Multiplexing

Multiplexing is the art of letting many conversations share one physical channel without trampling each other — packing thousands of phone calls onto a single fibre, or hundreds of users onto one cell tower. The classic ways carve up a different resource: frequency-division multiplexing (FDM) gives each user their own frequency band, like radio stations on a dial; time-division multiplexing (TDM) gives everyone the whole channel but only for brief, repeating time-slots, like speakers taking strict turns; code-division multiplexing (CDM) lets everyone talk at once but tags each stream with a unique code so the receiver can separate them.

Real systems blend these. A 4G cell splits users in both time and frequency on an OFDM grid; fibre-optic links pack dozens of colours of laser light down one strand (wavelength-division multiplexing, the optical form of FDM); and 5G adds spatial multiplexing (MIMO), aiming separate beams at separate users from the same antenna array. The opposite operation — peeling the combined stream back into its parts at the far end — is demultiplexing, done by a device aptly named a demux.