  Linear space-time block codes (STBCs) of unitary rate and full diversity,
systematically constructed over arbitrary constellations for any number of
transmit antennas are introduced. The codes are obtained by generalizing the
existing ABBA STBCs, a.k.a quasi-orthogonal STBCs (QO-STBCs). Furthermore, a
fully orthogonal (symbol-by-symbol) decoder for the new generalized ABBA
(GABBA) codes is provided. This remarkably low-complexity decoder relies on
partition orthogonality properties of the code structure to decompose the
received signal vector into lower-dimension tuples, each dependent only on
certain subsets of the transmitted symbols. Orthogonal decodability results
from the nested application of this technique, with no matrix inversion or
iterative signal processing required. The exact bit-error-rate probability of
GABBA codes over generalized fading channels with maximum likelihood (ML)
decoding is evaluated analytically and compared against simulation results
obtained with the proposed orthogonal decoder. The comparison reveals that the
proposed GABBA solution, despite its very low complexity, achieves nearly the
same performance of the bound corresponding to the ML-decoded system,
especially in systems with large numbers of antennas.
