Some simple observations on MISO codes

This paper considers certain aspects of some well-known multiple-input single-output (MISO) codes. In the first section it is proved how in some special cases the n + 1 MISO channel can be seen as consisting of several parallel MISO channels having less transmit antennas. It is also pointed out that unitary conjugation does not change the diversity-multiplexing tradeoff (DMT) of a code. These simple results are then applied to analyze the DMT of several well-known MISO codes. In particular its is proved that all the considered codes are DMT optimal. As a by-product of this study it is seen that the full-diversity quasi-orthogonal codes by Su and Xia are unitarily equivalent to division algebraic constructions. This relation is then used to place the constructions by Su and Xia into a wider context. In the latter part of the paper the 2 + 1 slow fading MISO channel is considered and it is proven that one of the previously proposed MISO multi-block codes (MB-codes) has a linear worst-case sphere decoding complexity.