Noise in Communication Systems

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CHAPTER 5

• Noise in Communication Systems

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Noise in Communication Systems

• Outline
• Introduction
• Thermal Noise
• Shot Noise
• Signal - to Noise Ratio
• Noise Factor Noise Figure
• Noise Temperature
• Cascaded Networks

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By the end of this chapter you should be 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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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

• 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.

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Sources of noise
Noise
Internal Noise
External Noise

• Man-made noise and natural resources
• External noise comes from sources over which we
  have little or no control
• Industrial sources such as motors, generators,
  and manufactured equipment
• Atmospheric sources / static electricity such as
  speaker when there is no signal present

• 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 noise/Johnson noise
• Shot noise

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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,
• the resistance of the circuit.

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Noise Effect

• The effects of noise are as follow
• 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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Types of Noise

• The are several types of noise, among them are
• 1. Thermal Noise/White Noise
• 3. Shot Noise
• 4. Noise Temperature
• 5. Quantization Noise

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Thermal Noise (Johnson Noise /white noise)
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Thermal Noise (Johnson Noise /white noise)
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).
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Thermal Noise (Johnson Noise /white noise)
Movement of the electrons will forms kinetic
energy in the conductor related to the
temperature of the conductor.
When the temperature increases the movement of
free electrons will increases and causes current
flows through the conductor.
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Thermal Noise (Johnson Noise) (Contd)
Thermal noise is often referred to as white
noise because it has a uniform spectral density
across the EM frequency spectrum. (analogous to
the colour white which consists of all the colour
spectrum)
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Thermal Noise (Johnson Noise) (Contd)

• Experimental results (by Johnson) and theoretical
  studies (by Nyquist) give the mean square noise
  voltage as

Where k Boltzmanns constant 1.38 x 10-23
Joules per K T absolute temperature (Kelvin)
B bandwidth noise measured in (Hz) R
resistance (ohms)
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Thermal Noise (Johnson Noise) (Contd)
In 1928, J. B. Johnson have proven that noise
power generated is proportional to the
temperature and the BW.

• In dB, it is defined as
• PdBm 10log(KTB/0.001)

Noise power can be modeled using voltage
equivalent circuit (Thevenin equivalent circuit)
or current equivalent circuit (Norton equivalent
circuit)
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Analysis of Noise In Communication Systems
Thermal noise may be represented by an equivalent
circuit as shown below
(mean square value , power) then VRMS
i.e. Vn is the RMS noise voltage.
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Resistors in Series
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Resistors in Series
Assume that R1 at temperature T1 and R2 at
temperature T2, then
The resistor in series at same temperature behave
as a single resistor
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Analysis of Noise In Communication Systems
Resistance in Parallel
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Analysis of Noise In Communication Systems
Resistance in Parallel
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Example 1
Given a 50 kO resistor at a temperature of
290 K, 3 kHz bandwidth. Find Vrms value of noise
NOTE Temperature unit conversion
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Example 2

• 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.

Remember to convert the temperature to Kelvin
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EXAMPLE 3

• A receiver has a noise power bandwidth of 10 kHz.
  A resistor that matches the receiver input
  impedance is connected across its antenna
  terminals. Determine the Noise Power if the
  resistor has temperature of 27 oC.

SOLUTION 1. Use Noise Power formula 2. P
(1.38 x 10-23 J/K)(273o 27oK)(10000 Hz)
4.14 x 10-17 W. 3. in dB, P(dB)
10log(4.14 x 10-17 W) / 0.001 -133.8
P KTB
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Shot Noise

• Shot noise is a type of electronic noise that
  occurs when there are 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)

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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 of filtering, 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

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Signal to Noise Ratio
The signal to noise ratio is given by
The signal to noise in dB is expressed by
for S and N measured in mW.
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Signal to Noise Ratio

• 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

• Solution
• To express in dB

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Signal to Noise Ratio

• Signal to noise power ratio can be expressed in
  terms of voltages and resistances.

If the input and output resistances of the
amplifier, receiver or network being evaluated
are equal

• Where
• Rin input resistance (ohms)
• Rout output resistance (ohms)
• Vs signal voltage (volts)
• Vn noise voltage (volts)

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Signal to Noise Ratio

• 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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Noise Factor- Noise Figure
Consider the network shown below,
Noise factor, F
lower the value of F, the better the network.

• F equals to 1 for noiseless and in general F gt
  1.

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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)
If every variable is a dB Noise figure
NF SNRin - SNRout
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Noise Factor- Noise Figure (Contd)

• Example
• The signal to noise ratio at the input to a
  communication receiver is 40 dB. If the receiver
  has a noise figure of 12 dB, calculate the output
  signal to noise ratio

• Solution

NF SNRin - SNRout
SNRout SNRin - NF 40 -12
28 dB
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Noise Temperature
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
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)
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Cascaded Network
A receiver systems usually consists of a number
of passive or active elements connected in
series. A typical receiver block diagram is shown
below, with example
In order to determine the (S/N) at the input, the
overall receiver noise figure or noise
temperature must be determined. In order to do
this all the noise must be referred to the same
point in the receiver, for example to A, the
feeder input or B, the input to the first
amplifier.
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Cascaded Network
Total noise factor is the accumulation of the
individual noise factors. Friiss formula is used
to calculate the total noise factor of several
cascade amplifiers.
F1
F3
F2
Where Fn Noise factor (dB) Gn Power gain ,
amplifier n
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System Noise Figure
Assume that a system comprises the elements shown
below,
Assume that these are now cascaded and connected
to an aerial at the input, with
from the aerial.

Now ,
Since
similarly
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System Noise Figure (Contd)
The overall system Noise Factor is

The equation is called FRIIS Formula.
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System Noise Temperature
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Attenuator, Transmission Loss

• All transmission medium will attenuate power and
  caused power loss gt Pout lt Pin.
• Power loss or power attenuation is given by

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Transmission Loss
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Summary

• Thermal Noise
• Signal - to Noise
• Noise Factor
• Noise Figure

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

• Noise Temperature
• Cascaded Networks

Te T(F-1)
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Example 1.1 Calculate signal power if its value
in dBm is 0 dBm.
dBm 10 log P2 / P1 10 log P2 / 1 mW 0 P2
1 mW
Example 1.2 Calculate signal power in dB if its
value is 1 mW.
dB 10 log P2 / P1 10 log P2 / 1 W 10 log 1
mW / 1 W - 30 dB
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Example

• For three cascaded amplifier stages, each with
  noise factor of 2 dB and power gains of 10 d,
  determine the total noise figure.

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Example Cascade Three amplifiers, ABC was
connected in series. Noise figure and power gain
of the amplifiers are given below Amplifier A
GA 20 dB FA 3 dB Amplifier B GB 10 dB FB
5 dB Amplifier C GC 5 dB FC 10 dB An
input signal of 50 dB higher than noise level was
fed at the input of the network.
Calculate (a) Total noise factor (b) SNR at the
output
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Solution
Amplifier A GA 20 dB FA 3 dB Amplifier B
GB 10 dB FB 5 dB Amplifier C GC 5 dB FC
10 dB
(a) Angka hingar 10 log10 2.03 3.05 dB
(b) Di beri, SNRmasukan 50 dB
FdB SNR masukan (dB) SNR keluaran
(dB) SNR keluaran 50 dB 3.05 dB 46.95
dB