Noise in Communication SystemsNoise in Communication SystemsChapter # 2Chapter # 2Lecture # 3Lecture # 3

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Noise in Communication SystemsNoise in Communication Systems

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Outline :

IntroductionThermal NoiseShot NoiseSignal - to – NoiseNoise Factor – Noise FigureNoise Temperature BER

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

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Learning OutcomesLearning Outcomes

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IntroductionIntroduction

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.

IntroductionIntroduction

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Noise is a general term which is used to describe an “unwanted signal which affects a wanted signal.”

Noise is a random signal that exists in a communication system.

Random signal cannot be represented with a simple equation.

Sources of noiseSources of noise

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Noise

Internal Noise External Noise

Due to random movement of electrons in electronic circuit.

Electronic components in a receiver such as resistors, diodes, and transistors are major sources of internal noise

• Thermal (agitation) noise• Shot noise•Transit time noise

Man-made noise and natural resources

External noise comes from sources over which we have little or no control

•Industrial sourcesmotors, generators, manufactured equipment

•Atmospheric sources / static electricity

speaker when there is no signal present

Introduction (Cont’d)Introduction (Cont’d)

• 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 EffectNoise Effect

• Degrade system performance for both analog and digital systems.

• 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. Atmospheric2. Extraterrestrial (Cosmic & Solar)3. Thermal Noise4. White Noise 5. Shot Noise6. 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)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 will increases and the current flows through the conductor.

Movement of the electrons will forms kinetic energy in

the conductor related to the temperature of the

conductor.

Thermal Noise (Johnson Noise) (Cont’d)Thermal Noise (Johnson Noise) (Cont’d)

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

the mean square noise voltage as

)(4 22_

voltTBRkV

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

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

Thermal Noise (Johnson Noise)Thermal Noise (Johnson Noise)

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For example :

50 kΩ resistor at a temperature of 290 K, 3 kHz bandwidth. Find Vrms value of noise:

from Kelvin to Kelvin

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

Fahrenheit [°F] = [K] × 9⁄5 − 459.67 [K] = ([°F] + 459.67) × 5⁄9

Vn = √ 4 x 1.38 x 10-23 x 290 x 3000 x 50 x 103

= 49 nV

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Example 1.4

One operational amplifier with a frequency range of (18-20) MHz has input resistance 10 k. Calculate noise voltage at the input if the amplifier 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

Analysis of Noise In Communication SystemsAnalysis of Noise In Communication Systems

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Thermal Noise (Johnson noise)Thermal Noise (Johnson noise)

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

)(4 2____

2 voltTBRkV

____2V nVkTBR 2

(mean square value , power)then VRMS =

i.e. Vn is the RMS noise voltage.

Analysis of Noise In Communication Systems (Cont’d)Analysis of Noise In Communication Systems (Cont’d)

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22

___2

1

_______2

nnn 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 SeriesResistors in Series

Shot NoiseShot Noise

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•Shot noise is a type of electronic 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 random fluctuations 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)

Flicker Noise

• Also known as low frequency noise• Also known as pink noise• Also known as 1/f noise• It occurs due to fluctuations in current desnity

of the carrier currents• RMS value is proportional to the square of the

current

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Transit Time Noise

• Due to time taken by the carriers to cross the junction

• Also known as high frequency noise

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How to determine noise level in communication How to determine noise level in communication system?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 of filtering, demodulation and amplification.

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Noise CalculationNoise Calculation

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

• Noise Figure, F

• Noise factor, NF

dBN

SSNR log10

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dBNS

NS

FNF

oo

iilog10

log10

oo

ii

NS

NSF dB

Signal to NoiseSignal to Noise

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)(,

)(,

wattsPnPowerNoise

wattsPsPowerSignal

N

S

The signal to noise ratio is given by

The signal to noise in dB is expressed by

dBmdBmdB NSN

S

for S and N measured in mW.

dBN

S

N

S10log10

• 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 NoiseSignal to Noise

• Solution :

To express in dB;

100001.0

10

Pn

Ps

N

S

dBN

S

N

SdB 30

01.0

10log10log10 10

• Example :For an amplifier with an output signal voltage of 4V, an output noise voltage of 0.005 V, and an input and output resistance of 50 ohm, determine the signal to noise power ratio.

Solution :

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Signal to NoiseSignal to Noise

dBV

V

N

S

n

sdB 06.58

005.0

4log20log20 10

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NoiseNoise Factor- Noise Figure (Cont’d)

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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NoiseNoise Factor- Noise Figure (Cont’d)

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;

NoiseNoise 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 temperature of the amplifier, but rather a theoretical construct that is an equivalent temperature that produces that amount of noise power

Transmission LossTransmission Loss

Transmission Medium Frequency Loss dB/km

Kabel Terpiuh (Twisted-pair Cable)

10kHz

100kHz

300kHz

2

3

6

Kabel Sepaksi (Coaxial Cable)

100kHz

1MHz

3MHz

1

2

4

Pandu Gelombang Empat Segi (Rectangular Waveguide)

10GHz 5

Kabel Fiber Optik (Fiber Optic 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.

BER = The number of erroneous bits received total number of bits transmitted

BER = The number of erroneous bits received total number of bits transmitted

Noise - Bit EnergyThe signal also measured in terms of the bit energy in

joules (J), Eb.

The enery per bit is simply the energy of a single bit of information, Eb .

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)Eb = CTb (J/bit)

Summary

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• Thermal Noise

• Signal - to – Noise

• 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)