Abstract: Based on cyclic diagonal codes, an improved design of unitary space-time codes (USTC) at low SNR is proposed. What makes the new design different from cyclic diagonal codes is that the nonzero entries of unitary matrices in the signal constellations appear at the leading diagonal and the minor diagonal alternately. Theoretical analysis shows that the union bound (UB) on symbol error probability (SER) of the proposed USTC is smaller than that of cyclic diagonal codes. To further improve performance of the proposed design, its parameters are optimized to minimize UB on SER. Simulation results demonstrate superiority of the proposed method over cyclic diagonal codes in a wide range of SNR.

Key words: unitary space-time codes, union bound on SER, MIMO system, differential modulation