OFDMA - Orthogonal FDMA

Orthogonal FDMA (OFDMA) is an advanced channelized resource sharing technology that has been widely adopted in very high speed packet data communications. It offers a wide range of deployment constellations such as on a single unpaired TDD (Time Division Duplex) carrier or on a single pair of FDD (Frequency Division Duplex) carrier with channel bandwidths varying from 1.4MHz to 20MHz. Achieving such flexibility is possible in a way thanks to the FDMA type of bandwidth sharing. In OFDMA the total carrier bandwidth is divided into a number of discrete subcarriers which, like in CDMA, are orthogonal to one another thus eliminating the need of guard bands between the subcarriers. To maximize the number of subcarriers as function of the preselected carrier bandwidth, the subcarrier bandwidth is kept to the minimum, or about 15kHz, by a total symbol rate near the Nyquist limit obtained through a slow modulation.

User differentiation is accomplished via the assignment a set of subcarriers, also subset, that corresponds to the OFDMA traffic channel. The number of subcarriers in the subset varies and is dependent on the type of requested service. Based on transmission channel conditions an adaptive user-to-subcarrier allocation can be triggered for a better spectral efficiency and to avoid frequency selective fading. Thus different users will perceive different channel qualities with the better off users being allocated on the best subcarrier spreads across the whole carrier bandwidth.

Further in a TDMA type of bandwidth sharing each OFDMA subset is engaged in statistical time multiplex as to allow for instantaneous bandwidth allocation to users with higher traffic demands over other users with slower data rates. A packet scheduler is charged with assigning of the appropriate OFDMA subset overhead in order to guarantee for proper reassembling of the data packets at the receiver end.

OFDMA is the predominant AI standard for wideband mobile access ecosystems and is commercially deployed on WLAN (Wireless Local Area Network), WiMAX (Worldwide Interoperability for Microwave Access) and LTE (Long Term Evolution) networks.

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