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Principles of Commun Communications ications
Stella Wen
Principles of Communications Fifth Edition E dition
Systems, Modulation, and Noise Ziemer Tranter
LECTURE NOTE Department of E.E. National Chiao Tung Univ.
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Principles of Communications
Stella Wen
Chap 1 Introduction ∗
A communication systems conveys information from it source to a destination (some distance away).
∗
dissemination of information
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combination of circuits and devices
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Transmission is the major result
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uncertainty is the major characteristic %
-Uncertainty is due to noise --Noise is due to ()a unwanted
()b
signal perturbation
unpredictable nature of information
----- NEED PROBABILITIES ∗
Contributors to communications: probabilities ϖ
Shannons’ theory:
C=Blog2(1+S/N) bps~ Blog2e (S/N) (for small S/N) ϖ
development of circuits and devices
1.1Block diagram of a general Communication System.
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Stella Wen
Main Body : Transmitter + Channel + Receiver %
Input transducer : Input information
V or I
Signal ( Analog or digital ) %
Transmitter : To couple the message to the channel via modulation, filtering & amplifier /
Purposes :
()1 Ease of radiation ()2 Reduce the noise and interference ()3 For channel assignment ()4
For multiplication signals
()5 Overcome equipment limitations. %
Channel : Channel effect
%
Receiver : Extract desired signal, convert signal for input transducer ( demodulator)
%
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Output transducer
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Principles of Communications
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Input
Stella Wen
FIGURE 1.1 Block diagram of a communication system
Message
Transmitted
Received
signal
signal
Output
signal
signal
Message Input transducer
Transmitter
Carrier
Channel
Receiver
Output Message Output transducer
Additive noise,interference, distortion resulting from bandlimiting and nonlinearities, switching noise in networks, electromagnetic discharges such as lightening, powerline corona discharge,and so on.
References: Slide show" D-Transceiver.ppt"
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Channel Characteristics.
Noise : A. External noise (Atmospheric, man-made, extraterrestrial sources) B. ∗
Internal noise
A. External noise ϖ
Atmospheric noise %
result from spurious radio waves
%
referred to as static of spherical
%
%
below 100 Mhz, field strength α 1/f characterized in time domain as large Amp, shortduration-burst, i.e. impulse.
%
ϖ
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has large impact on low-freq. signals.
Man-made noise source %
result from modern engines & RFI
%
impulsive
%
irritation factor for voice transmission
%
serious for digital data transmission
Extraterrestrial noise 1/5
Principles of Communications
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Stella Wen
%
result from hot heavily bodies (e.g. Sun)
%
source of wideband radio energy
Multiple transmission path interference %
one or two strong reflect : specula
%
numerous reflect component : diffuse
Fading: signal degradation, perturbation REFERENCE: " Progagation.ppt"
%
B. Internal noise ϖ
Thermal noise : caused by random motion of free electrons in a conductor or semiconductor excited by thermal agitation
ϖ
Shot noise : caused by random arrival of discrete charge carriers in thermornic tubes or semiconductor junction devices.
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Flicker noise: unknown source and is more severe the lower the frequency.
