  We focus on full-rate, fast-decodable space-time block codes (STBCs) for 2x2
and 4x2 multiple-input multiple-output (MIMO) transmission. We first derive
conditions for reduced-complexity maximum-likelihood decoding, and apply them
to a unified analysis of two families of 2x2 STBCs that were recently proposed.
In particular, we describe a reduced-complexity sphere decoding algorithm
suitable for QAM signal constellations. Next, we derive a novel
reduced-complexity 4x2 STBC, and show that it outperforms all previously known
codes with certain constellations.
