Submillimeter Microwave: Tending Toward Light

Beyond the SHF bands lies the extremely high frequency (EHF) region, a vast range of spectrum
extending from 30GHz to 300GHz. (Acute readers will have realized by now that radio
spectrum is arranged in “decades,” where the uppermost limit of a region is always ten times
the frequency of the lower limit.) Some 36GHz of this spectrum falls within the 802.16 standard.
The region above 40GHz is somewhat inaccurately termed the submillimeter microwave
region (in actuality wavelengths in the useful bands in this region are all above 1 millimeter)
and has only recently become an option for the broadband wireless operator.
EHF is the last frontier of high-speed RF communications. Much of the spectrum has not
been allocated by any government or standards body, and equipment manufacturers have not
made many products available for this region. Still, I see significant opportunities in these
bands, and I predict that activity there will increase considerably over time.
Currently in the United States two bands are in commercial use for high-speed data
transmissions: an unlicensed band at 59GHz to 64GHz and a licensed band extending discontinuously
from 71GHz to 95GHz that is known as the E band. The FCC is considering the allocation
of still other bands to be located in the region above 90GHz. More equipment is
currently manufactured for 59GHz–64GHz than any other (this is often referred to as the
60GHz band).
Interestingly, as you have seen, the water vapor attenuation at 60GHz is extremely high,
which would seem to make this spectrum a poor choice for outdoor airlinks, and in fact the
band was originally allocated for indoor use. Outdoors, the practical limit for transmissions is
less than 1,000 feet, though some operators see this as an advantage because it permits almost
total frequency reuse in adjacent cells.
The band between 71GHz and 95GHz is situated within a deep attenuation trough with
slightly more than 0.2dB total attenuation per kilometer at sea level—equivalent to that of the
popular 38GHz band. Since much of that attenuation is because of oxygen absorption rather
than water vapor absorption, attenuation drops precipitately at high elevations, and some
authorities have suggested that the band would be well suited to trunk-line links connecting
mountaintops or even links involving circling aircraft or stationary balloons. Until very recently this spectrum was only utilized on an experimental basis by Loea, a Hawaiian equipment
manufacturer with deep expertise in radio photography. Loea owned a nationwide
license and had plans to establish networks on a wide scale, though how quickly these can
actually be developed remains uncertain. Recently the E band has become subject to expedited
licensing by the FCC, which means that applicants can obtain licenses on a link-by-link basis,
and can be fairly certain of obtaining licensing if no prior license has been established in the
restricted geographical area irradiated by the narrow-beam point-to-point link.
All of these bands above 40GHz share one thing in common: allocations of bandwidth that
enable them to achieve truly fiberlike throughput speeds. And yet, rather curiously, actual
deployments remain quite uncommon. The largest I am aware of took place in Florida under
the auspices of CAVU-eXpedient, a now defunct high-speed access provider that operated
radio links at 38GHz and 60GHz and backed up the 60GHz links with free-air optical transceivers.
Basing its business plan on a rapid rollout over several southern states, CAVUeXpedient
was unable to obtain third-round funding to continue its expansion and declared
bankruptcy in 2002.
Nevertheless, I see considerable potential in EHF bands because of the relatively enormous
throughputs they support. Services operating in the SHF spectrum may claim to offer
fiberlike speeds, but only EHF services are truly capable of provisioning multigigabit pipes.