November 30, 2017
A Realistic Look at Link Distances of 70-80 GHz Links
There has been significant growth in backhaul products operating in millimeter frequency range recently. Being subject to rain attenuation, and because different environments can have different characteristic attenuations, careful and precise calculation of theoretical distances is critical.
The Aviat Networks data sheet, Maximum Distance 70-80 GHz, graphically outlines the relative distances for different combinations of link characteristics. The graphs represent maximum distances in different cities around the world. The variables used are 0.1 m/3.97”, 0.3 m/1ft., 0.6 m/2 ft. antenna sizes; 250 MHz and 2000 MHz channel sizes; and 99.9 percent and 99.99 percent availability requirements.
Note that the graphs show maximum theoretical distances, and that rain attenuation and other environmental factors native to a city or locale, which can limit transmission range, are not given any numerical environmental data. However, you can see the range of differences for the significantly different environments of the major cities represented in the charts.
Determining the maximum transmission distance for the 70-80GHz radios involves three main variables: antenna size, channel size, and availability objective. With these three parameters, you can calculate the maximum theoretical distance between antennas. Features such as antenna placement, or atmospheric conditions such as rain, humidity, or electromagnetic interference, are site-specific, and are not discussed or quantified here or in the data sheet.
Antenna size: The larger your antenna, of course, the greater the transmission distance your links can cover. However, the effective antenna range is also limited by channel size and availability objective—these three factors are interdependent. The three sizes used for comparison in the data sheet are 0.1 meters (3.97 inches), 0.3 meters (1 foot), and 0.6 meters (2 feet).
Channel size: The smaller the channel size, the greater the maximum transmission distance. Note that using 2000 MHz channels, 0.1 meter antenna, and an availability objective of 99.9 percent limits the maximum distance to less than 7 kilometers. Using the same antenna size and availability goal with 250 MHz channels increases the maximum distance to around 9 kilometers.
Availability objective: This is the level of availability you need to serve customer expectations. The two levels commonly used are 99.9 percent and 99.99 percent. Note that increasing the availability objective from 99.9 percent and 99.99 percent reduces the maximum distance from 9 kilometers to just over 3 kilometers, given the 0.1 meter antenna and 2000 MHz channels cited in the examples above.
Due to atmospheric absorption for millimeter waves, the frequency bands used by the 70-80GHz radios are more susceptible to rain attenuation than lower frequency bands. In general, cities with heavier annual rainfall will have a shorter range than cities with dryer climates. Because different environments can have different characteristic attenuations, careful and precise calculation of theoretical distances is critical.
The purpose of the graphs in the data sheet is to plot the maximum distance millimeter wave radios can achieve using different antenna combinations for different cities worldwide. These plots provide a general idea of what distance range and capacity you can expect based on representative cities with different rain rates.