Unitary Query for the MIMO Backscatter RFID Channel

A multiple-input multiple-output backscatter radio frequency identification (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 × L × N setting in RFID can bring spatial diversity and has been studied with the use of space-time code (STC) at the tag end. Current research generally has ignored query signaling as a means to improve performance. Here we propose a novel unitary query scheme, which creates time diversity within the channel coherent time and can yield significant performance improvements. To overcome the difficulty of evaluating the performance when 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 to show that unitary query has superior performance. Simulations show that unitary query can bring 5-10 dB gain in mid signal-to-noise ratio regimes. In addition, different from the conventional uniform query case, unitary query can also improve the performance of single-antenna tags significantly, enabling single-antenna tags with low complexity and small size to be used for high performance.