SNR as the radiofrequency channel; and for single sideband modulation, it has the same bandwidth as the information source. Once the SNR drops below 30 dB or so, transmission of telephone-quality voice, for instance, becomes impossible without a bandwidth-spreading modulation like FM. As the SNR drops into the 10–20 dB range encountered in satellite channels, the degree of bandwidth expansion must increase, and the analog transmitter and receiver components must be increasingly wideband. Eventually it becomes more efficient to convert analog signals to digital form and make use of a digital modulation method. New modulation methods will soon become available which, when combined with an efficient analog-to-digital conversion method like adaptive differential PCM, should be more efficient than analog FM over present-day commercial satellites.
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Another hallmark of the satellite channel is its nonlinearity. All rf power amplifiers must operate nonlinearly in order to be highly efficient. At the same time, dc power generation on board a satellite is a heavy contributor to launch weight. Satellites are thus compelled to operate as nonlinear amplifiers. The modulations that fare best under these conditions are constantamplitude modulations, those that have little or no envelope variation; and this narrows the choice to either analog FM or digital phase modulation. A second factor that promotes the use of constant-amplitude modulation is their apparent ability to suppress interference. Interference arises naturally in a satellite link from closely spaced adjacent channels, from the multiple carriers that appear in a multiple-access system, or from spurious carriers that sweep the satellite.
The low power and nonlinearity of the satellite channel require precisely what the channel has in abundance, bandwidth. Bandwidth reduction remains a worthwhile goal, however, since a reduction in signaling bandwidth per data symbol in any channel leads to a correspondingly higher rate of information transfer.
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Answer the following questions.
1. What does the satellite channel consist of ? 2. Why is the satellite channel used for digital transmission? 3. What are the advantages of the satellite channel? 4. What are the main features of the satellite channel?
Task 1. Find the main sentences which give the idea of each paragraph. Write down the key words to each paragraph.
Task 2. Find information from the text and tell about the role of bandwidth-spreading modulation in the satellite channel.
Task 3. Explain the importance of nonlinearity in the satellite channel.
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Grammar exercise № 3
Point out Adverbial Participle constructions and Absolute Constructions in the following sentences and translate them accordingly.
1.The basic concept of radar being relatively simple, its practical implementation is not.
2.A radar operates by radiating electromagnetic energy and detecting the echo returned from reflecting objects (targets) (отражение).
3.The nature of the echo signal provides information about the target, the range (or distance) to the target being found from the time it takes for the radiated energy to travel to the target and back.
4.Resolution being sufficiently high, a radar can discern (рассмотреть) target’s size and shape, radar resolution being obtained in range or angle, or both.
5.Radar resolution may be in range or angle, range resolution requiring large antennas.
6.Resolution in the cross — range (боковое отклонение) dimensions being not very good, it is possible to use the resolution in Doppler frequency to resole the cross — range dimensions.
7.Being not dependent on ambient radiation, radar can detect relatively small targets at near or for distances and can measure their range.
8.Though the location of a target in range and angle having been determined by radar, the echo signal also can provide information about the nature of the target.
9.Automatic detection and track (сопровождение) (ADT) being available, the operator is usually presented with the processed target track rather than raw radar detections.
10.The transmitter power being radiated into space by a directive antenna, the latter immediately concentrates the energy into a narrow beam.
11.Though deployment (развертывание) having been
delayed, 40-Gbit/s transmission will enable service providers (поставщик услуг) to deliver necessary signal bandwidth.
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12.Wireless signal being able to jump automatically to an available frequency, transmissions will be more reliable.
13.The service specific parameters having been studied, the linear system of equations can be solved.
14.The cost of a base station having been reduced due to semiconductor advances, the station could be a laptop or home computer.
15.Though conventional NRZ (non-return-to zero) modulation format being adequate for transmitting over relatively short distances, special formats may be required for longer routes.
16.Fiber with high effective mode area having been used, possible damage of the system was reduced.
17.A receiver being set to many cycles per second, you can tune the antenna circuit to any stations frequency.
18.Other transmitters interfering with your reception, your only real option will be to wait out the problem.
19.If provided with new adaptive software, nearly any wireless system will be able to locate and link to any locally available unused radio spectrum.
20.The Global Positioning System (GPS) is a space-based navigation satellite system (GNSS) that provides reliable location and time information, GPS satellites continually transmitting messages about the time the message was transmitted.
21.After many years of anticipation (ожидание) the end of conventional analog broadcasting is evident, digital broadcasts offering consumers many advantages such as crystal-clear pictures and new information services.
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