A microwave is a short radio wave that varies from 1 millimeter to 30 centimeters in length. Because microwaves can pass through the ionosphere, which blocks or reflects longer radio waves, microwaves are well suited for satellite communications. This reliability also makes microwave well suited to terrestrial communications as well, such as those delivered by Local Multipoint Distribution Service (LMDS) and Multichannel Multipoint Distribution Service (MMDS).

Much of the microwave technology in use today for pointto- point communications was derived from radar developed during World War II. Initially, microwave systems carried multiplexed speech signals over common carrier and military communications networks; but today they are used to handle all types of information—voice, data, facsimile, and video—in either an analog or digital format.

The first microwave transmission occurred in 1933, when European engineers succeeded in communicating reliably across the English Channel—a distance of about 12 miles (20 kilometers). In 1947, the first commercial microwave network in the United States came online. Built by Bell Laboratories, this was a New York to Boston system consisting of 10 relay stations carrying television signals and multiplexed voice conversations.

A year later, New York was linked to San Francisco via 109 microwave relay stations. By the 1950s, transcontinental microwave networks were routinely handling over 2000 voice channels on hops averaging 25 miles (41.5 kilometers) in length. By the 1970s, just about every single telephone call, television show, telegram, or data message that crossed the country spent some time on a microwave link.

Over the years, microwave systems have matured to the point that they have become major components of the nation’s Public Switched Telephone Network (PSTN) and an essential technology with which private organizations can satisfy internal communications requirements. Microwave systems can even exceed the 99.99 percent reliability standard set by the telephone companies for their phone lines.

Early technology limited the operations of microwave systems to radio spectrum in the 1-GHz range, but because of improvements in solid-state technology, today’s government systems are transmitting in the 153-GHz region, while commercial systems are transmitting in the 40-GHz region with FCC approval.

The 64- to 71-GHz band is reserved for intersatellite links. These frequency bands offer short-range wireless radio systems the means to provide communications capacities approaching those now achievable only with coaxial cable and optical fiber. These spectrum allocations offer a variety of possibilities, such as use in short-range, high-capacity wireless systems that support educational and medical applications, and wireless access to libraries or other information databases.

In addition to telecommunication service providers, shorthaul microwave equipment is used routinely by hotel chains, CATV operators, and government agencies. Corporations are making greater use of short-haul microwave, especially for extending the reach of local area networks (LANs) in places where the cost of local T1 lines is prohibitive. Common carriers use microwave systems for backup in the event of fiber cuts and in terrain where laying fiber is not economically feasible.

Cellular service providers use microwave to interconnect cell sites with each other as well as to the regular telephone net-work. Some interexchange carriers (and corporations) even use short-haul microwave to bypass Incumbent Local Exchange Carriers (ILECs) to avoid lengthy service provisioning delays and to avoid paying hefty local access charges.

There are more than 25,000 microwave networks in the United States alone. There are basically two microwave network configurations: point-to-point and point-to-multipoint. The first type meets a variety of low- and medium-density communications requirements, ranging from simple links to more complex extended networks, such as:

• Sub-T1/E1 data links
• Ethernet/Token Ring LAN extensions
• Low-density digital backbone for wide area mobile radio and paging services
• PBX/OPX/FX voice, fax, and data extensions
• Facility-to-facility bulk data transfer

Point-to-multipoint microwave systems provide communications between a central command and control site and remote data units. Atypical radio communications system provides connections between the master control point and remote data collection and control sites. Repeater configurations are also possible. The basic equipment requirements for a point-to-multipoint system include:

• Antennas For the master, an omnidirectional antenna; for the remotes, a highly directional antenna aimed at the master station’s location.
• Tower (or other structure, such as a mast) To support the antenna and transmission line.
• Transmission line Low-loss coaxial cable connecting the antenna and the radio.
• Master station radio Interfaces with the central computer; it transmits and receives data from the remote radio sites and can request diagnostic information from the remote transceivers. The master radio also can serve as a repeater.
• Remote radio transceiver Interfaces to the remote data unit; receives and transmits to the master radio.
• Management station A computer that can be connected to the master station’s diagnostic system either directly or remotely for control and collection of diagnostic information from master and remote radios.

Traditionally, cable system operators have used microwave transmission systems to link cable networks. These Cable Antenna Relay Services (CARS) have experienced declining usage as cable operators have deployed more optical fiber in their transmission systems. However, improvements in microwave technology and the opening of new frequencies for commercial use have contributed to the resurgence in shorthaul microwave.

In the broadcast industry, short-haul microwave is often referred to as “wireless cable,” which comes in the form of Local Multipoint Distribution Service (LMDS) and Multichannel Multipoint Distribution Service (MMDS). These wireless cable technologies have two key advantages.

One is availability—with an FCC license, they can be made available in areas of scattered population and other areas where it is too expensive to build a traditional cable station. The other is affordability—because of the lower costs of building a wireless cable station, savings can be passed on to subscribers.

The radio spectrum is the part of the natural spectrum of electromagnetic radiation lying between the frequency limits of 9 kHz and 400 GHz. In the United States, regulatory responsibly for the radio spectrum is divided between the FCC and the National Telecommunications and Information Administration (NTIA).

The FCC, which is an independent regulatory agency, administers spectrum for non-federal government use, and the NTIA, which is an operating unit of the Department of Commerce, administers spectrum for federal government use. Within the FCC, the Office of Engineering and Technology (OET) provides advice on technical and policy issues pertaining to spectrum allocation and use. This office manages the spectrum and provides leadership to create new opportunities for competitive technologies and services for the American public.

Microwave is now almost exclusively a short-haul transmission medium, while optical fiber and satellite have become the long-haul transmission media of choice. Short-haul microwave is now one of the most agile and adaptable transmission media available, with the capability of supporting data, voice, and video. It is also used to back up fiberoptic facilities and to provide communications services in locations where it is not economically feasible to install fiber.