Line-of-sight Microwave Links

A line-of-sight microwave link uses highly directional transmitter and receiver antennas to communicate via a narrowly focused radio beam. The transmission path of a line-of-sight microwave link can be established between two land-based antennas, between a land-based antenna and a satellite-based antenna, or between two satellite antennas. Broadband line-of-sight links operate at frequencies between 1 and 25 gigahertz (the centimetre wavelength band) and can have transmission bandwidths approaching 600 megahertz.

 In the United States, line-of-sight microwave links are used for military communications, studio feeds for broadcast and cable television, and common carrier trunks for inter-urban telephone traffic. A typical long-distance, high-capacity digital microwave radio relay system links two points 2,500 kilometres apart by using a combination of nine terrestrial and satellite repeaters. Each repeater operates at 4 gigahertz, transmitting seven 80-megahertz-bandwidth channels at 200 megabits per second per channel.

The maximum range of land-based line-of-sight systems is limited by the curvature of the Earth. For this reason, a microwave radio repeater with transmitter and receiver dishes mounted on 30-metre (100-foot) towers has a maximum range of approximately 50 kilometres (30 miles), whereas the maximum range will increase to approximately 80 kilometres if the towers are raised to 90 metres (300 feet).

Line-of-sight microwave links are subject to severe fading, owing to refraction of the transmitted beam along the propagation path. Under normal conditions the refractive index of the atmosphere decreases with increasing altitude. This means that upper portions of the beam propagate faster, so that the beam is slightly bent toward the Earth, producing transmission ranges that go beyond the geometric horizon. However, temporary atmospheric disturbances can change the refractive index profile, causing the beam to bend differently and, in severe cases, to miss the receiver antenna entirely. For example, a strong negative vapour gradient over a body of water, with vapour concentration increasing closer to the surface, can cause a bending of the beam toward the Earth that is much sharper than the Earth's curvature — a phenomenon called ducting.

UNIT 17

Satellite Links

A telecommunications satellite is a sophisticated space-based cluster of radio repeaters, called transponders, that link terrestrial radio transmitters to terrestrial radio receivers through an uplink (a link from terrestrial transmitter to satellite receiver) and a downlink (a link from satellite transmitter to terrestrial receiver). Most telecommunications satellites have been placed in geostationary orbit (GEO), a circular orbit 35,785 kilometres (22,235 miles) above the Earth in which the period of their revolution around the Earth equals the period of the Earth's rotation. Remaining thus fixed above one point on the Earth's surface (in virtually all cases, above the Equator), GEO satellites can view a stationary patch covering more than one-third of the globe. By virtue of such a wide area of coverage, GEO satellites can deliver a variety of telecommunications services, such as long-distance point-to-point transmission, wide area broadcasting (from a single transmitter to multiple receivers), or wide area report-back services (from multiple transmitters to a single receiver). Modern GEO satellites have several microwave transmitter and receiver antennas, which allow a single satellite to form a combination of large area-of-coverage beams for broadcasting and small area-of-coverage “spot beams” for point-to-point communications. By switching between these beams upon request — a process known as demand assigned multiple access (DAMA) — multibeam satellites can link widely distributed mobile and fixed users that cannot be linked economically by optical fibre cables or earthbound radio relays.

A typical modern GEO satellite such as Intelsat VII has more than 50 separate microwave transponders that service a number of simultaneous users based on a time-division multiple access (TDMA) protocol. Each transponder consists of a receiver tuned to a specific channel in the uplink frequency band, a frequency shifter to lower the received microwave signals to a channel in the downlink band, and a power amplifier to produce an adequate transmitting power. A single transponder operates within a 36-megahertz bandwidth and is assigned one of many functions, including voice telephony (at 400 two-way voice channels per transponder), data communication (at transmission rates of 120 megabits per second or higher), television and FM radio broadcasting, and VSAT service.

Many GEO satellites have been designed to operate in the so-called C band, which employs uplink/downlink frequencies of 6/4 gigahertz, or in the Ku band, in which uplink/downlink frequencies are in the range of 14/11 gigahertz. These frequency bands have been selected to exploit spectral “windows,” or regions within the microwave band in which there is low atmospheric attenuation and low external noise. Different microwave frequencies are used for the uplink and downlink in order to minimize leakage of power from on-board transmitters to on-board receivers. Lower frequencies are chosen for the more difficult downlink because atmospheric attenuation is less at lower frequencies.

Because of the huge growth in satellite telecommunication since the 1970s, there are very few remaining slots for GEO satellites operating at frequencies below 17 gigahertz. This has led to the development of satellites operating in the Ka band (30/20 gigahertz), despite the higher atmospheric attenuation of signals at these frequencies.

UNIT 18

Optical Transmission

Optical communication employs a beam of modulated monochromatic light to carry information from transmitter to receiver. The light spectrum spans a tremendous range in the electromagnetic spectrum, extending from the region of 10 terahertz (10⁴ gigahertz) to 1 million terahertz (10⁹ gigahertz). This frequency range essentially covers the spectrum from far infrared (0.3-millimetre wavelength) through all visible light to near ultraviolet (0.0003-micrometre wavelength). Propagating at such high frequencies, optical wavelengths are naturally suited for high-rate broadband telecommunication. For example, amplitude modulation of an optical carrier at the near-infrared frequency of 300 terahertz by as little as 1 percent yields a transmission bandwidth that exceeds the highest available coaxial cable bandwidth by a factor of 1,000 or more.

Practical exploitation of optical media for high-speed telecommunication over large distances requires a strong light beam that is nearly monochromatic, its power narrowly concentrated around a desired optical wavelength. Such a carrier would not have been possible without the invention of the ruby laser, first demonstrated in 1960, which produces intense light with very narrow spectral line width by the process of coherent stimulated emission. Today, semiconductor injection-laser diodes are used for high-speed long-distance optical communication.

Two kinds of optical channels exist: the unguided free-space channel, where light freely propagates through the atmosphere, and the guided optical fibre channel, where light propagates through an optical waveguide.

The free-space channel

The loss mechanisms in a free-space optical channel are virtually identical to those in a line-of-sight microwave radio channel. Signals are degraded by beam divergence, atmospheric absorption, and atmospheric scattering. Beam divergence can be minimized by collimating (making parallel) the transmitted light into a coherent narrow beam by using a laser light source for a transmitter. Atmospheric absorption losses can be minimized by choosing transmission wavelengths that lie in one of the low-loss “windows” in the infrared, visible, or ultraviolet region. The atmosphere imposes high absorption losses as the optical wavelength approaches the resonant wavelengths of gaseous constituents such as oxygen (O₂), water vapour (H₂O), carbon dioxide (CO₂), and ozone (O₃), as shown by the peaks of the solid line in Figure 2. On a clear day the attenuation of visible light may be one decibel per kilometre or less, but significant scattering losses can be caused by any variability in atmospheric conditions, such as haze, fog, rain, or airborne dust.

The high sensitivity of optical signals to atmospheric conditions has hindered development of free-space optical links for outdoor environments. A simple and familiar example of an indoor free-space optical transmitter is the handheld infrared remote control for television and high-fidelity audio systems. Free-space optical systems also are quite common in measurement and remote sensing applications, such as optical range-finding and velocity determination, industrial quality control, and laser altimetry radar (known as LIDAR).

Optical Fibre Channels

In contrast to wire transmission, in which an electric current flows through a copper conductor, in optical fibre transmission an electromagnetic (optical) field propagates through a fibre made of a nonconducting dielectric. Because of its high bandwidth, low attenuation, interference immunity, low cost, and light weight, optical fibre is becoming the medium of choice for fixed high-speed digital telecommunications links. Optical fibre cables are supplanting copper wire cables in both long-distance applications, such as the feeder and trunk portions of telephone and cable television loops, and short-distance applications, such as local area networks for computers and home distribution of telephone, television, and data services. For example, the standard Bellcore OC-48 optical cable, used for trunking of digitized data, voice, and video signals, operates at a transmission rate of up to 2.4 gigabits (2.4 billion binary digits) per second per fibre.