Block-Level Unitary Query: Enabling Orthogonal-Like Space-Time Code With Query Diversity for MIMO Backscatter RFID

Future backscatter RFID systems are required to be more reliable and data intensive for emerging fields such as Internet of Things (IoT). 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 MIMO backscatter RFID. Also a recently proposed scheme called unitary query was shown to be able to considerably improve the reliability of MIMO backscatter RFID by exploiting query diversity. Therefore, incorporating the classical OSTBC (at the tag end) with the unitary query (at the query end) seems promising. However, in this paper, we first show that simple direct employment of OSTBC together with unitary query incurs a linear decoding problem and eventually leads to severe performance degradation. As a redesign of the unitary query idea 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 MIMO backscatter RFID.