外文翻譯--移動通信

1、英文資料及中文翻譯Mobile CommunicationCellular Telephone SystemsA cellular telephone system provides a wireless connection to the PSTN for any user location within the radio range of the system. Cellular systems accommodate a lar

2、ge 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

3、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 distance away. A sophistic

4、ated switching technique called a handoff enables a handoff enables a call to proceed uninterrupted when the user moves from one cell to another.A basic cellular system consists of mobile stations, base stations and a mo

5、bile switching center (MSC). The Mobile Switching Center is sometimes called a mobile telephone switching office (MTSO),since it is responsible for connecting all mobiles to the PSTN in a cellular system. Each mobile com

6、municates via radio with one of the base stations and may beheaded-off to any number of base stations throughout the duration of a call .The mobile station contains a transceiver, an antenna, and control circuitry ,and m

7、ay be mounted in a cuticle or used as a portable hand-held unit .The base stations of several transmitters and receivers which simultaneously handle full duplex communications and generally have towers which support seve

8、ral transmitting and receiving antennas. The base station serves as a bridge between all mobile users in the cell and connects the simultaneous mobile calls vis telephone lines or microwave links to the MSC. The MSC coor

9、dinates of all of the base stations and connects the entire cellular system to the PSTN.A typical MSC handles 100000 cellular subscribers and 5000 simultaneous conversations at a time, and accommodates all billing and sy

10、stem maintenance functions, as well .In large cities, several MSCs are used by a single carrier.Cordless Telephone SystemsCordless telephone systems are full duplex communication systems that use radio to connect a porta

11、ble handset to a dedicated base station ,which is then connected to a dedicated telephone line with a specific telephone number on the pubic switched individually involves far more equipment because separate filters , mo

12、dulators 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 Division Multiplexing It is possible, with

13、pulse modulation systems, to use the between samples to transmit signals from other circuits .The technique is known as time division multiplexing (TDM).To do this it is necessary to employ synchronized switches at each

14、end of the communication link to enable samples to be transmitted in turn ,from each of several circuits .Thus several subscribers appear to use the link simultaneously . Although each user only has periodic short time s

15、lots, 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 , directly .Howev

16、er in digital modulation a stream of pulses ,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

17、of the two techniques commonly used.All pulse systems depend on the analog waveform being sampled at regular intervals. The signal created by sampling our analog speech input is known as pulse amplitude modulations .It

18、 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 digital modulation come from the transmitted pulses having two level

19、s only ,this being known as a binary system .In PCM the height of each sample is converted into a binary number .There are three steps in the process of PCM: sampling, quartering and coding .Optical Fiber Communications

20、Communication 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 s

21、ystem the information transfer is frequently achieved by superimposing or modulating the information on to an electromagnetic wave which acts as a carrier for the information signal .This modulated carrier is then transm