In this paper, we analyze the error rate performance of downlink coded multiple-input multiple-output orthogonal frequency division multiple access (MIMO OFDMA) systems under frequency selective fading channel conditions. The IEEE 802.16 standard, commonly known as WiMAX, is the latest technology that has promised to offer broadband wireless access over long distance. Since 2001 WiMAX has evolved from 802.16 to 802.16d for fixed wireless access and to the new IEEE 802.16e standard with mobility support. In WiMAX, multiple input multiple-output (MIMO) antenna technologies play an essential role in meeting the 4G requirements. The application of MIMO technologies is one of the most crucial distinctions between 3G and 4G. A large family of MIMO techniques has been developed for various links and with various amounts of available channel state information in IEEE 802.16e/m. In this paper, the performance of downlink STC (Space Time Coding) PUSC with Matrix B under ITU OIP-B outdoor-to-indoor pedestrian channels with the velocity of 3Km/h for various modulation systems. The outputs have been generated averaging over 1000 frames on MIMO channel.