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The methodology used to evaluate the performance of the various MIMO schemes in orthogonal frequency division multiplexing (OFDM) WiMAXWiMAX stands for Worldwide Interoperability for Microwave Access. systems is described. The per-tone signal-to-noise plus interference ratio (SINR) of a MIMO OFDM system is derived in a multi user, multicell and multisector communication system. The sector average spectral efficiency for a coded MIMO OFDM system is evaluated under a single frequency reuse deployment scenario. It is shown that the second antenna at the subscriber station receiver provides significant gains over single transmit single receive antenna (SISO) systems. The rate 2 spatial multiplexing MIMO scheme on the downlink improves the sector spectral efficiency by 10% over single transmit and two receive antenna (SIMO) system in a single frequency reuse deployment. It is also shown that in a time-division duplex (TDD) WiMAX system, the gain in downlink spectral efficiency from beamforming can be very large if the delay between the uplink channel estimate and the downlink beamforming is small compared to the coherence time of the channel. On the uplink, it is shown that the collaborative spatial multiplexing increases the spectral efficiency by 9% over a SIMO system with the same number of receive antennas at the base station (BS). Significant gains in uplink spectral efficiency are seen with more antennas at the BS and by using uplink beamforming as compared to SISO.