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外文资料原文 Mobile Communication Basic Knowledge of Communication Frequency Division Multiplexing Frequency division multiplexing technique is an analog of a voice signal in the frequency band 0-3 kHz, with an amplitude modulation sideband (SSB) can be moved to a new voice signal band. For example, using this modulation, moving 4-way signal similar to Germany, making sharing of 5-20 kHz band. The gap between the brightest Germany called the guard interval, the interval makes little practical system allows filtering, frequency error and the lack of defects Once this 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 groups 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 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 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 slots, the original analog signals between samples can be reconstituted at the receiver. Pulse Code Modul