  Full-rate space time codes (STC) with rate = number of transmit antennas have
high multiplexing gain, but high decoding complexity even when decoded using
reduced-complexity decoders such as sphere or QRDM decoders. In this paper, we
introduce a new code property of STC called block-orthogonal property, which
can be exploited by QR-decomposition-based decoders to achieve significant
decoding complexity reduction without performance loss. We show that such
complexity reduction principle can benefit the existing algebraic codes such as
Perfect and DjABBA codes due to their inherent (but previously undiscovered)
block-orthogonal property. In addition, we construct and optimize new full-rate
BOSTC (Block-Orthogonal STC) that further maximize the QRDM complexity
reduction potential. Simulation results of bit error rate (BER) performance
against decoding complexity show that the new BOSTC outperforms all previously
known codes as long as the QRDM decoder operates in reduced-complexity mode,
and the code exhibits a desirable complexity saturation property.
