OFDMA was the best choice to make it interoperable with coming technologies such as LTE and LTE-adv . As far as Interference between users are concerned then lot of successful work has been done to mitigate it and CoMP schemes have been developed to enhance the System Utilization and Throughput .
WiMAX uses CDMA in the uplink direction, just at ranging phase. at the first entry of Mobile Station to the Base Station, CDMA is employed. for eliminating interference between users, there are many techniques used, such as distributed permutation as well as beamforming and others.
Just compare the Spectral efficiency of a OFDMA system like LTE-A to a CDMA2000 system and one can easily find out why they went with OFDM. with proper frequency diverse scheduling and distributed permutation subchannelization they tried to minimize the interference.
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