WiMax Operator's Manual

CHAPTER 3 ■ STRATEGIC PLANNING OF SPECTRUM AND SERVICES 43
and simultaneous overexploitation of a scarce resource to the ultimate benefit of no one—the
Open Spectrum will be open only to users of various kinds of spread spectrum radios. The
users must have the following attributes:
They have the ability to tolerate high values of interference.
They are frequency agile within the band, distributing the signal among various coding
sequences, subbands, and time slots so that no slice of spectrum is exclusively occupied
by any individual user.
They utilize various mechanisms for ensuring fairness such as mutual power control,
network polling for controlling access, and the exchange of information among intelligent
radios via control channels.
The first commercial spread spectrum radios appeared in the early 1990s at just about the
same time that the FCC began to allocate unlicensed spectrum for miscellaneous uses. The
first band so designated was the ISM band situated at 902MHz to 928MHz. This was shortly
followed by the band extending from 2.4GHz to 2.4835GHz, the unlicensed band that is most
subject to use today in transmitting data. This in turn was followed by two contiguous bands
extending from 5.15GHz to 5.23GHz and another band between 5.725GHz and 5.825GHz. The
5GHz unlicensed frequencies are often referred to as the Unlicensed National Information
Infrastructure (U-NII) bands and are subject to more usage restrictions than the lower
frequencies, being explicitly designated for data transmissions only, not for the operation
of remote control devices. In November 2003, the FCC allocated 250MHz of new spectrum
extending from 5.470GHz to 5.725GHz, a band that is already in use elsewhere in the world. By
this action the FCC has harmonized U.S. spectral allocations with those in many other nations,
which should stimulate product development and bring down equipment prices.
Abroad, both 2.4GHz and 5.8GHz as well as 5.4GHz are commonly available as unlicensed
spectrum, though not on a worldwide basis.
A further unlicensed band occupies spectrum between 24.0 and 24.25GHz in the United
States, and another is situated between 59GHz and 64GHz. The 24GHz band is newly approved
and has seen little or no use as yet. The 60GHz band has been commercially exploited on a
small scale thus far, and the principal user is the now-defunct CAVU-eXpedient network in
Florida. Rumor has it that the FCC will probably allocate additional spectrum for unlicensed
usage in the midterm. Incidentally, the 60GHz band is in use in some other areas of the world
as well.
The general rule for operation within the unlicensed bands in the United States is that
users must be prepared to tolerate interference but must not generate undue interference
themselves that would prevent others from utilizing the bands. All users are essentially
enjoined to live and let live, and what is supposed to ensure that this will happen are rather
severe output power limitations placed upon individual radios.
Since for many prospective broadband wireless operators licensed spectrum is simply
unavailable, and unlicensed spectrum is the only option, such rather vague injunctions cannot
but be disturbing. Won’t interference inevitably increase as more and more entities use the
bands? What is to prevent everyone from interfering with everyone else at a certain point,
thereby rendering the bands useless and undermining the whole concept of Open Spectrum?
These are valid concerns, but in my experience of covering wireless broadband as a trade
journalist, which dates from the earliest days of the industry, I have encountered relatively few
instances where a network operator was severely hampered by high levels of interference. This