Another method is to transmit more than one channel on a radio link. This may be achieved either in the frequency or time domain, or a combination of the two. When the aggregation of channels is done before the modulation process, it is known as multiplexing. When several independent stations access a medium, is the term “multiple access” is applied. A well known example of multiple access is Ethernet LANs where all nodes have access to the same medium.

With frequency division multiplexing (FDM) or frequency division multiple access (FDMA) a carrier frequency is modulated with a number of sub-carriers. Each sub-carrier carries a data signal. The sub-carriers may be amplitude, frequency or phase modulated. The more common is narrow shift frequency modulation. Each channel is independent of the otherones, and may transmit with a different speed or use a different alphabet or system. One such system used in HF communications is the CODAN 16 carrier QPSK system.

With time division multiplexing (TDM) or time division multiple access (TDMA) each data source is allowed access to the aggregate channel (line or radio link) in well-defined time slots. To keep pace with the incoming bit stream, the aggregate channel speed must be the sum of the speed of the individual channels. All channels must have identical speeds. However, a channel subdivision scheme has been standardized so that up to four sub-channels may share one channel. An example is the ARQ-M2 and M4 modes.

In satellite systems a combination of FDMA and TDMA is often used to optimize channel usage. DAMA (Demand Assigned Multiple Access) satellite systems are an example of this type of multiple access.

Polarization division multiplexing is used in satellite communications, whereby differently polarized signals use the same frequency leading to frequency reuse. Inmarsat earth station – satellite links in C-band use this type of multiplexing.

The overwhelming majority of radio data systems will transmit the individual bits of a codeword one after the other in serial transmission. However real-time or high volume data systems, like digitized secure voice, computer network access and image or file transfer often uses parallel transmission. The serial code words are fed to a serial-to-parallel converter and then to the sub-carrier modulators of a FDM.