WiMax Operator's Manual

CHAPTER 3 ■ STRATEGIC PLANNING OF SPECTRUM AND SERVICES 35
exceeding 100GHz are currently used only for imaging (radio photography). Electromagnetic
waves occurring above 300GHz are conventionally considered infrared light, but there is no
arbitrary frequency where the characteristics of wave propagation change drastically and the
radio wave suddenly assumes the properties of radiant light.
Commercial broadcasts commence at hypersonic frequencies in the hundreds of kilohertz
(thousands of cycles per second), frequencies that are assigned to AM broadcast stations in the
United States. These frequency bands are wholly unsuitable for high-speed data for a number
of reasons. Wavelengths span literally hundreds of yards and require immense amounts of
power to propagate at detectable signal levels. And because a radio signal can convey only data
rates that are a few multiples of the carrier frequency, such low-frequency signals simply
cannot transmit data very quickly.
As you proceed up into the megahertz (millions of cycles per second), the bands become
increasingly well suited to the transmission of data, but most of these bands have long ago
been assigned to what are now well-entrenched commercial and governmental users and
therefore are effectively unavailable. Only as you approach the low microwave region from
1GHz to about 10GHz do bands become available that can, on the one hand, support highspeed
data traffic and, on the other hand, have not been assigned to users who are so influential
that they cannot be made to surrender the spectrum for new uses.
Much of the vast amount of radio spectrum located between 2GHz and approximately
100GHz lends itself to data transmission simply because high frequencies enable high data
throughputs. What is not useful are those regions of the spectrum where atmospheric conditions
conspire to limit range. In the following sections, you will examine the microwave region
much more closely, and I will discuss the characteristics of those bands that have already been
allocated for data use in the United States and elsewhere.
Beachfront Property: The Lower Microwave Frequencies
Spectrum available for high-speed data starts in the ultrahigh frequency (UHF) bands beginning
at 300MHz and extending to 3GHz. In the United States the lowest frequencies currently
available for broadband wireless transmissions reside between the 700MHz and 800MHz spectrum
formerly assigned to television. Further spectrum is available in the United States
between 902MHz and 928MHz, at 2.3GHz, at 2.4GHz, from 2.5GHz to 2.7GHz, and in several
bands from 5GHz to 6GHz. Bands located at 2.4GHz and at 5.8GHz are widely available across
the globe. Throughout most of the world, though not in the United States, a band centered at
3.5GHz is also available for public access data networks and is fairly widely used. Early in 2005
the Federal Communications Commission (FCC) approved new unlicensed spectrum for
broadband data services located between 3650MHz and 3700MHz.
The spectrum between 3GHz and 30GHz is termed super high frequency (SHF) but is not
all of a piece in regard to the characteristics of RF transmissions within this frequency range.
Transmissions occurring from 3GHz to approximately 10GHz and occupying the lower third of
the SHF region really have more in common with UHF in that they are relatively limited in
throughput, do not readily conduce to high degrees of frequency reuse, and, perhaps most
important, share a vulnerability to what is known as multipath distortion.
Multipath distortion is a condition in which the signal interferes with itself because reflections
off physical boundaries converge with the direct signal, causing the signal level at the
receiver to swell or fade depending on the phase alignments of the converging waveforms at
the moment they interact with one another. In addition, multipath results in errors in the