Anti-Collision

Anti-Collision in RFID

Anti-collision refers to the set of protocol mechanisms that prevent signal interference when multiple RFID tags attempt to respond to a reader simultaneously. In any deployment where more than one tag is within the reader's interrogation zone — which is virtually every real-world scenario — anti-collision is what makes reliable communication possible.

Why Collisions Happen

An RFID reader broadcasts a query, and every tag within range that meets the query criteria is entitled to respond. Because passive tags have no way to sense whether other tags are also transmitting, they respond independently. If two or more tags modulate their backscatter at the same time on the same channel, the reader receives a superposition of signals that it cannot decode. This is a collision.

Collisions waste time and energy. In a warehouse portal reader scanning a pallet of 100 tagged cases, unresolved collisions could mean missed items and inaccurate inventory counts.

Anti-Collision Algorithms

RFID systems use two primary families of anti-collision algorithms:

Probabilistic (ALOHA-based): EPC Gen2 employs a slotted ALOHA derivative controlled by the Q-algorithm. The reader defines 2^Q time slots; each tag randomly selects a slot and transmits only when its slot is active. Collisions still occur when two tags pick the same slot, but the probability decreases as Q increases. The reader dynamically adjusts Q to optimise throughput.

Deterministic (tree-walking): ISO 18000-63 supports an optional binary tree search where the reader progressively narrows the tag population by querying sub-ranges of tag identifiers. This guarantees every tag is eventually isolated, but it is slower for large populations and is rarely used in mainstream UHF deployments.

Session Flags and Target

EPC Gen2 extends anti-collision with session flags (S0-S3) and A/B target states. Once a tag is successfully read, it flips its session flag so it is excluded from the current inventory round. This prevents already-read tags from consuming slots and colliding with unread tags — a form of progressive population reduction.

Practical Implications

System integrators should consider tag population density, conveyor speed, and reader dwell time when tuning anti-collision parameters. A higher Q value reduces collisions but increases the time to complete an inventory round. For high-speed conveyor applications in logistics or retail, the reader firmware typically implements auto-Q tuning that adapts in real time.

Antenna design also affects collision rates. Near-field antennas with tightly controlled interrogation zones limit the number of tags in the field at any moment, reducing the anti-collision workload and enabling faster singulation.