Chapter 3 : Fundamentals of Transmission Systems: Technologies and Applications (Page Seven)

While all the various microwave frequency bands suffer from these limitations, the higher frequencies suffer to a greater extent. Those higher frequencies, set aside for digital microwave in the United States, suffer more from environmental interference including dust, smog, agricultural haze, and precipitation. Table 3.3 lists examples of frequency bands set aside by the Federal Communications Commission for commercial microwave [3-9].

The several frequency bands set aside for microwave are protected by regulatory authority in most countries and regions. Additionally, the placement of the antennae and the power level of transmission is regulated, with licenses granted to individual carriers and end users. However, difficulties have developed over time in certain areas (e.g., Europe and Asia) due to factors that include the small size of the individual nations; conflicting regulations, or the lack thereof; conflicting commercial and military applications; and unwillingness of national regulators to govern the use of radio frequencies on a coordinated, regional basis.

Configuration

Microwave radio consists of antennae centered within reflective dishes, that are attached to structures such as towers or buildings. Cables connect the antennae to the actual transmit/receive equipment.

Bandwidth

Microwave offers substantial bandwidth, often in excess of 6 Gbps. T1 (1.544 Mbps) capacity is routine, even in end user applications, with many private microwave networks operating at T3 (45 Mbps) rates.

Error Performance

Microwave, especially digital microwave, performs well in this regard, assuming proper design. However, such high frequency radio is particularly susceptible to environmental interference (e.g., precipitation, haze, smog, and smoke). Generally speaking, however, microwave performs well in this regard.

Distance

Microwave clearly is distance-limited, especially at the higher frequencies (see Table 3.2). This limitation can be mitigated through special and more complex arrays of antennae incorporating spatial diversity in order to collect more signal.

Security

As is the case with all radio systems, microwave is inherently not secure. Security must be imposed through encryption (scrambling) of the signal.

Cost

The acquisition, deployment and rearrangement costs of microwave can be high. However, it often compares very favorably with cabled systems, which require right-of-way, trenching, conduit, splicing, etc. Additionally, microwave is not affected by backhoe fade, as are cabled systems.

Regulation

Microwave, generally speaking, must be licensed on a case-by-case basis in order to avoid interference between adjacent systems; this process can be lengthy and costly. Additionally, local zoning ordinances and health and safety regulations may affect the placement of antennae.

Applications

Microwave originally was used for long haul voice and data communications. Competing long distance carriers, first in the United States, found microwave a most attractive alternative to cabled systems, due to the speed and low cost of deployment; where feasible, however, fiber optic technology is currently used in this regard. Contemporary applications include private networks, carrier bypass, temporary disaster recovery, interconnection of cellular radio switches, and as an alternative to cabled systems in consideration of difficult terrain.

Satellite Radio

Satellite radio, quite simply, is a nonterrestrial microwave transmission system utilizing a space relay station. The concept initially was offered in a letter published in Wireless World in February 1945 by Arthur C. Clarke, then a physicist at the British Interplanetary Society and since the author of 2001: A Space Odyssey and many other science fiction books. Since the launch of the Earlybird I satellite in 1965 proved the effectiveness of the concept of satellite communications, satellites have proved invaluable in extending the reach of voice, data, and video communications around the globe and into the most remote regions of the world. Exotic applications such as the Global Positioning System (GPS) would have been unthinkable without the benefit of satellites [3-10].

Geostationary Satellites

Contemporary satellite communications systems involve a satellite relay station which is launched into a geostationary, geosynchronous, or geostatic orbit, also known as a Clarke orbit. Such an orbit is approximately 22,237 miles (36,000 km.) above the equator (Figure 3.5). At that altitude and in an equatorial orbital slot, the satellite maintains its synchronization with the revolution of the earth. In other words, it maintains its relative position over the same spot of the earth’s surface. Consequently, transmit and receive earth stations (microwave dishes) can be pointed reliably at the satellite for communications purposes. Geosynchronous Earth Orbiting (GEO) satellites are also known as Fixed Satellite Systems (FSS).

The popularity of satellite communications has placed great demands on the international regulators (e.g., Intelsat) to manage and allocate available frequencies, as well as the limited number of orbital slots available for satellite positioning. As in the case of terrestrial microwave radio, there are a number of frequency bands assigned to satellite systems, most of which are in the MegaHertz (MHz) or GigaHertz (GHz) ranges. Due to the wide footprint, or area of coverage of a satellite, the frequencies must be carefully managed at national, regional and international levels. Generally speaking, geostationary satellites are positioned approximately 2º apart in order to minimize interference from adjacent satellites using overlapping frequencies [3-10].

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