【正文】 中文 6190字 英文資料及中文翻譯 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 acmodate 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 parable 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 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 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 municates via radio with one of the base stations and may beheadedoff to any number of 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 handheld unit .The base stations of several transmitters and receivers which simultaneously handle full duplex munications 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 stations and connects the entire cellular system to the typical MSC handles 100000 cellular subscribers and 5000 simultaneous conversations at a time, and acmodates all billing and system maintenance functions, as well .In large cities, several MSCs are used by a single carrier. Cordless Telephone Systems Cordless telephone systems are full duplex munication 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 work (PSTN).In first generation cordless telephone systems (manufactured in the 1980? s ), the portable unit municates 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 PSTN and are primarily for inhome 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 bined 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 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. Basic Knowledge of Communication Communication System A generalized munication system has the following ponents : (a) In formation 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 for transmission over the munication channel. (c) Communication Channel .The munication 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 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 puter data bank. Once this new baseboard signal ,a “group” of 4 channels , has been formed it is moved around the trunk work 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 groups are moved around as single units by the munications 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 promise as treating each channel individually 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 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 end of the munication link to enable samples to be trans mitted in turn ,from each of several circuits .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 or frequency of a carrier , directly .However in digital modulation a stream of pulses ,representing the original ,is created .This stream is then used