Quasi-orthogonal space-time coding for backscatter communications with multiple-antenna tags

Existing orthogonal space-time block coding (OSTBC) schemes for backscatter communication systems cannot achieve a full transmission code rate when the tag is equipped with more than two antennas. In this paper, we propose a quasi-orthogonal spacetime block code (QOSTBC) that can achieve a full transmission code rate for backscatter communication systems with a four-antenna tag and then extend the scheme to support tags with 2ⁱ antennas. Specifically, we first present the system model for the backscatter system. Next, we propose the QOSTBC scheme to encode the tag signals. Then, we provide the corresponding maximum likelihood detection algorithms to recover the tag signals. Finally, simulation results are provided to demonstrate that our proposed QOSTBC scheme and the detection algorithm can achieve a better transmission code rate or symbol error rate performance for backscatter communication systems compared with benchmark schemes.