Transmission Impairments

Reference: Digital Telephony, John C. Bellamy

Attenuation:

• 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 travelled distance.

Distortion• 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.

Distortion

Distortion

Types:• 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.

Noise

Reference: Electronic Communication System

Sanjeev Gupta

Noise

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.

Signal-to-noise ratio (SNR)

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

Noise figure

I/P O/P

where SNRin and SNRout are the input and

output signal-to-noise ratios

Comm Device

Noise-equivalent temperature

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 external

– Atmospheric noises – Extraterrestrial noises – Man-made noises or industrial noises.

• Internal noise in communication, i.e. noises which get, generated within the receiver or communication system.

– Thermal noise or white noise or Johnson noise – Shot noise. – Transit time noise

- Miscellaneous internal noise