The Wireless Spectrum

The Wireless Spectrum

- Methods for Spectrum Allocation
Most countries have government agencies responsible for allocating and controlling the use of the radio
spectrum. In the United States spectrum allocation is controlled by the Federal Communications Com-
mission (FCC) for commercial use and by the Office of Spectral Management (OSM) for military use. The
government decides how much spectrum to allocate between commercial and military use. Historically
the FCC allocated spectral blocks for specific uses and assigned licenses to use these blocks to specific
groups or companies. For example, in the 1980s the FCC allocated frequencies in the 800 MHz band
for analog cellular phone service, and provided spectral licenses to two companies in each geographical
area based on a number of criteria. While the FCC still typically allocates spectral blocks for specific
purposes, over the last decade they have turned to spectral auctions for assigning licenses in each block
to the highest bidder. While some argue that this market-based method is the fairest way for the gov-
ernment to allocate the limited spectral resource, and it provides significant revenue to the government
besides, there are others who believe that this mechanism stifles innovation, limits competition, and hurts technology adoption. Specifically, the high cost of spectrum dictates that only large conglomerates can
purchase it. Moreover, the large investment required to obtain spectrum delays the ability to invest in
infrastructure for system rollout and results in very high initial prices for the end user. The 3G spectral
auctions in Europe, in which several companies have already defaulted, have provided fuel to the fire
against spectral auctions.
In addition to spectral auctions, the FCC also sets aside specific frequency bands that are free to
use according to a specific set of etiquette rules. The rules may correspond to a specific communications
standard, power levels, etc. The purpose of these “free bands” is to encourage innovation and low-cost
implementation. Two of the most important emerging wireless systems, 802.11b wireless LANs and
Bluetooth, co-exist in the free National Information Highway (NIH) band set aside at 2.5 GHz. However,
one difficulty with free bands is that they can be killed by their own success: if a given system is widely
used in a given band, it will generate much interference to other users colocated in that band. Satellite
systems cover large areas spanning many countries and sometimes the globe. For wireless systems that
span multiple countries, spectrum is allocated by the International Telecommunications Union Radio
Communications group (ITU-R). The standards arm of this body, ITU-T, adopts telecommunication
standards for global systems that must interoperate with each other across national boundaries.

- Allocations for Existing Systems
Most wireless applications reside in the radio spectrum between 30 MHz and 30 GHz. These frequencies
are natural for wireless systems since they are not affected by the earth’s curvature, require only mod-
erately sized antennas, and can penetrate the ionosphere. Note that the required antenna size for good
reception is inversely proportional to the signal frequency, so moving systems to a higher frequency allows
for more compact antennas. However, received signal power is proportional to the inverse of frequency
squared, so it is harder to cover large distances with higher frequency signals. These tradeoffs will be
examined in more detail in later chapters.
As discussed in the previous section, spectrum is allocated either in licensed bands (which the FCC
assigns to specific operators) or in unlicensed bands (which can be used by any operator subject to
certain operational requirements). The following table shows the licensed spectrum allocated to major
commercial wireless systems in the U.S. today.
Note that digital TV is slated for the same bands as broadcast TV. By 2006 all broadcasters are
expected to switch from analog to digital transmission. Also, the 3G broadband wireless spectrum is
currently allocated to UHF TV stations 60-69, but is slated to be reallocated for 3G. Both analog and
2G digital cellular services occupy the same cellular band at 800 MHz, and the cellular service providers
decide how much of the band to allocate between digital and analog service.
Unlicensed spectrum is allocated by the governing body within a given country. Often countries try
to match their frequency allocation for unlicensed use so that technology developed for that spectrum is
compatible worldwide. The following table shows the unlicensed spectrum allocations in the U.S.
ISM Band I has licensed users transmitting at high power that interfere with the unlicensed users.
Therefore, the requirements for unlicensed use of this band is highly restrictive and performance is
somewhat poor. The NII bands were set aside recently to provide a total of 300 MHz of spectrum with
very few restrictions. It is expected that many new applications will take advantage of this large amount
of unlicensed spectrum.