MU-MIMO for Passive UHF RFID

Radio Frequency Identification (RFID) is frequently deployed in high tag density environments, where tag read rate can become a limiting factor. Current Class 1 Gen 2 (C1G2) RFID systems are limited in read rate by the Framed Slotted Aloha (FSA) scheduling algorithm and physical layer modulation parameters. We propose a multi-user MIMO (MU-MIMO) RFID system compatible with C1G2 which enables simultaneous communication with multiple tags, achieving greater read rates. Multiple monostatic reader antennas are exploited to recover collided tag data and perform channel estimation. These channel estimates are then used to precode the reader's ACK signals across multiple transmit antennas into spatial channels such that the tags will receive separated acknowledgements. To evaluate potential performance gains, we calculate theoretical throughput improvements and empirically measure the signal-to-interference ratio (SIR) required for commercial passive tags to respond to collided acknowledgements. Furthermore, we perform simulations to determine the effect of increasing number of tag responses on channel estimation accuracy, and hence the received SIR at tags. An experiment is carried out using two monostatic transceivers with two emulated tags, showing successful channel recoveries and uncollided reader acknowledgments commands at the tags, and hence compatability with C1G2 protocol provided a reader can be developed meeting the timing requirements.