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A MIMO backscatter RFID system consists of three operational ends: the queryend (with $M$ reader transmitting antennas), the tag end (with $L$ tagantennas) 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 beenstudied for STC at the tag end. Current understanding of the query end is thatit is only an energy provider for the tag and query signal designs cannotimprove the performance. However, we propose a novel \textit{unitary query}scheme, which creates time diversity \emph{within channel coherent time} andcan yield \emph{significant} performance improvements. To overcome thedifficulty of evaluating the performance when the unitary query is employed atthe query end and STC is employed at the tag end, we derive a new measure basedon the ranks of certain carefully constructed matrices. The measure impliesthat the unitary query has superior performance. Simulations show that theunitary query can bring $5-10$ dB gain in mid SNR regimes. In addition, theunitary query can also improve the performance of single-antenna tagssignificantly, allowing employing low complex and small-size single-antennatags for high performance. This improvement is unachievable for single-antennatags when the conventional uniform query is employed.

Unitary Query for the $M \times L \times N$ MIMO Backscatter RFID Channel