Tradeoffs between rate, puncturing and orthogonality in space-time block codes

We investigate methods to improve the rate of complex modulation space-time block codes for more than two Tx antennas. For concreteness we concentrate on 4 Tx antennas, for which the maximal rate for an orthogonal space-time block code is 3/4. We devise multi-modulation schemes, which increase the rate to 7/8 or 1. In these codes, the orthogonality of QPSK modulation is sacrificed for some symbols, but the orthogonality of the underlying space-time block code is kept intact. An increase in rate leads to a decrease in performance. The same is true if the rate increase is effected by sacrificing the orthogonality of the space-time block code itself. To assess the capacity enhancement due to the rate increase, space-time block codes with different rates are concatenated with turbo codes, with a puncturing scheme guaranteeing the same effective code rates for the concatenated systems. It is shown that with a constant rate for the concatenated scheme, increasing the space-time block code rate indeed improves the performance of the concatenated scheme. Also, it is shown that sacrificing the orthogonality of the space-time block code itself is more effective than sacrificing the orthogonality of the modulation