Microwave Link Networks
History
A microwave link is a communications system that uses a beam of radio
waves in the microwave frequency range to transmit information between
two fixed locations on the earth. They are crucial to many forms of
communication and impact a broad range of industries. Broadcasters use
microwave links to send programs from the studio to the transmitter
location, which might be miles away. Microwave links carry cellular
telephone calls between cell sites. Wireless Internet service
providers use microwave links to provide their clients with high-speed
Internet access without the need for cable connections. Telephone
companies transmit calls between switching centers over microwave
links, although fairly recently they have been largely supplanted by
fiber-optic cables. Companies and government agencies use them to
provide communications networks between nearby facilities within an
organization, such as a company with several buildings within a city.
One of the reasons microwave links are so adaptable is that they are
broadband. That means they can move large amounts of information at
high speeds. Another important quality of microwave links is that they
require no equipment or facilities between the two terminal points, so
installing a microwave link is often faster and less costly than a
cable connection. Finally, they can be used almost anywhere, as long
as the distance to be spanned is within the operating range of the
equipment and there is clear path (that is, no solid obstacles)
between the locations. Microwaves are also able to penetrate rain,
fog, and snow, which means bad weather doesn’t disrupt transmission.
“我找到了一个新的女朋友，名字叫宽带！”“还是个日本妞？”记得或者听
过这个广告的，还是单身狗的，请和我一起默哀三分钟。
你可听过一个从从天而降的卫星通讯链路？
A simple one-way microwave link includes four major elements: a
transmitter, a receiver, transmission lines, and antennas. These basic
components exist in every radio communications system, including
cellular telephones, two-way radios, wireless networks, and commercial
broadcasting. But the technology used in microwave links differs
markedly from that used at the lower frequencies (longer wavelengths)
in the radio spectrum. Techniques and components that work well at low
frequencies are not useable at the higher frequencies (shorter
wavelengths) used in microwave links. For example, ordinary wires and
cables function poorly as conductors of microwave signals. On the
other hand, microwave frequencies allow engineers to take advantage of
certain principles that are impractical to apply at lower
frequencies. One example is the use of a parabolic or “dish” antenna
to focus a microwave radio beam. Such antennas can be designed to
operate at much lower frequencies, but they would be too large to be
economical for most purposes.
In a microwave link the transmitter produces a microwave signal that
carries the information to be communicated. That information—the
input—can be anything capable of being sent by electronic means, such
as a telephone call, television or radio programs, text, moving or
still images, web pages, or a combination of those media.
The transmitter has two fundamental jobs: generating microwave energy
at the required frequency and power level, and modulating it with the
input signal so that it conveys meaningful information. Modulation is
accomplished by varying some characteristic of the energy in response
to the transmitter’s input. Flashing a light to transmit a message in
Morse Code is an example of modulation. The differing lengths of the
flashes (the dots and dashes), and the intervals of darkness between
them, convey the information—in this case a text message.
The second integral part of a microwave link is a transmission
line. This line carries the signal from the transmitter to the antenna
and, at the receiving end of the link, from the antenna to the
receiver. In electrical engineering, a transmission line is anything
that conducts current from one point to another. Lamp cord, power
lines, telephone wires and speaker cable are common transmission
lines. But at microwave frequencies, those media excessively weaken
the signal. In their place, engineers use coaxial cables and,
especially, hollow pipes called waveguides.
The third part of the microwave system is the antennas. On the
transmitting end, the antenna emits the microwave signal from the
transmission line into free space. “Free space” is the electrical
engineer’s term for the emptiness or void between the transmitting
and receiving antennas. It is not the same thing as “the
atmosphere,” because air is not necessary for any type of radio
transmission (which is why radio works in the vacuum of outer
space). At the receiver site, an antenna pointed toward the
transmitting station collects the signal energy and feeds it into the
transmission line for processing by the receiver.
Antennas used in microwave links are highly directional, which means
they tightly focus the transmitted energy, and receive energy mainly
from one specific direction. This contrasts with antennas used in many
other communications systems, such as broadcasting. By directing the
transmitter’s energy where it’s needed—toward the receiver—and by
concentrating the received signal, this characteristic of microwave
antennas allows communication over long distances using small amounts
of power.
一个重点，具体问题具体分析。什么时候用什么波段，什么时候用定向天线，
什么时候用全向天线。
Between the link’s antennas lies another vital element of the
microwave link—the path taken by the signal through the earth’s
atmosphere. A clear path is critical to the microwave link’s
success. Since microwaves travel in essentially straight lines,
man-made obstacles (including possible future construction) that might
block the signal must either be overcome by tall antenna structures or
avoided altogether. Natural obstacles also exist. Flat terrain can
create undesirable reflections, precipitation can absorb or scatter
some of the microwave energy, and the emergence of foliage in the
spring can weaken a marginally strong signal, which had been adequate
when the trees were bare in the winter. Engineers must take all the
existing and potential problems into account when designing a
microwave link.
Which, however, is IMPOSSIBLE. That means 80% is fine. 100% is out
of the question.