gps通信系统外文翻译---全球移动通信系统(编辑修改稿)

gps通信系统外文翻译---全球移动通信系统(编辑修改稿)内容摘要：

quency reuse and cell splitting. Frequency reuse es into play by using radio channels on the same frequency in coverage areas that are far enough apart not to cause cochannel interference. This allows handling of simultaneous calls that exceed the theoretical spectral capacity. Cell splitting is necessaty when the traffic demand on a cell has reached the maximum and the cell is then divided into a microcellular system. The shape of cell in a cellular system is always depicted as a hexagon and the cluster size can be seven, nine or twelve. The GSM system requires a number of functions to be created for a fully operational mobile system. The cell coverage area is controlled by a base station which is itself made up of two elements. The first element is the transmission system which municates out to the （论文） 12 mobile and also receives information from it to set up and maintain calls when actually in operation. The base Station transceiver (BST) is controlled by the base station controller( BSC ), which municates with the mobile switching center( MSC)the essential link to the local public switched telephone work(PSTN), and to the subscriber data which is stored in registers within the system. The subscriber registers allow the GSM system to check a subscriber who requests the use of the work, allow access and then set up the charging function, etc. The GSM system was allocated part of the 900 MHz band at the 1978 World Administration Conference (WAC), the actual bands being 890 to 915 MHz for the uplink transmission and 935 to 960 MHz for the downlink. The access method is time division multiple access (TDMA). The GSM system operates in a burst transmission mode with 124 radio channels in the 900 MHz band, and these bursts can carry different types of information. The first type of information is speech, which is coded at kb/s or 13 kb/s. The second type is data, which can be sent at kb/s, 6 kb/s or kb/s. There two forms of transmission are the useful parts of the transmission, but have to be supported by overhead information which is sent in control channels (CCH). The use of digital radio transmission and the advanced handover algorithms between radio cells in GSM work allows for significantly better frequency usage than in analogue cellular systems, thus increasing the number of subscribers that can be served. Since GSM provides a mon standard, cellular subscribers will also be able to （论文） 13 use their telephones over the entire GSM service area. Roaming is fully automatic between and within all countries covered by GSM system. In addition to international roaming, GSM provides new services, such as highspeed data munication, facsimile and short message service. The GSM technical specifications are designed to work in concert with other standards, . ISDN. Interworking between the standards is in this way assured. In the long term perspective cellular systems, using a digital technology will bee the universal method of telemunication. The third generation mobile munication system currently being developed in Europe is intended to integrate all the different services of the second generation systems and cover a much wider range of broadband services (voice, data, video, multimedia) consistent and patible with technology developments taking place within the fixed telemunication works. Radio Receiver A modern radio receiver ， whose input signals to this radio are amplitudemodulated radio waves. The basic electronic circuits include: antenna, tuner, mixer, local oscillator, IF amplifier, audio detector, AF amplifier, loudspeaker, and power supply. Any antenna system capable of radiating electrical energy is also able to abstract energy from a passing radio wave. Since every wave passing the receiving antenna includes its own voltage in the antenna conductor, it is necessary that the receiving equipment be capable of separating the desired signal from the unwanted signals that are also inducing voltages in the antenna. This separation is made on the basis of the （论文） 14 difference in frequency between transmitting stations and is carried out by the use of resonant circuits, which can be made to discriminate very strongly in favor of a particular frequency. It has already been pointed that, by making antenna circuit resonant to a particular frequency, the energy abstracted from radio waves of that frequency will be much greater than the energy from waves of other frequencies。 this alone gives a certain amount of separation between signals. Still greater selective action can be obtained by the use of additional suitably adjusted resonant circuits located somewhere in the receiver in such a way as to reject all but the desired signal. The ability to discriminate between radio waves of different frequencies is called selectivity and the process of adjusting circuits to resonance with the frequency of a desired signal is spoken of as tuning. Although intelligible radio signals have been received from the stations thousands of miles distant, using only the energy abstracted from the radio wave by the receiving antenna much more satisfactory to the radiofrequency currents before detection, in which case it is called radiofrequency amplification or it may be applied to the rectified currents after detection, in which case it is called audiofrequency amplification. The use of amplification makes possible the satisfactory reception of signals from waves that would otherwise be too weak to give an audible response. The process by which the signal being transmitted is reproduced from the radiofrequency currents present at the receiver is called detection, or sometimes demodulation. Where the intelligence is transmitted by varying the amplitude of the radiated wave, detection is acplished by rectifying the radio frequency cur。