Abstract: This paper presents the trellis decoding scheme of uncoded DUSTM system in the situation where its constellations form a group under the matrix multiplication.The system structure of DUSTM can be considered as a trellis structure which has full transition branches with the cardinality of the constellation as the number of the trellis states.Thus the soft Viterbi decoding algorithm can be performed.It is shown that more coding advantages can be gotten compared with the traditional uncoded DUSTM which is decoded by maximum likelyhood decoding based on single symbol.Since no constellation expansion，the scheme has higher spectral efficiency against the combined DUSTM at the expense of some performance.