A channel is also called a channel. A channel is the channel through which signals are transmitted in a communication system, and it is the transmission medium through which signals are transmitted from the transmitter to the receiver. This is the definition of a narrow channel. The definition of generalized channel includes not only transmission media, but also related equipment for signal transmission.
wireless channels mainly include wireless channels that radiate radio waves and underwater acoustic channels that propagate sound waves underwater.
the radio signal is radiated by the antenna of the transmitter to the whole free space for propagation. Radio waves in different frequency bands have different propagation modes, mainly including:
1. Ground wave transmission: < P > The earth and ionosphere form a waveguide, and medium-long wave, long wave and very long wave can propagate along the ground and bypass obstacles on the ground in this natural waveguide. Long wave can be applied to maritime communication, and medium wave AM broadcasting also uses ground wave transmission.
2. Sky wave transmission:
Short wave and ultrashort wave can propagate through the reflection channel formed by ionosphere and the scattering channel formed by troposphere. Short-wave radio stations use sky wave transmission. The maximum distance of sky wave transmission can reach about 4 kilometers.
3. Line-of-sight transmission:
For higher frequency electromagnetic waves such as ultrashort waves and microwaves, direct point-to-point linear transmission is usually adopted. Because the wavelength is too short to bypass obstacles, line-of-sight transmission requires that there is no obstacle between transmitter and receiver.
Extended data
Features
1. The channel always has an input signal terminal and an output signal.
2, the channel is generally linear, that is, the superposition principle is satisfied between the input signal and the corresponding output signal.
3, the channel is causal, that is, after the input signal passes through the channel, the response of the corresponding output signal is delayed.
4, the channel distorts the passing signal, that is, the corresponding output signal will be attenuated after the input signal passes through the channel.
5, there is noise in the channel, even if the input signal is zero, the output signal will still have a certain power.
therefore, the modulation channel can be described as a multi-port linear system. If the distortion of the signal through the channel is time-varying, then this is a linear time-varying system, and such a channel is called "random parameter channel"; If the distortion is independent of time, then this is a linear time-invariant system, and this channel is called "constant parameter channel".