Abstract: The new encoding algorithm for the unitary space-time scheme with four transmit antennas is proposed. By combining the unit matrix with the space-time block code matrix, maximum likelihood decoding at the receiver is simplified to maximum Euclidean distance decoding so that complexity for decoding is reduced greatly. On the basis of analysis on the receive SNR in the Rayleigh channel, the lower bound of the capacity is deduced. Analysis and simulation results show that full diversity gain is guaranteed by the scheme, the number of times of complex number calculation for decoding is reduced by 85.6％ while the bit error rate of this scheme is approximate to that of the maximum likelihood decoding algorithm. And the lower bound of channel capacity coincides with the one deduced theoretically.

Key words: MIMO, unitary space-time codes, singular value, capacity