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Channel Characteristics

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1. Noise Sources

Noise in a communication system can be classified into two broad categories, depending on its source. Noise generated by components within a communication system, such as resistors and solid-state active devices is referred to as internal noise. The second category, external noise, results from sources outside a communication system, including atmospheric, man-made, and extraterrestrial sources.

Atmospheric noise results primarily from spurious radio waves generated by the natural electrical discharges within the atmosphere associated with thunderstorms. It is commonly referred to as static or spherics. Below about 100 MHz, the field strength of such radio waves is inversely proportional to frequency. Atmospheric noise is characterized in the time domain by large-amplitude, short-duration bursts and is one of the prime examples of noise referred to as impulsive. Because of this inverse dependence on frequency, atmospheric noise affects commercial AM broadcast radio, which occupies the frequency range from 540 kHz to 1.6 MHz, more than it affects television and FM radio, which operate in frequency bands above 50 MHz.

Man-made noise sources include high-voltage power line corona discharge, commutator-generated noise in electrical motors, automobile and aircraft ignition noise, and switching-gear noise. Ignition noise and switching noise, like atmospheric noise, are impulsive in character. Impulse noise is the predominant type of noise in switched wireline channels, such as telephone channels. For applications such as voice transmission, impulse noise is only an irritation factor; however, it can be a serious source of error in applications involving transmission of digital data.

Yet another important source of man-made noise is radio-frequency transmitters other than the one of interest. Noise due to interfering transmitters is commonly referred to as radio-frequency interference (RFI). RFI is particularly troublesome in situations in which a receiving antenna is subject to a high-density transmitter environment, as in mobile communications in a large city.

Extraterrestrial noise sources include our sun and other hot heavenly bodies, such as stars. Owing to its high temperature (6000°C) and relatively close proximity to the earth, the sun is an intense, but fortunately localized source of radio energy that extends over a broad frequency spectrum. Similarly, the stars are sources of wide and radio energy. Although much more distant and hence less intense than the sun, nevertheless they are collectively an important source of noise because of this vast numbers. Radio stars such as quasars and pulsars are also be seen sources of radio energy. Considered a signal source by radio astronomers, such stars are viewed as another noise source by communications engineers. The frequency range of solar and cosmic noise extends from a few megahertz to a few gigahertz.

Another source of interference in communication systems is multiple transmission paths. These can result from reflection off buildings, the earth, airplanes, and ships or from refraction by stratification in the transmission medium. If the scattering mechanism results in numerous reflected components, the received multi path signal is noise-like and is termed diffuse. It the multi path signal component is composed of only one or two strong reflected rays, it is termed specular. Finally, signal degradation in a communication system can occur because of random changes in attenuation within the transmission medium. Such signal perturbations are referred to as fading, although it should be noted that specular multipath also results in fading due to the constructive and destructive interference of the received multiple signals.

Internal noise results from the random motion of charge carriers in electronic components. It can be of three general types: the first is referred to as thermal noise, which is caused by the random motion of free electrons in a conductor or semiconductor excited by thermal agitation; the second is called shot noise and is caused by the random arrival of discrete charge carriers in such devices as thermionic tubes or semiconductor junction devices; the third, known as flicker noise, is produced in semiconductors by a mechanism not well understood and is more severe the lower the frequency.

2. Types of Transmission Channels

There are many types of transmission channels. We will discuss the characteristics, advantages, and disadvantages of three common types: electromagnetic-wave propagation channels, guided electromagnetic-wave channels, and optical channels. The characteristics of all three may be explained on the basis of electromagnetic-wave propagation phenomena. However, the characteristics and applications of each are different enough to warrant considering them separately.

Electromagnetic-Wave Propagation Channels

The possibility of the propagation of electromagnetic waves was predicted in 1864 by James Clerk Maxwell (1831 - 1879), a Scottish mathematician who based his theory on the experimental work of Michael Faraday. Heinrich Hertz (1857 - 1894), a German physicist, carried out experiments between 1886 and 1888 using a rapidly oscillating spark to produce electromagnetic waves, thereby experimentally proving Maxwell’s predictions. Therefore, by the latter part of the nineteenth century, the physical basis for many modern inventions utilizing electromagnetic-wave propagation - such as radio, television, and radar - was already established.

The basic physical principle involved is the coupling of electromagnetic energy into a propagation medium, which can be free space or the atmosphere, by means of a radiation element referred to as an antenna. Many different propagation modes are possible, depending on the physical configuration of the antenna and the characteristics of the propagation medium. The simplest case - which never occurs in practice - is propagation from a point source in a medium that is infinite in extent. The propagating wave fronts (surfaces of constant phase) in this case would be concentric spheres. Such a model might be used for the propagation of electromagnetic energy from a distance spacecraft to earth. Another idealized model, which approximates the propagation of radio waves from a commercial broadcast antenna, is that of a conducting line perpendicular to an infinite conducting plane. These and other idealized cases have been analyzed in books on electromagnetic theory. Our purpose is to point out basic aspects of propagation phenomena in practical channels.

Except for the case of propagation between two spacecrafts in outer space, the intermediate medium between transmitter and receiver is never well approximated by free space. Depending on the distance involved and the frequency of the radiated waveform, a terrestrial communication link may depend on line-of-sight, ground-wave, or ionospheric skip-wave propagation (see the figure below).

The table below lists frequency bands from 3 kHz to 107 GHz, along with letter designations for microwave bands used in radar among other applications. Note that the frequency bands are given in decades; the VHF band has 10 times as much frequency space as the HF band. The 2nd table below shows some bands of particular interest.

 

General application allocations are arrived at by international agreement. The present system of frequency allocations is administered by the International Telecommunications Union (ITU), which is responsible for the periodic convening of Administrative Radio Conferences on a regional or a worldwide basis (WARC before 1995; WRC 1995 and after, standing for World Radiocommunication Conference). The responsibility of the WRCs is the drafting, revision, and adoption of the Radio Regulations, which is an instrument for the international management of the radio spectrum.

In the United States, the Federal Communications Commission (FCC) awards specific applications within a band as well as licenses for their use. The FCC is directed by five commissioners appointed to five-year terms by the President and confirmed by the Senate. One commissioner is appointed as chairperson by the President.

At lower frequencies, or long wavelengths, propagating radio waves tend to follow the earth’s surface. At higher frequencies, or short wavelengths, radio waves propagate in straight lines. Another phenomenon that occurs at lower frequencies is reflection (or refraction) of radio waves by the ionosphere (a series of layers of charged particles at altitudes between 30 and 250 miles above the earth’s surface). Thus, for frequencies below about 100 MHz, it is possible to have skip-wave propagation. At night, when lower ionospheric layers disappear due to less ionization from the sun (the E, F1, and F2 layers coalesce into one layer - the F layer), longer skip-wave propagation occurs as a result of reflection fro the higher, single reflecting layer of the ionosphere.

Above about 300 MHz, propagation of radio waves is by line of sight, because the ionosphere will not bend radio waves in this frequency region sufficiently to reflect them back to the earth. At still higher frequencies, say above 1 or 2 GHz, atmospheric gases (mainly oxygen), water vapor, and precipitation absorb and scatter radio waves. This phenomenon manifests itself as attenuation of the received signal, with the attenuation generally being more severe the higher the frequency (there are resonance regions for absorption by gases that peak at certain frequencies). The following figure shows specific attenuation curves as a function of frequency for oxygen, water vapor, and rain. One must account for the possible attenuation by such atmospheric constituents in the design of microwave links, which are used, for example, in transcontinental telephone links and ground-to-satellite communication links.

At about 23 GHz, the first absorption resonance due to water vapor occurs, and at about 62 GHz a second one occurs due to oxygen absorption. These frequencies should be avoided in transmission of desired signals through the atmosphere, or undue power will be expended (one might, for example, use 62 GHz as a signal for cross-linking between two satellites, where atmospheric absorption is no problem, and thereby prevent an enemy on the ground from listening in). Another absorption frequency for oxygen occurs at 120 GHz, and two other absorption frequencies for water vapor occur at 180 and 350 GHz.

Communication at millimeter-wave frequencies (that is, at 30 GHz and higher) is becoming more important now that there is so much congestion at lower frequencies (the Advanced Technology Satellite, launched in the mid-1990s, employs an uplink frequency band around 20 GHz and a downlink frequency band at about 30 GHz). Communication at millimeter-wave frequencies is becoming more feasible because of technological advances in components and systems. Two bands at 30 and 60 GHz, the LMDS (Local Multipoint Distribution System) and MMDS (Multichannel Multipoint Distribution System) bands, have been identified for terrestrial transmission of wideband signals. Great care must be taken to design systems using these bands because of the high atmospheric and rain absorption as well as blockage by objects such as trees and buildings.

Somewhere above 1 THz (1000 GHz), the propagation of radio waves becomes optical in character. At a wavelength of 10 μm (0.00001 m), the carbon dioxide laser provides a source of coherent radiation, and visible-light lasers (for example, helium-neon) radiate in the wavelength region of 1 μm and shorter. Terrestrial communications systems employing such frequencies experience considerable attenuation on cloudy days, and laser communications over terrestrial links are restricted to optical fibers for the most part. Analyses have been carried out for the employment of laser communications cross-links between satellites.

Guided Electromagnetic-Wave Channels

Up until the last part of the twentieth century, the most extensive example of guided electromagnetic-wave channels is the part of the long-distance telephone network that uses wire lines, but this has almost exclusively been replaced by optical fiber. Communication between persons a continent apart was first achieved by means of voice frequency transmission (below 10,000 Hz) over open wire. Quality of transmission was rather poor. By 1952, use of the types of modulation known as double-sideband and single-sideband on high-frequency carriers was established. Communication over predominantly multipair and coaxial-cable lines produced transmission of much better quality. With the completion of the first trans-Atlantic cable in 1956, intercontinental telephone service improved significantly.

Bandwidths on coaxial-cable links are a few megahertz. The need for greater bandwidth initiated the development of millimeter-wave waveguide transmission systems. However, with the development of low-loss optical fibers, efforts to improve millimeter-wave systems to achieve greater bandwidth ceased. The development of optical fibers, in fact, has made the concept of a wired-city - wherein digital data and video can be piped to any residence or business with a city - nearly a reality. Modern coaxial-cable systems can carry only 13,000 voice channels per cable, but optical links are capable of carrying several times this number (the limiting factor being the current driver for the light source).

Optical Links

The use of optical links was, until recently, limited to short and intermediate distances. With the installation of trans-Pacific and trans-Atlantic optical cables in 1988 and early 1989, this is no longer true. The technological breakthroughs that preceded the widespread use of light waves for communication were the development of small coherent light sources (semiconductor lasers), low-loss optical fibers or waveguides, and low-noise detectors.

A typical fiber-optic communication system has a light source, which may be either a light-emitting diode or a semiconductor laser, in which the intensity of light is varied by the message source. The output of this modulator is the input to a light-conducting fiber. The receiver, or light sensor, typically consists of a photodiode. In a photodiode, an average current flows that is proportional to the optical power of the incident light. However, the exact number of charge carriers (that is, electrons) is random. The output of the detector is the sum of the average current that is proportional to the modulation and a noise component. This noise component differs from the thermal noise generated by the receiver electronics in that it is “bursty” in character. It is referred to as shot noise, in analogy to the noise made by shot hitting a metal plate. Another source of degradation is the dispersion of the optical fiber itself. For example, pulse-type signals sent into he fiber are observed as “smeared out” at the receiver. Losses also occur as a result of the connections between cable pieces and between cable and system components.

Finally, it should be mentioned that optical communications can take place through free space.


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