In this paper we propose and analyze improved MIMO-OFDM techniques for wireless systems using QPSK modulation for four transmit and four receive antennas. Cases with frequency selective fading channels are considered. We first consider such a system employing two 16-state, 2-antenna space-time codes with successive interference cancellation and channel estimation, which was previously proposed to reduce the complexity of a 4-antenna space-time code system. We show that our recently proposed space-time code has a 2 dB improvement over a previously published code at 5 Hz fading. Furthermore, we propose a 4-antenna, 16-state code that achieves an additional 2 dB improvement with lower complexity and a 256-state code that achieves an additional 2 dB gain. The 256-state code performs within 3 dB of outage capacity (and within 2 dB with perfect channel estimation), which is better than any other published result, yet does not use complex iterative decoding.
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