Orthogonal frequency-division multiple access
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 simultaneous low-data-rate transmission from several users. Comparisons OFDMA is often compared to the combination of OFDM with statistical time-division multiplexing. The advantages and disadvantages summarized below are further discussed in the Characteristics and principles of operation section. See also the list of OFDM key features. Advantages Allows simultaneous low-data-rate transmission from several users. Pulsed carrier can be avoided. 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. Flexibility of deployment across various frequency bands with little needed modification to the air interface. Averaging interferences from neighboring cells, 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. Offers frequency diversity by spreading the carriers all over the used spectrum. Allows per-channel or per-subchannel power. Disadvantages Higher sensitivity to frequency offsets