Link Block Diagram 101iz

Link Block Diagram This diagram is from an NEC 500 series microwave link system (circa 1983) and shows one equipment block path. The "return direction" block is the reverse of that detailed in the main diagram.

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 inteference that may affect the microwave receiver. License costs are usually linked with the size of the spectrum occupied by the transmitter signal and are often directly influenced by some of the spectrum lease costs realized by the local regulator, eg FCC, ACMA, PTT's etc.

Microwave Radiation Safety A safety aspect of microwave radio EMR radiation is also defined by standards and guidelines, and often human exposure 'exclusion' zones exist around the front of microwave dish antennas, horns and dielectric antennas. Personnel safety must also be considered around open waveguide ends and waveguide switches with unterminated ports. Refer to other material at the GHN on EMR Safety. Treating microwave radiation safety in a conservative manner is always prudent, never look down open waveguide, never stand in front of a microwave antenna.

Frequency Planning

Sample extract of micrwave band frequency planning

In the older Frequency Division Multiplex (FDM) microwave radio link systems, only a single pair of frequencies were allocated to the whole link network, with an alternating polarisation isolation

arrangement from more distant stations in the network. This meant that at a single microwave repeater station, the link transmitters operate on the same frequency, but with antennas pointed in different directions, and with opposite antenna polarisation. The same frequency planning logic still applies to modern digital microwave radio links, with a close eye on the 'emission' bandwidth / designator. Frequency planning may also have restrictions from the Regulator (FCC / ACMA / Ofcom / PTT's), so extensive consultation is required before any commitments can be made.

Microwave Radio Link Planning The design and construction of a microwave radio link network is based on a number of factors. These include: 

Distance between microwave radio terminals;

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Terrain properties, eg bodies of water, cliffs, forests, snow;

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Frequency of operation, often governed by licensing costs, frequency availability, planned distances and even susceptibility to rain fading;

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Interference management to the microwave link receiver. Generally managed by allocating a clear frequency pair by the Regulator, but for frequency bands 'sold at auction' or with delegation, eg Defence communications and large carriers, this becomes the management responsibility of the band licensee/owner;

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Fading, dispersion and multipath distortion;

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Size of antennas, feedline properties, need for towers and masts, and for high gain antennas - even the stability (both tilt and torsional properties) of the ing mast must be engineered to avoid the antenna beam being mis-directed due to wind or ice on the structure;

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Management of moisture inside external waveguides;

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Management of equipment, power and security alarms, remote control switching and order wire systems.

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Council, Local Government, FAA, CASA and community development permissions governing visual and controlled airspace intrusions;

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Cost of equipment and cost benefit analysis including equipment maintenance;

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Satellite communication links are also classed as microwave radio links, but given their minimal exposure to atmospheric conditions, these type of microwave links can operate at minimal fade margins, ie having minimal contingency in the level of received signal strengths;

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Availability of equipment, spares, maintenance, test equipment and skilled staff;

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Sun transits for microwave link receivers facing at the eastern or western horizons. The issue here is that the "sun noise" will often overwhelm broadband microwave receivers, generating what is called a 'sun transit outage'. Same deal for satellite communication links as well.

Manufacturers of Microwave Link Equipment In no specific order, these include: 

- NEC

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- Ericsson

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- Nokia

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- Marelli

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- Marconi

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- GT&E

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- GE

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- Phillips

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- Rohde & Schwartz

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- Kuhne

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- Codan

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- Alcatel

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- Fujitsu

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- Siemens

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- ATI

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- Hughes