The term "Microwave" is a broad term that covers the UHF (Ultra High Frequency with frequencies between 300MHz and 3GHz) to the EHF (Extremely High Frequency with frequencies between 30GHz to 300GHz). Licensed microwave links and unlicensed wireless Ethernet bridges typically operate in the SHF (Super High Frequency with frequencies between 3GHz to 30GHz) and the EHF bands.
A general rule of thumb is that lower the frequency the farther the signal will travel. Also lower frequencies the lower the throughput and higher the frequency the higher the throughput. Again this is in general terms and depends on the wireless radio hardware used.
Primary advantages of microwave system
The primary advantage of microwave radio's that it can carry thousands of voice channels without physically connected cables between points of communication, thus avoiding the need for continuous right of way between points. Further radio is better able to span water, mountains, heavily wooded terrain that poses barriers to cable installation. Most long distance links today are coaxial cable or microwave radio.
The tall towers with large horns or dish antennas that can be seen in the country are repeater system for radio link systems. These systems are popular for carrying large quantities of data and voice traffic because they do not require right of way acquisition between towers, they just require the purchase or lease of only a small area of ground for installation of each tower. Because of their very high operating frequencies they can carry large quantities of information per radio systems.
Line-of-Sight Microwave Systems
"Line-of-sight" is a term used in radio system design to describe a condition in which radio device antennas can actually see each other. High frequency radios, such as those used in Spread Spectrum Radio require line-of-sight between antennas.
In many instances there may be obstacles to overcome such as buildings, trees, small hills and elevated roads, and it may not be possible to confirm that line-of-sight exists without additional aid. Keep in mind that even a "perfectly clear" visual path may not actually be so. When establishing line-of-sight, it is extremely important to plan for the future. In urban areas, new building construction may result in total path obstruction. In areas where construction is not anticipated, the rapid growth of trees or foliage may severely affect the path over time.
Assuming an appropriate line-of-sight path from radio site to radio site can be established, both the feasibility and viability of a point-to-point microwave radio link will be dependent upon the gains, losses and receiver sensitivity corresponding with the system. Gains are associated with the transmitter power output of the radio, and the gains of both the transmitting and receiving antennas.
Radio Transmitter Power
Radio transmitters are described in terms of power output expressed in watts. The power output may also be expressed in terms of decibels of gain (dB). Radio receivers are rated in terms of sensitivity (ability to receive a minimal signal). The rating is listed in terms of milliwatts (mW), or decibels of gain (dB). Antenna cable is rated in terms of signal loss per foot and expressed as dB of loss per foot. The antenna is rated in terms of gain (dB). There are a number of software programs that will calculate path loss by frequency and use the specifications of the system hardware to help determine the overall system feasibility.
Microwave backbone
For applications requiring an interface between a microwave radio backbone and a 2-way radio system, E&M interfaces can be used to interconnect the two systems. This allows remote communications between radios via the microwave radio backbone beyond the normal coverage area of the individual radio systems.
Cellular Networks
The mobile telephone service that preceded cellular service was known as Improved Mobile Telephone Service (IMTS), which operated in several frequency ranges: 35 to 44 MHz, 152 to 158 MHz, and 454 to 512 MHz. But IMTS suffered from call setup delay, poor transmission, limited frequency reuse, and lack of service areas. IMTS was supplanted by Advanced Mobile Phone Service (AMPS) that operates in the 800- to 900-MHz range. AMPS overcame the limitations of IMTS and set the stage for the explosive growth of cellular service which continues today.
Proposed by AT&T in 1971, AMPS is still the standard for analog cellular networks. It was trialed in 1978, and in the early 1980s cellular systems based on the standard were being installed throughout North America. Although AMPS was not the first system for wireless telephony, the existence of a single standard enabled the United States to dominate analog cellular.
A digital version of AMPS—referred to as D-AMPS—solves many of these problems, while providing increased capacity and a greater range of services. Both AMPS and D-AMPS operate in the 800-MHz band and can coexist with each other. D-AMPS can be implemented with time division multiple access (TDMA) as the underlying technology. TDMA provides 10 to 15 times more channel capacity than AMPS networks and allows the introduction of new feature-rich services such as data communications, voice mail, call waiting, call diversion, voice encryption, and calling line identification.
The primary wireless communications link established with the cellular telephone is to the nearest cell site. The cellular carrier's network consists of a number of cell sites, each typically covering a radius of approximately one to ten miles, which are in turn connected to an MTSO either via cable or microwave radio links (Figure above). The system is engineered so that the cell sites are located in close enough proximity to one another to provide seamless networking capability.
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