CDMA: Unpacking signals

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2025-01-17 23:00:04

There are a lot of cases where multiple devices have to communicate simultaneously over a single radio channel, or wire. Examples include mobile networks, 2 way radios, satellite communications and local area networking.

The simplest solution is to have the devices take turns, also known as TDMA (Time Division Multiple Access), but this requires precise synchronization.

Another is using a different carrier frequency for the device, effectively splitting the channel into many subchannels. (Frequency Division Multiple Access) This works, but limits the bandwidth available to each device to the total bandwidth divided by the total number of devices that can ever connect. With radio and moving stations, like satellites, this also creates the problem of large Doppler shifts ruining the channel spacing.

A particularly clever solution is CDMA (Code Division Multiple Access), which requires no synchronization, and is used by 2G, 3G, 2.4 GHz WiFi, GPS and more. 1.