Overview of Commercial Radio Spectrum Suitable for Broadband Data Applications

Radio transmissions occur at frequencies as low as 28 cycles per second (low audio frequencies)
and as high as a couple of hundred billion cycles per second (gigahertz). Audio frequency
radio transmissions have no commercial application and are currently utilized only by navies
for communicating with deeply submerged submarines. On the other hand, transmissions 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.