Radio Frequency Identification (RFID) is often used in high tag density scenarios where tag read rate becomes 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 is capable of communicating with more than one tag simultaneously allowing greater read rates. Multiple reader receiver antennas are exploited to recover collided tag data and perform channel estimation. The channel estimates are used to precode the reader ACK signals from multiple transmit antennas into spatial channels so that the tags will receive fully separated acknowledgements. 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.