Архитектура Аудит Военная наука Иностранные языки Медицина Металлургия Метрология
Образование Политология Производство Психология Стандартизация Технологии
Communication systems and information theory
Communication theory deals primarily with systems for transmitting information from one point to another. The source output might represent a voice waveform, a sequence of binary digits from a magnetic tape, he output of a set of sensors in a space probe, or a target in a radar system. The channel might represent a telephone line, a high frequency radio link, a space communication link, or a storage medium.
As it is known, in the early 1940’s a mathematical theory for dealing more fundamental aspects of communication systems was developed. The distinguishing characteristics of this theory are, first, a great emphasis on the theory of probability and, second, a primary concern with the encoder and decoder, both in terms of their functional roles and in terms of their achieving a given level of performance. In the past 20 years, information theory has been made more precise, has been extended and brought to the point where it is being applied in practical communication systems.
Much of modern communication theory stems from works of communication systems as well as from desirability of modelling both signal and noise as random processes. N. Wiener was interested in finding the best linear filter to separate the signal from additive noise with a prescribed delay. His work had an important influence on subsequent research in modulation theory.
Information technology (IT) is the study, design, development, implementation, support or management of computer-based information systems, particularly software applications and computer hardware. Information technology deals mainly with the use of electronic computers and computer software to convert, store, protect, process, transmit and securely retrieve information.
Today the term information technology includes many aspects of computing and technology and covers many fields. Information technology professionals perform a variety of duties that range from installing applications to designing complex computer networks and information databases. The duties of IT specialists may involve data management, networking, engineering computer hardware, database and software design, as well as the management and administration of the whole system. When computer and communication technologies are combined, the result is information technology, or “infotech”. Information technology describes any technology that helps to produce, manipulate, store, communicate and/or disseminate information.
Thus, Information Communication Technology (ICT) embraces all technologies for the communication of information. It includes any medium to record information (paper, pen, magnetic disc / tape, optical discs – CD / DVD, flash memory, etc.) and technology for broadcasting information – radio, television. It involves any technology for communicating through voice and sound or images – microphone, camera, loudspeaker, and telephone. At present it is apparently culminating to information communication with the help of Personal Computers (PCs) networked through the Internet, information technology that can transfer information using satellite system or intercontinental cables.
10. Revise the text and answer the questions:
1. What system does communication theory deal with? 2. When was the
mathematical theory for communication systems developed? 3. What are the distinguishing features of the theory? 4. What does modern communication theory stem from? 5. What was Wienner’s contribution into the development of communication theory? 6. What is IT? 7. What does it deal with? 8. What jobs are IT experts engaged in? 9. What do you understand by “infotech”? 9. What technologies does ICT include?
11. Memorize the following word combinations:
Communication theory; primarily; the source output; a voice wave form; a
sequence of binary digits; a set of sensors; a space probe; a high frequency radio link; a storage medium; distinguishing characteristics; a great emphasis on the theory of probability; a primary concern; encoder and decoder; in terms of; a given level of performance; to stem from; as well as; desirability; linear filter; additive noise; a prescribed delay; to have influence on subsequent research; implementation; support; management; particularly; software applications; computer hardware; to retrieve information securely; to cover many fields; perform a variety of duties; installing applications; to design complex computer networks and information databases; data management; networking; engineering computer hardware; database and software design; to disseminate information; to communicate through voice and sound or images; a loudspeaker; to network through the Internet.
12. Translate the following terms and word combinations into English:
Цифровая связь; технология организации дальней cвязи; методы
управления; беспроводная связь; дополнительная информация; передача в режиме реального времени; заданная задержка; установка прикладных программ; сигнал от аддитивных шумов; набор датчиков; отличительные свойства; высокочастотная радиосвязь; теория вероятности; беспорядочные процессы; охватывать многие области; автоматизированные информационные системы; последовательность двоичных цифр; управлять; размещать прикладные программы; видеосвязь; уровень пропускной способности.
Read the text and translate it into Russian:
Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequenciesfrom 300 GHz to as low as 3 kHz, and corresponding wavelengths ranging from 1 millimeter (0.039 in) to 100 kilometers (62 mi). Like all other electromagnetic waves, they travel at the speed of light. Naturally, occurring radio waves are made by lightning, or by astronomical objects. Artificially generated radio waves are used for fixed and mobile radio communication, broadcasting, radar and other navigation systems, communication satellites, computer networks and innumerable other applications. Different frequencies of radio waves have different propagation characteristics in the Earth's atmosphere; long waves may cover a part of the Earth very consistently, shorter waves can reflect off the ionosphere and travel around the world, and much shorter wavelengths bend or reflect very little and travel on a line of sight.
To prevent interference between different users, the artificial generation and use of radio waves is strictly regulated by law, coordinated by an international body called the International Telecommunications Union (ITU). The radio spectrum is divided into a number of radio bands based on frequency, allocated to different uses.
Radio waves were first predicted by mathematical work done in 1867 by Scottish mathematical physicist James Clerk Maxwell. Maxwell noticed wavelike properties of light and similarities in electrical and magnetic observations. He then proposed equations that described light waves and radio waves as waves of electromagnetism that travel in space, radiated by a charged particle as it undergoes acceleration. In 1887, Heinrich Hertz demonstrated the reality of Maxwell's electromagnetic waves by experimentally generating radio waves in his laboratory. Many inventions followed, making the use of radio waves to transfer information through space. The study of electromagnetic phenomena such as reflection, refraction, polarization, diffraction, and absorption is of critical importance in the study of how radio waves move in free space and over the surface of the Earth. Different frequencies experience different combinations of these phenomena in the Earth's atmosphere, making certain radio bands more useful for specific purposes than others.
Radio waves travel at the speed of light in a vacuum. When passing through an object, they are slowed according to that object's permeability and permittivity. The wavelength is the distance from one peak of the wave's electric field to the next, and is inversely proportional to the frequency of the wave. The distance a radio wave travels in one second, in a vacuum, is 299, 792, 458 meters (983, 571, 056 ft) which is the wavelength of a 1-hertz radio signal. A 1-megahertz radio signal has a wavelength of 299.8 meters (984 ft).
In order to receive radio signals, for instance from AM/FM radio stations, a radio antenna must be used. However, since the antenna will pick up thousands of radio signals at a time, a radio tuner is necessary to tune in a particular signal. This is typically done via a resonator (in its simplest form, a circuit with a capacitor and an inductor). The resonator is configured to resonate at a particular frequency, allowing the tuner to amplify sine waves at that radio frequency and ignore other sine waves. Usually, either the inductor or the capacitor of the resonator is adjustable, allowing the user to change the frequency at which it resonates. The etymology of " radio" or " radiotelegraphy" reveals that it was called " wireless telegraphy", which was shortened to " wireless" in Britain. The prefix radio- in the sense of wireless transmission was first recorded in the word radio conductor, a description provided by the French physicist Edouard Branly in 1897. It is based on the verb to radiate (in Latin " radius" means " spoke of a wheel, beam of light, ray" ).
The word " radio" also appears in a 1907 article by Lee De Forest. It was adopted by the United States Navy in 1912, to distinguish radio from several other wireless communication technologies, such as the photo phone. The term became common by the time of the first commercial broadcasts in the United States in the 1920s. (The noun " broadcasting" itself came from an agricultural term, meaning " scattering seeds widely." ) The term was adopted by other languages in Europe and Asia. British Commonwealth countries continued to use commonly the term " wireless" until the mid-20th century, though the magazine of the BBC in the UK has been called Radio Times ever since it was first published in the early 1920s.
In recent years, the more general term " wireless" has gained renewed popularity through the rapid growth of short-range computer networking, e.g., Wireless Local Area Network (WLAN), Wi-Fi, and Bluetooth, as well as mobile telephony, e.g., GSM and UMTS. Today, the term " radio" specifies the actual type of transceiver device or chip, whereas " wireless" refers to the lack of physical connections; one talks about radio transceivers, but other talks about wireless devices and wireless sensor networks.
13. Translate the verbs and their derivatives:
To communicate – communication; communicative; uncommunicative; communicator.
To transmit – transmitter; transmission; transmitted; transmissible; transmitting (coil).
To receive – receiver; reception; receptive; receptivity; receiving (coil).
To follow – follower; following.
To contribute – contribution; contributor; contributory.
To invent – inventor; invention; invented.
To implement – implementation; implemented.
To retrieve – retrieval; retrievable; irretrievable.
To improve – improvement; improver; improved; unimproved; improvable;
To appear – to disappear; appearance; disappearance.
To establish – to disestablish; established; establishment.
To predict – predicted; prediction; predictor.
To address – addressability; addressable; addressee; addressing; addressless; addressness.
Sequence – sequent; sequential; sequencer; consequently.
Function – functional; functionality; functionally/
14. Read the texts, study basic types of modulation and speak of them:
Basic types of modulation
Today vast amounts of information are communicated using radio communications systems. Both analogue radio communications systems and digital or data radio communications links are used.
However, one of the fundamental aspects of any radio communications transmission system is modulation, or the way in which the information is superimposed on the radio carrier.
In order that a steady radio signal or " radio carrier" can carry information, it must be changed or modulated in one way so that the information can be conveyed from one place to another.
There are very many ways in which a radio carrier can be modulated to carry a signal, each having its own advantages and disadvantages. The choice of modulation have a great impact on the radio communications system. Some forms are better suited to one kind of traffic whereas other forms of modulation will be more applicable in other instances. Choosing the correct form of modulation is a key decision in any radio communications system design.
There are three main ways in which a radio communications or RF signal can be modulated:
- Amplitude modulation, AM: as the name implies, this form of modulation involves modulating the amplitude or intensity of the signal.
Amplitude modulation was the first form of modulation to be used to broadcast sound, and although other forms of modulation are being increasingly used, amplitude modulation is still in widespread use.
- Frequency modulation, FM: this form of modulation varies the frequency
in line with the modulating signal.
Frequency modulation has the advantage that, as amplitude variations do
not carry any information on the signal, it can be limited within the receiver to remove signal strength variations and noise. As a result, this form of modulation has been used for many applications including high quality analogue sound broadcasting.
- Phase modulation, PM: as the name indicates, phase modulation varies
the phase of the carrier in line with the modulating signal.
Phase modulation and frequency modulation have many similarities and are linked - one is the differential of the other. However, phase modulation lends itself to data transmissions, and as a result, its use has grown rapidly over recent years.
Each type of modulation has its own advantages and disadvantages, and accordingly they are all used in different radio communications applications.
In addition to the three main basic forms of modulation or modulation techniques, there are many variants of each type. Again, these modulation techniques are used in a variety of applications, some for analogue applications, and others for digital applications.
Angle modulation is a name given to forms of modulation that are based on altering the angle or phase of a sinusoidal carrier. Using angle modulation there is no change in the amplitude of the carrier.
The two forms of modulation that fall into the angle modulation category are frequency modulation and phase modulation.
Both types of angle modulation, namely, frequency modulation and phase modulation are linked because frequency is the derivative of phase, i.e. frequency is the rate of change of phase.
Another way of looking at the link between the two types of modulation is that a frequency modulated signal can be generated by first integrating the modulating waveform and then using the result as the input to a phase modulator. Conversely, a phase modulated signal can be generated by first differentiating the modulating signal and then using the result as the input to a frequency modulator.
One key element of any signal is the bandwidth it occupies. This is important because it defines the channel bandwidth required, and hence the number of channels that can be accommodated within a given segment of radio spectrum. With pressure on the radio spectrum increasing, the radio signal bandwidth is an important feature of any type of radio emission or transmission.
The bandwidth is governed by two major features:
- The type of modulation: Some forms of modulation use their bandwidth
more effectively than others. Accordingly, where spectrum usage is of importance, this alone may dictate the choice of modulation.
- The bandwidth of the modulating signal: A law called Shannon's law
determines the minimum bandwidth through which a signal can be transmitted. In general, the wider the bandwidth of the modulating signal, the wider the bandwidth required.
Modulating signal type
Apart from the form of modulation itself, the type of signal being used to modulate the carrier also has a bearing on the signal. Analogue and data are two very different forms of modulating signal and need to be treated differently. While different formats of actual modulation may be used, the type of signal being applied via the modulator also have a bearing on the signal.
Signals for high quality stereo broadcasting will be treated differently to signals that provide digital telemetry for example. As a result, it is often important to know the signal type that needs to be carried by the RF carrier.
By Ian Poole
Последнее изменение этой страницы: 2016-03-17; Просмотров: 1843; Нарушение авторского права страницы