Low Complexity Unitary Differential Space-Time Modulation with Spherical Code

We show a new design of unitary differential space-time modulation (DSTM) with low decoding complexity, i.e. its decoding can be performed by two parallel decoders where each of the decoder has a search space of only radicN for a codebook with N codewords. The design is based on orthogonal space-time block code (O-STBC) and spherical code. We separate the symbols of an O-STBC into two groups and jointly modulate the symbols within a group using a joint constellation set constructed from spherical code. The proposed unitary DSTM scheme has a lower decoding complexity than many other DSTM schemes including those based on group codes and Sp(2) with better or comparable decoding performance. It also achieves a better performance than existing DSTM schemes with similar decoding complexity