Spectrum Allocation
Aviation, marine, trucking and emergency public agencies all use different portions of the spectrum, or airwaves, to communicate over wireless radio frequencies. The term spectrum refers to a range of frequencies or the portion of the radio waves allocated for particular applications such as satellites or cellular service. A frequency is the number of times each second that each radio wave completes a cycle. As mentioned in Chapter 1, each cycle looks like a resting letter S. A cycle is complete when a radio wave passes through the highest and lowest portions of the wave. The term hertz refers to one cycle of a radio wave.
The allocation of spectrum is administered on both an international and national level. The International Telecommunications Union (ITU) manages the allocation of spectrum for services such as satellite and television because these services cross national borders. Individual countries allocate spectrum for their own areas. For this reason, countries have assigned different frequencies to fixed wireless service. This drives up manufacturing costs as fewer economies of scale are achieved.
In the United States, the FCC has broken up spectrum into bands and assigned it for particular purposes. For example, residential cordless phones are assigned to the 800 million cycles (hertz) MHz per second, and 2.4 gigahertz bands. Citizens band radio is assigned to the 27, 462 and 467 MHz, or million hertz bands. The very high frequency (VHF) bands for TV channels 2 through 12 and mobile radio services for police and fire dispatch use are in the 30 MHz to 300 MHz range of frequencies. Thirty MHz means that each wave has 30 million cycles or hertz per second. It completes 30 million cycles; 30 million resting Ss in one second.
Higher frequencies have shorter wavelengths. (A wavelength is the distance between the highest point in one wave to the highest point in the next wave.) For example, a 3000-Hz wave has a longer wavelength than a 3,000,000-Hz wave. Small wavelengths are more susceptible to rain and weather conditions. A rain droplet can destroy a smaller wave more easily than a larger one. The raindrop is bigger in relationship to a small wavelength than to a large wavelength. Therefore, high-frequency microwave systems are more susceptible to weather conditions than lower frequency systems. Signals on higher frequency services such as personal communications services (PCS) also cannot be transmitted as far as lower frequencies before they fade. Therefore, PCS towers and antennas must be closer together than lower frequency, traditional cellular services. This is the reason that 3G (third generation) advanced wireless networks that use higher frequencies will need to add many more cellular towers. The towers will need to be closer together to accommodate short wavelengths.
If frequencies were not allocated to specific companies for specific uses, transmissions would overlap or interfere with each other. For example, if two telephone calls took place in the same airspace at the same frequency, the callers would be able to hear each other's conversations. This is interference.
The limiting factor in cellular network is spectrum. The advent of cellular and digital technologies discussed later enabled a fixed amount of spectrum to be used more efficiently by many more users. This is called spectral efficiency, the capability of using the same spectrum for more subscribers.
Spectrum for Higher Speed 3G Services
The World Radiocommunication Conference (WRC) 2000 of the International Telecommunications Union identified three bands of spectrum on which third generation cellular (3G) service should operate. (3G service for advanced voice and data services is reviewed later.) However, available spectrum is scarce in the United States and most of the spectrum in these bands is already occupied by other services:
1710 to 1885 MHz— 1850 to 1885 MHz is used for personal communications service (PCS), a form of digital cellular. The Department of Defense is the primary user of 1755 to 1850 MHz. The Commerce Department has stated that 45 MHz could be cleared in these ranges at a cost of $2.1 billion.
2500 to 2690 MHz— Instructional television and Multipoint Multichannel Distribution Service (MMDS) for fixed wireless.
PCS frequencies were auctioned off in 1996 to many companies including NextWave Telecom Inc. and Metro PCS. Both of these companies filed for bankruptcy and to date have not used the spectrum. The FCC repossessed the licenses and held a new auction for the licenses won by NextWave. Companies backed by Verizon, VoiceStream Wireless, Dobson Communications, AT&T Wireless and Cingular submitted the highest bids. However, in June 2001 the courts ruled that the FCC had no right to reclaim the licenses. Moreover, a bankruptcy court refused to allow the government to repossess the Metro PCS licenses. It's not known if the government will appeal these decisions or if NextWave and Metro PCS will sell the licenses to other cellular companies.
The lack of unassigned, usable spectrum is an enormous problem in the United States. Spectrum in lower frequencies was given out to public service agencies, television, police and radio. As higher frequencies became usable, they were assigned for applications such as fixed wireless and very small aperture terminal (VSAT) satellites without adequate planning. Moreover, free spectrum was given to television broadcasters for digital television. Broadcasters have spectrum they currently use for analog television plus spectrum for digital television. The government is planning to auction, for wireless service, the frequencies used for UHF (ultra high frequencies) channels 11 to 69, which TV broadcasters are scheduled to give up by 2006.
The auctions are scheduled to start with parts of channels 60 to 69 that are not in major markets in 2003. However, cellular providers prefer to wait until and if spectrum in major markets opens up. President Bush has proposed a two-year delay on these auctions, which cover the 700 MHz range.
The ITU has stated that each country is free to select the spectrum to be used for 3G. However, devices for roaming service will be simpler to manufacture if they operate on the same frequencies worldwide. The FCC originally stated that it would identify spectrum for 3G systems by July 2001 and hold an auction by September 30, 2002. In June 2001, it announced that it would not meet the July 2001 date and would continue working on the matter.