Introduction to FDM, OFDM, OFDMA, SOFDMA

OFDM has been in theory for decades but just entered real world applications in recent years thanks to the availability of modern chips that can handle complex digital signal processing. Wireline and wireless, fixed and mobile communications or networking technologies have chosen OFDM to achieve higher data rate (what is called broadband). Examples of such technologies are: ADSL, HomePlug AV, WiMedia UWB, Wi-Fi (802.11a/g), WiMAX.

Orthogonal Frequency Division Multiple Access (OFDMA)

Like OFDM, OFDMA employs multiple closely spaced sub-carriers, but the sub-carriers are divided into groups of sub-carriers. Each group is named a sub-channel. The sub-carriers that form a sub-channel need not be adjacent. In the downlink, a sub-channel may be intended for different receivers. In the uplink, a transmitter may be assigned one or more sub-channels.

Picture. Orthogonal Frequency Division Multiple Access
Sub-carriers with the same color represent a sub-channel.

Subchannelization defines sub-channels that can be allocated to subscriber stations (SSs) depending on their channel conditions and data requirements. Using subchannelization, within the same time slot a Mobile WiMAX Base Station (BS) can allocate more transmit power to user devices (SSs) with lower SNR (Signal-to-Noise Ratio), and less power to user devices with higher SNR. Subchannelization also enables the BS to allocate higher power to sub-channels assigned to indoor SSs resulting in better in-building coverage.

Picture. Uplink Subchannelization in WiMAX
In OFDM, only one SS transmits in one time slot.
In OFDMA, several SS's can transmit at the same time slot over several sub-channels.

Subchannelization in the uplink can save a user device transmit power because it can concentrate power only on certain sub-channel(s) allocated to it. This power-saving feature is particularly useful for battery-powered user devices, the likely case in Mobile WiMAX.