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Radio Frequency Identification (RFID) systems often are operated in environments with multiple RFID tags. In such an environment, a conventional RFID system resolves collisions of multiple tags on the medium access control layer, discarding the signals of the physical layer. This paper proposes a zero-forcing and an interference cancellation receiver architecture for an RFID reader, to recover from collisions of two tags on a physical layer and identify tags successfully even in case of a collision. The expected throughput increase is approximately 1.6 times the throughput of a conventional reader. We explore the signal properties of collisions and propose a model for the physical layer. Moreover, we present a method for estimating the signal constellation states in a collision. The entire structure, including channel estimation and both of the proposed receivers are verified with data generated during a measurement. Additionally, performance simulations of the two structures with different channels are shown.