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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.
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