Multiplexing techniques vary widely based on what is being
multiplexed. Modern telecommunications use a very wide array of
techniques including:
TDM - examples: TDMA, T-carrier
FDM - examples: DWDM
Spatial - example: MIMO
Code division - examples: CDMA
Phase or polarization division - cable/satellite TV
Statistcal - examples: packet mode (STS), FHSS etc etc.
This is far from a complete list. I think the question needs to
be more specific.