They make up part of the overall operating system a cellular carrier uses. No transmission scheme stands by itself, that is, these techniques are not by themselves operating systems. They are part of one. American PCS operating systems may use either TDMA or CDMA, two different transmission technologies. Analog cellular might use conventional frequency multiplexing division.
Wireless systems use many ways to transmit information.
It's much simpler than its name suggests. As we've seen , a carrier's assigned radio spectrum is divided into specific frequencies, each separated by space. Like AM radio, which is divided into 10 KHz chunks. That's all FDM is. Think of FDM as a single train running on a single track, pulling just one freight car.