WiMax Operator's Manual

CHAPTER 3 ■ STRATEGIC PLANNING OF SPECTRUM AND SERVICES 39
Nevertheless, the millimeter wave frequencies are not wholly problematic, and for a
certain customer base they are actually advantageous.
Generous spectral allocations are the norm in the higher bands, so the operator has more
inherent capacity with which to work. Now it is also true that bit-to-hertz ratios bear an inverse
relationship to frequency and are usually only unity in the millimeter wave bands, but with
bands spanning minimally several hundred megahertz and in some cases several gigahertz,
the loss in spectral efficiency is more than offset by the additional spectrum. Add to that the
fact that a millimeter wave airlink is generally much cheaper than a fiber link, and you begin to
see a business plan.
The higher frequencies lend themselves to narrow beam transmissions, which is also an
advantage in certain types of deployments. A tightly focused beam is ideal for a point-to-point
connection where the full spectral allocation is assigned to a single user because that same
spectrum can then be reused in another beam separated by only a few degrees with almost no
interference between the two. And because high-frequency signals are subject to rapid attenuation,
the spectrum can be reused in an adjacent cell whose center is as little as couple of
kilometers away.
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 discon-
tinuously 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