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:
-
Time Division Multiple Access (TDMA) – Each device can use the
entire available spectrum in the cell, but only for a short period of time.
-
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.
-
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.