Radio Frequency Identification (RFID) is a wireless identification technology which often operates in environments with multiple RFID tags. Already introduced multiple antenna receivers can recover from a collision of up to M tags as long as M is less than the number of receive antennas and the channel is known at the receiver. This paper proposes a Zero-Forcing (ZF) and a Minimum Mean Square Error (MMSE) receiver which allows the separation of up to M=2NR tags, where NR is the number of receiving antennas on the reader. The proposed algorithms are verified through simulations.

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In this work, we focus on framed slotted Aloha (FSA) and passive ultra high-frequency radio frequency identification multi-antenna systems with physical layer collision recovery. We modify the tags slightly by adding a so-called 'postpreamble' that facilitates channel estimation. Furthermore, we investigate the throughput performance of advanced receiver structures in collision scenarios. More specifically, we analyse the throughput of FSA systems with up to four receive antennas that can recover from a collision of up to eight tags on the physical layer and acknowledge all tags involved in that collision. Due to the higher collision recovery capabilities, the frame sizes can be significantly reduced, and thus, the throughput can be increased. We also derive analytically optimal frame sizes, given that a certain number of collisions can be resolved. We further study the constraints to the throughput due to the structure of our receiver and channel estimation for different collision scenarios. Furthermore, we propose a novel collision recovery method with two phases: first, a successive interference cancellation and, second, a projection of the constellation into the orthogonal subspace of the interference. Additionally, the inventory time, i.e. the number of slots necessary to successfully decode all tags in the reader range, is calculated and compared for different receiver types. A validation of our theoretical predictions is achieved by means of simulations. We show that by our proposed methods, we can realistically achieve more than ten times higher throughput or, equivalently, a reduction of the inventory time by more than 90%.

Radio Frequency Identification (RFID) systems usually operate in a multiple RFID tag environment. If multiple tags respond simultaneously, a collision occurs and the information is discarded. In this paper, we propose a novel physical layer collision recovery mechanism. We analyse theoretical limits of the inventory time and show how our new method can approach such theoretical maxima. We propose a method that resolves more colliding tags by making collisions less destructive. In this method a postprocessing of the received signal by beamforming is performed. Furthermore, we investigate the influence of the proposed method on the system performance by means of simulations. We show that our method considerably shortens the time necessary to interrogate tags.