OFDMA is essentially a hybrid of FDMA and TDMA: Users are dynamically assigned subcarriers (FDMA) in different time slots (TDMA) . The advantages of OFDMA start with the advantages of single-user OFDM in terms of robust multipath suppression and frequency diversity. In addition, OFDMA is a flexible multiple-access technique that can accommodate many users with widely varying applications, data rates, and QoS requirements. Because the multiple access is performed in the digital domain, before the IFFT operation, dynamic and efficient bandwidth allocation is possible. 
This allows sophisticated time- and frequencydomain
scheduling algorithms to be integrated in order to best serve the user population. Some of these algorithms are discussed in the next section. One significant advantage of OFDMA relative to OFDM is its potential to reduce the transmit power and to relax the peak-to-average-power ratio (PAPR) problem. The PAPR problem is particularly acute in the uplink, where power efficiency and cost of the power amplifier are extremely sensitive quantities. By splitting the entire bandwidth among many MSs in the cell, each MS uses only a small subset of subcarriers. 
Therefore, each MS transmits with a lower PAPR—recall that PAPR increases with the number of subcarriers—and with far lower total power than if it had to transmit over the entire bandwidth.Lower data rates and bursty data are handled much more efficiently in OFDMA than in single-user OFDM or with TDMA or CSMA, since rather than having to blast at high power over the entire bandwidth, OFDMA allows the same data rate to be sent over a longer period of time using the same total power.