Reference: Digital Telephony,
John C. Bellamy
• Attenuation affects the propagation of waves and
signals in electrical circuits, in optical fibers, as well as
in air (radio waves).
• Attenuation is reduction of signal strength during
transmission. Attenuation is the opposite of
amplification, and is normal when a signal is sent from
one point to another. If the signal attenuates too
much, it becomes unintelligible, which is why most
networks require repeaters at regular intervals.
Attenuation is measured in decibels.
• Naturally, attenuation happens exponentially with the
• Distortion is known as the alternation of the original
signal. This may happen due to the properties of the
medium. There are many types of distortion such as
amplitude distortion, harmonic distortion, and phase
distortion. For electromagnetic waves polarization
distortions also occurs. When the distortion occurs,
shape of the waveform is changed.
• For example, amplitude distortion happens if all the
parts of the signals are not equally amplified. This
happens in wireless transmissions because the
medium get changed by the time. The receivers should
be able to identify these distortions.
• Amplitude distortion
• Delay / Phase Distortion
• Frequency related distortion
Difference between attenuation and distortion
1. Although scaled down in amplitude, shape of
waveform does not change in attenuation
unlike in distortion.
2. Removal of the effects of attenuation is easier
than a removing the effects distortion.
3. If the attenuation happens in different
amounts for the different parts of the signal, it
is a distortion.
The random and unpredicted electrical signal
(coming from both internal or external portion
of the system) which interfere the reception
of actual required signal is called- noise.
Noise can be characterized by statistical
parameter such as averaged/ squared noise,
current/ voltage etc.
A receiver has an input signal power of l.2µW. The noise power
is 0.80µW. What is the signal to noise ratio?
• Signal to Noise Ratio = 10 Log (1.2/0.8)
= 10 log 1.5
= 10 (0.176)
= 1.76 dB
where SNRin and SNRout are the input and output
It is a reference measurement between a
minimum noise level due to thermal noise and
total (ext.+ int.) noise.
Teq= To (F-1)
Classification of Noise
• External noises, i.e. noise whose sources are
– Atmospheric noises
– Extraterrestrial noises
– Man-made noises or industrial noises.
• Internal noise in communication, i.e. noises
which get, generated within the receiver or
– Thermal noise or white noise or Johnson noise
– Shot noise.
– Transit time noise
- Miscellaneous internal noise