Introduction to Communication System

Syllabus Composing of Communication

System

Noise in Communication System

Communication Channel

Electric Wave Propagation Feature

in Mobile Communication

Composing of Communication System

Composing of Communication System

The usual signal sources are sound,image and data etc.The electronic signal conv

erted from original sound,image and data is transmitted to the signal sources enc

oder by suitable sensors like microphone and vidicon.

The signal source encoder transmits the data signal to the channel encoder

after performing ADC and compressed encoding.

The channel encoder can encode the digital signal properly ,such as Convolutiona

l codes,to increase its redundancy and have the error detecting and correcting ab

ility.Generally speaking,the signal after channel encoding is called symbol.

Modulation is to modulate the symbols properly ,such as FSK and PSK etc,to

some carrier with certain frequency according to the channel feature and

requirement after channel encoding.

Syllabus Composing of Communication

System

Noise in Communication System

Communication Channel

Electric Wave Propagation Feature

in Mobile Communication

Definition and Classification of Noise

Noise is the signal that confines the ability to distinguish the useful information correctly.

External Noise

Internal Noise

External noise comes from wireless environment such as light&thunder ,spurious ,industrial and other noise.

Electromagnetism interference like Radar and communication machines etc are also external noise.

Internal noise comes from the system itself,such as thermal noise generated by electrons’ free movement inside the electronic chips .

Emission,radiance noise and the non-linear interference etc are also internal noise.

Classification of Thermal Noise

White noise-from the spectrum

Gauss noise-from the probability distribution of amplitude

The thermal noise coming from free electrons’ thermal movement

inside the chips or conductors is called white noise.

What’s the probability density of thermal noise？ The thermal noise comes from lots of electrons’ free movement and every electron contributes very less,So its voltage should follow normal distribution

according to Central Limit Theorem.

Syllabus Composing of Communication

System

Noise in Communication System

Communication Channel

Electric Wave Propagation Feature

in Mobile Communication

Free Space channel—Satellite Communication

Suppose signal radiant source is a point named A and transits directional power in free space,then any point whose distance from p

oint A is R where the power density is as followed:

224

mwR

Pt

Multi-path channel — Electric wave

propagates along the ground

Syllabus Composing of Communication

System

Noise in Communication System

Communication Channel

Electric Wave Propagation Feature

in Mobile Communication

Electric Wave Propagation Feature in Mobile Communication

Fast Fading

The receiving antenna receives not only the direct electric

wave,but also the reflected wave by ground and spurious

wave.What’s more, Doppler Shift will be caused by the rapid

movement of Mobile Stations(Vehicles).All of these factors

make the amplitude and phase of the signals received by MS

change sharply with time(measured by ms).This phenomenon is

called fast fading or short-term fading.

Electric Wave Propagation Feature in Mobile Communication

Slow Fading

After real test, the signal’s average value will change slo

wly with time,situation and speed (measured by second) when

the signal value changes rapidly in mobile communication .Th

e phenomenon above is called slow fading.

Slow fading occurs because electric wave’s propagation

path and climate have changed during the movement.