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 and phase noise. Asynchronous data communication services such as web access are characterised by short communication bursts at high data rate. Few users in a base station cell are transferring data simultaneously at low constant data rate. The complex OFDM electronics, including the FFT algorithm and forward error correction, are constantly active, thus consuming power, independent of the data rate, while OFDM combined with data packet scheduling may allow FFT algorithm to hibernate at times. The OFDM diversity gain and resistance to frequency-selective fading may partly be lost if very few sub-carriers are assigned to each user, and if the same carrier is used in every OFDM symbol. Adaptive sub-carrier assignment based on fast feedback information about the channel, or sub-carrier frequency hopping, is therefore desirable. Dealing with co-channel interference from nearby cells is more complex in OFDM than in CDMA. It would require dynamic channel allocation with advanced coordination among adjacent base stations. The fast channel feedback information and adaptive sub-carrier assignment is more complex than CDMA fast power control. Characteristics and principles of operation Based on feedback information about the channel conditions, adaptive user-to-subcarrier assignment can be achieved. If the assignment is done sufficiently fast, this further improves