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One of the main problems which lower the efficiency of passive UHF RFID system is the collision between the tags. Current solutions are based on collision avoidance using MAC Layer Protocols (e.g. FSA, BT algorithms), these can be time consuming process because they discard the signals of Physical Layer and then require signal retransmission. It's therefore an issue of great importance to develop anti-collision algorithms based on physical layer. In this preliminary work, we show how to separate multiple UHF RFID tag signals based on physical layer characteristics. DAZF (Dual Antenna Zero forcing) and DAMMSE (Dual antenna receiver using Minimum -Mean-Square-Error) are two approaches which exploit the spatial signatures in order to separate the collided signals into the components corresponding to the single tags. Performance results are compared using different simulation scenarios.