Noise

Noise

For any data transmission event, the received signal will consist of the transmitted
signal, modified by the various distortions imposed by the transmission system, plus
additional unwanted signals that are inserted somewhere between transmission and
reception. These unwanted signals are referred to as noise. Noise is the major limiting
factor in communications system performance.
Noise may be divided into four categories:
• Thermal noise
• Intermodulation noise
• Crosstalk
• Impulse noise
Thermal noise is due to thermal agitation of electrons. It is present in all electronic
devices and transmission media and is a function of temperature. Thermal
noise is uniformly distributed across the frequency spectrum and hence is often
referred to as white noise. Thermal noise cannot be eliminated and therefore places
an upper bound on communications system performance. Because of the weakness
of the signal received by satellite earth stations, thermal noise is particularly significant
for satellite communication.
The amount of thermal noise to be found in a bandwidth of 1 Hz in any device
or conductor is

When signals at different frequencies share the same transmission medium,
the result may be intermodulation noise. Intermodulation noise produces signals at
a frequency that is the sum or difference of the two original frequencies or multiples
of those frequencies. For example, the mixing of signals at frequencies fl and h
might produce energy at the frequency fl + h. This derived signal could interfere
with an intended signal at the frequency fl + h.
Intermodulation noise is produced when there is some nonlinearity in the
transmitter, receiver, or intervening transmission system. Normally, these components
behave as linear systems; that is, the output is equal to the input times a constant.
In a nonlinear system, the output is a more complex function of the input. Such
nonlinearity can be caused by component malfunction, the use of excessive signal
strength, or just the nature of the amplifiers used. It is under these circumstances that
the sum and difference frequency terms occur.
Crosstalk has been experienced by anyone who, while using the telephone, has
been able to hear another conversation; it is an unwanted coupling between signal
paths. It can occur by electrical coupling between nearby twisted pairs or, rarely,
coax cable lines carrying multiple signals. Crosstalk can also occur when unwanted
signals are picked up by microwave antennas; although highly directional antennas
are used, microwave energy does spread during propagation. Typically, crosstalk is
of the same order of magnitude as, or less than, thermal noise. However, in the unlicensed
ISM bands, crosstalk often dominates.
All of the types of noise discussed so far have reasonably predictable and
relatively constant magnitudes. Thus it is possible to engineer a transmission
system to cope with them. Impulse noise, however, is noncontinuous, consisting
of irregular pulses or noise spikes of short duration and of relatively high
amplitude. It is generated from a variety of causes, including external electromagnetic
disturbances, such as lightning, and faults and flaws in the communications
system.
Impulse noise is generally only a minor annoyance for analog data. For
example, voice transmission may be corrupted by short clicks and crackles with no
loss of intelligibility. However, impulse noise is the primary source of error in digital
data transmission. For example, a sharp spike of energy of 0.01 s duration
would not destroy any voice data but would wash out about 560 bits of data being
transmitted at 56 kbps.