1、外文翻译 Communication System A generalized communication system has the following components: (a) Information Source. This produces a message which may be written or spoken words, or some form of data. (b) Transmitter. The transmitter converts the message into a signal, the form of which is suitable fo
2、r transmission over the communication channel. (c) Communication Channel. The communication channel is the medium used transmit the signal, from the transmitter to the receiver. The channel may be a radio link or a direct wire connection. (d) Receiver. The receiver can be thought of as the inverse o
3、f the transmitter. It changes the received signal back into a message and passes the message on to its destination which may be a loudspeaker, teleprinter or computer data bank. An unfortunate characteristic of all communication channels is that noise is added to the signal. This unwanted noise may
4、cause distorions of sound in a telephone, or errors in a telegraph message or data. Frequency Diversion Multiplexing Frequency Diversion Multiplexing(FDM) is a one of analog technologies. A speech signal is 03 kHz, single sideband amplitude (SSB) modulation can be used to transfer speech signal to n
5、ew frequency bands, four similar signals, for example, moved by SSB modulation to share the band from 5 to 20 kHz. The gaps between channels are known as guard spaces and these allow for errors in frequency, inadequate filtering, etc in the engineered system. Once this new baseband signal, a “group”
6、 of 4 channels, has been formed it is moved around the trunk network as a single unit. A hierarchy can be set up with several channels forming a “group”, several groups a “supergroup” and several “supergroup” either a “mastergroup” or “hypergroup”. Groups or supergroups are moved around as single un
7、its by the communications equipment and it is not necessary for the radios to know how many channels are involved. A radio can handle a supergroup provided sufficient bandwidth is available. The size of the groups is a compromise as treating each channel individually involves far more equipment beca
8、use separate filters, modulators and oscillators are required for every channel rather than for each group. However the failure of one module will lose all of the channels associated with a group. Time Diversion Multiplexing It is possible, with pulse modulation systems, to use the between samples t
9、o transmit signals from other circuits. The technique is known as time diversion multiplexing (TDM). To do this, it is necessary to employ synchronized switches at each end of the communication links to enable samples to be transmitted in turn, from each of several circuits. Thus several subscribers
10、 appear to use the link simultaneously. Although each user only has periodic short time slots, the original analog signals between samples can be reconstituted at the receiver. Pulse Code Modulation In analog modulation, the signal was used to modulate the amplitude or frequency of a carrier, direct
11、ly. However, in digital modulation a stream of pulse, representing the original, is created. This stream is then used to modulate a carrier or alternatively is transmitted directly over a cable. Pulse Code Modulation (PCM) is one of the two techniques commonly used. All pulse systems depend on the a
12、nalog waveform being sampled at regular intervals. The signal created by sampling our analog speech input is known as pulse amplitude modulation. It is not very useful in practice but is used as an intermediate stage towards forming a PCM signal. It will be seen later that most of the advantages of
13、digital modulation come from the transmitted pulses having two levels only, this being known as a binary system. In PCM the height of each sample is converted into a binary number. There are three step in the process of PCM: sampling, quantizing and coding. Optical Fiber Communications Communication
14、 may be broadly defined as the transfer of information from one point to another. When the information is to be conveyed over any distance a communication system is usually required. Within a communication system the information transfer is frequently achieved by superimposing or modulating the info
15、rmation on to an electromagnetic wave which acts as a carrier for the information signal. This modulated carrier is then transmitted to the required destination where it is received and the original information signal is obtained by demodulation. Sophisticated techniques have been developed for this
16、 process by using electromagnetic carrier waves operating at radio frequencies as well as microwave and millimeter wave frequencies. However,” communication” may also be achieved by using an electromagnetic carrier which is selected from the optical range of frequencies. In this case the information
17、 source provides an electrical signal to a transmitter comprising an electrical stage which drives an optical source to give modulation of the light-wave carrier. The optical source which provides the electrical-optical conversion may be either a semiconductor laser or light emitting diode (LED). Th
18、e transmission medium consists of an optical fiber cable and the receiver consists of an optical detector which drives a further electrical stage and hence provides demodulation optical carrier. Photodiodes (P-N, P-I-N or avalanche) and , in some instances, phototransistor and photoconductors are ut
19、ilized for the detection of the optical signal and the electrical-optical conversion. Thus there is a requirement for electrical interfacing at either end of the optical link and at present the signal processing is usually performed electrically. The optical carrier may be modulated by using either
20、an analog or digital information signal. Analog modulation involves the variation of the light emitted from the optical source in a continuous manner. With digital modulation, however, discrete changes in the light intensity are obtained (i.e. on-off pulses). Although often simpler to implement, ana
21、log modulation with an optical fiber communication system is less efficient, requiring a far higher signal to noise ratio at the receiver than digital modulation. Also, the linearity needed for analog modulation is not always provided by semiconductor optical source, especially at high modulation fr
22、equencies. For these reasons, analog optical fiber communications link are generally limited to shorter distances and lower bandwidths than digital links. Initially, the input digital signal from the information source is suitably encoded for optical transmission. The laser drive circuit directly mo
23、dulates the intensity of the semiconductor laser with the encoded digital signal. Hence a digital optical signal is launched into the optical fiber cable. The avalanche photodiode detector (APD) is followed by a fronted-end amplifier and equalizer or filter to provide gain as well as linear signal p
24、rocessing and noise bandwidth reduction. Finally, the signal obtained is decoded to give the original digital information. Mobile Communication Cordless Telephone Systems Cordless telephone system are full duplex communication systems that use radio to connect a portable handset to a dedicated base
25、station, which is then connected to a dedicated telephone line with a specific telephone number on the public switched telephone network (PSTN) .In first generation cordless telephone systems (manufactured in the 1980s), the portable unit communications only to the dedicated base unit and only over
26、distances of a few tens of meters. Early cordless telephones operate solely as extension telephones to a transceiver connected to a subscriber line on the PSTN and are primarily for in-home use. Second generation cordless telephones have recently been introduced which allow subscribers to use their
27、handsets at many outdoor locations within urban centers such as London or Hong Kong. Modern cordless telephones are sometimes combined with paging receivers so that a subscriber may first be paged and then respond to the page using the cordless telephone. Cordless telephone systems provide the user
28、with limited range and mobility, as it is usually not possible to maintain a call if the user travels outside the range of the base station. Typical second generation base stations provide coverage ranges up to a few hundred meters. Cellular Telephone System A cellular telephone system provides a wi
29、reless connection to the PSTN for any user location within the radio range of the system. Cellular systems accommodate a large number of users over a large geographic area, within a limited frequency spectrum. Cellular radio systems provide high quality service that is often comparable to that of the landline telephone systems. High capacity is achieved by limiting the coverage of each base station transmitter to a small geographic area called a cell so that the same radio channels may be reused by another base station located some
