NOISE IN COMMUNICATION SYSTEMS

• Noise is any unwanted electrical signals that are always present in electrical systems. The presence of noise superimposed on a signal tends to obscure or mask the signals. It limits the receiver’s ability to demodulate a modulated carrier perfectly.

• Noise arises from a variety of sources, both man-made and natural.• Man-made noise includes such sources as spark-plug ignition noise,

switching transient, spark from electrical motor, etc.• Natural noise includes electrical circuit and component noise,

atmospheric and galactic noise, etc.• Good engineering design can eliminate much of the noise or its

undesirable effect through filtering, shielding, the choice of modulation, and the selection of an optimum receiver site.

• There is a natural source of noise that cannot be eliminated, called thermal or Johnson noise.

THERMAL NOISE (1)

• A resistive conductor contains free electrons that have some random motion if it has a temperature above absolute zero ( > 0°K ). This random motion causes a noise voltage to be generated at the terminals of the conductor.

• Power spectral density of thermal noise generated in the conductor is:

1

2)(

kTfhtn

e

fhfS T : absolute temperature

K : Boltzmann constantH : Planck constant

THERMAL NOISE (2)

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THERMAL NOISE (3)

Maximum noise power is transferred if RL = R

21)(

L

L

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222

21)()()(

22 oo

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Thermal noise power in bandwidth B :

BkTBdfP oo

B

B

on

2

THERMAL NOISE (4)

• Thermal noise is also called as white noise, because it has uniform power distribution over wide frequency range ( > 1012 Hz ).

• ηo is noise power spectral density for positive frequency, so that if there are positive and negative frequencies ( for mathematical purposes), then the noise power spectral density is ηo/2

WHITE NOISE (1)

• The primary spectral characteristics of thermal noise is that its power spectral density is the same for all frequencies of interest in most communication systems.

• A thermal noise source emanates an equal amount of noise power per unit bandwidth at all frequencies, from dc to about 1012 Hz.

• A simple model for thermal noise assumes that its power spectral density Sn(f) is flat for all frequencies, as shown in the figure below.

• The absence of a delta function in the power spectral density at the origin of the figure means that no dc power, that is, its mean or average value is zero.

WHITE NOISE (2)

• The factor 2 is included to indicate that Sn(f) is a two-sided power spectral density.

• The noise has such a uniform spectral density, is called white noise.• The autocorrrelation function of white noise is given by the inverse

Fourier transform of the noise power spectral density.

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WHITE NOISE (3)

• The autocorrelation function of white noise consists of a delta function weighted by the factor ηo/2 and occurring at τ = 0.

• Rn(τ) = 0 for τ ≠ 0 → Any two different samples of white noise, no matter how close together in time, are uncorrelated.

EXAMPLE #1

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2

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EXAMPLE #2

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NOISE EQUIVALENT BANDWIDTH (1)

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NOISE EQUIVALENT BANDWIDTH (2)

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0

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In the same way, the noise equivalent bandwidth of a BPF can be derived

Bn is called the noise equivalent bandwidth of the LPF, if Pn1 = Pn2

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21 )(

NOISE EQUIVALENT BANDWIDTH (3)

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NOISE FIGURE (1)

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portinput at the psd noise :)(

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NOISE FIGURE (2)

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NOISE TEMPERATURE (1)

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NOISE TEMPERATURE (2)

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SYSTEM NOISE TEMPERATURE (1)

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SYSTEM NOISE TEMPERATURE (2)

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ANTENNA NOISE TEMPERATURE

n

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Antenna noise temperature

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TRANSMISSION LINE NOISE TEMPERATURE

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