Unitary Differential Space–Time Modulation With Joint Modulation

We develop two new designs of unitary differential space–time modulation (DSTM) with a low decoding complexity. Their decoder can be separated into a few parallel decoders, each of which having a decoding search space of less than√Nif the DSTM codebook containsNcodewords. 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 an orthogonal space–time block code (O-STBC) with joint constellation constructed from spherical codes. The second design is based on a quasi-orthogonal STBC (QO-STBC) with specially designed pairwise constellation sets. Both the proposed unitary DSTM schemes have a considerably lower decoding complexity than many prior DSTM schemes, including those based on group codes and Symplectic group Sp(2), which generally have a decoding search space ofNfor a codebook size ofNcodewords and a much better decoding performance than the existing O-STBC DSTM scheme. Between the two designs, the proposed DSTM based on the O-STBC generally has a better decoding performance, whereas the proposed DSTM based on the QO-STBC has a lower decoding complexity for eight transmit antennas.