Tag identification time in multiantenna collision scenarios

Framed Slotted Aloha (FSA) is often employed for scheduling tag´s transmissions in Radio Frequency Identification (RFID) systems. Tags choose one of the offered slots for transmission and it can happen that some of the slots are empty, some have just one tag active - singleton slots and in some of the slots several tags are transmitting - collision slots. A standard compliant reader is capable of acknowledging just the tags from singleton slots. If a collision recovery is employed, even some of the tags from the collision slots can be acknowledged. This requires certain changes in the standard as modification of a tag signal and consequently the change of slot durations. In this paper, we propose two different ways of acknowledging colliding tags and we derive analytical formulas for calculating the expected frame duration in collision scenarios and the expected number of identified tags. Furthermore, we evaluate the proposed schemes by means of Monte Carlo simulations. The obtained results are compared with the performance of a standard compliant reader.