Singulation

Singulation

Singulation is the process by which an RFID reader identifies and communicates with individual tags one at a time from a population of tags that are all within range simultaneously. Without singulation, multiple tags would respond at once, their signals would collide, and the reader would be unable to decode any individual response. The singulation protocol is therefore the backbone of reliable RFID inventory operations.

The Anti-Collision Problem

When a reader transmits a Query command, every tag in the field that meets the query criteria is eligible to respond. If two or more tags transmit their EPC simultaneously, the reader observes a garbled signal — an event called a collision. The singulation algorithm's job is to serialise these responses so that each tag gets an uncontested time slot to reply.

EPC Gen2 uses a slotted-random anti-collision approach built around the Q-algorithm. The reader broadcasts a Q value that determines the number of available slots (2^Q). Each tag independently picks a random slot number, counts down as the reader advances through slots, and responds when its counter reaches zero.

Inventory Rounds

An inventory round begins when the reader issues a Query command with a target flag (A or B) and a session value. Tags matching the criteria enter the round. As each tag is successfully singulated and acknowledged, it flips its session flag so it will not respond again in the current round. The reader continues issuing QueryRep commands to advance through remaining slots until it detects enough empty slots to conclude the round.

Efficient singulation depends on choosing the right Q value. If Q is too low, many collisions occur; if Q is too high, too many slots are empty and time is wasted. The Q-algorithm dynamically adjusts Q based on observed collision and empty-slot rates.

Impact on System Design

Singulation speed directly determines the aggregate read rate of the system. A well-tuned reader reading a population of 500 tags can complete a full inventory round in under two seconds using FM0 encoding, or five to eight seconds in dense reader mode with Miller-4. For portal reader applications where items pass through the field quickly, the singulation window may be only 500 ms, requiring careful tuning of Q, session, and transmit power.

Understanding singulation mechanics is essential for anyone designing conveyor tunnels, dock-door portals, or high-density item-level tagging deployments where thousands of tags must be inventoried within tight time constraints.