Here’s one illustration of radio propagation in the millimeter wave bands from 60 GHz to 80 GHz. Siklu says that at 60 GHz, 99.9 percent availability can be obtained at a distance of 1.2 km (about 0.7 miles).
Signal propagation at that same level of availability, using 70 GHz or 80 GHz frequencies, is about 1.1 km (about 0.66 miles).
Others might model lower coverage limits, either to boost availability higher (which implies trading distance for signal strength), or to increase the number of potential “line of sight” paths.
If, as some others expect, millimeter wave small cells have a transmission radius of about 50 meters (165 feet) to 200 meters (perhaps a tenth of a mile), it is easy to predict that an unusually-dense backhaul network will have to be built (by mobile network standards).
In the past, mobile operators have only required backhaul to macrocells to towers spaced many miles apart. All that changes with new small cell networks built using millimeter wave spectrum (either for 5G mobile or fixed use, or for ISP fixed access).
Keep in mind that street lights are spaced at distances from 100 feet (30.5 meters) to 400 feet (122 meters) on local roads.
In some urban areas that might imply small cell deployment density of roughly “fiber to every other light pole,” or perhaps a cell every fourth light pole on a road.
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