Company name: foreign name of channel: location of Pindao headquarters: China, date of establishment: 1978-5-6, scope of business: nature of channel company: annual business channel turnover:100000, classification: narrow concept wired channel, wireless channel, storage channel, wired channel, wireless channel, storage channel, broad concept, modulation channel and coding channel. Coding channel model, capacity, narrow concept, according to the transmission medium, narrow channels can be divided into three categories: wired channels, wireless channels and wired channels. Wired channels use wires as the transmission medium, and signals are transmitted along the wires, and the energy of the signals is concentrated near the wires, which has high transmission efficiency, but the deployment is not flexible enough. The transmission media used in this channel include overhead wire, telephone line, twisted pair, symmetrical cable and coaxial cable. And an optical fiber for transmitting the modulated optical pulse signal. Wireless channels Wireless channels mainly include wireless channels that radiate radio waves and underwater acoustic channels that propagate sound waves underwater. Radio signals are radiated to the whole free space by the antenna of the transmitter for propagation. Radio waves in different frequency bands have different propagation modes, mainly including: ground wave propagation: the earth and ionosphere form a waveguide, and medium-long wave, long wave and very long wave can propagate along the ground in this natural waveguide, bypassing obstacles on the ground. Maritime communication can be broadcast by long wave and medium wave AM and transmitted by ground wave. Sky wave propagation: 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 waves for transmission. The maximum distance of sky wave transmission can reach about 400 kilometers. The reflection and scattering of ionosphere and troposphere form a number of time-varying propagation paths from transmitter to receiver, and radio signals form constructive or destructive superposition at the receiving end through these paths, so that the amplitude and phase of the received signal change randomly. This is the fading of multipath channel, which is called fading channel. Line-of-sight transmission: For high-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 no obstacles between transmitter and receiver. Due to the influence of the curvature of the earth, the distance of line-of-sight transmission is limited, and the farthest transmission distance d is equal to the height h of the transmitting antenna from the ground. If long-distance transmission is needed, ground relay stations or satellite relay stations must be set up for relay transmission, that is, microwave line-of-sight relay and satellite relay transmission. The line-of-sight transmission of optical signals also belongs to this category. Because of the great transmission loss of electromagnetic waves in water, underwater acoustic channel is usually used for transmission. Underwater acoustic channel is also a multipath fading channel due to the reflection and refraction of water layers with different densities and salinities and the scattering of underwater objects. Wireless channels propagate signals in free space (air and space for wireless channels and water for underwater acoustic channels), with scattered energy, low transmission efficiency, easy interception by others and poor security. However, the communication through wireless channel gets rid of the bondage of wired communication, so wireless communication has a high degree of flexibility that wired communication does not have. Storage channel In a sense, data storage media such as magnetic tape, optical disk and magnetic disk can also be regarded as a communication channel. The process of writing data into the storage medium is equivalent to the process of the transmitter sending signals to the channel, and the process of reading data from the storage medium is equivalent to the process of the receiver receiving signals from the channel. Block diagram of generalized concept modulation channel and coding channel. For digital communication systems, the modulation channel is a part of the coding channel. Generalized channels can be divided into modulation channels and coding channels according to their functions. Modulation channel modulation channel refers to the part through which the signal is transmitted 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 relationship between the input and output 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. Coding channel The coding channel is the part where the exponential 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. Modulation channel model additive noise channel model. The channel composed of coaxial cable, optical fiber and other media conforms to this model. Linear time-varying channel model. Linear time-invariant channel model. The modulation channel model describes the mathematical relationship between the output signal and the input signal of the modulation channel. The characteristic channel always has an input signal terminal and an output signal. The channel is generally linear, that is, the superposition principle is satisfied between the input signal and the corresponding output signal. The channel is causal, that is, after the input signal passes through the channel, the echo of the corresponding output signal is delayed. The channel distorts the passing signal, that is, after the input signal passes through the channel, the corresponding output signal will be attenuated. 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". Mathematical model y (t) = x (t) * h (t; τ)+n(t) where x(t) is the input signal of the modulation channel at time t, that is, the modulation signal. Y(t) is the output signal of the modulation channel at time t, and h (t; τ) is the impulse response of the channel, τ represents the time delay, and h (t; τ) represents the echo of the channel to the pulse function τ (t) at time t, and the delay is τ, which describes the distortion and delay of the channel to the input signal. * is the convolution operand. N(t) is the additive noise in the modulation channel, which has nothing to do with the input signal x(t), and is also called "additive interference". Due to the linear nature of the channel and considering the channel noise, x (t) * h (t; τ)+n(t) means that x(t) consists of channel echo h (t; τ) describes the output of the modulation channel. A modulation channel can have multiple input signals and multiple output signals at the same time. In this case, x(t) and y(t) are vector signals. H(t) makes the amplitude of the output signal y(t) of the modulation channel change with time t, so it is called "multiplicative interference". Multiplicative interference h(t) is a function of t, which is influenced by channel characteristics and usually changes randomly with time. Therefore, its statistical characteristics can only be described by stochastic process, and this kind of channel is called "random parameter channel". However, the multiplicative interference of some channels basically does not change with time, and its h(t) can be regarded as a constant, which is called "constant parameter channel". The modulation channel composed of short-wave ionospheric reflection, ultrashort wave and microwave ionospheric scattering, ultrashort wave line-of-sight diffraction and other media belongs to parametric channel. The modulation channel composed of overhead open wire, symmetrical cable, coaxial cable, optical cable, microwave line-of-sight propagation, light wave line-of-sight propagation and other media belongs to constant parameter channel. N(t) is the additional noise of the channel, which is independent of the input signal X and therefore independent of the output signal Y. Even if the input signal of the channel is zero, the channel still has energy output from the noise. The main sources of additive noise are: thermal noise and shot noise inside the circuit, external cosmic noise and so on. Coded channel model digital communication uses a finite number of symbols to represent information through an encoder. After these symbol signals pass through the coded channel, there will be errors in the symbol signals reconstructed by the decoder at the output end of the coded channel due to channel distortion and noise interference. The coded channel model describes the mathematical relationship between the input symbol signal and the output symbol signal of the coded channel. The coded channel model describes the transition probability between the input and output symbol signals of the coded channel. Let the used symbol set of the coded channel be, and m be the codebook size. The output signal of the encoder is, and the output signal of the decoder is. Then the transition probability P(Yj | Xi) describes the probability that the input signal Xi is detected as Yj after passing through the coded channel. Capacity channel is a channel for transmitting information, and channel capacity describes the maximum capacity of error-free transmission of information, which can be used to measure the quality of the channel. Shannon gave the definition and calculation of channel capacity in his famous paper Mathematical Principles of Communication, that is, channel capacity is the upper bound of mutual information between input signal and output signal. For the additive white Gaussian noise channel with S/N signal-to-noise ratio and B bandwidth, the channel capacity is log2( 1+S/N), which is the spectral efficiency of the channel transmission information, that is, the amount of information that can be transmitted per unit time and bandwidth, and the unit is. Improving the signal-to-noise ratio can improve the channel capacity, which can be achieved by suppressing noise or improving the transmission power. If the SNR is infinite, the channel capacity tends to infinity. However, because there is always noise in the channel, the power of the transmitter cannot be infinite, so this will not happen. Increasing the channel bandwidth can also increase the channel capacity, but this increase is not infinite. If the noise power spectral density of the channel is N0, the noise power N = BN0 will also increase with the increase of the channel bandwidth b, remember that the maximum signal power is Es, and when the bandwidth is infinite, the limit of the channel capacity can be seen. Increasing bandwidth is not a good way to improve channel capacity. Channel capacity is the theoretical limit of channel's ability to transmit information. In various communication technologies, the actual channel throughput is far less than this limit.