EEE 309 Communication Systems I
Semester: Jan 2022
Suzit Hasan Nayem
Lecturer
Department of EEE, BUET
Email: suzit@eee.buet.ac.bd
Office: ECE 915(C), ECE Building
Part 02:
Noise in Communication
Systems
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Noise
Noise:
❑ Any unwanted signal, whether audible or not
❑ Noise gets added to the signal and degrades the quality of signal
Noise and Interference:
Although they play a somewhat similar role in electrical systems, they are dissimilar in
nature in one important aspect
Noise is usually composed of randomly occurring voltages, which are unrelated in
phase or frequency and may sometimes be of a very peaky nature
Interference, on the other hand, is usually more structured than noise since it arises
as unwanted coupling from just a few signals (e.g., from other users) in the network
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Source of Noise
Artificial or man-made sources:
❑ Commutator motors
❑ Spark plugs of vehicles
❑ Faulty switches
❑ Fluorescent lights
❑ Electric shavers
❑ Power lines
Natural sources:
❑ Cosmic (galactic) radiation
❑ Atmospheric (e.g., lightning
discharge, rain attenuation)
❑ Intrinsic circuit noise
❑ etc.
Solid lines:
Man-made noise
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White Noise
❑Both thermal and shot noise are characterized as white noise
❑White noise means it contains noise of all frequency with a flat PSD
❑This is approximately what you hear in empty AM radio channels , you see in empty TV
channel
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Types of Noise
1. Thermal Noise (Johnson noise):
❑It is produced by the random motion of ‘free’ electrons in a conductor
❑ Any substance with temperature above zero Kelvin (absolute zero) contains some
electrons that are free to move about in that substance. The amount of energy
contained by these electrons increases as the temperature increases, and an
increase in energy means an increase in the average speeds of the free electrons.
However, moving electrons constitute an electric current as electrons randomly
collide with lattice atoms. Since the currents increase with temperature, the noise
power likewise increases with temperature.
❑ Properties:
1. Thermal noise is present in any conductor
2. The only predictable property of thermal noise is its average power
3. In the case of thermal noise, the power is spread uniformly up to very high
frequency (about a 10% drop at 2 THz)
Thermal Noise characteristic is white
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Thermal Noise
The mean-square thermal noise voltage at the terminal of an open-circuit
resistor of value Rs is:
where, (Vn)2 = Mean-square value of thermal voltage
K = Boltzmann’s constant = 1.38 x 10 –23 J/K
T = Absolute temperature of the resistor in Kelvin
B = Equivalent noise bandwidth (system bandwidth) in Hz
Rs = Resistance of the conductor
Thermal noise power (Maximum):
Vn
Rs
Unit: dBm
T = 300 K
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Types of Noise
2. Shot Noise:
❑ Generated in active electronic components due to discrete and random
emission of electrons
❑ Shot Noise normally occurs when there is a potential barrier (voltage
differential). PN junction diode is an example that has potential barrier.
When the electrons and holes cross the barrier, shot noise is produced.
❑ In simple terms, shot noise is caused by random fluctuations and random
emission of electrons, particularly In transistors. In effect, it means that in a
communication system, in addition to the main signal, there is a background
amount of random emissions, which create « noise ».
❑ In a high power system, this noise is so small in comparison with the main signal,
so it can be mostly ignored.
❑ In a very low power system, shot noise can be significant. Many modern systems
try to use as little power as possible
❑ Unlike the Thermal Noise, it is not related to Temperature.
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Types of Noise
2. Shot Noise:
❑A resistor normally does not produce shot noise since there is no potential
barrier built within a resistor
❑ If the active device provides amplification, the noise also gets amplified
along with the signal
❑ Shot noise characteristic is white
in = RMS value of shot noise current
e = Charge of an electron
Ia = Average current
B = Bandwidth of the system
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Types of Noise
Shot Noise
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Types of Noise
3. Impulse Noise:
❑ Impulse noise can occur from switching transients in electromechanical
switching offices or from rotary dial telephones
❑ Step-by-step switching is the most frequent source
❑ Impulse noise is usually measured in terms of number of pulses per
second
4. Quantization Noise:
❑ Quantization noise arises during the
digitization process as the sampled
values are different than the quantized
value
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Signal To Noise ration (SNR)
❑ SNR is a measurement parameter in use that compares the
level of the desired signal to the level of background noise.
❑ it is the ratio of signal power to the noise power,
❑ Often it is expressed in decibels.
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Noise Figure (NF)
Noise Factor , F relates the SNR at the input of a network (or device) to the
SNR at the output of the network (or device)
Noise Figure , NF the difference in decibels (dB) between input of a network (or
device) to the output of the network (or device)
30d
B
NF = 10
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Noise Figure (NF)
Si = Signal power at the amplifier input
Ni = Noise power at the amplifier input
No = Noise power at the amplifier output
= GNi + Na = G(Ni + Nai)
Nai = Amplifier noise referred to the input
G = amplifier gain
For n-stage cascaded system:
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Noise Figure (NF)-Problem
dBm (decibel-milliwatts) is a unit of level used to indicate that a power level is
expressed in decibels (dB) with reference to one milliwatt (mW).
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Noise Temperature
Any white noise source can be specified in terms of an effective noise
temperature
Here,
T00 is the reference temperature of the noise
source, i.e., customarily taken as 290K
T0R is called the effective (equivalent) input
noise temperature of a device (e.g., an
amplifier)
❑
❑
Noise temperature is an alternative of NF, but equivalent characterization of
noiseness of a device
In general, applications involving very low noise devices seem to favor the
effective temperature measure over the NF
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Encountering the Problem of Noise
Alternate Options:
1. Increasing transmit power: Not feasible
2. Increasing bandwidth: very expensive
3. Amplifier at the receiver side: Not useful as it amplifies the noise as well
4. Amplifiers along the line
5. Regenerative repeaters along the line
Amplifiers
along the line
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Regenerative Repeater vs Amplifier
Regenerative
Repeater along the
line
Repeater regenerates the signal so
that the noise can be reduced or
eliminated.
Amplifier increases the amplitude of the
signal with the noise.
Amplifiers
Amplifiers: Amplifiers boost the strength of a signal without significantly altering its content. They are beneficial when the signal weakens over
distance due to factors like resistance or attenuation. However, amplifiers don't distinguish between the signal and the noise, so they can
amplify both. If the noise is significant, it might degrade the signal quality further.
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dB, dBm, dBW
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Example
Consider a system as shown in the figure operates at T = 230C with a
bandwidth of 2GHz. The length of the transmission line is 10 km.
Signal attenuation and noise gain along the transmission line are 1
dB/km and 0.5 dB/km respectively. The gain and the noise figure of the
amplifier is 100 and 10 respectively. If the signal power at the input of
the amplifier is 1mW, calculate (i) noise power at the output of the amplifier
(ii) SNR at the end of the transmission line
10 km
Channel Capacity
❑
Channel Reliability of a system is expressed in terms of Bit Error Rate(
BER).
❑ Is it possible, to transmit message with a zero BER in noisy channel?
❑ Before Shannon's theory no one believed that.
❑ Shannon's Capacity Formula (1948):
C = B log2 (1 + SNR), bps
C = capacity (bps), B = channel bandwidth (Hz),
❑ Capacity increases linearly with bandwidth, but only logarithmically with
signal strength
❑ Shannon's limit tells us what can be achieved. But, it tells nothing on
how to accomplish it
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Channel Capacity
Shannon's Capacity Formula (1948):
C = B log2 (1 + SNR), bps
C = capacity (bps), B = channel bandwidth (Hz)
Observation
❑
Two primary resources in communications:
Transmitted power (should be green, i.e., lower energy requirement)
Channel bandwidth (very expensive)
We can trade Bandwidth and Signal Power to maintain accuracy. But this
not one to one trade. Doubling the SNR will not compensate halving the
Bandwith.
This is the upper limit of throughput. If there is no noise, the capacity
would be infinite, C=
∞
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Channel Capacity-Practical example
Case-1:
Softspoken & fast Person ( low signal Power) , 🡪
Difficult to Understand
( As a receiver, our Bandwidth is limited , along with Low SNR 🡪 Capacity is
Low)
Case-2:
Fast & Loud spoken Person ( high signal Power)🡪Likely more easy to
Understand
( as signal Power is raised, and so the SNR)
Will Doubling the speaker volume allow us to speak twice faster?
Case-3:
Slow & Loud spoken Person ( high signal Power)🡪Best scenario for receiver
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