  A MIMO backscatter RFID system consists of three operational ends: the query
end (with $M$ reader transmitting antennas), the tag end (with $L$ tag
antennas) and the receiving end (with $N$ reader receiving antennas). Such an
$M \times L \times N$ setting in RFID can bring spatial diversity and has been
studied for STC at the tag end. Current understanding of the query end is that
it is only an energy provider for the tag and query signal designs cannot
improve the performance. However, we propose a novel \textit{unitary query}
scheme, which creates time diversity \emph{within channel coherent time} and
can yield \emph{significant} performance improvements. To overcome the
difficulty of evaluating the performance when the unitary query is employed at
the query end and STC is employed at the tag end, we derive a new measure based
on the ranks of certain carefully constructed matrices. The measure implies
that the unitary query has superior performance. Simulations show that the
unitary query can bring $5-10$ dB gain in mid SNR regimes. In addition, the
unitary query can also improve the performance of single-antenna tags
significantly, allowing employing low complex and small-size single-antenna
tags for high performance. This improvement is unachievable for single-antenna
tags when the conventional uniform query is employed.
