NOISESUBMITTED TO: SUBMITTED BY:

RAJBIR KAUR ANSHU ARORA ROLL NO:10902002

BATCH- E1

COMMUNICATION:

Communication is the process of establishing connection or link between two points for

information exchange. Communication is simply the process of conveying message at a

distance or communication is the process of exchanging information. The electronic

equipments which are used for communication purpose are called communication

equipments. Different communication equipments when assembled together form a

communication system [1].

Communication is the activity of conveying meaningful information. Communication

requires a sender, a message, and an intended recipient, although the receiver need not be

present or aware of the sender's intent to communicate at the time of communication; thus

communication can occur across vast distances in time and space [2].Typical examples of

communication system are line telephony and line telegraphy ,radio telephony and radio

telegraphy, radio broadcasting , point- to-point communication and mobile

communication , computer communication, radar communication , television

broadcasting, radio telemetry, radio aids to navigation, radio aids to aircraft landing etc.

In the most fundamental sense, communication involves the transmission of information

from one point to another through a succession of process as listed ahead:

i. The generation of a thought pattern or image in mind of an originator.

ii. The description of that image, with a certain measure of precision, by a set of oral

visual symbols.

iii. The encoding of these symbols in a form that is suitable for transmission over a

physical medium of interest.

iv. The transmission of encoded symbols to desired destination.

v. The decoding and reproduction of original symbols.

vi. The recreation of original thought pattern or image, with a definable degradation

in quality, in the mind of a recipient.

The purpose of communication system is to transmit an information bearing signal, from

a source, located at one point, to a user or destination, located at another point some

distance away. [1]

NOISE:

Noise may be defined as an unwanted from of energy which tend to interfere with proper

reception and reproduction of transmitted signals.In both analog and digital

electronics, noise is an unwanted perturbation to a wanted signal; it is called noise as a

generalization of the audible noise heard when listening to a weak radio transmission.

Signal noise is heard as acoustic noise if played through a loudspeaker; it manifests as

"snow" on a television or video image. Noise can block, distort, change or interfere with

the meaning of a message in human, animal and electronic communication. In signal

processing or computing it can be considered unwanted data without meaning; that is,

data that is not being used to transmit a signal, but is simply produced as an unwanted by-

product of other activities. "Signal-to-noise ratio" is sometimes used to refer to the ratio

of useful information to false or irrelevant data in an exchange. [3]

AC waveform with noise added from electric motor[4]

In pulse communication, noise may produce unwanted pulses or cancel the required

pulses. In other words, we can say that noise may limit the performance of a

communication system. Noise may interfere with signal at any point in communication

systems. In communication systems, the noise is an error or undesired random

disturbance of a useful information signal, introduced before or after the detector and

decoder. The noise is a summation of unwanted or disturbing energy from natural and

sometimes man-made sources. [5]

Figure 2: (a) A signal without noise and (b) a signal with noise.[6]

CLASSIFICATION OF NOISE

Noise may be classified in two broad groups as under:

i. External noise

ii. Internal noise

EXTERNAL NOISE:

External noise may be defined as that type of noise which is generated external to a

communication system i.e. whose sources are external to the communication systems.

External noise cannot be analyzed quantitatively. In addition of this, for a given

geographical point or location, external noise cannot be controlled. Thus, to reduce the

effect of external noise, the only way is to shift the communication system to other place

or location which has comparatively smaller external noise. Thus due to this reason, the

satellite earth stations are generally located in noise- free valleys.[1]

External Noise may be classified as under:

i. Atmospheric Noise

ii. Extraterrestrial Noise

iii. Industrial Noise

ATMOSPHERIC NOISE:

Atmospheric noise, which is also called static, is produced by lightning discharges in

thunderstone and other natural electrical disturbances which occur in the atmosphere.

These electrical impulses are random in nature. Atmospheric noise contains spurious

radio signals which are distributed over a wide frequency range. These spurious radio

waves constituting the noise propagate over the earth in manner as required radio waves

of same frequency. Due to this reason, the receiving antenna picks up not only the

required signal but also the static from all thunder storms.

The field strength of atmospheric noise varies inversely with frequency. Large

atmospheric noise is produced in low and medium frequency bands where as very small

noise is produced in VHF and UHF bands.

Thus atmospheric noise becomes less severe at frequencies above about 30 MHz

EXTRATERRESTRIAL NOISE:

 It is mainly radio disturbances from sources other than those related to the earth. [7]

Extraterrestrial noise may be divided into two sub groups as under:

i. Solar noise

ii. Cosmic noise

SOLAR NOISE:

Solar noise is an electrical noise emanating from the sun.

Under steady conditions, there is regular radiation of noise from sun. this radiation of

noise from sun is due to fact that sun is big body at an extremely high temperature and it

radiated electric energy in form of noise over a wide frequency spectrum including also

the frequency spectrum which is occupied by radio communication. The condition of sun

varies and follows an eleven years cycle. Hence at the peak of this eleven years cycle, the

electrical disturbances are caused. This form of noise from electrical disturbance is in

addition to the quite noise from the sun. Although this additional noise is produced from a

small portion of sun’s surface it can be larger than sun noise. The solar cycle repeats this

type of electrical disturbances nearly every eleven years.

COSMIC NOISE:

Distant stars can also be considered suns. These distant stars have high temperature and

therefore radiate noise in same manner as the sun. the noise received from these distant

stars is thermal noise and is distributed over the entire sky. The noise is also received

from the center of our own galaxy, for other distant galaxies and from other virtual point

sources such as quasers and pursers. This type of galactic noise is quite intense but the

angle s subtended by earth is very small. Therefore strength of galactic noise received on

the earth gets diminished. We can also hear the fall of a meteorite in a radio receiver; as

the falling object burns from friction with the Earth's atmosphere, ionizing surrounding

gases, thereby producing radio waves.[8]

The space noise is quite, significant at frequencies in 1.43 GHz. In the frequency range of

20 to 12 MHz, the space noise becomes the strongest noise component next only to

industrial noise. Below 20 MHz the space noise doesn’t penetrate much through

ionosphere to reach the earth.

INDUSTRIAL NOISE:

The industrial noise or manmade noise is that type of noise which is produced by such

sources as automobile s and aircraft ignition, electrical motors, switch gears and leakage

from high voltage transmission lines and several other heavy equipments. Such type of

noise is produced by the arch discharge taking place during operation of all these machine

or equipments. Industrial or manmade noise is quire intensive in industrial areas, densely

populated urban areas.

Noise can not only cause hearing impairment (at long-term exposures of over

85 decibels (dB), known as an exposure action value), but it also acts as a causal factor

for stress and raises systolic blood pressure.

Additionally, it can be a causal factor in work accidents, both by masking hazards and

warning signals, and by impeding concentration.

Noise also acts synergistically with other hazards to increase the risk of harm to workers.

In particular, noise and dangerous substances (e.g. some solvents) that have some

tendencies towards toxicity may give rise to rapid ear damage.[9]

Since industrial noise is highly variable and hence can be analyzed only statistically.

INTERNAL NOISE:

Internal noise is that type of noise which is generated internally or within the

communication system or receiver. Internal noise may be treated quantitatively and can

also be reduced or minimized by proper design. Since internal noise is widely distributed

over entire frequency spectrum, the noise present in a given bandwidth B is the same at

any frequency in the frequency spectrum. Hence, this random noise power is proportional

to the bandwidth over which it is measured. Internal noise may be classified as under:

i. Shot noise

ii. Partition noise

iii. Low frequency or flicker noise

iv. High frequency or transit-time noise

v. Thermal noise

SHOT NOISE:

Shot noise is a type of electronic noise that may be dominant when the finite number of

particles that carry energy (such as electrons in an electronic circuit or photons in an

optical device) is sufficiently small so that uncertainties due to  Poisson distribution,

which describes occurrence of independent random events, are of significance. It is

important in electronics, telecommunications, optical detection, fundamental physics. [10]

This noise arises in active device due to random behavior of charge carriers. In electron

tubes, shot noise is generated due to random emission of electrons from cathodes,

whereas in semiconductor devices shot noise is generated due to random diffusion of

minority carriers or simply random generation and d recombination of electron-hole pairs.

The current in electron devices (i.e. tubes or solid state device) flows in the form of

discrete pulses, every time in a charge carrier moves from one point to other. Hence

although current appears to be continuous it is still a discrete phenomena.

For large numbers the Poisson distribution approaches a normal distribution, typically

making shot noise in actual observations indistinguishable from true Gaussian

noise except when the elementary events (photons, electrons, etc.) are so few that they are

individually observed. Since the standard deviation of shot noise is equal to the square

root of the average number of events N, the signal-to-noise ratio is given by:

.

Thus when N is very large, the signal-to-noise ratio is very large as well, and

any relative fluctuations in N due to other sources are more likely to dominate over shot

noise.

The number of photons that are collected by a given detector varies, and follows a

Poisson, depicted here for averages of 1, 4, and 10. [11]

PARTITION NOISE:

Partition noise is generated in a circuit when a current has to divide between two or more

paths. This means that the partition noise results from the random fluctuations in the

division. Hence, it is expected that a diode must be less noisy than a transistor, all else

being equal. Due to this reason, the inputs of microwave receivers are generally taken

directly to the diode mixers. For partition noise, the spectrum is a flat spectrum. Also,

the active three terminal components in which the control terminal draws less current are

less noisy. In recent time, Metal-Semiconductor Field Effect Transistors have been

developed which draws almost zero gate bias current. Due to this reason, these devices

have low partition noise and therefore find applications in low noise microwave

amplification.[1]

FLICKER NOISE, OR LOW FREQUENTY NOISE:

At low frequencies (below few kHz), a particular type of noise appears. The power

spectral density of this noise increase as the frequency decreases. This noise is called as

flicker noise or (1/f) noise. In case of vacuum tubes, the main causes of flicker noise are

slow changes which take place in the oxide structure of oxide coated cathodes and

migration of impurity ions.

In semiconductor devices, flicker noise is generated from the fluctuations

in the carrier density and creates more problems in semiconductor amplifying device than

vacuum tubes at low frequencies.

Actually, the fluctuations in the carrier density generate fluctuations in the

conductivity of the material. This produces a fluctuating voltage drop when a direct

current flows. This fluctuating voltage drop is called as the flicker noise voltage. The

power density spectrum of the flicker noise is inversely proportional to frequency.

Mathematically,

S (w) ∝ 1/f.

It occurs in almost all electronic devices, and can show up with a variety of other effects,

such as impurities in a conductive channel, generation and recombination noise in a

transistor due to base current, and so on. 1/f noise in current or voltage is always related

to a direct current because it is a resistance fluctuation, which is transformed to voltage or

current fluctuations via Ohm's law. Flicker noise is found in carbon composition resistors,

where it is referred to as excess noise, since it increases the overall noise level above

the thermal noise level, which is present in all resistors. In contrast, wire-wound resistors

have the least amount of flicker noise. [12]

TRANSIT-TIME NOISE OR HIGH FREQUENCY NOISE:

It is genially observed in semiconductor devices, when the transit- time of charge-carries

crossing a junction is comparable with the time period of the signal, some charge- carries

diffuse back to the source or emitters. This process given rise to an input admittance in

which the conductance component increases with frequency. This conductor has a noise

current source which is associated with it in parallel. Because this conductance increases

with frequency, the power spectral density will also increase.

THERM AL NOISE:

The thermal noise or white noise or Johnson noise is the random noise which is generated

in a resistor or the resistive component of complex impedance due to rapid and random

motion of the molecules, atoms and electrons.

Accordingly to the kinetic theory of thermodynamics, the temperature of a particle

denotes its internal kinetic energy. This means that the temperature of a body expresses

the rms value of the velocity of motion of the particles in body. As per this kinetic

theory, the kinetic energy of these particles becomes approximately zero (i.e. zero

velocity) at absolute zero.

Therefore, the noise power produced in a resistor is proportional to its

absolute temperature. Also the noise power is proportional to the bandwidth over which

the noise is measured.

Therefore, the expression for maximum noise power output of a resistor may be

given as

Pn ∝ T.B

Or Pn = k.T.B

Where k= Boltzmann’s constant

= 1.38 * 10-23 joule/ deg.K

T= absolute temperature

B= bandwidth of interest in Hz.

Thermal noise is distinct from shot noise, which consists of additional current fluctuations

that occur when a voltage is applied and a macroscopic current starts to flow. For the

general case, the above definition applies to charge carriers in any type of

conducting medium (e.g. ions in an electrolyte), not just resistors. It can be modeled by a

voltage source representing the noise of the non-ideal resistor in series with an ideal noise

free resistor.[13]

We can draw an equivalent circuit of a resistor as a noise voltage generator as shown:

Voltage model of a noisy resistor

Pn= V2/R

Pn= (Vn/2)2/R

Pn= (Vn)2/4R

Pn = k.T.B

(Vn)2/4R= k.T.B

Vn= √4KTBR

From this result we conclude that square of rms noise voltage associated with a resistor is

proportional to absolute temperature T of the resistor, value R of the resistor and

bandwidth B over which noise is measured.

[14]

References:

[1] Sanjay Sharma, ‘Signals and systems’, 2009 edition

[2] Communication, Wikipedia: http://en.wikipedia.org/wiki/Communication

[3] Noise, Wikipedia: http://en.wikipedia.org/wiki/Noise

[4] Noise waveform:

http://www.electronicsteacher.com/succeed-in-physical-science/wave-motion/waveform-

noise.php

[5] Noise (electronics), Wikipedia: http://en.wikipedia.org/wiki/Noise_(electronics)

[6] Noise waveform, http://cnx.org/ ,

http://cnx.org/content/m13859/latest/SignalNoise.png

[7] The free dictionary by farlex:

http://encyclopedia2.thefreedictionary.com/extraterrestrial+noise

[8] Cosmic noise, Wikipedia: http://en.wikipedia.org/wiki/Cosmic_noise

[9] Industrial noise, Wikipedia: http://en.wikipedia.org/wiki/Industrial_noise

[10] Shot noise, Wikipedia: http://en.wikipedia.org/wiki/Shot_noise

[11] Shot noise, Poisson distribution:

http://upload.wikimedia.org/wikipedia/commons/1/16/Poisson_pmf.svg

[12]Flicker noise, noise (electronics), Wikipedia:

http://en.wikipedia.org/wiki/Flicker_noise

[13] J.S.Chitode, ‘Communication Systems – I’,2004 edition

[14] Thermal noise as a function of resistance and bandwidth, Noise Power & Voltage: http://www.rfcafe.com/references/electrical/noise-power.htm