Communication Assignment - Sources of impairments

SOURCES OF IMPAIRMNTS IN A COMMUNICATION SYSTEM

v Identify major sources of impairments in a communication system?

v How the communication analog and digital is affected by these impairments? 

v  Explain major sources of noise and how it is countered in a communication system? 

What is transmission impairment

Transmission impairment is a property of a transmission medium which causes the signal to be degraded, reduced in amplitude, distorted or contaminated. Impairment can introduce errors into digital signals and in analog signal.

With any communications system, the signal that is received may differ from the signal that is transmitted due to various transmission impairments. For analog signals, these impairments can degrade the signal quality. For digital signals, bit errors may be introduced, such that a binary 1 is transformed into a binary 0 or vice versa.

Important of transmission impairment

It is important to understand transmission impairments for several reasons. Understanding the source of a transmission impairment like attenuation or dispersion will enable the user to partially correct for (equalize the signal) these effects. Understanding the source of transmission impairments (dispersion, attenuation, impulse noise, and thermal noise) can also help the user understand some of the constraints placed on the transmission of data as a result of these effects. Such constraints include the maximum length of network links, the choice of physical transmission media, the choice of encoding methods, and the data rate supported by the medium.

Major sources of impairment

a)    Attenuation

b)    Delay Distortion

·         Attenuation

Attenuation means the loss of energy. When a signal, simple or composite, travels through medium, it loses some of its energy so that it can overcome the resistance of the medium. That is why a wire carrying electrical signals gets worm, if not hot, after a while. Some of the electrical energy in signal is converted to heat. To compensate for this loss, the amplifiers are used to amplify the signal.

  Attenuation is a property of the transmission medium. It measures how much energy is absorbed and/or radiated from the traveling signal due to its interaction with the transmission medium. Attenuation is measured as a function of the distance traveled through the transmission medium. The transmission medium absorbs energy because the signal is influenced by small impurities within it. Such impurities have different sizes and distributions depending on the type of medium. Impurities of different sizes effect different frequencies in the signal. The effect of attenuation is, therefore, a function of frequency. The frequency variation of attenuation can be partially corrected, or equalized, by applying corrections based on a physical model. When a signal is attenuated it’s amplitude is reduced. The interpretation of a received signal depends on being able to tell the difference between different signal levels. If the amplitude is reduced too much by attenuation it becomes impossible to accurately tell the difference between the different signal levels, and the information in the signal is lost. To prevent this from happening repeaters (digital) or amplifiers (analog) are used. These devices increase the amplitude of the signal by decoding and retransmitting the signal or increasing the received amplitudes respectively. By inserting amplifiers or repeaters in the transmission media, the maximum signal propagation distance (a property of the attenuation of the medium) is increased.

Attenuation introduces three considerations for the transmission engineer. First, a received signal must have sufficient strength so that the electronic circuitry in the receiver can detect the signal. Second, the signal must maintain a level sufficiently higher than noise to be received without error. Third, attenuation varies with frequency. The first and second problems are dealt with by attention to signal strength and the use of amplifiers or repeaters. The third problem is particularly noticeable for analog signals. To overcome this problem, techniques are available for equalizing attenuation across a band of frequencies. This is commonly done for voice-grade telephone lines by using loading coils that change the electrical properties of the line; the result is to smooth out attenuation effects. Another approach is to use amplifiers that amplify high frequencies more than lower frequencies.

·        Delay distortion

Distortion means that signal changes its form or shape. Distortion occurs in a composite signal that is made of different frequencies. Each signal component has its own propagation speed through a medium and, therefore its own delay in arriving at the final destination.

Delay distortion occurs because the velocity of propagation of a signal through a guided medium varies with frequency. For a band limited signal, the velocity tends to be highest near the centre frequency and fall off toward the two edges of the band. Thus various frequency components of a signal will arrive at the receiver at different times, resulting in phase shifts between the different frequencies. Delay distortion is particularly critical for digital data, because some of the signal components of one bit position will spill over into other bit positions, causing intersymbol interference. This is a major limitation to maximum bit rate over a transmission channel.

•         Similar to attenuation distortion.

•         Delay distortion is Dependent on frequency.

•         The higher the frequency, the faster it travels through cables.

•         Different frequencies arrive at different times.

•         Resultant combined signal suffers some distortion.

•         Usually not critical for analog signals because human hear cannot distinguish slight distortion.

•         Can be critical for digital information spill over imposes maximum bit rate on system.

What is noise and major sources of noise

Noise

 Noise can be defined as an unwanted signal that interferes with the communication or measurement of another signal. A noise itself is a signal that conveys information regarding the source of the noise.

The noise from a car engine conveys information regarding the state of the engine. The sources of noise are many, and vary from audio frequency acoustic noise emanating from moving, vibrating or colliding sources such as revolving machines, moving vehicles, computer fans, keyboard clicks, wind, rain, etc. to radio-frequency electromagnetic noise that can interfere with the transmission and reception of voice, image and data over the radio-frequency spectrum. Signal distortion is the term often used to describe a systematic undesirable change in a signal and refers to changes in a signal due to the non–ideal characteristics of the transmission channel, reverberations, echo and missing samples. Noise and distortion are the main limiting factors in communication and measurement systems.

Therefore the modeling and removal of the effects of noise and distortion have been at the core of the theory and practice of Communications and signal processing. Noise reduction and distortion removal are important problems in applications such as cellular mobile communication, speech recognition, image processing, medical signal Processing, radar, sonar, and in any application where the signals cannot be isolated from noise and distortion.

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, referred to as noise, that are inserted somewhere between transmission and reception. Noise is a major limiting factor in communications system performance. Noise may be divided into four categories.

Noise may enter the system from external sources (e.g. interference generated by a motor next to the receiver system) or may be generated from spontaneous fluctuations internal to a circuit.

Sources:

·        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 bandwidths typically used in communications systems and hence is often referred to as white noise. Thermal noise cannot be eliminated and therefore places an upper bound on communications system performance; and. is particularly significant for satellite communication.

·        Intermodulation noise.

The effect of intermodulation noise is to produce signals at a frequency that is the sum or difference of the two original frequencies or multiples of those frequencies, thus possibly interfering with services at these frequencies. It is produced by nonlinearities in the transmitter, receiver, and/or intervening transmission medium.

·        Crosstalk

 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. It can also occur when microwave antennas pick up unwanted signals; 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.

·        Impulse noise

  Is non continuous, 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. It is generally only a minor annoyance for analog data. However impulse noise is the primary source of error in digital data communication. 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.