Novel Rotated Quasi-Orthogonal Space-Time Block Codes With the Fixed Nearest Neighbor Number

In this letter, we consider a coherent communication system equipped with multiple transmitter antennas and a single receiver antenna, i.e., a MISO system. For such a system, quasi-orthogonal STBC designs with fast maximum likelihood (ML) decoding have been proposed [1]-[3]. However, full diversity cannot be achieved by such codes. The rotated quasi- orthogonal STBC [15] overcomes such shortcomings and enables full diversity and optimal coding gain. However, for large constellations, the performance of such codes deteriorates due to the increase of the number of the nearest neighbor per symbol. In this letter, we propose to add the number of nearest neighbor as a design criterion. We show that for the rotated quasi- orthogonal code proposed in [4], the number of nearest neighbor tends to infinity when the size of constellation becomes infinite. Furthermore, we propose to have a particular value of rotation and show that this value not only achieves full diversity and maximum coding gain, but also has only a small number of nearest neighbors even for very large constellations.