MSc Digital Communications

University of Baghdad-2011

Lecture -1

Introduction to Communications System Parameters

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Basic Digital Communications System

Transmitter Channel Receiver

______________________________________________________________________________________________

Information Source Source encoder Channel encoder Digital Modulation

________________________________________________________________

Transmitter

Channel

_____________________________________________________________________

Destination Source decoder Channel decoder Digital Demod

_____________________________________________________________________

Receiver

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Definitions

• Information: Binary (0, 1)

• Source encoder: remove any redundancy in

binary data (data compression)

• Channel encoder: add redundancy so it can be

used to detect / correct errors

• Digital modulator: map digital data to

analogue signal sent to destination by wire or wireless.

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Communication Channel

We can model the linear Gaussian channel in the form:

Input Output

+

Random Variables of Gaussian distribution

Equivalent Gaussian Communication Channel

Output = Input + Noise

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Digital transmission

Noise corrupt data:

_________ _________ • _______________ t _______________ t

• Digital demodulator: convert received analogue signal into digits

• Channel decoder: use added redundancy to estimate original data with possibly some errors

• Source decoder: Decode the compression at transmitter.

Communication systems always have certain, fast processors, filters, data storage.

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Performance of digital

communication system

System performance is dependable on the following system parameters:

Speed of information transmission given by Shannon channel capacity which is limited by the available spectrum (bits/sec/Hz)

Available transmission power which is expressed by energy / bit ( ) dB

Data transmission codes used

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bE

Shannon channel capacity

Shannon theorem specifies the limit on therate of error free transmission for a power-limited, band-limited channel in additivewhite Gaussian noise.

channel capacity expressed in terms ofthree key parameters: channel bandwidthB, average received power S and noisepower spectral density at the channeloutput.

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0N

Shannon channel capacity

Shannon limit for channel capacity C is

C = B ( 1 + ) bits / sec

C = 3.32 B ( 1 + ) bits / sec

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2log

10log

BN

S

0

BN

S

0

Shannon channel capacity

Show that

Where are energy/bit, data rate, and channel capacity.

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B

RN

E

b

BbR

b 2

0

CRE bb ,,

Electric field

Space that surrounds an electric charge.

An electric field or E-field exerts a force on

charged objects.

The concept of electric field was introduced

by Michael Faraday

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Electric field forces

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Magnetic field

Magnetic fields or H-field exists when there

is a changing electric field or an electric

current or permanent magnet.

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Magnetic field forces

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Radio waves

• Radio is the wireless transmission of

signals by electromagnetic (EM) waves

with frequencies below those of light.

• An EM has H & E field orthogonal to each

• A wave is a disturbance that propagates

through space and transfer energy from

one point to another.

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EM waves

Electromagnetic waves travel in space with speed of light. It has electric and magnetic components and can travel in vacuum (without media).

The electric and magnetic components of EM wave oscillate at right angle to each other and to the direction of energy propagation and are in phase with each other.

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EM Radiation

EM radiation occurs in radio waves,

microwaves, infrared radiation, visible

light, ultraviolet radiation, X-rays, and

gamma rays.

• Wave length ( ) is the distance between

successive crests (or troughs) of a wave

pattern.

•

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.f

lightofspeedmx sec/103 8

Data speed

Data rate is the speed in bits per second (bits/sec) at which information is being communicated.

• Bandwidth

• Spectrum occupied by the transmission in Hz. Bandwidth and data rate are related by system efficiency

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durationpulseR

1ateData

HzbitsB

Rb sec//

Noise and errors

• Noise is the unwanted signal accompanies the useful signal. Noise is mainly due to thermal noise but it may include interference from other transmitters.

• Error rate is the rate at which transmission error occur. When an error occurs binary ‘1’ is received as ‘0’ and binary ‘0’ is received as binary ‘1’.

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Nyquist Bandwidth

Nyquist bandwidth is the minimum

bandwidth at which data can be

transmitted and recovered correctly. In

ideal case (no noise or interference) it is

equal

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DECIBEL: A tenth of a Bel

The Bel is mostly used in acoustics invented by Bell Telephone Lab (founder Alexander Graham Bell) in 1924 to quantify the reduction in audio level over a 1 mile (1.6 km) length of standard telephone cable.

• The Bel was too large for everyday use, so the decibel (dB), equal to 0.1 Bel (B), became more commonly used.

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Power in dB

• A decibel is a relation between two values

of POWER levels.

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Definitions of dBs

• dBm power A in dB when power of

reference transmitter B = 1 mW.

• Example 1 watt in dB = 30 dBm

• dBi is gain of antenna when its radiation is

equal in all directions (omni directional

antenna). Omni directional antenna = 10

dBi when it has gain of 10 dB in all

directions

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Signal to noise ratio (SNR)

• Signal to noise ratio is the power ratio

between the information signal power and

the background noise power where signal

and noise are measured within the same

bandwidth.

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SNR in dB

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Multimedia

• Multimedia is media that uses multiple forms

of information content and processing (e.g.

text, audio, graphics, animation, video and

interactive applications such as gaming) to

inform or entertain the user or audience.

• Multimedia Systems then support the

interactive use of text, audio, still images,

video, and graphics.

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Thermal noise floor

Thermal noise floor at room temperature T is

kTB

k=Boltzmann’s Constant= J/K

T=Temperature = 300 K

kT= =-174 dBm

kTB= -174 + dB

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231038.1 x

wattsx 211014.4

)(log10 10 HzinB

Radio Link Design

Worked example• A radio system operating at centre frequency 850 MHz is detected by a

sensitive receiver located at 550 metres away and experiencing clear line of sight. You may assume the following system parameters:

• Transmitter power 15 dBm

• Transmitter antenna gain 2 dBi

• Receiver antenna gain 3.2 dBi

• Receiver filter noise bandwidth (B) 120 Hz

• Thermal noise floor -174+10 dB

• Receiver noise figure 3 dB

• Receiver threshold 5dB

• Calculate the signal margin in dB. You may assume the free line of sight loss in dB is 20 .

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