Abstract: A fast searching decoder algorithm of the Unitary Space-Time Modulation is presented for the constellations created by the systematic method. The receiver creates a constellation and multiplies it with the received signal by each dimension separately. Then all the elements are added up according to a certain mode, and the receiver can get four correlations. Only by some additions and inner norms, can the new scheme save the multiplications in the Maximum Likelihood (ML) algorithm when the rotating matrix is generated and the multiplication between the rotating matrix and the base constellation, and it can also save the multiplication between the three constellations and the received signal. It is proved that the new scheme can reduce the searching scope of the ML algorithm by about 75％ without degradation of the performance.

Key words: unitary space-time modulation, maximum likelihood detection, decoder