Multiple Access and Bandwidth
Multiple access to shared medium

A fundamental problem in wireless communications is that the atmosphere is a shared medium. The question of how two or more users could access the same medium without collisions must be addressed.

One way to deal with shared access is to have an official authority such as the FCC or ETSI set fixed frequencies to be used. Multiple stations may transmit simultaneously without collisions as long as they use their assigned carrier frequencies and they follow the rules on power and interference. Receivers must tune to the carrier frequency, to obtain broadcasts from a specific station. A good example of this is commercial FM broadcast radio .

The cellular telephone networks have, at various times, used several different methods for sharing their medium. There are three primary techniques that have been used to share the airwaves:

  1. Time Division Multiple Access (TDMA) – Each device can use the entire available spectrum in the cell, but only for a short period of time.
  2. Frequency Division Multiple Access (FDMA) – Each device can use a portion of the available spectrum, for as long as the device needs to, while in the cell.
  3. Code Division Multiple Access (CDMA) – This technique is really a combination of the previous two. This is the most advanced scheme and the one that is leading to Third Generation (3G) mobile wireless technologies.