An OFDMA TDD model extension of the NS-3 WiMAX module was created. The necessary background in 802.16 WiMAX standards is presented. Additions supporting OFDMA are presented along with the challenges encountered during development. The addition of an extra dimension of sub-channels required new data structures to manage allocations. This required significant changes to portions of the NS-3 WiMAX OFDM schedulers. The model was extended to use the CDMA ranging required by the OFDMA WiMAX standard. Many message structures were extended or added. The basic upload scheduler design from the NS-3 WiMAX model and the enhanced version are extended to work with the OFDMA implementation. The OFDMA model performance of various cases are compared and discussed. Possible extensions and improvements to the current model are offered.