  Because of the emerging field of Internet of Things (IoT), future backscatter
RFID is required to be more reliable and data intensive. Motivated by this,
orthogonal space-time block code (OSTBC), which is very successful in mobile
communications for its low complexity and high performance, has already been
investigated for backscatter RFID. On the other hand, a recently proposed
scheme called unitary query was shown to be able to considerably improve the
reliability of backscatter radio by exploiting query diversity. Therefore
incorporating the classical OSTBC (at the tag end) with the recently proposed
unitary query (at the query end) seems to be promising. However, in this paper,
we show that simple, direct employment of OSTBC together with unitary query
incurs a linear decoding problem and eventually leads to a severe performance
degradation. As a re-design of the recently proposed unitary query and the
classical OSTBC specifically for MIMO backscatter RFID, we present a
BUTQ-mOSTBC design pair idea by proposing the block-level unitary query (BUTQ)
at the query end and the corresponding modified OSTBC (mOSTBC) at the tag end.
The proposed BUTQ-mOSTBC can resolve the linear decoding problem, keep the
simplicity and high performance properties of the classical OSTBC, and achieve
the query diversity for the $M \times L \times N$ MIMO backscatter RFID
channel.
