Microwave Radio and based systems:

Microwave frequencies range from 300 MHz to 30 GHz, corresponding to wavelengths of 1 meter to 1 cm.  These frequencies are useful for terrestrial and satellite communication systems, both fixed and mobile.  In the case of point-to-point radio links, antennas are placed on a tower or other tall structure at sufficient height to provide a direct, unobstructed line-of-sight (LOS) path between the transmitter and receiver sites. In the case of mobile radio systems, a single tower provides point-to-multipoint coverage, which may include both LOS and non-LOS paths.  LOS microwave is used for both short- and long-haul telecommunications to complement wired media such as optical transmission systems.  Applications include local loop, cellular back haul, remote and rugged areas, utility companies, and private carriers.   Early applications of LOS microwave were based on analog modulation techniques, but today’s microwave systems used digital modulation for increased capacity and performance. 

Microwaves are widely used for point-to-point communications because their small wavelength allows conveniently-sized antennas to direct them in narrow beams, which can be pointed directly at the receiving antenna. This allows nearby microwave equipment to use the same frequencies without interfering with each other, as lower frequency radio waves do. Another advantage is that the high frequency of microwaves gives the microwave band a very large information-carrying capacity; the microwave band has a bandwidth 30 times that of all the rest of the radio spectrum below it. A disadvantage is that microwaves are limited to line of sight propagation; they cannot pass around hills or mountains as lower frequency radio waves can.

Microwave radio transmission is commonly used in point-to-point communication systems on the surface of the Earth, in satellite communications, and in deep space radio communications. Other parts of the microwave radio band are used for radars, radio navigation systems, sensor systems, and radio astronomy.

Standards:

In the United States, radio channel assignments are controlled by the Federal Communications Commission (FCC) for commercial carriers and by the National Telecommunications and Information Administration (NTIA) for government systems.

The FCC's regulations for use of spectrum establish eligibility rules, permissible use rules, and technical specifications. FCC regulatory specifications are intended to protect against interference and to promote spectral efficiency. Equipment type acceptance regulations include transmitter power limits, frequency stability, out-of-channel emission limits, and antenna directivity.

The International Telecommunications Union Radio Committee (ITU-R) issues recommendations on radio channel assignments for use by national frequency allocation agencies. Although the ITU-R itself has no regulatory power, it is important to realize that ITU-R recommendations are usually adopted on a worldwide basis.

The Digital Microwave System supports and interconnects the following two way-radio services:  

• Local Base - Provides instantaneous two-way radio communications between a dispatch center and employees working in the immediate area or close proximity. As an example a State Parks Dispatcher at Park headquarters could contact a Park Ranger working on the other side of the Park.
• Remote Base - Provides instantaneous two-way radio communications between a dispatch center and employees working long distances from the center. As an example a State Police Dispatcher could contact an officer on patrol possibly hundreds of miles from the dispatch center.
• Single Channel Console - Provides a convenient, functional, and inexpensive interface between a dispatcher and one local or remote base station.
• Multi Channel Console - Allows a dispatcher to control multiple local and/or remote base stations from a single dispatch center.
• Mobile Radio - Usually installed in a service vehicle, allows employees in the field to communicate to a dispatch center through a local or remote base station.
• Mobile Radio with Repeater - Allows an employee, typically police officers, to operate their mobile radio when they are out of the vehicle. An office on foot can instantaneously communicate from a hand held radio to a dispatch center hundreds of mile away.
• Radio Pager - A radio pager functions as a hand held radio and can also receive pages from any telephone.
• Portable Radio - Provides communication between a small handheld device and another portable radio in the area or through a base station to a dispatch center. 

Advantage: Able to Transmit Large Quantities of Data

According to "Microwave Communication," microwave radio systems have the capacity to broadcast great quantities of information because of their higher frequencies. They use repeaters (a device that receives the transmitting signal through one antenna, converts it into an electrical signal and retransmits it) to transmit large volumes of data over great distances. Microwave radio communication systems propagate signals through the earth's atmosphere. These signals are sent between transmitters and receivers that lie on top of towers. This allows microwave radio systems to transmit thousands of data channels between two points without relying on a physical transmitting medium (optical fibers or metallic cables).

Advantage: Relatively Low Costs

Microwave communication systems have relatively low construction costs compared with other forms of data transmission, such as wire-line technologies. A microwave communication system does not require physical cables or expensive attenuation equipment (devices that maintain signal strength during transmission). Mountains, hills and rooftops provide inexpensive and accessible bases for microwave transmission towers.

Disadvantage: Line of Sight Technology

Microwave radio systems are a line of sight technology, meaning the signals will not pass through objects (e.g., mountains, buildings and airplanes). This drawback limits microwave communication systems to line of sight operating distances. Signals flow between one fixed point to another, provided no solid obstacle disrupts the flow.

Disadvantage: Subject to Electromagnetic and Other Interference

 According to "Rural America at the Crossroads: Networking for the Future," microwave radio signals are affected by electromagnetic interference (EMI). EMI is any disturbance that degrades, obstructs or interrupts the performance of microwave signals. Microwave signal disruption EMI is caused by electric motors, electric power transmission lines, wind turbines, television/radio stations and cell phone transmission towers. Wind turbines, for instance, scatter and diffract TV, radio and microwave signals when placed between signal transmitters and receivers. Microwave radio communication is also affected by heavy moisture, snow, vapor, rain and fog due to rain fade (the absorption of microwave signals by ice, snow or rain, causing signal degradation and distortion).

The Statewide Digital Microwave System also supports broadband data services and telephone to agency field offices: 

• T-1 or greater bandwidth connecting isolated field offices to headquarter data networks and Internet services.
• Telephone service to remote stations lacking commercial providers.
• Collocation agreements with cell providers to expand cellular service to rural areas.
• Security alarming and video monitoring of critical State communications facilities. 

Dispatch operations include monitoring, gathering, processing and disseminating all radio transmissions from field units of all agencies as well as all related incoming and outgoing telephone communications for each agency served. 

 

http://www.eogogics.com/talkgogics/tutorials/microwave-line-of-sight-systems

 

http://en.wikipedia.org/wiki/Microwave_transmission

 

http://www.ehow.com/list_6137210_microwave-radio-communications-advantages-disadvantages.html

 

http://www.doit.state.nm.us/service_catalog/radio.html