Usage areas: wireless applications, WiMAX, LTE, wideband digital communication
Orthogonal frequency-division multiple access (OFDMA) is a multi-user version of the popular orthogonal frequency-division multiplexing (OFDM) digital modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual users. This allows for simultaneous low-data-rate transmission from several users. Used in communications networks such as WiMAX and LTE, OFDMA provides air interfaces that are superior to CDMA and TDMA. See SC-FDMA, WiMAX, LTE, CDMA, and TDMA. The advantages and disadvantages summarized below are further discussed in the Characteristics and principles of the operation section. See also the list of OFDM key features.
Claimed advantages over OFDM with time-domain statistical multiplexing :
- It allows simultaneous low-data-rate transmission from several users.
- It can avoid the pulsed carrier
- Lower maximal transmission power for low-data-rate users.
- Shorter delay and constant delay.
- Contention-based multiple access (collision avoidance) is simplified.
- Further improves OFDM robustness to fading and interference.
- Combat narrow-band interference.
Claimed OFDMA advantages
- The flexibility of deployment across various frequency bands with little needed modification to the air interface.
- Averaging interferences from neighboring cells by using different basic carrier permutations between users in different cells.
- Interferences within the cell are averaged by using allocation with cyclic permutations.
- Enables single-frequency network coverage, where coverage problem exists and gives excellent coverage.
- It offers frequency diversity by spreading the carriers all over the used spectrum.
- Allows per-channel or per-subchannel power.
- Recognized disadvantages of OFDMA
- Higher sensitivity to frequency offsets and phase noise.
Asynchronous data communication services such as web access are characterized by short communication bursts at the high data rate. Few users in a base station cell are transferring data simultaneously at a low constant data rate.