统计时分复用.ppt

* A simple example of transmitting three voice signals simultaneously over a medium. The bandwidth of a voice signal is 4kHz. If such a signal is used to amplitude-modulate a 64kHz carrier, the spectrum is as figure (b). We elect to transmit only the lower sideband(下边带）. If three voice signals are used to modulate carriers at 64, 68, 72kHz, and only the lower sideband of each is taken , the spectrum of figure (c) results. * A number of analog or digital signals [mi (t) ,i=1,n] are to be multiplexed onto the same transmission medium. Each signal mi (t) is modulated onto a carrier fi . The resulting analog, modulated signals are then summed to produce a composite baseband (基带) signal mb (t). Figure (b) shows the result. The spectrum of signal mi (t) is shifted to be centered on fi. fi must be chosen so that the bandwidths of the various signals do not significantly overlap. The composite signal may then be shifted as a whole to another carrier frequency by an additional modulation step. The FDM signal s(t) has a total bandwidth B, where At the receiving end ,the FDM signal is demodulated to retrieve (to regain) mb (t), which is then passed through n bandpass filters, each filter centered on fi and having a bandwidth Bi Baseband: is used to designate the band of frequencies of the signal delivered by the source and potentially used as a modulating signal.数据源传递的信号频带，这些信号是将要被调制的信号。 * At the first level of the ATT hierarchy, 12 voice channels are combined to produce a group signal with a bandwidth of 12×4kHz=48kHz, in the range 60 to 108kHz. The next basic block is the 60-channel supergroup, which is formed by FDM multiplexing five group signals. Group: 基群 supergroup: 超群 mastergroup: 主群 jumbogroup: 巨群 * Multiple digital signals can be carried on a single transmission path by interleaving portions of each signal in time. The interleaving can be at the bit level or in blocks of bytes or larger quantities. Interleaving：交叉(存取)，交错，插入 Synchronous: occurrin