At the beginning of the frame, the reader initializes Q with
bit strings of CQ and makes CQ empty. If CQ does not have any bit string (e.g., when the
reader resets), Q is initialized with two 1 bit strings, 0 and 1. The reader dequeues a bit
string from Q and transmits a query at a time. The reader enqueues some of the used
bit strings into CQ according to the result of receiving tag responses. The identi?¬?cation
frame continues until Q is empty.
8.5.1 Query Insertion
Let q1q2 . . . qx be the bit string of the transmitted query. According to the number of tags
responses, the reader acts as follows:
Adaptive Tag Anticollision Protocols for RFID Passive Tags 145
. Idle query (number of tag responses??0): The reader enqueues q1q2 . . . qx into CQ.
. Readable query (number of tag responses??1): The reader enqueues q1q2 . . . qx
into CQ.
. Collision query (number of tag responses 2): The reader enqueues q1q2 . . . qx0
and q1q2 . . . qx1 into Q.
The reader enqueues idle queries as well as readable queries into CQ. Therefore, CQ has
all the leaf nodes in the tree at the end of the frame. By maintaining CQ, the tree search of
AQS begins at the leaf nodes in the tree of the last frame and skips nodes whose queries
caused tag collisions in the last identi?¬?cation frame. Since some arriving tags may not
match any nodes of readable cycles of the last frame, the tree search of AQS starts not only
at the nodes of readable cycles but also at the nodes of idle cycles of the last frame.
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