A hybrid transmit antenna/code selection scheme using space-time block codes

Complex orthogonal space-time block codes for more than two transmit antennas cannot achieve full rate over the fading channel. By relaxing the orthogonality constraint, however, it is possible to achieve full rate or even rates higher than one for any number of transmit antennas at the expenses of loosing some degree of diversity advantage. On the other hand, if a feedback channel is available and the fading coefficients are known at the transmitter, then maximum diversity advantage can be achieved with transmit antenna selection. In this paper, a new full-rate nonorthogonal space-time block code for three transmit antennas is proposed that needs only two RF chains, has decoding delay equal to two, and whose maximum likelihood decoder based on linear processing is the same as that of the Alamouti code. We then introduce hybrid transmit antenna/code selection. Based on the instantaneous fading coefficients, the transmitter selects either the proposed code with the best three antennas or the Alamouti code with the best pair of antennas. Simulations results for up to six transmit antennas and one receive antenna indicate that full diversity is achieved in spite of using a nonorthogonal code. With this simple example, coding gains of up to 0.4 dB over the pure antenna selection with Alamouti, proposed by Gore and Paulraj (2002), is also observed.