Principles of Communications

Principles of
Communications
Chap 1 Introduction

* A communication systems conveys information from it source to a destination (some distance away).
* dissemination of information
* combination of circuits and devices
* Transmission is the major result
* uncertainty is the major characteristic
* -Uncertainty is due to noise
–Noise is due to
(a) unwanted signal perturbation
(b) unpredictable nature of information
—– NEED PROBABILITIES(
* Contributors to communications:
* probabilities
* Shannons’ theory: (
C=Blog2(1+S/N) bps~ Blog2e (S/N) (for small S/N)
* development of circuits and devices

1.1Block diagram of a general Communication System.

* 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)
* Output transducer

* FIGURE 1.1 Block diagram of a communication system

1.2 Channel Characteristics.
Noise :
A. External noise (Atmospheric, man-made, extraterrestrial sources)
B. Internal noise
* A. External noise
* Atmospheric noise
* result from spurious radio waves
* referred to as static of spherical
* below 100 Mhz, field strength **1/f
* characterized in time domain as large Amp, short-duration-burst, i.e. impulse.
* 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
* 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
* Thermal noise : caused by random motion of free electrons in a conductor or semiconductor excited by thermal agitation

* Shot noise : caused by random arrival of discrete charge carriers in thermornic tubes or semiconductor junction devices.

* Flicker noise: unknown source and is more severe the lower the frequency.

Types of transmission channels :
* EM-wave propagation channels: FM radio, TV.
* Guided EM-wave channels: telephone line, coaxial cable.
* Optical Links

* EM-wave Propagation Channel
* 1864 Maxwell (Scottish) : Prediction of propagation of EM Wave
* 1886 Hertz ( German ) : Experimentally Provement

* Basic Idea: the coupling of EM energy into a propagation medium (free space or atmosphere) by means of a radiation element referred to as an antenna.
* Propagation modes dependent on
* (1) configuration of antenna
* (2) Propagation medium.
* (3) frequency of transmitted signal (Table 1.2)

* Ideal cases ( Well studied cases) :
* (1) ideal point propagation
* (2) Propagation of radio of a conducting line perpendicular to an infinite conduction plane.
e.g. propagation of commercial broadcast antenna
* Intermediate media transmission :
* Hard to model (Fig.1.2,3)

* 1.3 Summary of Systems-analysis Techniques
* Time & Frequency-Domain Analysis
* Modulation & communication Theories

* 1.4 Probabilistic approaches to System Optimization
* Statistical Signal Detection and Estimation Theory
* Information Theory and Coding

SUMMARY:
* Analog commu. system: messages are analog signals.
* Digital commu. system: messages are digital signals.
Digital vs. Analog
signal
time
amplitude
Analog X(t)
continuous
continuous
Discrete time X[n]
(sampled signals)
discrete
continuous
Digital X[n]
discrete
discrete