Space-time-frequency (STF) coding for MIMO-OFDM systems

We consider the capacity of multiple-input-multiple-output (MIMO) systems that use OFDM as the modulation format. We point out a basic equivalence between antennas and OFDM-tones. This similarity immediately allows us to essentially reuse all space-time codes designed for flat-fading channels in MIMO-OFDM systems operating in frequency-selective channels. An optimum code would thus code across all antennas and tones (as well as time) simultaneously. Since this can become very complex, we propose a method for grouping antennas and codes in such a way that the inherent diversity is retained, while the complexity is greatly reduced. Capacity computations between the full-complexity and the reduced-complexity systems illustrate this tradeoff.