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  • Publicado : 4 de abril de 2010
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200611612 | Cony Asenath Juárez Franco |
200714584 | Krysta Maria Salazar Tale |


Suppose that two people, separated by a considerable distance, wish to communicate with one another. If there is a pair of conducting wires extending from onelocation to another, and if each place is equipped with a microphone and earpiece, the communication problem may be solved. The microphone, at one end of the wire communications channel, impresses and electrical signal voltage on the line, which voltage is then received at the other end. The received signal, however, will have associated with an erratic, random, unpredictable voltage waveformwhich is described by the term noise. Because of the length of the wire link, the received message signal voltage will be greatly attenuated in comparison with its level al the transmitting end of the link. As a result, the message signal voltage may not be very large in comparison with the noise voltage, and the message will be perceived with difficulty or possibly not at all. An amplifier at thereceiving end will not solve the problem, since the amplifier will amplify signal and noise alike. The amplifier may be source of additional noise.

A principal concern of communication theory is the study of methods to suppress the effect of noise. With this purpose, it may be better not to transmit directly the original signal. Instead, the original signal is used to generate a differentsignal waveform, which new signal waveform is then impressed on the line. This processing of the original signal to generate the transmitted signal is called encoding or modulation. At the receiving end an inverse process called decoding or demodulation is required to recover the original signal.

It may well be that there is a considerable expense in providing the wire communication link. We are,therefore, naturally led to inquire whether we may use the link more effectively by arranging for the simultaneous transmission over the link of more than just a single waveform. It turns out that such multiple transmissions is indeed possible and may be accomplished in a number of ways. Such simultaneous multiple transmissions are called multiplexing and are again a principal area of concern ofcommunication theory. The communications medium is the free space.

A branch of mathematics which is of inestimable value in the study of communications systems is the spectral analysis. It concerns itself with the description of waveforms in the frequency domain and with the correspondence between the frequency-domain description and the time-domain description.

A waveform can be expressed asan explicit function of time v (t). The waveforms encountered in telecommunications are in many instances unpredictable. The waveform can be called signal. If the signal were predictable, the transmission would be unnecessary, and the entire communications system would serve no purpose.

One of the basic problems of communications engineering is the design and the analysis of systems whichallow many individual messages to be transmitted simultaneously over a single communication channel. A method by which such multiple transmission, called multiplexing, may be achieved consists in translating each message to a different position in the frequency spectrum. Such multiplexing is called frequency multiplexing. The individual message can eventually be separated by filtering. Frequencymultiplexing involves the use of an auxiliary waveform, usually sinusoidal called carrier.

A basic telecommunication system consists of three elements:
* A transmitter that takes information and converts it to a signal;
* A transmission medium that carries the signal; and,
* A receiver that receives the signal and converts it back into usable information.
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