11 - Multiple access techniques

Summary

Physics places no cooperation requirements on wireless communication. Any time electrons are moving within a conductor there is some electromagnetic propagation from that conductor. If this propagating wave is intentional, we call the conductor an antenna and the wave is intended for wireless communication. If the EM propagation is unintentional we have many names for it, including interference, leakage, and a large number of other names which are not printable.

We apply wireless communication with the expectation that it will work. So not only will the transmitter and receiver be within range of each other, at the time we want to send a message we expect that the channel will be available.

In order for our wireless communications to be useful then there must be cooperation in how we use this EM propagation. The electromagnetic spectrum is only really useful for DWC if we share it with open cooperation. Over the past century there have developed several sharing methods, called multiple access (MA) techniques, to effect a useful sharing of the EM spectrum. In order of their development in history, the main ones are frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). For some applications the technique of carrier sense multiple access (CSMA) is also proven to be effective.

Less common are the techniques of space division multiple access (SDMA) and polarization division multiple access (PDMA). Of these, SDMA continues to be actively developed.