On the search for high-rate quasi-orthogonal space-time block code

Quasi-Orthogonal Space-Time Block Code (QO-STBC) may be preferred to Orthogonal STBC as it can achieve a higher code rate and to non-orthogonal STBC as it can achieve a lower decoding complexity. In this paper, the algebraic structure of QO-STBC is derived. The search of QO-STBC with rate greater than one for four transmit and at least two receive antennas is carried out. With the aids of graph theory, it is found that if the transmit diversity level is four, the maximum code rate is limited to 5/4; while if the transmit diversity level is lowered to two, the maximum code rate can go up to 4. The maximum likelihood decoding of these QO-STBCs can be computed in two separated groups, hence lead to a lower decoding complexity as compared to other non-orthogonal STBCs.