Noise in

Communications

Noise is unwanted random signals which cause interference with the signal being

transmitted, resulting in degradation and errors. The noise signals may be electrical,

electromagnetic or acoustic.

Noise, in its broadest definition, consists of any undesired signal in a

communication circuit.

Noise in communication systems originate both in the channel and in the

communication equipment.

Noise consists of undesired(Usually random)variations that interfere with the

desired signals and inhibit communication. It cannot be avoided completely. but its effects

Can be reduced by various means, such as reducing the signal bandwidth, increasing the

transmitter power, and using low-noise amplifiers to weak signals.

Noise (in Glossary of Telecommunications Terms)

(1) Unplanned energy introduced onto a communications path, resulting in

transmission errors. Undesirable signals bearing no desired information.

(2) The unpredictable difference between the observed data and the true process.

Characteristics of noise

Noise is usually classified as white or coloured depending on the spectral density

of the noise power with frequency.

0 White Noise is defined as having a flat power spectral density over all

frequencies of interest, with a value usually denoted as N0 Watts/Hz.

0 Coloured noise has a non-uniform spectral distribution; however, over a finite

bandwidth corresponding perhaps to a single communications channel, the

power spectral density might appear flat, hence the term bandlimited white

Gaussian noise.

Not only is it necessary for the spectrum to be flat, but the statistics of the noise must be

such that the envelope distribution of the bandlimited noise must be Gaussian in nature to

fully satisfy the Shannon condition. Fortunately this holds approximately true for the

majority of practical narrowband communications systems.

Sources of noise

In general, sources of noise can be grouped into two main categories.

0 The correlated noise, generates noise that is correlated with the desired signal. Such

noise is normally only generated in the presence of the information signal, from

non-linear processing such as non-linear amplification which causes the generation

of harmonics. Signals mixing also generate noise due to inter-modulation products

that create a lot of distortion. For periodic signals, the correlated noise is also

periodic.

0 The uncorrelated noise, generate noise that bears no relation to the information

signal and the noise presence is independent of the presence of the signal. This noise

can be generated by either external or internal process. The external process could

be due to atmospheric effects such as static electricity or lightning. However, the

atmospheric noise is insignificant at frequencies above 30 MHz. Another external

source of noise is due to extraterrestrial influences due to deep space effects, such as

sun’s heat noise and black body noise. The extraterrestrial noise contribution is

significant up to about 1.5 GHz. Finally, a possible external source of noise could be

man-made, i.e. generated by motors, ignition systems and switching equipment.

Internal uncorrelated noise is due to the electron’s random motion in the atomic

level of metal molecules causing collision with each other. Such random motion

generates noise voltage that increases with heat but does not depend on the value of

the metal electrical resistance. This noise is commonly encountered in most

communication systems and is appropriately called thermal noise. The thermal noise

process is defined by Gaussian distribution and it has power spectral density (N0)

which is almost constant over a very large frequency spectrum and hence is called

white Gaussian noise. The one-sided spectral density (positive frequencies only) of

the thermal noise, denoted by (N0), in watts/Hz, and the thermal Noise Power (N) in

dBm at room temperature (290 degrees Kelvin) are

N0 = KT

N = −174 + 10 log10 (B) dBm

where B is the bandwidth in Hz, T is the absolute temperature in degrees Kelvin and

K is Boltzmann’s constant=1.38×10−23 joules/Kelvin. The two-sided (positive and

negative frequencies) noise spectral density is given by N0/2.

Another way noise sources can be grouped into three categories:

0 Intrinsic noise sources

0 Man-made noise sources

0 Noise due to natural disturbances

Type of Noise

Noise can be divided into three types:

0 External noise, which is a property of the channel and originating from outside

sources.

Ex. Space Noise

Atmospheric noise

Equipment Noise and so on.

0 Internal noise, which originates within the communication equipment. Noise is

generated in all electronic equipment. Both passive components (such as

resistors and cables) and active devices (such as diodes, transistors, and tubes)

can be noise sources.

Ex. Thermal noise

Partition noise and so on.

0 Man-made noise, which made for a specific purpose.

Ex. Noise diode and so on .

Examples of Noise

0 Thermal or Johnson Noise. Thermal noise occurs in all transmission media and

all communication equipment, including passive devices. It arises from random

electron motion and is characterized by a uniform distribution of energy over the

frequency spectrum with a Gaussian distribution of levels. Every equipment

element and the transmission medium proper contribute thermal noise to a

communication system if the temperature of that element or medium is above

absolute zero. Thermal noise is the factor that sets the lower limit of sensitivity of

a receiving system and is often expressed as a temperature, usually given in units

referred to absolute zero. These units are kelvins. Thermal noise is a general

expression referring to noise based on thermal agitations. The term “white

noise” refers to the average uniform spectral distribution of noise energy with

respect to frequency. Thermal noise is directly proportional to bandwidth and

temperature.

0 Intermodulation Noise. Intermodulation (IM) noise is the result of the

presence of intermodulation products. If two signals with frequencies F1 and F2

are passed through a nonlinear device or medium, the result will contain IM

products that are spurious frequency energy components. These components may

be present either inside and/or outside the band of interest for a particular device.

IM products may be produced from harmonics of the desired signals in question,

either as products between harmonics or as one of the signals and the harmonic

of the other(s) or between both signals themselves. The products result when two

(or more) signals beat together or “mix.” Devices passing multiple signals

simultaneously, such as multichannel radio equipment, develop intermodulation

products that are so varied that they resemble white noise.

Intermodulation noise may result from a number of causes:

Improper level setting. If the level of input to a device is too high, the

device

is driven into its nonlinear operating region (overdrive).

Improper alignment causing a device to function nonlinearly.

Nonlinear envelope delay.

Device malfunction.

To summarize, intermodulation noise results from either a nonlinearity or a

malfunction that has the effect of nonlinearity. The cause of intermodulation

noise is different from that of thermal noise. However, its detrimental effects and

physical nature can be identical with those of thermal noise, particularly in

multichannel systems carrying complex signals.

0 Impulse Noise. Impulse noise is noncontinuous, consisting of irregular

pulses or noise “spikes” of short duration, broad spectral density, and relatively

high amplitude. In the language of the trade, these spikes are often called

“hits.” A technician may say that the circuit is getting “hit up.” Impulse noise

degrades telephony ordinarily only marginally, if at all. However, it may

seriously degrade data error performance on data or other digital waveforms.

The effects of impulse noise can be alleviated by the use of a wide band clipping

circuit followed by a band limiting filter. This procedure first reduces the

amplitudes of the spectral components and then reduces the number of

components.

0 Partition Noise . Partition noise occurs in multi-electrode devices such as

transistors and valves. It is due to the current through the device being divided

between the various electrodes.

0 Shot Noise . Shot noise is the name given to noise generated in active devices

such as valves, transistors and integrated circuits. It is caused by the random

varying velocity of electron movement under the influence of externally applied

potentials or voltages at the terminals or electrodes. It is similar to thermal

noise in that it has a Gaussian distribution and a flat power spectrum. It differs,

however in that it is not directly affected by temperature. Its magnitude is

proportional to the square root of the direct current through the device and thus

may be a function of signal amplitude.

0 Flicker Noise . Flicker noise is sometimes known as low frequency (1/f) noise

or excess noise. It is because of its unusual increase at very low frequencies

that it is also called low frequency noise. Its cause is associated with contact

and surface irregularities in cathodes of valves and semiconductors and it

appears to be caused by fluctuations in the conductivity of the medium. Because

of the advances in cleaning and passivation techniques, which are employed

during component manufacturing processes, a good device will exhibit

negligible flicker noise above 1kHz.

0 Atmospheric noise. Atmospheric noise is a general term given to noise arising

from electromagnetic radiation from solar and galactic sources. Certain stars,

for example, emit definite and regular amounts of noise which are best avoided

by pointing the antenna away from the noise source. The compound effect of

this noise is usually expressed as an equivalent sky noise temperature and is

generally much less than thermal noise. The level of noise varies considerably

with frequency, with the higher levels of noise occurring in the microwave

region of the spectrum.

0 Space Noise. Space noise, the sun is a powerful source of radiation over a wide

range of frequencies, including the radio-frequency spectrum. Other stars also

radiate noise called cosmic, stellar, or sky noise, Its intensity when received on

the earth is naturally much less than for solar noise because of the greater

distance. Solar noise can re a serious problem for satellite reception, which

becomes impossible when the satellite is in a line between the antenna and the

sun. Space noise is more important at higher frequencies (VHF and above)

because atmospheric noise dominates at lower frequencies and because most of

the space noise at lesser frequencies is absorbed by the upper atmosphere.

0 Equipment Noise. Equipment Noise is generated by equipment that produces

sparks. Examples include automobile engines and electric motors with brushes.

Any fast-risetime voltage current can also generate interference, even without

arcing. Light dimmers and computer’ fall into this category. Noise of this type

has a broad frequency spectrum. but its energy is not equally distributed over

the frequency range. This type of interference is generally more severe at lower

frequencies. hut the exact frequency distribution depends upon the source itself

and any conductors to which it is connected. Computers. for instance, may

produce strong signals at multiples and submultiples of their clock frequency

and little energy elsewhere.

0 Man-made noise. Man-made noise can propagate through space or along

power lines, It is usually easier to control it at the source than at the receiver. A

typical solution for a computer, for instance, involves shielding and grounding

the case and all connecting cables and installing a low-pass filter on the power

line where it enters the enclosure.

0 Contact noise . Contact noise is caused by fluctuating conductivity due to an

imperfect contact between tow materials. It occurs anywhere when tow

conductors are joined together. When appear in resistors it is referred to as

access noise and when observed in vacuum tubes it is referred to as flicker

noise . Due to its unique frequency characteristic it is often called (1/f) noise or

low-frequency noise.

0 Popcorn noise . Popcorn noise also called burst noise, was first discovered in

semiconductor diodes and has recently reappeared in integrated circuits. It

caused due to a manufacturing defect and can be eliminated by improved

manufacturing processes.

0 White noise. White Noise that has a continuous, fairly uniform frequency

spectrum over a limited frequency range. It is often experienced as background

hiss, a form of interference on telephone or radio channels, and is caused by

the temperature-dependent thermal agitation of electrons. It is also known as

Gaussian noise and Random noise.

0 Gaussian noise. Noise which occurs in electronic systems and is spread over a

wide frequency range, occurring randomly in this range. It is often seen as

ambient noise, background noise and hiss.

0 Intrinsic noise. Intrinsic noise The noise which is inherent in a system, such as

a transmission line, and has not been caused by other external factors.

Some

Definitions related to Noise

0 Noise bandwidth(effective noise bandwidth): effective noise bandwidth: It is defined

as the width of a rectangular frequency response curve having a height equal to the

maximum height of this curve and corresponding to the same total noise power.

0 Noise current: The current generated in a circuit by the presence of a noise

voltage.

0 Noise diode: A semiconductor diode which can be used as a noise source.

The output is mainly shot noise, in a broad frequency band up to about 5

MHz, which may be adjusted by varying the input current to the diode.

0 Noise Equivalent Power (NEP): The radiant power which, under specified

conditions, produces a Signal to Noise Ratio (SNR) of one when measured by a

photodetector, i.e. it is the amount of radiant power which produces a signal

equivalent to the noise level.

0 Noise factor: A measure of the noise introduced by a system, it is given by

the ratio of the Signal to Noise Ratio (SNR) at the input of the system to

the Signal to Noise Ratio at the output from the system. So if these are

equal to SNRi and SNRo then the noise factor (F) is given by F = SNRi/SNRo.

0 Noise Figure (NF): The Noise Figure is equivalent to the noise factor

stated in decibels. Therefore if the Signal to Noise Ratio (SNR) at the

input and output to a system are SNRi and SNRo respectively, then the Noise Figure

(NF) is give by NF = 10log SNRi – 10log SNRo. The Noise Figure is therefore the

number of decibels by which the signal is degraded by the system.

0 Noise level: The noise power, usually when compared to a reference value, and

measured in decibels.

0 Noise margin: The maximum amplitude of the noise signal which can be applied to a

system without its performance being adversely affected.

0 Noise meter: An instrument which is used to measure noise. Instruments are

available for measurement of the different types of noise, both electrical and

acoustic.

0 Noise power: The total power contained in the noise signal.

0 Noise Power Density (NPD): The noise power per hertz, i.e. the noise power in a

bandwidth of one hertz.

0 Noise resistance: (1) The ability of a circuit or a system to prevent or reduce the

occurrence of noise. (2) In thermal noise the hypothetical equivalent resistance

which has the same effect as the noise.

0 Noise suppressor: A device which eliminates or reduces the noise in a system.

0 Noise temperature: The temperature of a source of thermal noise, in degrees Kelvin,

which gives the same noise power as the device being considered.

0 Noise voltage: Voltage which is generated by the noise signal and causes

interference within the circuit being considered.

0 Noise weighting: System, produced by regulatory bodies, which defines the effect of

noise on a telephone user, taking into account the frequency and amplitude of the

noise source and the attenuation characteristics of the telephone handset.

0 Line noise weighting: Noise on the line has a different effect on telephone users

depending on the frequency involved. Experimental line noise weighting curves exist

which capture this and standard line weighting networks have been produced which

also take into account factors such as the effect of attenuation caused by the

telephone handset.

0 Noise cancelling microphone: A microphone which has been designed to

compensate for the presence of acoustic noise. It has two sound ports

which give access to the front and back of the microphone diaphragm. It

reacts to the difference in sound pressure on the diaphragm, this being

made up of the near field sound and the far field sound. The noise

cancellation of the microphone is given by the difference between the

near field and far field measurements.