advance communication system lectures part 5

8/2/2019 Advance Communication System Lectures Part 5

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Noise in CommunicationSystems

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Noise in Communication Systems

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Outline :

Introduction

Thermal NoiseShot Noise

Signal - toNoise

Noise FactorNoise Figure Noise Temperature

BER


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Student Able to :

Define noise and describe the prominent

sources of electrical noise

Explain and calculate the most common types

of noise in communication system

3

Learning Outcomes


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4

Introduction

Noise is the static you hear in the speaker when you tune any AM or FM

receiver to any position between stations. It is also the snow or confetti that

is visible on a TV screen.


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Introduction

5

Noise is a general term which is used to describe an unwantedsignal which affects a wanted signal.

Noise is a random signal that exists in a communicationsystem.

Random signal cannot be represented with a simpleequation.


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Sources of noise

6

Noise

Internal Noise External Noise

Due to random movement ofelectrons in electronic circuit.

Electronic components in areceiver such as resistors,

diodes, and transistors aremajor sources of internal noise

Thermal (agitation) noise Shot noise

Transit time noise

Man-made noise and naturalresources

External noise comes fromsources over which we have littleor no control

Industrial sourcesmotors, generators,manufactured equipment

Atmospheric sources / staticelectricity

speaker when there is nosignal present


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Introduction (Contd)

The noise level in a system is proportional to

temperature and bandwidth, the amount of

current flowing in a component, the gain of the

circuit, and the resistance of the circuit.

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Noise Effect

Degrade system performance for both analog and digitalsystems.

The receiver cannot understand the original signal.

The receiver cannot function as it should be.

Reduce the efficiency of communication system.

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Noise - Type of Noise

The are several types of noise, among them are:

1. Atmospheric

2. Extraterrestrial (Cosmic & Solar)

3. Thermal Noise

4. White Noise

5. Shot Noise

6. Quantization Noise

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Atmospheric Noise (Static)

Results due to spurious radio waves inducing

voltages at antenna creating spurious

waveforms

Reasons

Weather conditions (moisture, lightening and thunder)

Dominant upto 30 MHz

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Extraterrestrial

Solar

Due to radiation from sun

Cosmic

Due to radiations from other heavenly bodies

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Industrial

Created by man due to several reasons

Line passing near by a transformer

Interference by other coexisting equipment

(TV remotes and IR equipments)

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Thermal Noise (Johnson Noise /white noise)

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Thermal noise is the result of the random motion of charged particles

(usually electrons) in a conducting medium such as a resistor.

This type of noise is generated by all resistances (e.g. a resistor,

semiconductor, the resistance of a resonant circuit, i.e. the real part of the

impedance, cable etc).

When the temperature increases the movement of free electrons willincreases and the current flows through the conductor.

Movement of the electronswill forms kinetic energy in

the conductor related to the

temperature of the

conductor.


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Thermal Noise (Johnson Noise) (Contd)

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Experimental results (by Johnson) and theoretical studies (by Nyquist) give

the mean square noisevoltage as

)(4 2

2_

voltTBRkV

Where k = Boltzmanns constant = 1.38 x 10-23 Joules per KT = absolute temperature (Kelvin)

B = bandwidth noise measured in (Hz)R = resistance (ohms)


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Thermal Noise (Johnson Noise)

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For example :

50 k resistor at a temperature of 290 K, 3 kHz bandwidth. Find Vrmsvalue of noise:

from Kelvin to Kelvin

Celsius [C] = [K] 273.15 [K] = [C] + 273.15

Fahrenheit [F] = [K] 95 459.67 [K] = ([F] + 459.67) 59

Vn = 4 x 1.38 x 10-23 x 290 x 3000 x 50

= 49 nV
http://en.wikipedia.org/wiki/Celsiushttp://en.wikipedia.org/wiki/Fahrenheithttp://en.wikipedia.org/wiki/Fahrenheithttp://en.wikipedia.org/wiki/Celsius


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Example 1.4

One operational amplifier with a frequency range of (18-20) MHz hasinput resistance 10 k. Calculate noise voltage at the input if theamplifier operate at ambient temperature of 270C.

Vn2 = 4KTBR

= 4 x 1.38 x 10-23 x (273+ 27) x 2 x 106 x 104

Vn = 18 volt


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Analysis of Noise In Communication Systems

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Thermal Noise (Johnson noise)

This thermal noise may be represented by an equivalent circuit as shown below

)(4 2____

2voltTBRkV

____

2V nVkTBR 2

(mean square value , power)

then VRMS =

i.e. Vn is the RMS noise voltage.


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Analysis of Noise In Communication Systems (Contd)

18

22

___

21

_______

2nnn VVV

11

____2

1 4 RBTkVn

22

____2

2 4 RBTkVn

)(4 2211

____

2 RTRTBkVn

)(4 21

____2

RRBkTVn

Assume that R1 at

temperature T1 and R2 at

temperature T2, then

i.e. The resistor in series at same temperature behave as a

single resistor

Resistors in Series


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Shot Noise

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Shot noise is a type ofelectronic noise that occurs when the finite

number of particles that carry energy, such as electrons in an electronic

circuit or photons in an optical device

Shot noise was originally used to describe noise due to randomfluctuations in electron emission from cathodes in vacuum tubes

(called shot noise by analogy with lead shot).

Shot noise also occurs in semiconductors due to the release of charge

carriers.

Shot noise is found to have a uniform spectral density as for thermal

noise (White noise)


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How to determine noise level in communication

system?

Noise effect can be determined by measuring:

- Signal to Noise Ratio, SNR for analog system

- Noise Factor, F

- Noise Temperature, Te .

- probability of error or bit error rate, BER for digital system

To determine the quality of received signal at the receiver or an antenna,SNRi is used.

SNR o is always less than SNRi , due to the facts that the existence of

noise in the receiver itself. In the receiver usually constitute a process offiltering, demodulation and amplification.

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Noise Calculation

SNR is a ratio of signal power,S to noise power,N.

Noise Figure,F

Noise factor,NF

dBN

SSNR log10

21

dBNS

NS

FNF

oo

iilog10

log10

oo

ii

NS

NSF dB


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Signal to Noise

22

)(,

)(,

wattsPnPowerNoise

wattsPsPowerSignal

N

S

The signal to noise ratio is given by

The signal to noise in dB is expressed by

dBmdBmdBNS

N

S

for S and N measured in mW.

dBN

S

N

S10log10


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Example :For an amplifier with an output signal power of 10 W and an

output noise power of 0.01 w, determine the signal to noise

power ratio

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Signal to Noise

Solution :

To express in dB;

100001.0

10

Pn

Ps

N

S

dBN

S

N

SdB 30

01.0

10log10log10 10


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Example :

For an amplifier with an output signal voltage of 4V, an output noisevoltage of 0.005 V, and an input and output resistance of 50 ohm, determinethe signal to noise power ratio.

Solution :

24

Signal to Noise

dBV

V

N

S

n

sdB 06.58005.0

4

log20log20 10


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Noise Factor-Noise Figure (Contd)

Noise factor, F =

OUT

IN

NS

NS

F equals to 1 for noiseless and in general F > 1.

lower the value of F, the better the network.

Consider the network shown below,


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Noise Factor-Noise Figure (Contd)

Noise figure (NF) is the Noise factor converted to dB

Noise Figure (NF) dB = 10 log10 (F)

NF = SNRin SNRout

If every variable is a dB Noise figure;


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Noise Temperature

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Noise temperature (Te) is expressed as :

Where;

Te = equivalent noise temperature (Kelvin)

T = environmental temperature (reference value of 290 K)

F = Noise factor

Te = T(F-1)

Equivalent noise temperature Te is not the physical temperatureof the amplifier, but rather a theoretical construct that is an

equivalenttemperature that produces that amount of noise power


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Transmission Loss

Transmission Medium Frequency Loss dB/km

Kabel Terpiuh (Twisted-

pair Cable)10kHz

100kHz

300kHz

2

3

6

Kabel Sepaksi (CoaxialCable)

100kHz1MHz

3MHz

12

4

Pandu GelombangEmpat Segi (Rectangular

Waveguide)

10GHz 5

Kabel Fiber Optik (FiberOptic Cable)

3.6 x 1014Hz

2.4 x 1014Hz

1.8 x 1014Hz

2.5

0.5

0.2

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What is Error Rate?

The error rate is the degree of errors in the

transmission of data due to bad hardware or

noisy links. The higher the error rate the less

reliable the connection or data transfer will

be.

It occurred in digital communication.


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BER = The number of erroneous bits received

total number of bits transmitted


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Noise - Bit Energy

The signal also measured in terms of the bit energy injoules (J), Eb.

The enery per bit is simply the energy of a single bitof information, E

b

.

It is defined as below:

Eb = energy of a single bit (joules per bit)Tb = time of a single bit (seconds)C = carrier power (watts)

Eb = CTb (J/bit)


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Summary

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Thermal Noise

Signal - toNoise

Noise Factor

Noise Figure

Noise Temperature

)(4 22_

voltTBRkV

)(,

)(,

wattsPnPowerNoise

wattsPsPowerSignal

N

S

OUT

IN

NS

NS

Noise Figure (NF) dB = 10 log10 (F)

Te = T(F-1)

advance communication system lectures part 5

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