hjr-Communication Principle (vi): Channel

Before saying the various signals of transmission and noise, and then say the channel

As the name implies is the channel of information transmission. Divided into cable (cable, optical cable ... ) and wireless channel (electromagnetic wave)

The mathematical model of the modulation channel is as follows

K (t) multiplicative disturbance n (t) additive disturbance

Channel classification:

constant parameter channel : multiplicative disturbance K (t) does not change with time or basically does not change

with reference channel : multiplicative disturbance K (t) random rapid change

The constant parameter channel is a channel composed of overhead wire, cable, medium and long wave propagation, ultrashort and microwave sight propagation, satellite relay, optical fiber, and the transmission medium of light wave distance.

The accompanying channel includes a modulation channel composed of short wave ionospheric reflection, ultrashort wave meteor remnant scattering, ultrashort sweep microwave tropospheric scattering, ultrashort shortwave ionospheric scattering, and ultrashort wave ultra-distance diffraction transmission medium.

The following three features are included with the reference channel:

(1) Change in signal decay over time

(2) Delay of transmission varies with time

(3) Multipath propagation

channel noise is categorized by source :

(1) Artificial noise: electric spark, lighting, electrical electromagnetic radiation, etc.

(2) Natural noise: electromagnetic radiation, atmospheric noise, cosmic noise, etc.

The thermal noise is more important in the natural noise, and all electronic components generate thermal noise due to the electronic thermal motion.

Channel noise is classified by nature:

(1) Impulse noise

(2) narrowband noise

(3) The fluctuation noise is important to the channel knowledge is Shannon Formula, in the case of the default noise is thermal noise, the channel capacity can be obtained by Shannon Formula

C: Channel capacity = information rate

B: Channel Bandwidth

S: Signal Power

N: noise Power spectral density

When the signal-to-noise ratio is 30db: 1+s/n = 1001