Unitary Differential Space-Time Modulation with Joint Modulation
  We develop two new designs of unitary differential space-time modulation (DSTM) with low decoding complexity. Their decoder can be separated into a few parallel decoders, each of which has a decoding search space of less than sqrt(N) if the DSTM codebook contains N codewords. Both designs are based on the concept of joint modulation, which means that several information symbols are jointly modulated, unlike the conventional symbol-by-symbol modulation. The first design is based on Orthogonal Space-Time Block Code (O-STBC) with joint constellation constructed from spherical codes. The second design is based on Quasi-Orthogonal Space-Time Block Code (QO-STBC) with specially designed pair-wise constellation sets. Both the proposed unitary DSTM schemes have considerably lower decoding complexity than many prior DSTM schemes, including those based on Group Codes and Sp(2) which generally have a decoding search space of N for a codebook size of N codewords, and much better decoding performance than the existing O-STBC DSTM scheme. Between two designs, the proposed DSTM based on O-STBC generally has better decoding performance, while the proposed DSTM based on QO-STBC has lower decoding complexity when 8 transmit antennas. 
Chau Yuen, Yong Liang Guan, T. T. Tjhung