3.4 Channels of telecommunication

A communication channel is a set of means that ensure the transmission of a signal from a certain point of the transmitter to a certain point of the receiver. The part of the transmitter that is not part of the channel is the source of the signal for this channel and in the same communication system it is possible to distinguish different entry points into different channels.

A channel is called continuous if the input and output signals of the channel are continuous, and discrete if the signals input to the channel and taken from its output are discrete. There are also discrete-continuous and continuous-discrete channels, the input of which receives discrete signals, and continuous output is removed from the output, or vice versa. Note that the channel can be discrete or continuous, regardless of the nature of the transmitted messages. Moreover, in the same communication system both discrete and continuous channels can be distinguished. It all depends on how the entry and exit points of the channel are selected.

A communication channel can be characterized in the same way as a signal by three parameters: the time of the TC during which the channel is being transmitted; FK channel bandwidth and DK dynamic range. Under the dynamic range of the channel understand the ratio of the permissible power of the transmitted signal to the power of interference, inevitably present in the channel. The types of channels through which messages are transmitted are numerous and varied. Channels of wired communications, shortwave radio communications using the reflection from the ionosphere, ultrashortwave communications of ionospheric and tropospheric scattering, meteor communications, space communications, etc. are widely used. The characteristics of these channels differ significantly from each other. A generalized characteristic of a continuous channel is its capacity (volume): VK = TKFKDK.
A necessary condition for undistorted transmission over a channel of signals with a volume of VС should be

VC <VK (3.1)

To match the signal with the channel, a secondary transformation of the primary signal is performed. In the simplest case, the signal is matched with the channel in all three parameters, i.e. achieve the following conditions:

TC <TC; FC <FK; DC <DK. (3.2)

Under these conditions, the signal volume completely fits into the channel volume. However, inequality (3.1) can also be satisfied when one or two of inequalities (3.2) are not satisfied. This means that it is possible to “exchange” the duration for the width of the spectrum or the width of the spectrum for the dynamic range, etc. For example, a tape recorded signal can be played at reduced speed. In this case, the frequency range of the original signal will decrease by as many times as the transmission time will increase. The received signal is also recorded on tape, and then reproduced at a higher speed to restore the original signal. The recorded signal can also be transmitted at a higher speed. Widely used is the exchange of dynamic range for bandwidth. Thus, the use of noise-resistant broadband modulation types allows you to send a message over channels with an increased level of interference. But this requires a channel bandwidth wider than the spectrum of the message.

According to the method of propagation of electromagnetic energy, channels with open and closed propagation are distinguished. In channels with closed propagation, electromagnetic collars are propagated along guide lines (wire, cable, waveguide paths, etc.). In open-spread channels, radio waves are used in the frequency range from 30 to 10 · 10 ¹² Hz.