EECS20N: Signals and Systems

Radio Spectrum

Emission of radio signals is easy enough that anyone with minimal knowledge of electronics can do it. In fact, it's fairly difficult to avoid, and most electronic devices emit radio signals of some sort. In the United States, the FCC (Federal Communications Commission) regulates the emission of radio signals, dictating what is allowed and what it not. Most electronic devices have to be tested for compliance with FCC regulations.

Even with FCC compliance, electronic devices can interfere with one another through radio signals. The FAA (Federal Aviation Administration) prohibits the use of all electronic devices on airplanes within 10 minutes of takeoff or landing for fear that these devices may interfere with critical navigation and control electronics on the aircraft. This is a legitimate fear with some aircraft, but not with others, so human factors considerations cause the universal application of the rule.

For most electronic devices, FCC compliance is achieved by emitting a minimal amount of radio power. Some devices, however, deliberately emit radio power. These include all devices that use radio waves for communication, including two-way radios (of course), radio broadcast, television broadcast, cordless telephones, cellular telephones, microwave links in the telephone network, satellites, garage door openers, and countless other devices.

A major task of the FCC is to ensure that these legitimate emissions of radio signals do not interfere with one another. To do this, the FCC issues licenses that allow emission of radio signals within a certain frequency range, at a specified power level, within a certain geographic region. This use of frequency range to divide the radio space is convenient, but by no means absolute. It is in part dictated by the state of early radio technology, which was better able to deal with frequency ranges than with other ways to divide up the space.

At a very coarse level, the spectrum is shown below:

This shows the spectrum from about 10kHz to above 10GHz. The full details are given in a detailed frequency chart, valid as of 1996. The detailed chart shows the following fairly familiar uses of the radio spectrum:

  • 535 kHz - 1.6 MHz: AM Radio stations.
  • 54 MHz - 72 MHz: Television (channels 2-4).
  • 76 MHz - 88 MHz: Television (channels 5-6).
  • 88 MHz - 108 MHz: FM radio.
  • 174 MHz - 216 MHz: Television (channels 7-13).
  • 470 MHz - 806 MHz: Television (channels 14-69).

In the chart, "land mobile" includes cellular telephony, cordless telephones, and a suite of older services including police, taxi dispatch, and ambulance services.

The system of frequency allocation is fairly archaic, and tradition dictates much of its design. The lower frequencies are allocated to the oldest services, such as maritime communication and navigation, aeronautical communication and navigation, and AM radio. Many regions of the spectrum, such as those devoted to radio navigation, have been technologically supplanted (in this case by GPS, the global positioning system, a satellite-based global navigation system). Huge portions of the spectrum are wasted on television broadcasts, and even more have been recently allocated to allow for simultaneous digital and analog transmission during a transition phase to an all digital television system. But it is difficult to reclaim these regions of the spectrum because residual uses remain. Partly as a consequence, it has become extremely difficult to find unused radio spectrum for new services. Cellular telephony, for example, has undoubtedly had its growth constrained by the shortage of available bandwidth.