A microwave link is a communications system that uses a beam of radio waves (> 1GHz) in the microwave frequency range to transmit information between two fixed locations on the earth.
Microwaves are short-wavelength, high-frequency signals that occupy the electromagnetic spectrum 1,000 MHz (1 GHz) to 1,000 GHz (1 terahertz). This is just above the radio frequency range and just below the infrared range.
Broadcasters use microwave links to send programs from the studio to the transmitter location, which might be miles away. Wireless Internet service providers use microwave links to provide their clients with high-speed Internet access without the need for cable connections.
Microwave systems are used to relay multiplex signals from point to point. A simplex relay system provides one-way communications and consists of a transmitting terminal, a certain number of repeaters.
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.
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.
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.
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. 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. 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.
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.
At the end of the link is the final component, the receiver. Here, information from the microwave signal is extracted and made available in its original form. To accomplish this, the receiver must demodulate the signal to separate the information from the microwave energy that carries it. The receiver must be capable of detecting very small amounts of microwave energy, because the signal loses much of its strength on its journey.
Regulatory and Licensing
Each country has a varying requirement for the licensing of microwave radio links. In most cases this license only addresses the transmitter, but in the same instance, it offers regulatory protection to any interference that may affect the microwave receiver.
Ref:
http://www.ieeeghn.org/wiki/index.php/Microwave_Link_Networks
http://www.itblogs.in/communication/technology/microwave-communication-an-introduction/
http://www.trangosys.com/products/point-to-point-wireless-backhaul/encryption-ipsec-tunnel-application-diagram.shtml
http://www.nec.co.jp/press/en/0810/0201.html
An example of 4-bit pulse code modulation showing quantization and sampling of a signal (red).
