1、中文 6190 字 英文资料及中文翻译 Mobile Communication Cellular Telephone Systems A 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 large number of users over a large geographic area, within a limited f
2、requency 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
3、be reused by another base station located some distance away. A sophisticated 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 mobile s
4、witching 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 communicates via radio with one of the base stations and may beheaded-off to any number o
5、f base stations throughout the duration of a call .The mobile station contains a transceiver, an antenna, and control circuitry ,and may 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 commu
6、nications and generally have towers which support several 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 coordinates of all of the base
7、 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 system maintenance functions, as well .In large cities, several MSCs are used by a single carrier. Cordless
8、Telephone Systems Cordless telephone systems are full duplex communication systems that use radio to connect a portable handset to a dedicated base station ,which is then connected to a dedicated telephone line with a specific telephone number on the pubic switched telephone network (PSTN).In first
9、generation cordless telephone systems (manufactured in the 1980 s ), the portable unit communicates only to the dedicated base unit and only over 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
10、 PSTN and are primarily for in-home use. Second generations cordless telephones have recently been introduced which allow subscribers to use their handsets at many outdoor locations within urban centers such as London or Hong Kong. Modern cordless telephones are sometimes combined with paging receiv
11、ers so that a subscriber may first be paged and then respond to the page using the cordless telephone. Cordless telephone systems provide the user 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 seco
12、nd generation base stations provide coverage ranges up to a few hundred meters. Basic Knowledge of Communication Communication System A generalized communication system has the following components : (a) In formation Source .This produces a message which may be written or spoken words, or some form
13、of data. (b) Transmitter .The transmitter converts the message into a signal ,the form of which is suitable for 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
14、may be a radio link or a direct wire connection. (d) Receiver. The receiver can be thought of as the inverse of the transmitter .It changes the received signal back into a message and passes the message on to its destination which may be a loudspeaker, teleprompters or computer data bank. Once this
15、new baseboard signal ,a “group” 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 “super group” and several “super group” either a “mastergroup”or “hyper group”. Groups or super g
16、roups are moved around as single units by the communications equipment and it is not necessary for the radios to know how many channels are involved .A radio can handle a super group provided sufficient bandwidth is available .The size of the groups is a compromise as treating each channel individua
17、lly involves far more equipment because 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 Division Multiplexing It is possible, with pulse modulation s
18、ystems, 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 end of the communication link to enable samples to be transmitted in turn ,from each of several ci
19、rcuits .Thus several subscribers 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
20、or frequency of a carrier , directly .However 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 of the two techniques commonly used.
21、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 is not very useful in practice but is used as an intermediate stage towards forming a PCM signal .It will be seen lat
22、er that most of the advantages of 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 steps in the process of PCM: sampling, quartering and coding . Optica
23、l Fiber Communications 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 system 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
