Figure 6-6 is a block diagram of a typical digital baseband signal transmission system. It is mainly composed of sending filter (channel signal generator), channel, receiving filter and sampling decision. In order to ensure the reliable and orderly work of the system, there should also be a synchronous system.
The functions of each box in the figure and the physical process of signal transmission are described as follows:
(1) Channel signal generator (transmit filter). Its function is to generate baseband signal waveform suitable for channel transmission. Because the input is generally a transmission code generated by the code encoder, the corresponding basic waveform is usually a rectangular pulse, and its spectrum is very wide, which is not conducive to transmission. The transmission filter is used to compress the input signal frequency band and transform the transmission code into the baseband signal waveform suitable for channel transmission.
(2) Channel. It is a medium that allows baseband signals to pass through, usually a wired channel, such as twisted pair, coaxial cable, etc. The transmission characteristics of the channel generally do not meet the conditions of distortion-free transmission, so it will cause distortion of the transmission waveform. In addition, the channel introduces noise n(t) and assumes that it is Gaussian white noise with a mean of zero.
(3) Receiving filter. It is used to receive the signal, filter out the channel noise and other interference as much as possible, balance the channel characteristics, and make the output baseband waveform conducive to sampling decision.
(4) Sampling decision. Under the background of unsatisfactory transmission characteristics and noise, the output waveform of the receiving filter is sampled at a specified time (controlled by a bit timing pulse) to recover or regenerate the baseband signal.
(5) Timing pulse and synchronous extraction. The bit timing pulse used for sampling is extracted from the received signal by the synchronous extraction circuit, and the accuracy of the bit timing will directly affect the decision effect.
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