1. Noise in Communication SystemNoise in Communication System

2. 2
IntroductionIntroduction
“How do you want to send data/information to
someone who is far from you?”
“If the information that you want to send is your
voice, how to make sure that what your words
are understood by your friend?”
“What is the source and technology available
surround you that can help?”

3. 3
NoiseNoise
• Practically, we cannot avoid the existence of
unwanted signal together with the modulated signal
transmitted by the transmitter.
• This unwanted signal is called noise.
• Noise is a random signal that exists in a
communication system.
• Random signal cannot be represented with a simple
equation.
• The existence of noise will degrade the level of
quality of the received signal at the receiver.

4. 4
Types of noiseTypes of noise
Noise
Internal Noise External Noise
Due to random movement of
electrons in electronic circuit
• Thermal noise/Johnson noise
• Shot noise
Man-made noise and
natural resources
• Lightning
• Solar noise
• Ignition
• Crosstalk
any phenomenon by which a signal transmitted on one frequency
creates an undesired effect on another frequency.
an undesired coupling
(energy transfer) from
one circuit or medium to another

5. 5
Noise EffectNoise Effect
• Degrade system performance for both analog and
digital systems.
• The receiver cannot understand the sender.
• The receiver cannot function as it should be.
• Reduce the efficiency of communication system.

6. 6
Thermal NoiseThermal Noise
• Johnson–Nyquist noise (thermal noise, Johnson noise, or
Nyquist noise) is the Electronic noise - generated by the
thermal agitation of the charge carriers (the electrons) inside
an electrical conductor in equilibrium, which happens
regardless of any applied voltage.
• 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
increase and the current flows through the conductor.
• Current flows due to the free electrons will create noise voltage, n(t).
• Noise voltage, n(t) is influenced by the temperature and therefore it is
called thermal noise.
• Also known as Johnson noise or white noise.

7. 7
kTBP
TBP
n
n
=
∝
Watt
where
Pn = noise power (Watt)
k = Boltzmann’s constant (1.38 x 10-23
J/K)
T = Temperature (K)
B = BW spectrum system (Hz)
In 1928, J. B. Johnson have proven that noise power generated is
proportional to the temperature and the BW.
Noise power can be modeled using voltage equivalent circuit (Thevenin
equivalent circuit) or current equivalent circuit (Norton equivalent circuit)
This type of noise was first measured by John B. Johnson at Bell Labs in 1928. He described his
findings to Harry Nyquist, also at Bell Labs, who was able to explain the results.

8. 8
• Noise source will be connected to a system with the input
resistance RL.
• Therefore, total noise power is Pn.
• With the concept of maximum power transfer ie when Rn = RL , all
the power will be transferred to the load.
• Also called as impedance matching.
Rn, Noise
source
Vn, Noise
voltage source
Rn, noise
free
=
(b) Thevenin equivalent circuit(a) Noise source circuit
It can be modeled by a voltage source representing the noise of the non-ideal
resistor in series with an ideal noise free resistor.
revise on circuit theory

9. 9
RL, system input
resistance
Vn, Noise
voltage source
Rn, Noise
free
(c) Thevenin equivalent circuit with the load
VL
RRR Ln ==
R
V
R
V
R
V
P n
n
L
L
4
2 2
2
2
=






==
kTBPP Ln ==2
n
n
Ln
L
L
V
V
RR
R
V
=
+
=
Note: Vn = Vrms
kTBRV
kTBRV
kTB
R
V
n
n
n
4
4
4
2
2
=
=
=
Given
Voltage across RL :
=>
and
=>
voltage divider rule

10. 10
Example 1
One operational amplifier with a frequency range of (18-20) MHz has
input resistance 10 kΩ. Calculate noise voltage at the input if the
amplifier operate at ambient temperature of 270
C.
Vn
2
= 4KTBR
= 4 x 1.38 x 10-23
x (273+ 27) x 2 x 106
x 104
Vn
= 18 µvolt
BW = fh – fl = (20-18) MHz
= 2 MHz

11. 11
How to determine noise level inHow to determine noise level in
communication system?communication system?
• Noise effect can be determined by measuring:
- Signal to Noise Ratio, SNR for analog system
- probability of error or bit error rate, BER for digital
system
• To determine the quality of received signal at the
receiver or an antenna, SNRiis used.
• SNR o is always less than SNRi, due to the facts that
the existence of noise in the receiver itself.
• Another parameters that can be used is Noise
Factor, F and Noise Temperature, Te.
output=receiver

12. 12
Noise CalculationNoise Calculation
• SNR is a ratio of signal power, S to noise power, N.
• Noise Figure, F
• Noise factor, NF
dB
N
S
SNR log10=
dB
NS
NS
FNF
oo
ii
log10
log10
=
=
oo
ii
NS
NS
F = dB

13. 13
Noise calculation in AmplifierNoise calculation in Amplifier
• To simplify the analysis, two types of noise model are used.
• - Amplifier with noise
• - Amplifier without noise
aio NGNN +=
G
Na
Ni No
G
Ni
No
Nai
(a) Amplifier with noise (b) Amplifier without noise
( )aiio NNGN +=
where
G
N
N a
ai = BkTNP iin ==and
input noise, Ni
artificial noise source, Nai
Gain

14. 14
Analysis Amplifier with NoiseAnalysis Amplifier with Noise
io GSS =
( )aii
a
i
aio
NNG
G
N
NG
NGNN
+=






+=
+=
G
Na
Model Amplifier with noise
Ni
So
No
Si
(1)
( )
i
ai
i
aii
aii
i
i
i
o
i
N
N
N
NN
NNG
GS
N
S
SNR
SNR
+=
+
=
+
=
1
i
ai
N
N
F +=1
io SNRSNR <<
(2)
( ) ie TFT 1−=
i
e
T
T
F +=1
Noise Figure: Noise Temperature:
BkT
BkT
F
i
e
+=1
We have:
BkTN ii = BkTN eai =and
=>
(3)
Noise Figure, F

15. 15
Analysis Amplifier Without NoiseAnalysis Amplifier Without Noise
G
Si So
NoNi+Nai
Model Amplifier without noise
( )aiio
io
NNGN
GSS
+=
=
( )
i
ai
i
aii
aii
i
i
i
o
i
N
N
N
NN
NNG
GS
N
S
SNR
SNR
+=
+
=
+
=
1
i
ai
N
N
F +=1
io SNRSNR <<
(2)
( ) ie TFT 1−=
i
e
T
T
F +=1
Noise Figure: Noise Temperature:
BkT
BkT
F
i
e
+=1
We have:
BkTN ii = BkTN eai =and
=>
(3)
(1)

16. 16
Example 2
Noise generated in amplifier of 5 MHz bandwidth is represented by
amplifier input noise power of 0.082 pW. Calculate noise factor and
noise figure if the amplifier was fed with the
(a) source input signal match the temperature of 300 K
(b) source input signal match the temperature of 100 K
No
Ni
Ne = 0.082PW
(a) Noise power from the source input = KTiB
= 1.38 x 10-23
x 300 x 5 x 106
= 0.021 pW
9.4
021.0
103.0
021.0
082.0021.0
FigureNoise ==
+
=
+
=
Ni
NeNi
Noise Factor = 10log10
4.9 = 6.9 dB
Ni

17. 17
(b) Noise power from the source input = KTiB
= 1.38 x 10-23
x 100 x 5 x 106
= 0.007 pW
7.12
007.0
103.0
007.0
082.0007.0
ureFaktor Fig ==
+
=
+
=
Ni
NeNi
Noise Factor = 10log10
12.7 = 11.04 dB
Noise factor and noise figure were less when operated at room
temperature.
No
Ni
Ne = 0.082PW

18. 18
Example 3
An antenna is connected to an amplifier with noise temperature, Te
= 125 o
K,
gain, G = 108
. Given the bandwidth, B = 10 MHz and output receiver noise, No
= 10 µW. Determine the antenna temperature, Ti and noise figure, F of the
receiver.
( )
( )
( )
( )
KT
T
GTTKB
GBKTBKT
GNNN
i
i
ei
ei
eio
o
8623
600
1012510101038.110
=∴
+×××=
+=
+=
+=
−
µ
2.1
600
125
11 =+=+=
i
e
T
T
F or 2.1
8.82
100
==
+
=
i
ei
N
NN
F
Thanks to Samura
(600+125)/600