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Microwave transmission is thetransmission of information byelectromagnetic waves withwavelengths in themicrowave frequency range of 300 MHz to 300 GHz (1 m - 1 mm wavelength) of theelectromagnetic spectrum. Microwave signals are normally limited to theline of sight, so long-distance transmission using these signals requires a series of repeaters forming amicrowave relay network. It is possible to use microwave signals in over-the-horizon communications usingtropospheric scatter, but such systems are expensive and generally used only in specialist roles.
Although an experimental 40-mile (64 km) microwavetelecommunication link across theEnglish Channel was demonstrated in 1931, the development ofradar in World War II provided the technology for practical exploitation of microwave communication. During the war, the British Army introduced the Wireless Set No. 10, which used microwave relays tomultiplex eighttelephone channels over long distances. A link across the English Channel allowed GeneralBernard Montgomery to remain in continual contact with his group headquarters in London.
In the post-war era, the development of microwave technology was rapid, which led to the construction of several transcontinental microwave relay systems in North America and Europe. In addition to carrying thousands of telephone calls at a time, these networks were also used to send television signals for cross-country broadcast, and later, computer data.Communication satellites took over the television broadcast market during the 1970s and 80s, and the introduction of long-distancefibre optic systems in the 1980s and especially 90s led to the rapid rundown of the relay networks, most of which are abandoned.
In recent years, there has been an explosive increase in use of the microwave spectrum by new telecommunication technologies such aswireless networks, anddirect-broadcast satellites which broadcasttelevision andradio directly into consumers' homes. Larger line-of-sight links are once again popular for handing connections between mobile telephone towers, although these are generally not organized into long relay chains.
Microwaves are widely used forpoint-to-point communications because their smallwavelength allows conveniently-sizedantennas to direct them in narrow beams, which can be pointed directly at the receiving antenna. This use of tightly-focused direct beams allows microwave transmitters in the same area to use the same frequencies, without interfering with each other as lower frequency radio waves would. Thisfrequency reuse conserves scarce radio spectrum bandwidth. Another advantage is that the high frequency of microwaves gives the microwave band a very large information-carrying capacity; the microwave band has abandwidth 30 times that of all the rest of theradio spectrum below it. A disadvantage is that microwaves are limited toline of sight propagation; they cannot pass around hills or mountains as lower frequency radio waves can.
Microwave radio transmission is commonly used inpoint-to-pointcommunication systems on the surface of the Earth, insatellite communications, and indeep space radio communications. Other parts of the microwave radio band are used forradars,radio navigation systems, sensor systems, andradio astronomy.
The next higher frequency band of theradio spectrum, between 30 GHz and 300 GHz, are called "millimeter waves" because their wavelengths range from 10 mm to 1 mm. Radio waves in the millimeter wave band are stronglyattenuated by the gases of theatmosphere, which limits their practical transmission distance to a few kilometers, not enough for long-distance communication. The electronic technologies needed in the millimeter wave band are also in an earlier state of development than those of the microwave band.
More recently, microwaves have been used forwireless power transmission.
Microwave radio relay is a technology widely used in the 1950s and 1960s for transmitting information, such as long-distancetelephone calls andtelevision programs between two terrestrial points on a narrow beam of microwaves. In microwave radio relay, a microwavetransmitter anddirectional antenna transmits a narrow beam of microwaves carrying many channels of information on aline of sight path to another relay station where it is received by adirectional antenna and receiver, forming a fixed radio connection between the two points. The link was often bidirectional, using a transmitter and receiver at each end to transmit data in both directions. The requirement of a line of sight limits the separation between stations to the visual horizon, about 30 to 50 miles (48 to 80 km). For longer distances, the receiving station could function as a relay, retransmitting the received information to another station along its journey. Chains of microwave relay stations were used to transmit telecommunication signals over transcontinental distances. Microwave relay stations were often located on tall buildings and mountaintops, with their antennas on towers to get maximum range.
Beginning in the 1950s, networks of microwave relay links, such as theAT&T Long Lines system in the U.S., carried long-distance telephone calls and television programs between cities.[1] The first system, dubbed TDX and built by AT&T, connected New York and Boston in 1947 with a series of eight radio relay stations.[1] Through the 1950s, they deployed a network of a slightly improved version across the U.S., known asTD2. These included longdaisy-chained links that traversed mountain ranges and spanned continents. The launch ofcommunication satellites in the 1970s provided a cheaper alternative. Much of the transcontinental traffic is now carried by satellites andoptical fibers, but microwave relay remains important for shorter distances.
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Because in microwave transmission the waves travel in narrow beams confined to a line-of-sight path from one antenna to the other, they do not interfere with other microwave equipment, so nearby microwave links can use the same frequencies. The antennas must therefore be highlydirectional (highgain), and are installed in elevated locations such as large radio towers in order to be able to avoid the obstructions closer to the ground and transmit across long distances. Typical types of antenna used in radio relay link installations areparabolic antennas, dielectric lens, andhorn-reflector antennas, which have a diameter of up to 4 m (13 ft). Highly directive antennas permit an economical use of the available frequency spectrum, despite long transmission distances.
Because of the high frequencies used, aline-of-sight path between the stations is required. Additionally, in order to avoid attenuation of the beam, an area around the beam called the firstFresnel zone must be free from obstacles. Obstacles in the signal field cause unwantedattenuation. High mountain peaks or ridges are often ideal positions for the antennas.
In addition to the use of conventional repeaters with back-to-back radios transmitting on different frequencies, obstructions in microwave paths can also be dealt with by usingPassive repeater or on-frequency repeaters.
Obstacles, the curvature of the Earth, the geography of the area and reception issues arising from the use of nearby land (such as in manufacturing and forestry) are important issues to consider when planning radio links. In the planning process, it is essential that "path profiles" are produced, which provide information about the terrain and Fresnel zones affecting the transmission path. The presence of a water surface, such as a lake or river, along the path also must be taken into consideration since it can reflect the beam, and the direct and reflected beam can interfere with each other at the receiving antenna, causingmultipath fading. Multipath fades are usually deep only in a small spot and a narrow frequency band, so space and/or frequencydiversity schemes can be applied to mitigate these effects.
The effects of atmospheric stratification cause the radio path to bend downward in a typical situation so a major distance is possible as the earth equivalent curvature increases from 6,370 km (3,960 mi) to about 8,500 km (5,300 mi) (a 4/3 equivalent radius effect). Rare events of temperature, humidity and pressure profile versus height, may produce large deviations and distortion of the propagation and affect transmission quality. High-intensity rain and snow makingrain fade must also be considered as an impairment factor, especially at frequencies above 10 GHz. All of the detrimental factors mentioned in this section, collectively known aspath loss, make it necessary to compute suitable power margins, in order to maintain the link operative for a high percentage of time, like the standard 99.99% or 99.999% used in 'carrier class' services of most telecommunication operators.
The longest known microwave radio relay crosses theRed Sea with a 361 km (224 mi) hop between Jebel Erba (20°44′47″N36°50′18″E / 20.74639°N 36.83833°E /20.74639; 36.83833, Sudan, 2,109.8 m (6,922 ft) a.s.l.) and Jebel Dakka (21°5′36″N40°17′28″E / 21.09333°N 40.29111°E /21.09333; 40.29111, Saudi Arabia, 2,650.65 m (8,696.4 ft) a.s.l.). The link was built in 1979 byTelettra to transmit 300 telephone channels and one TV signal, in the 2.4 GHz frequency band. (Hop distance is the distance between two microwave stations).[2][unreliable source?]
Previous considerations represent typical problems characterizing terrestrial radio links using microwaves for the so-called backbone networks: hop lengths of a few tens of kilometers (typically 10 to 60 km (6.2 to 37.3 mi)) were largely used until the 1990s. Frequency bands below 10 GHz, and above all, the information to be transmitted, were a stream containing a fixed capacity block. The target was to supply the requested availability for the whole block (Plesiochronous digital hierarchy, PDH, orsynchronous digital hierarchy, SDH). Fading and/or multipath affecting the link for short time period during the day had to be counteracted by the diversity architecture. During 1990s microwave radio links begun widely to be used for urban links incellular network. Requirements regarding link distance changed to shorter hops (less than 10 km (6.2 mi), typically 3 to 5 km (1.9 to 3.1 mi)), and frequency increased to bands between 11 and 43 GHz and more recently, up to 86 GHz (E-band). Furthermore, link planning deals more with intense rainfall and less with multipath, so diversity schemes became less used. Another big change that occurred during the last decade was an evolution towardpacket radio transmission. Therefore, new countermeasures, such asadaptive modulation, have been adopted.
The emitted power is regulated for cellular and microwave systems. These microwave transmissions use emitted power typically from 0.03 to 0.30 W, radiated by a parabolic antenna on a narrow beam diverging by a few degrees (1 to 3-4). The microwave channel arrangement is regulated by International Telecommunication Union (ITU-R) and local regulations (ETSI,FCC). In the last decade the dedicated spectrum for each microwave band has become extremely crowded, motivating the use of techniques to increase transmission capacity such as frequency reuse,polarization-division multiplexing,XPIC,MIMO.
The history ofradio relay communication began in 1898 with the publication by Johann Mattausch in the Austrian journal, Zeitschrift für Elektrotechnik.[3][4] But his proposal was primitive and not suitable for practical use. The first experiments withradio repeater stations to relay radio signals were done in 1899 by Emile Guarini-Foresio.[3] However thelow frequency andmedium frequency radio waves used during the first 40 years of radio proved to be able to travel long distances byground wave andskywave propagation.[citation needed]
In 1931, an Anglo-French consortium headed by Andre C. Clavier demonstrated an experimental microwave relay link across theEnglish Channel using 10-foot (3 m) dishes.[5] Telephony, telegraph, andfacsimile data was transmitted over the bidirectional 1.7 GHz beams 40 miles (64 km) betweenDover, UK, andCalais, France. The radiated power, produced by a miniatureBarkhausen–Kurz tube located at the dish's focus, was one-half watt. A 1933 military microwave link between airports at St. Inglevert, France, and Lympne, UK, a distance of 56 km (35 miles), was followed in 1935 by a 300 MHz telecommunication link, the first commercial microwave relay system.[6]
The development ofradar duringWorld War II provided much of the microwave technology which made practical microwave communication links possible, particularly theklystron oscillator and techniques of designing parabolic antennas. Though not commonly known, the British Army used theWireless Set Number 10 in this role during World War II.[citation needed] The need for radio relay did not really begin until the 1940s exploitation ofmicrowaves, which traveled byline of sight and so were limited to a propagation distance of about 40 miles (64 km) by the visual horizon.[citation needed]
After the war, telephone companies used this technology to build large microwave radio relay networks to carry long-distance telephone calls. During the 1950s a unit of the US telephone carrier,AT&T Long Lines, built a transcontinental system of microwave relay links across the US which grew to carry the majority of USlong distance telephone traffic, as well astelevision network signals.[7] The main motivation in 1946 to use microwave radio instead of cable was that a large capacity could be installed quickly and at less cost.[citation needed] It was expected at that time that the annual operating costs for microwave radio would be greater than for cable. There were two main reasons that a large capacity had to be introduced suddenly: Pent-up demand for long-distance telephone service, because of the hiatus during the war years, and the new medium of television, which needed more bandwidth than radio.[citation needed] The prototype was called TDX and was tested with a connection between New York City and Murray Hill, the location of Bell Laboratories in 1946.[citation needed] The TDX system was set up between New York and Boston in 1947. The TDX was upgraded to the TD2 system, which used [the Morton tube, 416B and later 416C, manufactured by Western Electric] in the transmitters, and then later to TD3 that usedsolid-state electronics.[citation needed]
Remarkable were the microwave relay links toWest Berlin during theCold War, which had to be built and operated due to the large distance betweenWest Germany and Berlin at the edge of the technical feasibility. In addition to the telephone network, also microwave relay links for the distribution of TV and radio broadcasts. This included connections from the studios to the broadcasting systems distributed across the country, as well as between the radio stations, for example for program exchange.[citation needed]
Military microwave relay systems continued to be used into the 1960s, when many of these systems were supplanted withtropospheric scatter orcommunication satellite systems. When theNATO military arm was formed, much of this existing equipment was transferred to communications groups. The typical communications systems used by NATO during that time period consisted of the technologies which had been developed for use by the telephone carrier entities in host countries. One example from the USA is the RCA CW-20A 1–2 GHz microwave relay system which utilized flexibleUHF cable rather than the rigidwaveguide required by higher frequency systems, making it ideal for tactical applications. The typical microwave relay installation or portable van had two radio systems (plus backup) connecting twoline of sight sites. These radios would often carry 24 telephone channelsfrequency-division multiplexed on the microwave carrier (i.e. Lenkurt 33C FDM). Any channel could be designated to carry up to 18teletype communications instead. Similar systems from Germany and other member nations were also in use.[citation needed]
Long-distance microwave relay networks were built in many countries until the 1980s, when the technology lost its share of fixed operation to newer technologies such asfiber-optic cable andcommunication satellites, which offer a lower cost per bit.[citation needed]
During the Cold War, the US intelligence agencies, such as theNational Security Agency (NSA), were reportedly able to intercept Soviet microwave traffic using satellites such asRhyolite/Aquacade.[8] Much of the beam of a microwave link passes the receiving antenna and radiates toward the horizon, into space.[citation needed] By positioning a geosynchronous satellite in the path of the beam, the microwave beam can be received.
At the turn of the 21st century, microwave radio relay systems were used increasingly in portable radio applications. The technology is particularly suited to this application because of lower operating costs, a more efficientinfrastructure, and provision of direct hardware access to the portable radio operator.[citation needed]
Amicrowave link is a communications system that uses a beam of radio waves in the microwave frequency range to transmitvideo,audio, ordata between two locations, which can be from just a few feet or meters to several miles or kilometers apart. Microwave links are commonly used by television broadcasters to transmit programmes across a country, for instance, or from anoutside broadcast back to a studio.
Mobile units can be camera mounted, allowing cameras the freedom to move around without trailing cables. These are often seen on the touchlines of sports fields onSteadicam systems.
Terrestrial microwave relay links are limited in distance to the visual horizon, a few tens of miles or kilometers depending on tower height.Tropospheric scatter ("troposcatter" or "scatter") was a technology developed in the 1950s to allow microwave communication links beyond the horizon, to a range of several hundred kilometers. The transmitter radiates a beam of microwaves into the sky, at a shallow angle above the horizon toward the receiver. As the beam passes through thetroposphere a small fraction of the microwave energy is scattered back toward the ground by water vapor and dust in the air. A sensitive receiver beyond the horizon picks up this reflected signal. Signal clarity obtained by this method depends on the weather and other factors, and as a result, a high level of technical difficulty is involved in the creation of a reliable over horizon radio relay link. Troposcatter links are therefore only used in special circumstances where satellites and other long-distance communication channels cannot be relied on, such as in military communications.
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