On the ML Decoding of Quasi-Orthogonal Space-Time Block Codes via Sphere Decoding and Exhaustive Search

For the large family of quasi-orthogonal space-time block codes, it is not clear when is best to decode using an exhaustive search, and when the sphere decoding algorithm (SDA) should be applied. In investigating this practical issue, a generic maximum-likelihood metric expression is utilized which is more conducive to lowering the decoding complexity. It is then shown that for smaller constellations and/or number of transmit antenna, the complexity advantage of smart exhaustive search in comparison with sphere decoding can be significant.