Wireless Technologies and the National Information Infrastructure.
Chapter 12 - Electromagnetic Interference and Wireless Devices:
Background

BACKGROUND

Any short length of wire in an electronic circuit or in an integrated chip can act as an antenna when exposed to radio waves and give rise to electric currents that may interfere with the normal operation of the circuit. (see footnote 2) This potential electromagnetic interference (EMI) is an inherent property of radio or television transmissions, electric motors, and household switches, as well as natural phenomena such as lightning, aurora borealis, and sunspot activity. (see footnote 3) To protect against it, shielding--either in a metal case or special shielded wire-- or better circuit design is necessary. Most of the time designers anticipate problems, and build devices not subject to interference when used as directed. However, there are cases in which devices are not shielded adequately against EMI, many involving medical devices.

While many of the reported EMI problems involve older analog radio transmitting devices, the wireless industries are increasingly turning to digital transmission formats to improve quality and increase capacity. This transition may pose new EMI problems because digital equipment may interact with other devices in unpredictable ways. For example, digital Global System for Mobile Communications (GSM) handsets and Time Division Multiple Access (TDMA) telephones emit higher strength peak electric fields than do analog telephones, while maintaining the same average power levels. (see footnote 4) This scheme results in better transmission and reception at a lower average power output--extending battery life--but it may also cause greater interference than analog phones. The increasing use of spread spectrum, including Code Division Multiple Access (CDMA) technologies, has also led some engineers to predict that, with a large number of users, interference between competing devices may make the systems unusable.