Channel refers to the channel through which signals are transmitted in the communication system, and it is the transmission medium through which signals are transmitted from the sender to the receiver.

The broad definition includes not only transmission media, but also related equipment used for signal transmission.

Channels are also called channels.

Generalized concept:

1. modulation channel: the modulation channel refers to the part of the signal from the output of the modulator to the input of the demodulator. For researchers of modulation and demodulation, the transmission medium and transformation equipment that the signal passes through on the modulation channel transform the signal in some form.

Researchers only care about the input-output relationship of these transformations, and do not care about the specific physical process of realizing this series of transformations. The input-output relationship of this series of transformations is usually described by multi-port time-varying network as the mathematical model of modulation channel.

2. Coding channel: The coding channel refers to the part where the digital signal is transmitted from the encoder output to the decoder input.

For the researchers of coding and decoding, the digital sequence output by the encoder becomes another set of digital sequences at the input of the decoder after a series of transformations on the coding channel. Researchers only care about the transformation relationship between two sets of digital sequences, but don't care about the specific physical process of this series of transformations, or even the specific changes of signals on the modulation channel.

The relationship between the digital sequence output by the encoder and the digital sequence input by the decoder is usually described by the transfer probability of the multi-port network as the mathematical model of the coding channel.