horn-reflector antennas on the roof of a telephone switching center in
Seattle, Washington, part of the U.S.
AT&T Long Lines microwave relay network in
Hamburg, Germany
Microwave radio relay is a technology widely used in the 1950s and 1960s for transmitting information, such as long-distance
telephone calls and
television programs between two terrestrial points on a narrow beam of microwaves. In microwave radio relay, a microwave
transmitter and
directional antenna transmits a narrow beam of microwaves carrying many channels of information on a
line of sight path to another relay station where it is received by a
directional 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 . 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 the
AT&T Long Lines system in the U.S., carried long-distance telephone calls and television programs between cities. 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. 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 ) 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, or
synchronous 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 in
cellular network. Requirements regarding link distance changed to shorter hops (less than , typically ), 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 toward
packet radio transmission. Therefore, new countermeasures, such as
adaptive 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.
History portable microwave relay station, 1945. Microwave relay systems were first developed in World War II for secure military communication. The history of
radio relay communication began in 1898 with the publication by Johann Mattausch in the Austrian journal, Zeitschrift für Elektrotechnik. But his proposal was primitive and not suitable for practical use. The first experiments with
radio repeater stations to relay radio signals were done in 1899 by Emile Guarini-Foresio. Telephony, telegraph, and
facsimile data was transmitted over the bidirectional 1.7 GHz beams between
Dover, UK, and
Calais, France. The radiated power, produced by a miniature
Barkhausen–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 , was followed in 1935 by a 300 MHz telecommunication link, the first commercial microwave relay system. The development of
radar during
World War II provided much of the microwave technology which made practical microwave communication links possible, particularly the
klystron oscillator and techniques of designing parabolic antennas. Though not commonly known, the British Army used the
Wireless Set Number 10 in this role during World War II. The need for radio relay did not really begin until the 1940s exploitation of
microwaves, which traveled by
line of sight and so were limited to a propagation distance of about by the visual horizon. 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 US
long distance telephone traffic, as well as
television network signals. 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. 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. 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. 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 used
solid-state electronics. Remarkable were the microwave relay links to
West Berlin during the
Cold War, which had to be built and operated due to the large distance between
West 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. Military microwave relay systems continued to be used into the 1960s, when many of these systems were supplanted with
tropospheric scatter or
communication satellite systems. When the
NATO 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 flexible
UHF cable rather than the rigid
waveguide 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 two
line of sight sites. These radios would often carry 24 telephone channels
frequency-division multiplexed on the microwave carrier (i.e. Lenkurt 33C FDM). Any channel could be designated to carry up to 18
teletype communications instead. Similar systems from Germany and other member nations were also in use. 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 as
fiber-optic cable and
communication satellites, which offer a lower cost per bit. During the Cold War, the US intelligence agencies, such as the
National Security Agency (NSA), were reportedly able to intercept Soviet microwave traffic using satellites such as
Rhyolite/Aquacade. Much of the beam of a microwave link passes the receiving antenna and radiates toward the horizon, into space. 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 efficient
infrastructure, and provision of direct hardware access to the portable radio operator.
Microwave link A
microwave link is a communications system that uses a beam of radio waves in the microwave frequency range to transmit
video,
audio, or
data 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 an
outside 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 on
Steadicam systems.
Properties of microwave links • Involve
line of sight (LOS) communication technology • Affected greatly by environmental constraints, including
rain fade • Have very limited penetration capabilities through obstacles such as hills, buildings and trees • Sensitive to high
pollen count • Signals can be degraded during
Solar proton events • Propagation delays are lower than in fiber optic networks because the speed of light in air is faster than in optical cable
Uses of microwave links • In communications between
satellites and base stations • As backbone carriers for cellular systems • In short-range indoor communications • Linking remote and regional telephone exchanges to larger (main) exchanges without the need for copper/optical fibre lines • Measuring the intensity of rain between two locations • To give financial advantage to high frequency traders at one stock exchange via faster knowledge of price changes at a distant exchange == Troposcatter ==